# Seminars (QIS)

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Event When Speaker Title Presentation Material
QIS 18th August 2004
14:00 to 15:00
Quantum measurements and finite geometry
QIS 19th August 2004
14:00 to 15:00
M Ben-Or Fast quantum Byzantine agreement
QISW01 23rd August 2004
10:10 to 11:00
Picturing qubits in phase space

In this talk I present a discrete-phase-space description of a system of qubits, in which each phase-space axis is labeled by the elements of a finite field and the state of the system is represented by a real function on phase space--a discrete Wigner function. Each set of parallel lines in the phase space corresponds to an orthogonal basis for the state space, and bases corresponding to different sets of parallel lines are mutually unbiased. I discuss the representation of quantum gates in this framework, and the problem of recognizing entanglement

QISW01 23rd August 2004
11:30 to 12:20
Entanglement loss along renormalization group flows
QISW01 23rd August 2004
14:00 to 14:50
Secure key from bound entanglement

The general class of quantum states which are unconditionally private are introduced. This allows us to consider the production of a secure key within the same paradigm as entanglement theory by making one change -- instead of distilling singlets we distill these private states. It is then shown that an arbitrarily secure key can be distilled from bound entangled states. In general the amount of distillable key is no greater than the relative entropy of entanglement.

QISW01 23rd August 2004
15:30 to 16:20
Quantum spin chains from the perspective of quantum information theory

We study the concept of valence bond states from the perspective of entanglement theory. This leads to several generalizations with powerful applications for the simulation of quantum spin systems. In particular, we extend the powerful Density Matrix Renormalization Group (DMRG) simulation method to the cases of periodic boundary conditions, finite-T, time-evolution, and 2 dimensions. We also discuss the relationship between entanglement and correlations in those systems, leading to a natural definition of entanglement length.

QISW01 23rd August 2004
16:20 to 17:10
L Vaidman Quantum versus classical, qubits versus bits
Methods for measuring an integral of a classical field via local interaction of classical bits or local interaction of qubits passing through the field one at a time are analyzed. A quantum method, which has an exponentially better precision than any classical method we could see, is described. An alternative quantum method using only one particle is proposed.
QISW01 24th August 2004
09:20 to 10:10
Robust polynomials and quantum algorithms

We define and study the complexity of robust polynomials for Boolean functions and the related fault-tolerant quantum decision trees, where input bits are perturbed by noise. We show that, in contrast to the classical model of Feige et. al., every Boolean function can be computed by O(n) quantum queries even in the model with noise. This implies, for instance, the somewhat surprising result that every Boolean function has robust degree bounded by O(n).

This joint work with Ilan Newman, Hein Roehrig, and Ronald de Wolf

QISW01 24th August 2004
10:10 to 11:00
J Kempe The complexity of local Hamiltonians

This is joint work with Alexei Kitaev and Oded Regev.

Complexity theory formalises the notion of an _efficient_ algorithm. A major challenge for complexity theory is to understand the interrelation between classical and the newly emerging quantum complexity classes.

The k-local Hamiltonian problem is a natural complete problem for the complexity class QMA, the quantum analog of NP. It had been known that 3-local Hamiltonian is QMA-complete; whereas 1-local Hamiltonian is in P, efficiently solvable and hence not believed to be QMA-complete. The exact complexity of the 2-local problem has so far been unknown.

We will introduce rigorous perturbation theory techniques to complexity theory and show that 2-local Hamiltonian is QMA complete. Our techniques also show that 2-local _adiabatic_ computation on qubits is equivalent to standard quantum computation. We hope that our physics inspired technique might prove useful elsewhere in (quantum) computer science. We outline some open problems and future directions.

QISW01 24th August 2004
11:30 to 12:20
Quantum algorithms and phase estimation

QISW01 24th August 2004
15:30 to 16:20
R Laflamme NMR quantum information processing

To follow

QISW01 25th August 2004
09:20 to 10:10
Entanglement \& area

We revisit the question of the relation between entanglement, entropy, and area for harmonic lattice Hamiltonians which are the discrete version of real free scalar fields. For the cubic harmonic lattice Hamiltonian which yields the real Klein Gordon field in the continuum limit we establish a strict relationship between the surface area of a distinguished hypercube and the degree of entanglement between the hypercube and the rest of the lattice, without resorting to numerical means. We outline extensions of these results to longer ranged interactions, finite temperatures and for classical correlations in classical harmonic lattice systems.

QISW01 25th August 2004
10:10 to 11:00
Proposal for a loophole-free Bell test using homodyne detection

In their seminal 1935 paper, Einstein, Podolsky, and Rosen advocated that if "local realism" is taken for granted, then quantum theory is an incomplete description of the physical world. The EPR argument gained a renewed attention in 1964 when John Bell derived his famous inequalities, which must be satisfied within the framework of any local realistic theory. The violation of Bell inequalities, predicted by quantum mechanics, has since then been observed in many experiments, thereby disproving the concept of local realism. So far, however, all these tests suffered from "loopholes" allowing a local realistic explanation of experimental observations by exploiting either the low detector efficiency or the time-like interval between the two detection events.

Here, I will report on an experimentally feasible optical setup that potentially allows for a loophole-free Bell test by using highly efficient homodyne detectors. A non-gaussian entangled state of two light beams is generated from a two-mode squeezed vacuum by subtracting a single photon from each mode with the help of standard single-photon detectors.

A Bell violation exceeding 1% is achievable for a 6dB squeezed light source using single-photon detectors with an efficiency as low as 10%, provided that the homodyning efficiency lies around 95%. Given the recent demonstration of photon subtraction from pulsed single-mode squeezed states, we envision that this proposal may lead to the first complete test of Bell violation in a foreseeable future.

QISW01 25th August 2004
11:30 to 12:20
Entanglement and decoherence in coined quantum walks

Quantum walks, both discrete (coined) and continuous time, form the basis of several recent quantum algorithms. We review the specific quantum properties of quantum walks and their sensitivity to decoherence. We then examine the entanglement properties of quantum walks in various dimensions (lines, 2D lattices and trees) using entropic characterizations generated from the subsystem density matrices of the walker position-coin state correlations.

QISW01 26th August 2004
09:20 to 10:10
Quantum and secret correlations

Quantum and secret correlations are two valuable resources in Quantum Information Theory and Cryptography, respectively. They both have the property of being monogamous: the more two parties share secret or quantum correlations, the less they are coupled to the environment. The analogies between these two resources become more evident if one compares the usual quantum (secret) correlation manipulation scenario: N parties share a quantum state rho_A1AN (probability distribution P(A1,...,AN)) that is also entangled (correlated) to the environment (Eve). From an operational point of view, one would like to know 1) how many entangled bits (secret bits) are required for the preparation of the state (probability distribution) and 2) how many entangled bits (secret bits) can be extracted, or distilled, from the state (probability distribution). Exploiting these analogies, the following results can be proven: 1) All two-qubit entangled states allow a secure key distribution when Alice, Bob and Eve perform operations at the single-copy level. 2) The preparation of a probability distribution requires entanglement if and only if secret bits are consumed in an alternative preparation using only classical means. This implies that all the entangled states, independently of their distillability properties, can be mapped into probability distributions containing secret correlations. 3) There exists a cryptographic analog of bound entanglement, known as bound information. As it happens for bound entanglement, bound information can be activated: the mixture of non-distillable probability distributions can lead to a distillable one.

QISW01 26th August 2004
10:10 to 11:00
Decoy state quantum key distribution

Quantum key distribution (QKD) allows two parties to communicate in absolute security based on the fundamental laws of physics. Up till now, it is widely believed that unconditionally secure QKD with the standard Bennett-Brassard (BB84) protocol is only possible at rather low key generation rate and short distances. Previously proposed methods (including single-photon sources) to extend the distances of BB84 and increase key generation rate are mostly experimental and present daunting experimental challenges. Here, we present a simple theoretical idea that will achieve these goals by using only current technology. Our method is to develop substantially the decoy state idea of Hwang and combine it with standard entanglement distillation approach to security proofs. Our results show that secure QKD is possible at a key generation rate as high as $O (\eta)$ (as opposed to $O(\eta^2)$ in prior art) where $\eta$ is the overall transmission probability of the channel and fiber-based QKD can be made unconditionally secure over 100km.

In summary, we can have the best of both worlds---making the best use of our imperfect experimental apparatus and yet getting the strongest level of security--- unconditional security, which is the Holy Grail of quantum cryptography. Our method is, therefore, a significant step in bridging the big gap between the theory and practice of QKD.

QISW01 26th August 2004
11:30 to 12:20
N Gisin Simulation of singlet correlation without any communication, but using a weaker resource: a "non-local machine"

The importance of quantum entanglement is by now widely appreciated as a resource for quantum information applications. A unit of entanglement has been identifies and named e-bit; it consists of a pair of maximally entangled qubits, e.g. of a singlet: the same singlet that Bohm used in his version of the EPR paradox. A few years ago connection with communication complexity started to be studied, with question like how much communication is required to simulate an e-bit? From Bell inequality we know that it is impossible to simulate a singlet without any communication even if one assumes that both parties share local hidden variables, or in modern terminology, share randomness. Recently, Tonner and Bacon proved that actually a single bit of communication suffice for perfect simulation. Independently from the above story, Popescu and Rochlich raised the following question: can there be correlation stronger than the quantum mechanical ones that do not allow one to signal? They answered by showing a hypothetical non-local machine that does not allow signaling, yet violates the CHSH-bell inequality by the absolute maximal value of 4 (while quantum correlation achieve at most $2\sqrt{2}$. They concluded asking why Nature is non-local, but not maximally non-local, where the maximum would be only limited by the no-signaling constraint? It is straightforward to simulate the PR machine with a single bit of communication. Consequently, the PR nonlocal machine is a strictly weaker resource than a bit of communication. We show that singlets can be simulated using only one instance of the PR non-local machine. Hence, assuming that Nature is sparing with resources, one is be tempted to conclude that she is using something like the non-local machine. Finally, we raise the question whether correlations arising from partially entangled qubits can be simulated using only an e-bit?

