ABSTRACTS

Quantum repeaters based on atomic ensembles - Ignacio Cirac

Quantum repeaters are useful to extend quantum communication over long distances. They can be constructed if one has some kind of quantum memory, and once one can purify and teleport states. In this talk I will report on a proposal to build quantum repeaters using ensembles of atoms at room temperature which are excited by laser light.

Quantum information with trapped ions - Ferdinand Schmidt-Kaler

Single ions in Paul traps are investigated for quantum information processing. Superpositions of long lived states are used to implement a qubit. Single ions are either held in a spherical Paul trap or alternatively, in a linear Paul trap [1,2]. The ions are optically cooled and fluorescence light is monitored for a state measurement by the "electron shelving" technique. Cooling is achieved in all three dimensions simultaneously as well as ground state cooling of two ions. In the latter case, all modes of vibration were cooled resulting in a ground state population of more than 95% [3]. A novel type of ground state laser cooling [4,5] of a single trapped ion is achieved using a technique which tailors the absorption profile for the cooling laser by exploiting electromagnetically induced transparency in the Zeeman structure of a dipole transition. This new method is robust, easy to implement and proves particularly useful for cooling several motional degrees of freedom simultaneously, which is of great practical importance for the implementation of quantum logic schemes with trapped ions [6]. Starting from a Fock state |n=0>, coherent quantum state manipulation is achieved. The data show that decoherence is negligible on the time scale required for a quantum gate operation [2,7]. In second experiment, we aim to realize a coupling between stored qubits, based on trapped ions, and qubits transported with single photons. In a first experiment, the qubit transition of single trapped Ca+ ion is coupled to a high finesse optical cavity. The dynamical response of the cavity electromagnetic field is observed in the ion´s excitation spectrum. Furthermore, the ion is used to probe the longitudinal standing wave intracavity field [8]. The ability to interface atomic and photonic qubits is important for the implementation of quantum logic schemes with trapped ions.

[1] H.C. Nägerl, et. al., Phys. Rev. A 60, 145 (1999)
[2] Ch. Roos et. al., Phys. Rev. Lett., 83, 4713 (1999)
[3] F. Schmidt-Kaler et. al., Journal of Modern Optics 47, 2573 (2000), H. Rohde et. al., J. Opt. B: Quant. Semiclass. Opt. 3 (2001) 34
[4] G. Morigi, J. Eschner, C. H. Keitel, Phys. Rev. Lett. 85, 4458 (2000)
[5] C. F. Roos et. al., Phys. Rev. Lett., 85, 5547 (2000)
[6] F. Schmidt-Kaler et. al., quant-ph/010787
[7] A. Steane et. al., Phys. Rev. A 62, 042305 (2000)
[8] A. Mundt et. al., to be published

Entanglement engineering with atoms and photons - Michel Brune

IManipulation of tailored entangled states seats at the heart of the physics of quantum nformation. Using "circular Rydberg atoms" strongly interacting one by one with a zero or one photon field stored in a high finesse microwave superconducting cavity, we prepare a tailored entangled state in a controlled sequence of quantum gate operation.

In our experiment, the entanglement originates from the Rabi oscillation experienced by a single excited atom emitting a photon in an initially empty cavity. By adjusting the atom-cavity interaction time so that atoms experience a p/2, p or 2p pulse, we realize various quantum logic operation. The p pulse allows to write or detect a 0 and 1 photon superposition field state in the cavity. The p/2 pulse prepares a maximally entangled atom-cavity EPR pair. The 2p pulse provides the conditional dynamic corresponding to the operation of a two qubit quantum phase gate or equivalently of a C-Not gate. By combining these elementary operation, we have prepared a three qubit (two atoms and a 0 or 1 photon field) "Greenberger Horne Zeilinger" entangled state by using a programmed sequence of resonant atom cavity interactions.

By using the non-resonant atom-cavity interaction, we have operated a new quantum gate, directly providing entanglement between two atoms in a cavity assisted collision process. This entanglement method is insensitive to cavity decay.

We will also show that by controlling the state of two non-degenerate cavity modes it is possible to control entanglement between two cavity qubits. These experiments will be presented and new perspectives will be discussed.

Quantum computation with single photons and coherent pulses - Gerard Milburn

I will discuss a simplified experimental scheme to implement a CNOT gate using single photons and conditional measurement. I will also describe a conditional scheme for implementing simple quantum gates using coherent pulse encoding of logical states. The error sources of each scheme will be discussed, together with suggestions to mitigate them.

