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Timetable (QISW02)

Entanglement and Transfer of Quantum Information

Monday 27th September 2004 to Friday 1st October 2004

Monday 27th September 2004
09:00 to 09:45 Y Hirayama ([NTT Basi Research Labs])
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.

09:45 to 10:30 L Oberbeck ([New South Wales])
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).

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11:00 to 11:45 A Greentree ([Melbourne])
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.

11:45 to 12:30 F Jelezko ([Stuttgart])
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.

14:00 to 14:45 C Marcus ([Harvard])
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.

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14:45 to 15:30 C Smith ([Cambridge])
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

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16:00 to 16:45 A Ardavan ([Oxford])
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.

16:45 to 17:30 P Lindelof ([Copenhagen])
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.

Tuesday 28th September 2004
09:00 to 09:45 F Meier ([UC, Santa Barbara])
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.

09:45 to 10:30 L Vandersypen ([Delft])
Electron spin qubits in quantum dots
11:00 to 11:45 L Tian ([Innsbruck])
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.

11:45 to 12:30 C Monroe ([Michigan])
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.

14:00 to 14:45 M Plenio ([Imperial])
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.

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14:45 to 15:30 S Benjamin ([Oxford])
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).

16:00 to 16:45 G Burkard ([IBM])
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.

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16:45 to 17:30 T Osborne ([Bristol])
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.

Wednesday 29th September 2004
09:00 to 09:45 A Steane ([Oxford])
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.

09:45 to 10:30 RB Blatt ([Innsbruck])
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).

11:00 to 11:45 E Hinds ([Imperial])
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.

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11:45 to 12:30 P Knight ([Imperial])
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

14:00 to 14:45 IA Walmsley ([Oxford])
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.

14:45 to 15:30 W Munro ([HP Labs])
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.

15:30 to 16:15 J Eisert ([Potsdam])
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.

16:45 to 17:30 JG Rarity ([Bristol])
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).

17:30 to 18:15 G Pryde ([Queensland])
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.

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Thursday 30th September 2004
09:00 to 09:45 H Mooij ([Delft])
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.

09:45 to 10:30 T Schaetz ([NIST])
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.

11:00 to 11:30 GJ Milburn ([Queensland])
Quantum closed-loop control and error correlation
11:45 to 12:30 N Gisin ([Geneva])
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.

University of Cambridge Research Councils UK
    Clay Mathematics Institute London Mathematical Society NM Rothschild and Sons