QISW01 26th August 2004
14:00 to 14:50
A Kent Remarks on mistrustful quantum and relativistic cryptography

I review some ideas on what cryptographic tasks might or might not be implementable with security guaranteed by special relativity and/or quantum theory.

QISW01 26th August 2004
15:30 to 16:20
Provably secure experimental quantum bit string generation

Coin Tossing is the problem in which two parties who do not trust each other want to generate a random coin. Quantum communication allows the parties to generate a coin with bias less than ½, which is impossible classically. However even quantum communication cannot guarantee that the bits are perfectly random. Bit string generation is the generalisation of coin tossing when the two parties want to generate a large number n of coins. We discuss the theory of quantum bit string generation and show that much better security is possible than for quantum coin tossing. We also report on an experiment in which a string of bits is generated which more random than is possible classically.

QISW01 26th August 2004
16:20 to 17:10
A new inequality for the von Neuman entropy

Strong subadditivity of von Neumann entropy, proved in 1973 by Lieb and Ruskai, is a cornerstone of quantum coding theory. All other known inequalities for entropies of quantum systems may be derived from it. I will describe some work with Andreas Winter in which we prove a new inequality for the von Neumann entropy which is independent of strong subadditivity. This work sheds light on extremal types of entanglement for multi-party quantum states.

QISW01 27th August 2004
09:20 to 10:10
Vacuum entanglement

The properties of vacuum entanglement of a free relativistic field and of a Linear Harmonic-chain will be discussed. We shall also discuss a gedanken-experiment for detecting Bells inequalities violation in the vacuum, and a scheme for detecting ground-state entanglement in a linear ion trap.

QISW01 27th August 2004
10:10 to 11:00
AS Kholevo Additivity problem: an overview

A class of problems in quantum information theory, having simple formulation but still resisting general solution, concerns additivity properties of various quantities characterizing quantum channels, notably the "classical capacity", the "minimal output entropy" and the "entanglement of formation". All known results, including numerical work, are consistent with the conjecture that these quantities are additive with respect to tensor products of channels. A proof of this conjecture would have important consequences in quantum information theory. In particular, according to this conjecture, the classical capacity cannot be increased by using entangled inputs of the quantum channel. In this talk we review the current status of the additivity/multiplicativity problems, discuss relations between them and give a survey of relevant results and approaches.

QISW01 27th August 2004
11:30 to 12:20
S Lloyd Quantum computation and quantum gravity

This talk reviews known connections between quantum gravity and quantum information, including (a) entanglement and Hawking radiation, (b) teleportation and black hole evaporation, (c) bit creation during inflation, and (d) the holographic principle. After the review, I will present a theory of quantum gravity based on quantum computation that provides explicit models for all these phenomena within a framework that allows quantitative calculations.

QISW01 27th August 2004
14:00 to 14:50
Generic behaviour in quantum information theory: applications of the concentration of measure phenomenon

In that branch of quantum information theory which generalises Shannon's communication theory, it is the rule (just as with Shannon) that information theoretically optimal constructions are not actually "constructive". Instead, the probabilistic method is used to show that some random way to build a code or protocol will succeed with positive probability (so that there actually exists a good code) - in fact, this probability will usually be close to 1.

Thus, it is not surprising that quantitative laws of large numbers play a significant role in the theory, and in fact the most important ones are those which give exponential probability bounds on "large deviations": Cramer's and Azuma's inequality for the convergence of empirical means and martingales, as well as relatives of Talagrand's inequality have been used in classical information theory.

This talk will describe the status of these tools and their applications in quantum information theory: I will show how the concentration of measure phenomenon on Euclidean spheres leads to strong statements for the unitary group, and to strong results regarding state randomisation, remote state preparation, data hiding, and information on the typical entanglement properties of random states. [Several collaborative papers with Abeyesinghe, Bennett, Hayden, Leung, Shor and Smith on quant-ph.]

Furthermore, I will outline operator versions of Cramer's theorem [joint work with Ahlswede, IEEE IT 2002] and of (a simple variant of) Talagrand's isoperimetric inequality [work in progress], and illustrate those with some applications such as the quantum reverse Shannon theorem and a generalisation of a "local converse to the channel coding theorem" due in the classical case to Ahlswede and Dueck.

QISW01 27th August 2004
15:30 to 16:20
Applications of randomized techniques in quantum information theory

Quantum information theory is concerned with the asymptotic manipulation of quantum systems to perform useful information processing tasks. This talk will survey various communication capacities that have been proved in the past two years. Of particular focus is the usefulness of randomized techniques in these proofs.

QISW01 27th August 2004
16:20 to 17:10
High temperature macroscopic entanglement

The main subject of my talk will be investigation of the possibility of having high temperature entanglement in the macroscopic (thermodynamical) limit. I will first introduce the notions of entanglement and classical correlations for a general state involving any number of subsystems. I will then develop the theory for calculating both classical and quantum correlations for totally symmetric pure states of any number of qubits, as well as any subset and mixture of these. These states are important as they feature in some high T superconducting models. In the second half of the talk, I will be discussing the role of entanglement in macroscopic solid-state systems providing examples from simple models such as the Ising and Heisenberg one-dimensional spin chains. I will argue that these models cannot sustain high temperature entanglement  entanglement only exists close to the very low (critical) temperature. Then I will look at the electron pairing of Yangs (in the Hubbard Model and related models) used in high T superconductivity. Electron pairing is described by totally symmetric states mentioned above and I plan to discuss the relationship between the existence of off diagonal long range order  which ensures typical superconducting behaviour - and the existence of entanglement and classical correlations. The question of having macroscopic entanglement at high temperature is not only important practically, for information processing, but also has a fundamental significance, which  I believe - may have a long term effect on fundamental physics.

QISW03 6th September 2004
11:30 to 12:15
Decoy state quantum key distribution: the best of both worlds?

The standard weak coherent state implementation of quantum cryptography has non-trivial security issues because an eavesdropper can, in principle, suppress single-photon signals relative to multi-photon signals. This leads to fundamental limits to the key generation rate [of order O (\eta2), where \eta is the transmission probability of the channel] and distance of uncondiitonally secure quantum key distribution [of order 20km for Telecom fibers]. Here, we show that the decoy state, first suggested by Hwang, can be used to prove unconditional security in the standard entanglement distillation approach. Our result shows that rather surprisingly, by using only current technology, unconditionally secure quantum key distribution can be achieved with a key generation rate of order O (\eta) and substantially longer distances (more than double). In our method, Alice simply turns the power of her laser up and down and prepares decoy states. The statistical properties of the transmitted decoy states are then analyzed to derive constraints on the signal states.

QISW03 6th September 2004
12:15 to 13:00

Privacy amplification is the art of shrinking a partially secret string Z to a highly secret key S. We show that, even if an adversary holds quantum information about the initial string Z, the key S obtained by two-universal hashing is secure, according to a universally composable security definition. Additionally, we give an asymptotically optimal lower bound on the length of the extractable key S in terms of the adversary's (quantum) knowledge about Z. Our result has applications in quantum cryptography. In particular, it implies that many of the known quantum key distribution protocols are universally composable.

QISW03 6th September 2004
14:15 to 15:00
A generic security proof for quantum key distribution

Quantum key distribution allows two parties, traditionally known as Alice and Bob, to establish a secure random cryptographic key if, firstly, they have access to a quantum communication channel, and secondly, they can exchange classical public messages which can be monitored but not altered by an eavesdropper, Eve. Quantum key distribution provides perfect security because, unlike its classical counterpart, it relies on the laws of physics rather than on ensuring that successful eavesdropping would require excessive computational effort. However, security proofs of quantum key distribution are not trivial and are usually restricted in their applicability to specific protocols. In contrast, we present a general and conceptually simple proof which can be applied to a number of different protocols. It relies on the fact that a cryptographic procedure called privacy amplification is equally secure when an adversary's memory for data storage is quantum rather than classical.

QISW03 7th September 2004
11:30 to 12:15
N Gisin 1. A new protocol for high bit rates 2. A new type of security proofs

Main message: Q crypto is entering the industrial age, but basic research is still needed. Examples: 1. New Q crypto protocols should be designed taking into account realistic constraints. Examples are "continuous variables" and the SARG protocol [PRL 92,057901,2004]. In this talk we present another new protocol that aims at Gbit/sec rates.

2. It is desirable to simplify proofs of security against the most general coherent attack. We take advantage of some recent results in classical information based cryptography and on the symmetry of the 6-state protocol to demonstrate that coherent attacks are not more powerfull than collective attacks. Using this we present a new kind of security proof that improves over the Shor-Preskill result.

QISW03 7th September 2004
12:15 to 13:00
Relating formal security for classical and quantum protocols

Using a simple example we demonstrate the necessity of a formal modelling of security. We explain the established notion of simulatable security (e.g. reactive simulatability by Backes, Pfitzmann, Waidner, or universal composability by Canetti) and show how this model can be extended to encompass quantum security (cf. also the work of Ben-Or, Mayers). We conclude with a Quantum Embedding Theorem that allows to securely use quantum protocols as sub-protocols in classically secure protocols.

QISW03 7th September 2004
14:15 to 14:35
S Wehner Quantum anoymous transmissions

We consider the problem of hiding sender and recipient of classical and quantum bits, even if all physical transmissions can be monitored. We present a quantum protocol for sending and receiving classical bits anonymously, which unlike all classical protocols, prevents later reconstruction of the sender. It appears that entangled quantum states are uniquely suited for anonymous transmissions. We then extend this protocol to provide sender and recipient untraceability for transmissions of qubits as well. In the process we also introduce a new primitive called anonymous entanglement, which may be useful for other protocols as well.