Mirrors, waveguides and chips for cold atoms - Ed Hinds

Clouds of cold atoms, collected and refrigerated by laser light, can be cooled to nanoKelvin temperatures. This has created the new field of atom optics where cold atoms are manipulated, much as photons are controlled in traditional optics using mirrors, lenses, and waveguides. I will show some of the first movies of atom clouds being manipulated.

From the viewpoint of basic science these clouds are a fantastic new tool for studying the quantum physics of gases close to absolute zero. It is now becoming possible to confine and manipulate atoms in extremely small structures, where the atomic wavepackets are only ~100 nm across. Atoms flowing in such tubes could provide the basis for a new technology similar to electronics but based on the flow and interaction of neutral atoms rather than on electricity in wires. I will describe how atom "chips" are being realised and will indicate how they might eventually be used for quantum information processing.

Geometrical strategies for Quantum Computation: the Holonomic approach - Paolo Zanardi

Holonomic Quantum Computation (HQC) is an all-geometrical approach to quantum information processing. In the HQC strategy information is encoded in degenerate eigen-spaces of a parametric family of Hamiltonians. The computational network of unitary quantum gates is realized by driving adiabatically the Hamiltonian parameters along loops in a control manifold. By properly designing such loops the non-trivial curvature of the underlying bundle geometry gives rise to unitary transformations i.e., holonomies that implement the desired unitary transformations. In view of their geometrical nature the holonomic gates are believed to be robust against several kind of perturbations and imperfections.

Simulation of Hamiltonians - representation-theoretical and combinatorial approaches - Martin Rotteler

We consider the problem of simulating Hamiltonians in quantum networks. Given a quantum system with a non-trivial Hamiltonian, simulation of any other Hamiltonian evolution can be achieved provided that a sufficiently large set of unitary control operations is available. In the first part of this talk we show that there exist finite groups having this property and present a sufficient condition for universality in terms of group characters. We give examples of such groups in small dimensions. The nodes of a multi-node quantum network are qudits which are coupled by pair-interactions. In this setting techniques from combinatorial theory can be used. We present efficient schemes based on orthogonal arrays for the problems of decoupling and inversion.

Bounds on quantum ordered searching and sorting - Peter Hoyer

A total multivariate boolean function is said to be symmetric if it is invariant under permutation of its inputs. By the seminal work of Beals, Buhrman, Cleve, Mosca, and de Wolf, we have by now a reasonable good understanding of the quantum complexities of total Boolean symmetric functions. Examples include the functions OR, AND, PARITY, MAJORITY, and THRESHOLD.

When we relax the condition of being symmetric or total even slightly, our knowledge is much more limited. A function that possesses many symmetries and for which we know at least something, is the ordered search function: of the boolean cube, it is defined only on the ordered bit-strings on which it equals the PARITY function. I will discuss the known upper and lower bounds on ordered searching, most of which have explanations via geometrical arguments. Ordered searching seems to be a good starting point for studying the quantum complexities of non-total functions.

A one-way quantum computer - Robert Raussendorf

We demonstrate that a class of highly entangled multi-particle states, the cluster states, can serve as quantum computers. Cluster states can be created efficiently with a quantum Ising-type interaction between two-state particles in a lattice configuration. It is shown that any quantum logic network, composed of CNOT-gates and 1-qubit rotations, can be implemented on a cluster state only by performing one-qubit measurements. A cluster state can be viewed as a one-way quantum computer, with the set of performed measurements forming the program. The one-way quantum computer has the property that any quantum logic network can be simulated on it. Conversely, not all ways of information processing that are possible with the one-way quantum computer can be explained within the network model. Two examples for the non-network behaviour of the one-way quantum computer are given. First, all circuits in the Clifford group can be performed in a single time-step. Many circuits for encoding are in this class. Second, not all measurement patterns to realize a quantum algorithm can be understood as a quantum logic network. An example for this is the bit-reversal gate which reverses the order of a number of qubits. The description of the one-way quantum computer in network terms is not adequate in every respect. A computational model underlying the one-way quantum computer is presented.

Topology and quantum algorithms - Mario Rasetti

The possibility is discussed that resorting to (topological) quantum field theory and its combinatorial structure might lead to quantum algorithms capable of handling in polynomial time non-polynomial enumeration problems.

Photonic Quantum Entanglement in Experiment - Anton Zeilinger

Entanglement can either be onserved through (a) creation of the entangled state in a source or (b) projection into entangles state. For independent photons the latter method is a necessary part of any teleportation protocol and it is the method of choice in the observation of entanglement of more than two photons such as in GHZ correlations. I will also briefly discuss the recent observations of violations of Bell's inequality in teleportation (entanglement swapping) and for photon states with orbital angular momentum. Finally I will argue that all information is classical in the sense that the quantum state can be seen as an expression of our knowledge of the apparatus.