QISW03 7th September 2004
14:35 to 14:55
R Colbeck Variable bias coin tossing

I will introduce the task of variable bias coin tossing, and describe two protocols for achieving it.

QISW03 7th September 2004
14:55 to 15:15
A simple no-go lemma

I describe a simple general no-go result in mistrustful quantum cryptography.

QISW03 8th September 2004
11:30 to 12:15
Non-local correlations and key distribution

We present a quantum key distribution scheme that is secure even against eavesdroppers who can break the laws of quantum mechanics, as long as superluminal signalling is impossible. The scheme is based on violation of a Bell inequality, and its security stems from the fact that non-local correlations are monogamous, in analogy with the monogamy of entanglement.

QISW03 8th September 2004
14:15 to 15:00
Graph states - stability under decoherence, entanglement purification and possible applications

We will consider a specific family of multipartite entangled states, namely (weighted) graph states. We investigate the influence of decoherence on the entanglement properties of the states for various decoherence models. We show that the lifetime of entanglement of GHZ states decreases with the number of particles, while cluster (and similar) graph states the lifetime is independent of the size of the system. We discuss several multiparty entanglement purification protocols which allow one to create and/or maintain two colorable graph states with high fidelity. We compare direct multiparty entanglement purification protocols with protocols based on bipartite purification and show that the former are more efficient. We investigate the influence of imperfections in local control operations, where we find a remarkable robustness against errors for protocols which allow one, e.g., to purify cluster states. The achievable fidelity using direct multiparty purification is higher than purification based on bipartite protocols. We discuss possible applications of multiparty entanglement purification protocols for secure multiparty communication.

QISW03 8th September 2004
15:30 to 16:15
On the key-uncertainty of quantum ciphers and the computational security of one-way quantum transmission

We consider the scenario where Alice wants to send a secret (classical) $n$-bit message to Bob using a classical key, and where only one-way transmission from Alice to Bob is possible. In this case, quantum communication cannot help to obtain perfect secrecy with key length smaller then $n$. We study the question of whether there might still be fundamental differences between the case where quantum as opposed to classical communication is used. In this direction, we show that there exist ciphers with perfect security producing quantum ciphertext where, even if an adversary knows the plaintext and applies an optimal measurement on the ciphertext, his Shannon uncertainty about the key used is almost maximal. This is in contrast to the classical case where the adversary always learns $n$ bits of information on the key in a known plaintext attack. We also show that there is a limit to how different the classical and quantum cases can be: the most probable key, given matching plain- and ciphertexts, has the same probability in both the quantum and the classical cases. We suggest an application of our results in the case where only a short secret key is available and the message is much longer. Namely, one can use a pseudorandom generator to produce from the short key a stream of keys for a quantum cipher, using each of them to encrypt an $n$-bit block of the message. Our results suggest that an adversary with bounded resources in a known plaintext attack may potentially be in a much harder situation against quantum stream-ciphers than against any classical stream-cipher with the same parameters.

QISW03 9th September 2004
11:30 to 12:15
Cheat sensitive bit commitment

We define cheat sensitive cryptographic protocols between mistrustful parties as protocols which guarantee that, if either cheats, the other has some nonzero probability of detecting the cheating. We give an example of an unconditionally secure cheat sensitive non-relativistic bit commitment protocol which uses quantum information to implement a task which is classically impossible.

QISW03 9th September 2004
12:15 to 13:00
Experimental quantum bit string generation

I will discuss the theory of quantum bit string generation, present an experimental realisation of quantum bit string generation with security better than can be achieved classicaly, and discuss how to improve on this first experimental realisation.

QISW03 9th September 2004
14:15 to 15:00
Applications of quantum universal composability theorem

The quantum universal composability theorem will first be reviewed. Next, applications to a new security definition of QKD and security proofs will be given. Finally, we prove composable security for various variants of the quantum message authentication scheme proposed in quant-ph/0205128 with key recycling.

QISW03 9th September 2004
16:00 to 22:00
PROSECCO meeting (joint with RESQ)
QISW03 10th September 2004
11:30 to 12:15
Locking of entanglement

We study the loss of entanglement of bipartite state subjected to discarding or measurement of one qubit. Examining the behavior of different entanglement measures, we find that entanglement of formation, entanglement cost, and negativity are lockable measures in that they can decrease arbitrarily after measuring one qubit. Relative entropy of entanglement is shown to be a non-lockable measure.

QISW03 10th September 2004
12:15 to 13:00
Quantum correlations in quantum cryptology

In basic quantum communication protocols one party creates quantum states and uses a quantum channel to transmit it to another party that performs immediately some measurement on it. This means, we effectively create correlated (classical) data between distant parties. In order to use the power of quantum mechanics, these correlation must show effects of quantum mechanics.

In the specific example of quantum key distribution one uses the correlations to distill a secret key in (classical) public discussion protocols e.g. via sifting, error correction and privacy amplification. We give a necessary condition for the success of any public discussion protocol: the observed correlations should allow to prove the presence of an internal, virtual state of entanglement in the distribution. This poses a first test whether any presented real quantum key distribution is indeed useful for the desired purpose. Moreover, a gap between the parameter regime of proven security of given realistic schemes and the regime of proven presence of vitual entanglement furthers the search for the optimal public discussion protocol.

QISW03 10th September 2004
14:15 to 15:00
Distillation of secret key and entanglement from quantum states

Based on techniques from classical information theory, which we generalise to the quantum domain, we show how to distill secret key between two parties by local operations and public communication from given states, at an asymptotically optimal rate. By implementing this protocol "coherently", we then show how it yields actually entanglement, at the same rate, proving the long-conjectured "hashing inequality": it states that from a given quantum state, once can, using one-way LOCC, distill entanglement with rate given by the coherent information.

This is joint work with Igor Devetak; it is contained in eprints quant-ph/0306078 and quant-ph/0307053.

QISW03 10th September 2004
15:30 to 16:15
CH Bennett Unlocking of a quantum channel's forward classical capacity by classical communication

Classical back communication does not increase a classical channel's forward capacity, but for quantum channels the situation is more complicated. It is well known that classical back communication can increase quantum capacity, but no example was known where it increased a quantum channel's classical capacity. We have found channels whose Holevo capacity is almost zero, but whose quantum and classical capacities in the presence of classical back communication are quite large. The greater complexity of the situation with quantum channels stems from the fundamental difference between entanglement and its closest classical analog, shared secret randomness. Joint work with Igor Devetak, Peter Shor, and John Smolin. quant-ph/0406086.

QIS 14th September 2004
11:00 to 12:00
Entanglement in spin systems
QIS 15th September 2004
11:00 to 12:00
Invariants and multi-qubit systems
QIS 23rd September 2004
11:00 to 12:00
M Santha Quantum algorithm for finding a triangle
QIS 24th September 2004
11:00 to 12:00
Quantum cellular automata
QISW02 27th September 2004
09:00 to 09:45
Y Hirayama Experimental implementation of semiconductor qubit

Coherent quantum system control, i.e, qubit operation, has received much interest from a viewpoint of quantum information processing, especially quantum computing. A small-scale test-bed for a quantum computer has been demonstrated by using solution NMR. However, from the viewpoint of scalability, a solid-state quantum computer is desirable. Among the many candidates for solid-state qubits, semiconductor systems have advantages in that they use existing cutting-edge IC technologies. The coherent control of charge, spin, nuclear spin, and exciton has been studied in semiconductor systems. In this presentation, we will discuss coherent control of electron charge and nuclear spin in semiconductor systems. A semiconductor charge qubit was embedded in a coupled quantum-dot structure [1]. In this charge qubit, electron occupation in either of the two coupled dots operates as a quantum two-level system. It is noteworthy that this charge qubit can be controlled all-electrically in semiconductor systems. We have demonstrated a modulation of the coherent oscillation frequency by electrical control of the coupling between two dots and achieved arbitrary control of pseudospin rotation on the Bloch sphere by designing the electrical pulse applied to the system [2]. Interactions between electron and nuclear spins have been studied in semiconductor heterostructures in the fractional-quantum-Hall regime. The nuclear spin polarization was observed in the situation where different fractional-quantum-Hall states with different spin configuration coexist in the system. All electrical control has been achieved for nuclear spin polarization and relaxation [3]. Recently, we have succeeded in controling nuclear spin polarization in a point contact regime with a mesoscopic scale [4]. Coherent control of nuclear spin polarization has been demonstrated by flowing radio-frequency pulse current along a micro-strip line near the point contact [5]. These experimental achievements represent the first step towards semiconductor qubit systems for quantum information processing.

[1] T. Hayashi, T. Fujisawa, H. D. Cheong, Y. H. Jeong and Y. Hirayama, Phys. Rev. Lett. 91, 226804 (2003). [2] T. Fujisawa, T. Hayashi, H. D. Cheong, Y. H. Jeong and Y. Hirayama, Physica E21, 1053 (2004). [3] K. Hashimoto, K. Muraki, T. Saku, and Y. Hirayama, Phys. Rev. Lett. 88, 176601 (2002). [4] G. Yusa, K. Hashimoto, K. Muraki, T. Saku, and Y. Hirayama, Phys. Rev. B69, 161302(RC) (2004). [5] G. Yusa, K. Hashimoto, K. Muraki, and Y. Hirayama, unpublished.

QISW02 27th September 2004
09:45 to 10:30
The fabrication of a silicon based quantum computer at the atomic-scale

Quantum computers have the potential to dramatically reduce computing time for problems such as factoring [1] and database searching [2]. In particular a silicon-based quantum computer [3] shows promise for its potential to scale to a large number of qubits and for its compatibility with standard CMOS processing.

Our group has designed a fabrication strategy for the realisation of a scaleable quantum computer based in silicon using a combination of scanning probe microscopy for single qubit placement and silicon molecular beam epitaxy to encapsulate the qubit array [4]. In order to achieve this goal we have demonstrated the following key steps: we have been able to incorporate single P atoms as the qubits in silicon with atomic precision [5]; we have been able to minimise P segregation and diffusion during Si encapsulation [6] and we have imaged the array of buried P atoms using scanning tunneling microscopy to prove that the array remains intact after the encapsulation stage. Recently we have been able to fabricate a robust electrical device in silicon using the scanning tunneling microscope to lithographically pattern the dopants [7] and have demonstrated that this device can be contacted and measured outside the ultra-high vacuum environment.

We highlight the importance of our results for the fabrication of a Si-based quantum computer and discuss the final stages of the fabrication process required to realize a functional device, including the formation of an electrical isolation barrier and the alignment of surface metal electrodes to the buried P atom array.

[1] P. W. Shor, Proc. of the 35th Annual Symposium on Foundations of Computer Science, Editor: S. Goldwasser (IEEE Computer Society Press, USA, 1994), p. 124. [2] L. K. Grover, Phys. Rev. Lett. 79, 325 (1997). [3] B. E. Kane, Nature 393, 133 (1998). [4] J. L. OBrien et al., Phys. Rev. B 64, 161401(R) (2001). [5] S. R. Schofield et al., Phys. Rev. Lett. 91, 136104 (2003). [6] L. Oberbeck et al., accepted for publication in Appl. Phys. Lett. (2004). [7] F.J. Ruess et al., submitted to Nano Letters (2004).

QISW02 27th September 2004
11:00 to 11:45
Towards an integrated photonic quantum information platform in diamond

We present concepts relating to efforts to create an integrated platform for quantum information processing based on the properties of NV diamond. NV diamond is ideally suited for the production of single photons and entangled photons and for investigating the interconversion of flying and stationary qubits. Furthermore the diamond matrix can be used to generate waveguides for the generated light to propogate through. With the ability to fabricate single crystal diamond into complicated photonic structures, the possibility arises to generate optical circuitry to perform a range of tasks, including non-deterministic quantum logic elements.

We will present an overview of the theoretical efforts of the device modelling group, and then discuss plans for fabricating diamond waveguides, transform limited single photon sources in diamond using the NV Centre and photon discriminator using NV diamond.

QISW02 27th September 2004
11:45 to 12:30
A two-qubit conditional quantum gate with single spins in solid

Quantum computers promise to increase substantially the efficiency of solving certain computationally demanding problems like searching atabases and factoring large integers. One of the greatest challenges ow is to implement basic quantum computational elements in a physical system and to demonstrate that they can be reliably controlled. Single spins in semiconductors, in particular associated with defect centers, are promising candidates for practical and scalable implementation of quantum computing even at room temperature . Such an implementation may also use the reliable and well known gate constructions from bulk nuclear magnetic resonance (NMR) quantum computing. This paper report an implementation of a quantum logical NOT and a conditional two-qubit gate (CROT) with single spins in a solid. As quantum bits a single electron spin and a single carbon thirteen nuclear spin of a single nitrogen vacancy defect center in diamond are used. The quantum state of the electron spin can be read out optically. Owing to long decoherence and relaxation times the systems meets the requirements of hardware for quantum computation. Density matrix tomography of the CROT gate shows that the achieved performance of the two bit conditional quantum gate is promising. The gate fidelity achieved in our experiments is up to 0.9, good enough to be used in quantum algorithms. Further on, the system may allow for solid-state room temperature quantum computation.

QISW02 27th September 2004
14:00 to 14:45
Prospects and challenges for quantum information processing using carbon nanotubes

This talk will review experimental progress toward the use of carbon nanotubes as a basis for solid-state quantum information processing. This topic is in its infancy, to say the least, but some modest accomplishments can be presented. These include local gate control of conductance and tunneling, the formation of multiple quantum dots, and demonstrated signatures of spin physics, including the Kondo effect.

The motivation to use nanotubes in this way is that these systems have very weak spin orbit coupling, mostly zero nuclear spin, and confined phonon spectra. One difficulty, of course, is that nanotubes are one dimensional. While schemes for universal quantum computing with nearest-neighbor-only coupling in one dimension are known, these appear to require unrealistic precision in control of exchange coupling. We expect that by the time control of nanotube spin and charge are sufficiently under control to provide precise qubit control, comparable advancements toward encorporating nanotubes into higher-dimensional structures will have also developed. For now, we concentrating on precise control of exchange and tunnel coupling in one dimension.

QISW02 27th September 2004
14:45 to 15:30
C Smith Erasable electrostatic lithography for quantum components

Erasable Electrostatic Lithography for Quantum Components C G Smith, R Crook, A C Graham, I Farrer, H E Beere, and D A Ritchie Department of Physics, University of Cambridge, Cambridge CB3 0HE, United Kingdom

Erasable electrostatic lithography (EEL) is a new lithographic technique where patterns of charge are drawn on a GaAs surface with a low-temperature scanning probe [1]. The surface charge locally depletes electrons from a subsurface 2D electron system to define a quantum component ready for measurement in the same low-temperature high-vacuum environment, enabling short lithography to measurement cycles and high productivity. Charge patterns are erased locally with the scanning probe or globally by illuminating the sample with red light. We demonstrate how ballistic 1-D channels can be created at 100 mK then erased and replaced by a small and the large quantum dot. We provide background and characterization data for the EEL technique and then describes the construction and measurement of a quantum billiard. A quantum billiard is a large open quantum dot which exhibits both chaotic behavior and classical orbits. Scanning probe images, made with the same apparatus in the same environment, reveal features associated with the classical closed-loop electron trajectories inside the quantum billiard. This new low temperature technique is ideally suited to the fabrication of the complex quantum architectures required for quantum computation. On of the big problems found when trying to fabricate QBITs using semiconducting quantum dots is that random impurity potentials make each dot different. In order to solve this problem each dot needs several dedicated gates to tune the system. With 10 000 QBITs it is possible that 30-50,000 gates would be required. With our EEL technique it is possible to use deposited charge to tune each QBIT to be in an ideal configuration. This greatly reduces the complexity of the resulting structure. 1. R. Crook, A. C. Graham, C. G. Smith, I. Farrer, H. E. Beere, D. A. Ritchie. NATURE 424 (6950): 751-754 AUG 14 (2003) Erasable electrostatic lithography for quantum components

2. R Crook, C G Smith, A C Graham, I Farrer, H E Beere, and D A Ritchie, 2003 Phys. Rev. Lett. 91, 246803 (2003) Imaging Fractal Conductance Fluctuations and Scarred Wave Functions in a Quantum Billiard

QISW02 27th September 2004
16:00 to 16:45
Endohedral fullerenes for electron-spin-based quantum computing

A nitrogen atom encapsulated in a C60 buckyball, N@C60, carries an electron magnetic moment that is well isolated from the environment. It exhibits extremely long spin decoherence times, comparable with the longest measured in any solid state system. We explain the potential advantages of exploiting endohedral fullerenes as qubits and describe approaches to employing N@C60 and related molecules in multi-qubit structures. Using pulsed electron spin resonance we examine the capability of existing spectrometers to perform high-fidelity single-qubit unitary transformations, and find that the current state of the art is adequate for simple quantum computations.

QISW02 27th September 2004
16:45 to 17:30
P Lindelof Field effect transistor behaviour in a single wall carbon nanotubes and peapods

We report on a comparative study of electron transport properties in single-walled carbon nanotubes (SWNTs) and SWNTs filled with Buckminster fullerenes, C60[at]SWNTs[dot] The single wall carbon nanotubes exhibit field effect induced by a n+Si gate separated from the SWNT by a 300 nm SiO layer. At low temperatures the metallic SWNTs exhibit Coulomb blockade effects and Luttinger liquid behaviour. In order to asses the effect of C60 inserted in SWNTs we have prepared defect-free SWNTs with a narrow diameter distribution 13.9-15.1Å, which allowed the assembly of C60@SWNTs in high yield (~90%). Systematic transport measurements from room temperature to liquid He temperatures in individual C60-filled SWNTs revealed unchanged Luttinger behaviour but a reduced transmission compared to the empty controls.

QISW02 28th September 2004
09:00 to 09:45
Quantum information processing using quantum dots

The electron spin of quantum dots is among the most promising candidates for quantum information processing in the solid state. Optical selection rules make it possible to control and measure spins in isolated quantum dots optically. The implementation of large-scale quantum algorithms requires architectures of coupled quantum dots to implement two-qubit gates. Constructing an efficient interface between spin and photon quantum states is crucial for applications in quantum communication.

We will discuss recent experimental and theoretical work on molecularly coupled quantum dots [1,2] and quantum dot cavity-QED [3]. Using time-resolved Faraday rotation, we have recently demonstrated coherent transfer of electron spin states between quantum dots coupled by conjugated molecules [1]. A simple transfer-Hamiltonian ansatz allows one to derive analytical expressions for the Faraday rotation signal of coupled quantum dots as a function of the probe frequency and captures many of the essential experimental features [2]. Recent progress in solid state microcavity design has led to mode volumes close to the theoretical limit and Q-factors of order 5000, approaching the strong-coupling regime for quantum dot cavity-QED. A quantum dot interacting with two resonant cavity modes is described by a two-mode Jaynes-Cummings model. Depending on the quantum dot energy level scheme, the interaction of a singly doped quantum dot with a cavity photon generates entanglement of electron spin and cavity states or allows one to implement a SWAP gate for spin and photon states [3]. This system thus provides a natural interface between quantum information schemes based on electron spins and linear optics, respectively, and potentially enables the integration of computation and communication.