Coherent control of a single cooper-pair qubit - Yasunobu Nakamura

Cooper-pair box is a small superconducting device which works as an effective two-level system. I will report our experimental activities on the coherent properties of the solid-state qubit.

Macroscopic Quantum Coherence in superconducting persistent-current loops: spectroscopy and decoherence - Kees Harmans

We present experiments on supercurrents in a micron-sized loop containing three Al-AlOx-Josephson junctions. The two quantum states of the loop are related to classically clockwise and anticlockwise circulating currents. The resulting up and down flux of the qubit is detected by a very underdamped DC-SQUID. We measured the ground state of the loop in dependence of the (external flux controlled) level splitting, and the level splitting spectrum by applying resonant (magnetic) microwaves.

To study the quantum dynamics we measured the relaxation time between the excited state and ground state by applying resonant microwaves bursts (Ton), separated by a varying time interval (Toff), and the qubit flux is measured in dependence of Toff. The relaxation time seems to depend on the level splitting, the electromagnetic environment and the applied microwave power.

Fig. 1. Ground state flux and spectroscopic
Fig. 2. The qubit flux relaxation peak and dip of the qubit versus applied flux. for 3 level splittings.

1. Y, Nakamura, Yu.A. Paskin and J.S. Tsai, Nature 398, 786 (1999)
2. Caspar v.d. Wal et.al., Science 290, 773 (2000) and refs therein.

Quantum information processing with semiconductor nanostructures - Fausto Rossi

A review of semiconductor based implementations of quantum information processing will be presented. As a first application, we shall discuss an all-optical scheme based on exciton-exciton interaction in semiconductor macroatoms/molecules [1]. We shall then review a transport-like scheme based on ballistic electrons on coupled quantum wires; the latter comes out to be particularly suited for testing Bell's inequality in a condensed-matter environment [2]. Finally, we shall propose a novel spin-based scheme, which combines the ultrafast time-scale of charge excitations with the microsecond time-scale of spin decoherence in semiconductor quantum dots.

1. E. Biolatti et al., Phys. Rev. Lett. 85, 5647 (2000).
2. R. Ionicioiu et al., Phys. Rev.A 63, 50101 (2001).

Computing with Waves: All-optical single-query 50-element database search - Ian Walmsley
Christophe Dorrer, Matt Anderson, Pablo Londero, Sascha Wallentowitz, Konrad Banaszek,
J. H. Eberly and Ian Walmsley

We demonstrate a 50-element single-query database search using interference, and show that all single-particle quantum computers are equivalent to classical wave-based processors. 2000 Optical Society of America

The Institute of Optics, University of Rochester, Rochester, NY, 14627, United States Phone : 716 275 2328, Fax : 716 244 4936, email walmsley@optics.rochester.edu

The continuity of quantum channel capacity - Reinhard Werner

We show that sufficiently small errors in a quantum channel can be corrected with negligible loss of transmission rate. In particular, the channel capacity is positive for a set of channels with positive measure. Moreover, the channel capacity is lower semicontinuous. The proof relies on quantum error correcting codes based on random graphs.

Some aspects of dynamical description of quantum computer as an open system - Michal Horodecki

So far, in descriptions of decohering quantum computer, the description of decoherence was independent of self-evolution of the computer. We point out from the theory of quantum open systems it follows that the memory of the reservoir implies dependence of decoherence on self-evolution. As a result there are multiqubit errors even in first order of approximation.

Integrated optics for practical Quantum Cryptography systems - Gabriele Bonfrate
G Bonfrate & P. D. Townsend

Since its first laboratory demonstration in 1989, Quantum Cryptography, a technique enabling secure communications between legitimate parties, has undergone significant progress. In particular, the focus has shifted from proof-of-principle demonstrations towards the realisation of Quantum Key Distribution (QKD) systems with enhanced practicality and improved operating characteristics. Most optical fibre based QKD systems employ phase encoding of the key bits using Mach-Zender interferometers. However these interferometers are typically expensive, bulky and relatively complex devices to fabricate. In addition, Mach-Zender designs often show a limited environmental stability due to the relatively long separate fibre paths employed (typically several metres). Fibre-based interferometers may not be suitable candidates, therefore, for practical QKD systems. In this presentation we describe the results of the work carried out within the EU EQUIS project, at Corning Research Centre on a new miniaturised version of channel waveguide Asymmetric Mach-Zender (AMZ) interferometers, which are suitable for the afore mentioned phase encoding in an integrated rather than an optical fibre structure. The use of integrated optics devices allows improved compactness, stability and ruggedness. The device design and fabrication process will be discussed, addressing the advantages and the novelties involved and the latest results on system tests will be presented.