[1] M. Ouyang and D. D. Awschalom, Science 301, 1074 (2003). [2] F. Meier, V. Cerletti, O. Gywat, D. Loss, and D. D. Awschalom, Phys. Rev. B 69, 195315 (2004). [3] F. Meier and D. D. Awschalom, cond-mat/0405342.

QISW02 28th September 2004
09:45 to 10:30
Electron spin qubits in quantum dots
QISW02 28th September 2004
11:00 to 11:45
Interfacing quantum optical and solid state qubits

We present a generic model of coupling quantum optical qubits and solid state qubits, and the corresponding transfer protocols. The example discussed is a trapped ion coupled to a superconducting charge qubit (single Cooper pair box). To enhance the coupling and achieve compatibility between the different experimental setups we introduce a superconducting cavity as the connecting element.

QISW02 28th September 2004
11:45 to 12:30
Interfacing a single trapped ion qubit with a single photon flying quibit

We report a series of experiments linking an ideal quantum memory with an ideal quantum communication channel. A single trapped ion probabalistically emits a single photon in such a way that the internal state of the trapped ion is entangled with the polarization of the photon. The entanglement is directly verified via measurements of correlations and Bell inequality violations. While the process is probabalistic, it can nevertheless be scaled to large numbers of qubits following known quantum repeater protocols.

QISW02 28th September 2004
14:00 to 14:45
M Plenio Entanglement propagation in interacting quantum systems

We consider how quantum information can be propagated in interacting quantum systems. Theoretical schemes will be presented and practical realiszations will be discussed.

QISW02 28th September 2004
14:45 to 15:30
Easing the experimental burden: global control, perpetual coupling and the like

The most commonly described paradigm for quantum computation involves switching individual qubit-qubit interactions 'on' and 'off' at will. This is of course an enormous challenge for many of the proposed experimental implementations. I will discuss alternative models that may allow one to (a) give up all local manipulations in favour of global control, (b) leave the underlying physical interactions on continuously, or (c) both.

Related papers: quant-ph/0407063 Phys. Rev. Lett. 90, 247901 (2003). Phys. Rev. Lett. 88, 017904 (2002).

QISW02 28th September 2004
16:00 to 16:45
G Burkard Production and detection of entangled electrons in small solid-state structures

The production and detection of entangled and spatially separated electrons in small solid-state structures has recently attracted considerable theoretical interest and represents a major challenge for experiments. In this talk, we review various theoretical ideas for producing spin-entangled electrons in mesoscopic superconductor-normal junctions or coupled semiconductor quantum dots and discuss the problem of spatial separation. The possibility of detecting and quantifying electron spin entanglement without spin-sensitive probes using electronic beam splitters will be discussed. The situation for electrons is quite different from entangled photons, because in solids, entangled electrons are typically surrounded by many indistinguishable and interacting electrons. We discuss the complications arising from the presence of the other electrons (the Fermi liquid) and some implications of spin decoherence.

QISW02 28th September 2004
16:45 to 17:30
T Osborne The propagation of quantum information through a spin system

It has been recently suggested that the dynamics of a quantum spin system may provide a natural mechanism for transporting quantum information. I'll show that one dimensional rings of qubits with fixed (time-independent) interactions, constant around the ring, allows high fidelity communication of quantum states. I'll then show that the problem of maximising the fidelity, in a restricted subspace of a single up spin, of the quantum communication is related to a classical problem in fourier wave analysis. By making use of this observation I'll argue that if both communicating parties have access to limited numbers of qubits in the ring (a fraction that vanishes in the limit of large rings) it is possible to make the communication arbitrarily good. I'll then show how to extend our results beyond the restricted 1-spin subspace. These results provide a novel interpretation of a spin systems as a second-quantised optical fibre or waveguide.

QISW02 29th September 2004
09:00 to 09:45
Cooling and controlled entanglement experiments using single and pairs of trapped calcium ions

I will present recent experimental work in which we have cooled calcium ions to near the ground state of motion by three varieties of Raman sideband cooling. We cooled the motion of single ions and pairs of ions in one dimension, obtaining mean vibrational quantum number n << 1. We studied a pulsed method and also two recently proposed continuous methods which I will describe.

I will also discuss experiments involving the joint spin and motional state of single ions and pairs of ions. We performed the Schrodinger Cat' experiment in which a superposition of coherent states of motion of a single ion, entangled with the spin state, is prepared by resonantly driving the motion. The quantum superposition (as opposed to a mixture) is confirmed by observing an interference effect when the states are recombined. This technique can also be used to create a 2-ion quantum logic gate, entangling the internal states of a pair of ions. I will report on progress towards this important milestone in our laboratory.

QISW02 29th September 2004
09:45 to 10:30
RB Blatt Entanglement and transfer of quantum information with trapped Ca+ ions

Trapped strings of cold ions provide an ideal system for quantum information processing. The quantum information can be stored in individual ions and these qubits can be individually prepared, the corresponding quantum states can be manipulated and measured with nearly 100% detection efficiency. With a small ion-trap quantum computer based on two and three trapped Ca+ ions as qubits we have generated in a pre-programmed way genuine quantum states. In particular, entangled states of two particles, i.e. Bell states [1], and of three particles, i.e. GHZ and W states [2], were generated using an algorithmic procedure and their decoherence was investigated. These states are of particular interest for the implementation of a three-ion quantum register: we have demonstrated selective read-out of single qubits (while protecting the other qubits) and manipulation of single qubits of the register conditioned on the read-out results. The generated states have been measured experimentally using a technique known as state tomography allowing the population and phase of the quantum system to be mapped. Moreover, quantum teleportation with trapped ions was implemented [3] and can be used as resource for the transfer of quantum information as well as for quantum information processing.

[1] C. F. Roos, G. P.T. Lancaster, M. Riebe, H. Häffner,W. Hänsel, S. Gulde, C. Becher, J. Eschner, F. Schmidt-Kaler, and R. Blatt, Phys. Rev. Lett. 92, 220402 (2004). [2] C. F. Roos, M. Riebe, H. Häffner, W. Hänsel, J. Benhelm, G. P. T. Lancaster, C. Becher, F. Schmidt-Kaler, and R. Blatt, Science 304, 1478 (2004). [3] M. Riebe, H. Häffner, C. F. Roos, W. Hänsel, J. Benhelm, G. P. T. Lancaster, T. W. Körber, C. Becher, F. Schmidt-Kaler, D. F. V. James, R. Blatt, Nature 429, 734 (2004).

QISW02 29th September 2004
11:00 to 11:45
Atom chips: a vision for QIP with neutral atoms

Cold quantum gases trapped above microstructured atom chips provide a very promising basis for storing and manipulating quantum information. Our early explorations of this have led to studies of decoherence effects, some fundamental and some due to the vaguaries of real meterials. I will discuss the experimental study of fluctuating rf magnetic fields and the implications of these for atom chips. We have also investigated the properties of magnetic traps based on current-carrying wires compared with those using permanent magnetic materials. I will discuss the results. An important new phase in the development of atom chips is the integration of microscopic optical structures. Small optical standing wave structures offer the prospect of building qubit registers based on "self-assembled" atom strings. Small high-Q optical cavities provide a way to transfer quantum information between photons and atoms. I will discuss the progress toward realising these optical structures and the vision for using them.

QISW02 29th September 2004
11:45 to 12:30
Decoherence issues for cold atoms near surfaces

I will discuss how cold atoms can be manipulated by atom chips to form quantum registers, and discuss the heating and loss of coherence of such atoms close to the guiding structures. My talk will be theoretical, but will relate to the experimental programme underway at Imperial College led by Prof E A Hinds

QISW02 29th September 2004
14:00 to 14:45
IA Walmsley Efficient conditional preparation of single photons for quantum optica

In this talk I will explore connections between quantum error correction and closed-loop quantum feedback. A simple model, based on detected measurement errors, in which the measurements are continuous in time will be presented. Error correction is implemented using the Poisson process feed-back model. I will also discuss a new scheme based on continuous measurement and feed-back that can increase the coherence time for a single encoded qubit. Finally I will discuss schemes that explicitly use decoherence for ancilla qubits, without reference to an explicit measured signal. These show how quantum closed-loop control may be used to increase the coherence time of encoded qubits.

QISW02 29th September 2004
14:45 to 15:30
Linear and nonlinear quantum optical information processing

In recent years we have seen signs of a new technological revolution in information processing, a revolution caused by a paradigm shift to information processing using the laws of quantum physics. There have been significance developments in all optical quantum information processing (QIP) following the recent discovery by Knill, Laflamme and Milburn that passive linear optics, photo-detectors, and single photon sources can be used to create massive reversible nonlinearities. Such nonlinearities are an essential requirement for optical quantum computation and many communication applications. These nonlinearities allow efficient gate operations to be performed. However, such operations are relatively inefficient (they have a probability of success significantly less than 50 percent) and hence are not scalable themselves. This is primarily due to the current state of the art in single photon sources and detectors. Before {\it true} optical universal quantum computation and information processing can be achieved, the efficiency of such detectors must be significantly improved. This is likely to require a drastic change in the approach to detection technology. We discuss a new and novel approach to the problem of creating a photon number resolving detector using the giant Kerr nonlinearities available in electromagnetically induced transparency. Our scheme can implement a photon number quantum non-demolition measurement with high efficiency (>99%) using only a few hundred atoms, and can distinguish 0, 1 and 2 photons. We discuss various applications of this detector and indicate how it can be used to significantly improve the success probability for the linear optical gates.

QISW02 29th September 2004
15:30 to 16:15
J Eisert Optimizing linear optics quantum gates

In this talk, the question of finding optimal success probabilities of linear optics quantum gates is linked to the theory of convex optimization. This question of optimalsuccess probability is important in the framework of quantum computation with linear optics and selective photon number measurements only, in order to assess the scalability of a specific scheme. Based on earlier work by other authors, it is shown that by exploiting this link, upper bounds for the success probability of gates involving single modes and arbitrary photon numbers can be derived that hold in all generality, and restrictions do not have to be imposed such as the requirement of a certain finite number of modes, of optical elements in the network or of photon numbers. The concept of Lagrange duality provides then rigorous proofs for bounds on success probabilities, without the need to resort to numerical means. As a corollary, the previously formulated conjecture is proven that the optimal success probability of a non-linear sign shift gate is exactly one quarter. In an extended outlook, other applications of the tools of semi-definite programming in quantum information theory will be sketched, and complete hierarchies of efficient criteria for multi-particle entanglement will be presented.

QISW02 29th September 2004
16:45 to 17:30
Experimental linear optical logic

Coding data bits in the phase or polarisation state of single photons allows us to exploit wave particle duality for novel communication protocols. Fibre and free-space quantum cryptography apparatus used for secure exchange of keys exploit this discovery [1,2]. Key sharing schemes are easily realised because they involve only single qubit manipulation. Further developments such as quantum relays and other few qubit applications require that pairs of qubits interact. To avoid the inevitably weak non-linear interactions between photons conditional linear optics logic has been developed to demonstrate CNOT operation albeit with limited efficiency [3,4]. We are developing suitable single photon and pair photon sources in order to demonstrate a teleportation based scalable CNOT gate. Key aims are to demonstrate efficient sources and optical circuits with high success probability. We also aim to exploit the low decoherence of photons to demonstrate high fidelity (QBER <10-4) operation. The presentation will summarise our progress towards these aims. [1] N. Gisin, G. Ribordy, W. Tittel and H. Zbinden Rev. Mod. Phys. 74, 145 (2002). [2] C. Kurtsiefer, et al, Nature 419, 450 (2002). [3] J.G.Rarity, Roy. Soc. Phil. Trans. 361, 2003, 1507-18 [4] J.L. O'Brien et al, Nature 426, 264 (2003).

QISW02 29th September 2004
17:30 to 18:15
G Pryde Generating and harnessing photonic entanglement using linear optics

Photonic entanglement is central to optical quantum computing schemes, and is also important for quantum communication, quantum metrology, and in fundamental quantum mechanics. I will discuss the results of several linear optics experiments for generating, characterising, and utilising entangled states in the context of quantum information.

Controlled-NOT gates are the archetypal two-qubit entangling gate. We have constructed a two-photon CNOT and fully characterized it using quantum process tomography. As well as being a proof-of-principle for entanglement schemes working via measurement-induced nonlinearity, two-photon CNOT and related circuits can also be used to make generalised quantum measurements, explore error correction protocols, and build prototypical cluster states for quantum computation.

In certain areas of quantum information, optical qudits (d-level quantum systems) hold advantages over photonic qubits. One type of qudit encoding uses the transverse spatial mode of a photon. We have fully characterised the two-qutrit spatial mode entanglement from a downconverted photon pair, and find a highly entangled state with only slight mixture. These kind of studies are important for characterising the requirements for applications such as quantum bit commitment, where qutrits should offer a higher degree of security compared with qubits.

QISW02 30th September 2004
09:00 to 09:45
H Mooij Superconducting quantum bits

Fabricated superconducting circuits can be used to develop quantum bits for a scalable quantum computer. The circuits contain weak Josephson junctions that allow for tunneling of Cooper pairs and weak coupling between superconductors. The number of Cooper pairs and the phase of the superconductor wave function are conjugate quantum variables and they can both be used to define states for the quantum bits. Three types have been developed so far: charge qubits, phase qubits and flux qubits. The status and prospects for further development will be reviewed. Research in Delft on flux qubits will be addressed in more detail.

QISW02 30th September 2004
09:45 to 10:30
T Schaetz Distributing entanglement in a multi-zone ion trap

We discuss experiments devoted to realizing the elements of quantum information processing using trapped ions. We use a multi-zone trap for Be+ ions, where ions can be entangled in one trap zone, then separated and distributed to separate zones where subsequent single- and two-ion gates, and/or detection can be performed. Recent work includes (1) demonstration of a dense-coding protocol, (2) demonstration of enhanced qubit detection efficiency using quantum logic, (3) generation of GHZ states and their application to enhanced precision in spectroscopy, (4) the realization of teleportation with atomic qubits, and (5) the implementation of a three qubit error correction code.

We also discuss work devoted to alternative trap fabrication methods and incorporation of sympathetic cooling in a multiplexed trap structure.

* Supported by the U. S. National Security Agency (NSA) and Advanced Research and Development Activity (ARDA) under Contract No. MOD 7171.04 and by NIST.

QISW02 30th September 2004
11:00 to 11:30
GJ Milburn Quantum closed-loop control and error correlation
QISW02 30th September 2004
11:45 to 12:30
N Gisin 2- 3- and 4-photon quantum communication in telecom fibers

Using time-bin encoding in standard telecom fibers demonstrations of entanglement based quantum cryptography, of quantum teleportation and of entanglement swapping will be presented and perspectives for future experiments in quantum communication discussed.

QIS 30th September 2004
14:30 to 16:00
Presentations on recent developments
QIS 30th September 2004
16:30 to 18:00
Presentations on recent developments
QIS 30th September 2004
18:00 to 19:30
Small group discussions between collaborators
QIS 1st October 2004
09:00 to 11:00
Presentations on recent developments
QIS 1st October 2004
11:30 to 12:30
Presentations on recent developments
QIS 5th October 2004
11:00 to 12:00
The laplacian of a graph as a density matrix
QIS 7th October 2004
11:00 to 12:00
Topological quantum computing for beginners
QIS 8th October 2004
11:00 to 12:00
Quantum speed-up of Markov chain based algorithms
QIS 14th October 2004
11:00 to 12:00
The Pangloss Universe
QIS 19th October 2004
11:00 to 12:00
MB Ruskai Recent progress on additivity and multiplicativity channels
QIS 21st October 2004
11:00 to 12:00
R Cleve Entangled Provers
QIS 26th October 2004
11:00 to 12:00
Quantum Bell inequalities (the quantum marginal problem)
QIS 28th October 2004
11:00 to 12:00
R Jozsa An introduction to measurement-based quantum computation
QIS 2nd November 2004
11:00 to 12:00
CH Bennett The reverse Shannon theorem
QIS 4th November 2004
11:00 to 12:00
Security of quantum key distribution using coherent states
QIS 5th November 2004
11:00 to 12:00
Quantum circuits for the Schur transform
QIS 9th November 2004
11:00 to 12:00
Aspects of generic entanglement
QIS 11th November 2004
11:00 to 12:00
A family of quantum protocols
QISW04 15th November 2004
10:30 to 11:15
On convex structures of states, POVM's and channels, and their mutual relations

After briefly reviewing the structure of the convex sets of POVM's and channels in finite dimensions, we will consider maps between different types of convex sets, corresponding to different kinds of quantum information processing, e. g. quantum calibration, programmable channels and POVM's, universal POVM's, pre-processing and post-processing of POVM's. In particular, we will focus attention on programmability of POVM's and pre-processing, introducing the problem of "clean POVM's", and concluding with a list of open problems.

QISW04 15th November 2004
11:15 to 12:00
G Chiribella Extremel covariant POVM's

The random choice between two different apparatuses measuring the same physical parameter can be viewed as a convex combination in the space of quantum measurements. In particular, the set of positive operator valued measures (POVM) pertaining to a given parameter is a convex set. The aim of this work is to characterize the extreme points of the set of POVM's which are covariant with respect to a finite dimensional representation of a Lie group. Necessary and sufficient conditions are given, also relating extremality with uniqueness and stability of measurements arising in concrete optimization problems.

QISW04 15th November 2004
14:00 to 14:45
Efficient minimax rates for Wigner function estimation from quantum homodyne tomography data

It was just recently that the problem of quantum tomography has been called to the attention of the statistical community throughout the proof of consistency results for Projection Pattern Function and Sieve Maximum Likelihood estimators of the density matrix and the Wigner function of the quantum state of light. Proving consistency means that the estimator converges to the desired state in appropriate norm. A step further in this study is to prove how fast (rate of convergence) an estimator converges as the number of samples increases. We show pointwise rates of convergence of the minimax error for the classical Kernel estimator of the Wigner function from quantum homodyne tomography data, assuming the Wigner function is in certain class. As a whole, we also give an overview of previous works and motivate future directions.

QISW04 15th November 2004
14:45 to 15:30
Local copying of orthogonal maximally entangled states and its relationship to local discrimination

In the quantum system, perfect copying is impossible without prior knowledge. But, perfect copying is possible, if it is known that unknown states to be copied is contained by the set of orthogonal states, which is called the copied set. However, if our operation is limited to local operations and classical communications, this problem is not trivial. Recently, F. Anselmi, A. Chefles and M.B. Plenio constructed theory of local copying when the copied set consists of maximally entangled states. They also classified the copied set when it consists of two orthogonal states (quant-ph/0407168). In this paper, we completely classify the copied set of local copying of the maximally entangled states in the prime dimensional system. That is, we prove that, in the prime dimensional system, the set of locally copiable maximally entangled states is equivalent to the set of Simultaneously Schmidt decomposable canonical form Bell states. As a result, we conclude that local copying of maximally entangled states is much more difficult than local discrimination at least in prime dimensional systems.

QISW04 16th November 2004
10:30 to 11:15
Optimal qubit mixed state estimation

Given a number N of identically prepared qubit mixed states, I analyse the optimal estimation protocol, based on collective measurements. I discuss two scenarios: completely general states (3D), and states known to lie on the equatorial plane of the Bloch sphere (2D). I will derive the optimal POVMs for finite number of copies and obtain the analytical expressions of the fidelities in the large N regime. In 3D, I will show that the optimal POVM's are independent of the prior distributions, provided they are isotropic. In 2D this is only true asymptotically. However, for the latter, the fidelity is F=1-1/(2N), independently of the prior.