"Stopping" of light and quantum memories for photons - TBC

The theory and first experimental results for a controlled and reversible transfer of the coherence and the quantum state of a photonic wavepacket to collective atomic spin excitations are presented. Potential applications to quantum memories for photons are discussed and the role of decoherence processes in systems with collective excitations are analyzed.

Active teleportation and entangled state information technology - Francesco De Martini

The structure if the newly born I.S.T. Network ATESIT is presented. A detailed description of the scientific highlights of the Program, of the most relevant research under investigation and, possibly, of the results already achieved will be given. At last, the relevant scientific and organization problems related to the ATESIT will be presented to the Community.

Quantum communication techniques with bright pulsed light - Natalia Korolkova

Quantum noise engineering and quantum entanglement of fiber optical solitons can be used as basic requisites for experimental quantum information. We present schemes for the generation and evaluation of continuous variable entanglement of bright optical beams and give a brief overview of the variety of optical techniques and quantum communication applications on this basis.

Fabrication of a silicon-based solid state quantum computer - Bob Clark

The fabrication of a scalable silicon-based quantum computer, in which the qubits are nuclear spin states of single phosphorus atoms embedded in isotopically pure silicon registered to surface control gates, is a significant challenge. The Australian program is approaching this in two ways. In our 'bottom-up' program the embedded phosphorus array is fabricated using advanced STM lithography techniques followed by Si MBE overgrowth. In our 'top down' strategy, a detailed process has been developed in which single phosphorus atoms are implanted (with on-chip verification) self-aligned to the surface control gates and fast single electron transistor readout devices. The fabrication pathways each have their list of associated problems. An outline will be given of the practical issues that have to be overcome, together with a view on how this might be achieved including progress to date. Our strategy in the top-down program is to concentrate on fabricating the simplest few-qubit test structures that will enable us to access the critical physics. However we have approached this from the viewpoint of developing a reliable, reproduceable process which, for linear phosphorus arrays, can then be readily scaled up to multi-qubit devices.

Acoustoelectric nanocircuits for quantum optics and computing - Valery Talyanskii

Our previous experiments have shown that a powerful surface acoustic wave (SAW) can transport electrons along a quasi-1D semiconductor channel in packets confined to the minima of the SAW electrostatic potential. Electron-electron interaction can suppress fluctuations in number of electrons in the packet, and the regimes with each SAW minimum containing one, two, and so on electrons have been realised.

These experiments lead to concept of acoustoelectric nanocircuits that comprise a network of quasi-1D channels serving as wires along which the SAW transports electrons. The nanocircuits allow control over charge movement down to a single electron, and over the photon emission down to a single photon. The acoustic transfer of single electrons from n- to p-type areas and subsequent single photon emission can be used in quantum optics and cryptography. Control over the electron spin in the SAW nanocircuits may open a way for implementation of quantum computation schemes that encode a qubit by the electron spin. In quantum cryptography it will give control over the polarisation of the photons emitted by the SAW single photon source.

Our current work towards implementation of the acoustoelectric nanocircuits will be discussed.

Entanglement of electron-hole pairs in quantum dot molecules - Manfred Bayer

Recently it has suggested that a a pair of vertically aligned QDs could be used as the optically driven quantum gate (1): The quantum bits are the states of individual carriers which can be either on dot zero or dot one. The different dot indices play the same role as a "spin", therefore we call them "isospin". Quantum mechanical tunneling between the dots rotates the "isospin" and leads to a superposition of the quantum dot states. The quantum gate is constructed when an electron and a hole are injected into the molecule. The two particles form entangled isospin states. The entanglement can be controlled by applying an electric field along the heterostructure growth direction. In this contribution results will be presented which support the feasibility of this proposal: We have studied single quantum dot molecules by injecting electron hole pairs optically. In particular, the evolution of the emission spectrum as function of the vertical separation between the dots has been addressed. In agreement with theory, we find a tunnel coupling induced splitting between the entangled exciton states that increases strongly with decreasing barrier width and exceeds the thermal energy at room temperature for dot separations < 5 nm resulting in an insensitivity on thermal perturbations. Experiments in magnetic field demonstrate that the entanglement can be controlled by electromagnetic fields. Further, the problem of exciton decoherence by interaction with other excitons and with phonons will be addressed. In particular we will demonstrate that the homogeneous linewidth is lifetime limited corresponding to a decoherence time of about a ns. We will also show that the spontaneous emission of excitons in quantum dots can be suppressed up to an order of magnitude by placing the dots in a resonator that exhibits a three-dimensional confinement of light.

(1) P. Hawrylak, S. Fafard, Z. R. Wasilewski, Cond. Matter News 7, 16 (1999).