QISW04 16th November 2004
11:15 to 12:00
Estimation of qubit mixed states with local measurements

I discuss the problem of estimating an unknown qubit mixed state from a large sample of N identically prepared states with local measurements. I use as a prior the Bures distribution and focus, for simplicity, on states belonging to the equatorial plane of the Bloch sphere. I will show that the average fidelity for a large sequence of fixed measurements tends to 1-1/N^{3/4}. This is a very suprising result that contrast with both, estimation of pure states and estimation of mixed states with collective measurements, where the fidelity tends to 1 as 1/N.

QISW04 16th November 2004
14:00 to 14:45
State estimation on a circle

Gisin and Popescu [PRL, 83, 432 (1999)] showed that more information about the direction of the Bloch vector of a pure qubit |\psi(\theta, \phi)> = cos(\theta/2)|0> + exp[i\phi] sin(\theta/2)|1> can be obtained from anti-parallel states |{\Psi}_{11}(\theta, \phi)> = |\psi(\theta, \phi)> X |\psi(\pi-\theta, \pi+\phi)>, compared to parallel states |{\Psi}_{20}(\theta, \phi)> = |\psi(\theta, \phi)> X |\psi(\theta, \phi)>, where (\theta, \phi) is distributed uniformly over [0, \pi] x [0, 2\pi]. As a cause behind this difference, they pointed out the difference between the dimensions of the subspaces spanned by parallel and anti-parallel states separately. When \theta = \pi/2, there is no difference in the amount of information as in that case (and only in that case) exact spin-flip is possible. For any fixed \theta, the dimension of the space spanned by N no. of same and M no. of its orthogonal qubits is (N+M+1). We found that whenever we fix \theta, anti-parallel states always give more information if \theta is different from 0 or \pi/2 or \pi, in the case when we estimate the direction of the Bloch vector of the qubit. We generalized this to the case of N no. of same and M no. of its orthogonal qubits. Here the measurement basis for optimal estimation strategy always turns out to be a quantum Fourier transform. But in the case of estimating the direction of the Bloch vector of the qubit |\psi(\theta, \phi)> = cos(\theta/2)|0> + exp[i\phi]sin(\theta/2)|1>, we found that both the sets S_P(\theta) = {|{\Psi}_{20}(\theta, \phi)> : \phi \in [0, 2\pi]} U {|{\Psi}_{20}(\pi-\theta, \pi+\phi)> : \phi \in [0, 2\pi]} and S_A(\theta) = {|{\Psi}_{11}(\theta, \phi)> : \phi \in [0, 2\pi]} U {|{\Psi}_{11}(\pi-\theta, \pi+\phi)> : \phi \in [0, 2\pi] give the same information.

QISW04 16th November 2004
14:45 to 15:30
M Hayashi Asymptotic theory of quantum estimation

We discuss the asymptotic bound the accuracy of the estimation when we use the quantum correlation in the measuring apparatus. It is also proved that this bound can be achieved in any model in the quantum two-level system and in the general case. Moreover, we show that in several specific model this bound cannot be attained by any quantum measurement with no quantum correlation in the measuring apparatus. That is, in such a model, the quantum correlation can improve the accuracy of the estimation in an asymptotic setting.

QISW04 16th November 2004
16:15 to 17:00
Quantum measurements that maximise the likelihood and optimal reference frame transmission

We determine the covariant measurements that maximize the likelihood according to the symmetries of quantum states, and apply our results to provide optimal schemes for reference frames transmission.

QISW04 17th November 2004
10:30 to 11:15
Some aspects of quantum statistical inference

In classical parametric statistical inference, an important question is What parts of the data are informative about the parameters of interest?'. Key concepts here are those of sufficient statistic, ancillary statistic and cut. Some analogous concepts for quantum instruments will be outlined.

QISW04 17th November 2004
11:15 to 12:00
D Petz Sufficiency in quantum statistical inference

Coarse-grining is a basic concept in mathematical statistics and its analogue may be defined in the formalism of quantum mechanics. In the lecture sufficiency of a coarse-graining is discussed with respect to a family of states. Charaterizations of sufficiency are given. The equality case for the strong subadditivity of the von Neumann entropy and the Koashi-Imoto theorem are among the applications. The lecture is partially based on joint work with Anna Jencova.

QISW04 17th November 2004
14:00 to 14:45
Quantum state estimation and large deviations

In this talk we propose a method to estimate the density matrix \rho of a d-level quantum system by measurements on the N-fold system. The scheme is based on covariant observables and representation theory of unitary groups and it extends previous results concerning the estimation of the spectrum of \rho. We show that it is consistent (i.e. the original input state is recovered with certainty for N \to infinity) and analyze its large deviation behavior. In addition we calculate explicitly the corresponding rate function which describes the exponential decrease of error probabilities in the limit of infinitely many input systems. For pure input states, or if \rho is mixed but only information about its spectrum is required, we then show that the proposed scheme is optimal in the sense that it provides the fastest possible decay of error probabilities. In the general case, however, the optimality question remains open.

QISW04 17th November 2004
14:45 to 15:30
M Guta On the relation between information and disturbance in quantum measurements

The state of a quantum system is perturbed due to the interaction with the environment. In quantum control one tries to undo the perturbation by using measurement results to perform correction operations. The correction is perfect if the measurement results do not contain any information about the state of the system which is considered unknown. If some information is obtained then inevitably the system cannot be brought back in the initial state. We show however that the perturbation is bounded by a power of the information. We then look at the difference between in the Fisher information before and after the measurement and show that it is bounded by the perturbation. An application of these ideas is the use of squeezed states for controlling a system shown recently by L. Bouten.

QISW04 18th November 2004
10:30 to 11:15
A Fujiwara Differential geometry of quantum channel estimation

Since almost every quantum protocol assumes a priori knowledge of the behavoir of the quantum channel under consideration, there is no doubt that identifying the channel is of fundamental importance in quantum information theory. In this talk, I will review some recent developments in quantum channel estimation theory, putting emphasis on an active interplay between noncommutative statistics and information geometry.

QISW04 18th November 2004
11:15 to 12:00
F De Martini Optimal realisation of non-unitary maps for quantum information

It is well known that several state transformations, allowed in the framework of classical information theory, cannot be realized exactly in the quantum realm. Precisely, these correspond to the non Completely Positive (Non-CP) Maps wich play a substantial role in fundamental issues as the superluminal signaling in Einstein-Podolsky-Rosen correlations or within any universal spin flip process. We outline the behaviour of the most important Non-CP Maps of general relevance in quantum information and the strategy adopted to implement them optimally and universally. All that will be substantiated by corresponding experiments based on two different methods of modern quantum optics: the photon stimulated emission process in a quantum injected optical parametric amplifier or the Ou-Mandel quantum-state photon symmetrization procedure.

QISW04 18th November 2004
14:00 to 14:45
Quantum estimation for non-differentiable models

State estimation is a classical problem in quantum information. In optimization of estimation scheme, to find a lower bound to the error of the estimator is a very important step. So far, all the proposed tractable lower bounds use derivative of density matrix. However, sometimes, we are interested in quantities with singularity, e.g. concurrence etc. In the paper, lower bounds to a Mean Square Error (MSE) of an estimator are derived for a quantum estimation problem without smoothness assumptions. Our main idea is to replace the derivative by difference, as is done in classical estimation theory. We applied the inequalities to several examples, and derived optimal estimator for some of them. (quant-ph/0207150)

QISW04 18th November 2004
14:45 to 15:30
Z Hradil Maximum likelihood methods in quantum mechanics

The principle of Maximum Likelihood (MaxLik) is not a rule that requires justification. It does not need to be proved and nowadays it is widely used in many applications. What makes this technique so attractive and powerful is it efficiency and versatility. MaxLik estimation may be advanatageously applied to the inverse problems in quantum mechanics. Variables, which cannot be directly measured may always be estimated obeying the rules of quantum theory. Such a tight relationship will be demonstrated on the estimation of the phase shift and number-phase uncertainty relations. This motivates the application of the MaxLik for the quantum state reconstruction, the so called quantum tomography. Extremal equation for the MaxLik estimate of quantum state will be found and interpreted as the closure relation for quantum state measurement. Though the operator equation is nonlinear, it may be solved by iterations. The accuracy may be evaluated by means of Fisher information matrix. MaxLik estimation may be easily modified in order to treat the insufficient data. In this sense the MaxLik reconstruction represents an advantageous alternative to the linear reconstruction techniques based for example on the Radon transformation, which are prone to artifacts of various origin. MaxLik estimation will be demonstrated on several examples including the operational phase concepts, reconstruction of spin and entangled spin states, reconstruction of higher dimensional states of photons with angular momentum, reconstruction of photon statistics counted by inefficient detectors or absorption and phase X-ray tomography.

QISW04 18th November 2004
16:15 to 17:00
Estimation of SU(d) using entanglement the d $>$ 2 case

In recent papers (Refs [1-4]) it is shown that if N copies of an SU(2) gate are available, one can estimate it with a square error that goes to 0 as 1/N^2 (instead of 1/N as one normally expects in statistics). This is achieved by using an Nfold entangled state as input. In my talk I will try to show that this is also possible for SU(d) with d > 2.

[1] M. Hayashi, (2004), quant-ph/0407053. [2] E. Bagan, M. Baig, and R. Munoz-Tapia, Phys. Rev. A 69, 050303 (2004), quant-ph/0303019. [3] E. Bagan, M. Baig, and R. Munoz-Tapia, Phys. Rev. A 70, 030301 (2004), quant-ph/0405082. [4] G. Chiribella, G. D.Ariano, P. Perinotti, and M. Sacchi, (2004), quant-ph/0405095.

QISW04 19th November 2004
10:30 to 11:15
J-A Larsson The Bell inequality and the coincidence time loophole

This paper analyzes effects of time dependence in the Bell inequality. A generalized inequality is derived for the case when coincidence and non-coincidence (and hence whether or not a pair contributes to the actual data) is controlled by timing that depends on the detector settings. Needless to say, this inequality is violated by quantum mechanics and could be violated by experimental data provided that the loss of measurement pairs through failure of coincidence is small enough, but the quantitative bound is more restrictive in this case than in the previously analyzed "efficiency loophole".

QISW04 19th November 2004
11:15 to 12:00
E Loubenets Class of quantum states satisfying the original Bell inequality

We introduce the analytic property of a quantum state (separable or nonseparable) to satisfy the perfect correlation form of the original Bell inequality under any quantum measurements of Alice and Bob.

QISW04 19th November 2004
14:00 to 14:45
Repeatable measurements without eigenstates

We show that, contrarily to the widespread belief, in quantum mechanics repeatable measurements are not necessarily described by orthogonal projectors-the customary paradigm of observable. Nonorthogonal repeatability, however, occurs only for infinite dimensions. We also show that when a non orthogonal repeatable measurement is performed, the measured system retains some "memory" of the number of times that the measurement has been performed.

QISW04 19th November 2004
14:45 to 15:30
Universal uncertainty principle

In 1927 Heisenberg [1] proposed a reciprocal relation for measurement noise and disturbance, or equivalently for joint measurement noises, by the famous gamma ray microscope thought experiment and claimed that the relation is a straightforward mathematical consequence of the commutation relation. However, his proposed proof did not really consider the measurement noise or disturbance, and was immediately reformulated by Kennard [2] as the famous inequality for the standard deviations of position and momentum. Kennard's relation was soon generalized by Robertson [3] to arbitrary pairs of observables. In spite of the above development, text books of quantum mechanics have treated Robertson's relation as a mathematical expression of Heisenberg's original relation for noise and disturbance.

In this talk, we discuss a new relation (the universal uncertainty principle) found and rigorously proven recently by the present speaker [8] for the well-defined root-mean-square noise and disturbance in all the physically possible quantum measurements. In the previous work [4], it has been shown that jointly unbiased measurements satisfy the reciprocal joint measurement noise relation analogous to Robsertson's. However, it should be pointed out that there is no jointly unbiased spin measurements for two different components, and that no projective measurements of finite level systems satisfy the reciprocal noise disturbance relation. Counter examples of the original Heisenberg relation also include the contractive state position measurement and the indirect position measurement for the EPR pair [6,8]. The universal uncertainty relation reveals the correct bound for the noise in non-disturbing measurements. Using this noise bound, we construct a proof of a quantitative generalization of the Wigner-Araki-Yanase theorem, setting a lower bound for the accuracy of measurement under conservation laws [5,9]. Then, this relation is used to give a lower bound for the accuracy of quantum gate operations realized by interactions between qubits and the controller (or the ancilla) obeying given conservation laws [7,9]. The relations to the no-cloning theorem and to security arguments for quantum cryptography will be also discussed briefly.

References. 1. W. Heisenberg, Z. Phys. 43, 172 (1927). 2. E. H. Kennard, Z. Phys. 44, 326 (1927). 3. H. P. Robertson, Phys. Rev. 34, 163 (1929). 4. M. Ozawa, in Lecture Notes in Physics 378 (Springer, Berlin), 3 (1991); S. Ishikawa, Rep. Math. Phys. 29, 257 (1991). 5. M. Ozawa, Phys. Rev. Lett. 88, 050402 (2002). 6. M. Ozawa, Phys. Lett. A 299, 1 (2002). 7. M. Ozawa, Phys. Rev. Lett. 89, 057902 (2002); 91, 089802 (2003). 8. M. Ozawa, Phys. Rev. A 67, 042105 (2003); Phys. Lett. A, 318, 21 (2003); Phys. Lett. A 320, 367 (2004); Ann. Phys. 31, 350 (2004). 9. M. Ozawa, Intern. J. Quant. Inf. (IJQI) 1, 569 (2003).

QIS 23rd November 2004
11:00 to 12:00
M Shirokov The Holevo capacity of infinite dimensional channels and the additivity problem
QIS 30th November 2004
11:00 to 12:00
High fidelity to low weight
QIS 1st December 2004
14:15 to 15:30
Entanglement and decoherence in semi-quantal gases
QIS 2nd December 2004
11:00 to 12:00
P Horodecki Quantum states and quantum channels
QIS 7th December 2004
11:00 to 12:00
L Kauffman Spin networks and anyonic topological quantum computing
QIS 9th December 2004
11:00 to 12:00
Quantum voting
QIS 9th December 2004
16:00 to 16:30
Uncertainty, monogamy, and implications for entanglement
QIS 9th December 2004
16:30 to 17:00
Communication complexity and fault tolerance
QIS 13th December 2004
15:00 to 17:00
Tutorial for QIS participants
QISW05 14th December 2004
10:00 to 11:00
Model of a big crunch/big bang transition
QISW05 14th December 2004
11:30 to 12:30
Black hole analogues and the universality of the Hawking effect
QISW05 14th December 2004
16:30 to 17:30
Bohr, Penrose and Hawking

No Phenomenon is a phenomenon until it is an observed phenomenon. What limits does quantum measurement theory place on black hole physics?

QISW05 15th December 2004
10:00 to 11:00
Information loss, determinism and quantum mechanics

Since the probabilistic interpretation of Quantum Mechanics requires unitarity, conventional Quantum Mechanics does not allow for a notion such as information loss. However, it is possible to interpret Quantum Mechanics in terms of an underlying deterministic theory, and it is here, in the classical sense, that information loss can be introduced. We argue that information loss then indeed may be an essential ingredient of the theory, helping us to understand how the notion of holography can be reconciled with locality. Thus, determinism and information loss may become crucial ingredients of Planck Scale Physics, and we explain why exactly this should lead to the unitary quantum mechanical framework of the Standard Model of the subatomic world.

QISW05 15th December 2004
11:30 to 12:30
Limits of quantum mechanics from general relativity: a clash of principles

The subject of "quantum gravity" is generally studied from the point of view of how our classical space-time picture might become modified when it is made subject to the principles of quantum (field) theory. Yet there are several good reasons to believe that, conversely, the very principles of quantum mechanics are also limited in their scope, and that Einstein's principle of equivalence may supply a limitation to the basic quantum principle of linear superposition. Such a limitation is suggested from a study of the Galilean limit of the Unruh effect, and appears to be accessible to forseeable experimental test.

QISW05 15th December 2004
16:00 to 17:00
Universe from sub-Planckian bits
QISW05 16th December 2004
10:00 to 11:00
D Gottesman Black hole evaporation, unitarity and final state projection

What happens when a black hole evaporates? Does it leave behind anything more than a spray of Hawking radiation, and perhaps more importantly, how is the state of that radiation related to objects that fell into the black hole? In particular, is the overall behavior of the system unitary, or is information about the system's initial state irrevocably lost? I will give an overview of proposed resolutions to the black hole information-loss problem aimed towards those studying quantum information. I will then focus on a recent proposal by Horowitz and Maldacena to solve the problem by introducing a final state projection at the black hole singularity, and an objection to this solution by Preskill and myself.

QISW05 16th December 2004
11:30 to 12:30
C Galfard The path integral approach to the black hole information problem

The black hole information loss problem has been the focus of much debate for the past thirty years. I will review the path integral approach to the problem: how it can be seen that black holes evaporate (Hartle-Hawking) thus apparently destroying information and how the sum over (Euclidean) space-time topologies may lead to a recovery of the information (Maldacena, Hawking). I will also point out why this may not be the end of the story (Barbon-Rabinovici) and, time permitting, mention a related holographic black hole non-formation argument to solve the problem which I am working on with Germani and Ishibashi.

QISW05 16th December 2004
16:30 to 17:30
When entanglement met black hole...

There are two ways of doing quantum information theory beyond qubits. One task is to investigate its basic notions in the new situations. The familiar entities there acquire some unusual properties. Entropy is observer-dependent, but not always so. Entanglement measures may be observer-dependent as well. We discuss the reasons for this dependence and the implications of both invariance and non-invariance, illustrating by the geometric entropy in quantum field theory and black hole entropy.

On the other hand, we may apply quantum information to the problems in other areas. For example, the logarithmic corrections for the black hole entropy may be related to the entanglement between parts of the spin network that describes its horizon. We also speculate that the entropy production during the black hole creation and evaporation is actually an expression of entanglement between gravity and matter.

QISW05 16th December 2004
17:30 to 18:30
Quantum information theory and the low energy problem of quantum gravity
QISW05 17th December 2004
10:00 to 11:00
S Lloyd Quantum gravity and quantum computation

This talk proposes a theory of quantum gravity and elementary particle physics based on quantum computation. In this theory, fundamental processes are described in terms of quantum information processing: the structure of space-time and of quantum fields are constructs, derived from the underlying quantum computation.

QISW05 17th December 2004
11:30 to 11:50
Entanglement and area
QISW05 17th December 2004
11:50 to 12:10
Locked information and the black hole information loss problem

Recently, a new and purely quantum effect has been discovered in which a small amount of information acts as a key that can unlock a much larger amount of information. Classically, the accessible mutual information about a random variable is never less than the total amount of entropy in the random variable minus the size of the key. Quantum-mechanically, the accessible information without the key can vanishing compared to the total entropy available when the key is known.

This effect may help resolve the black hole infomration loss problem: If the information leaks out of an evaporating black hole while appearing random until the hole is of Plank size, then the plank scale black hole must someone "encrypt" all the information that has radiated away so far while itself having very small entropy. This was thought to be impossible but is precisely the case in the locking effect.

QISW05 17th December 2004
12:10 to 12:30
Black holes, simulated time travel and the scalability of complexity