Long range transport of quantum information is of huge importance to the physical realisation of large scale quantum computers. This letter introduces a transport bus that deterministically mediates entanglment pairwise between isolated data qubits, while the bus itself never carries information. We demonstrate how this scheme generates standard two qubit operator measurements and its application to the preparation of linear cluster states and teleportation based universal computation.
Long range transport of quantum information is of huge importance to the physical realisation of large scale quantum computers. This letter introduces a transport bus that deterministically mediates entanglment pairwise between isolated data qubits, while the bus itself never carries information. We demonstrate how this scheme generates standard two qubit operator measurements and its application to the preparation of linear cluster states and teleportation based universal computation.
Processing information quantum mechanically is known to enable new communication and computational scenarios that cannot be accessed with conventional information technology (IT). We present here a new approach to scalable quantum computing---a "qubus computer"---which realizes qubit measurement and quantum gates through interacting qubits with a quantum communication bus mode. The qubits could be "static" matter qubits or "flying" optical qubits, but the scheme we focus on here is particularly suited to matter qubits. Universal two-qubit quantum gates may be effected by schemes which involve measurement of the bus mode, or by schemes where the bus disentangles automatically and no measurement is needed. This approach enables a parity gate between qubits, mediated by a bus, enabling near-deterministic Bell state measurement and entangling gates. Our approach is therefore the basis for very efficient, scalable QIP, and provides a natural method for distributing such processing, combining it with quantum communication.
In this paper, we present results illustrating the power and flexibility of one-bit teleportations in quantum bus computation. We first show a scheme to perform a universal set of gates on continuous variable modes, which we call a quantum bus or qubus, using controlled phase-space rotations, homodyne detection, ancilla qubits, and single-qubit measurement. Within our comparison criteria, the resource usage for this scheme is lower than any previous scheme to date. We then illustrate how one-bit teleportations into a qubus can be used to encode qubit states into a quantum repetition code, which in turn can be used as an efficient method for producing Greenberger-Horne-Zeilinger states that can be used to create large cluster states. Each of these schemes can be modified so that teleportation measurements are post-selected to yield outputs with higher fidelity without changing the physical parameters of the system.
There are numerous proposals for the physical realization of a quantum computer.However, distributed approaches, making use both of flying and stationary qubits, seem to constitute the most promising route towards a truly scalable device.Such systems guarantee extendibility, they incorporate the interface with communication applications and relax the physical realization of the device, allowing for defect tolerance.Flying qubits are included in the more general concept of a quantum bus, a mediating system which can be of higher dimension.Such a quantum bus can be used in the straightforward preparation of a standard multi-qubit resource enabling measurement based quantum computation, the cluster state.This constitutes the framework for the results presented in this thesis.We begin by investigating the effects of dissipation in the continuous variable bus scheme known as the qubus scheme.By considering loss in the bus as it mediates interactions between the stationary qubits, we obtain analytical results for the effective action of the induced quantum gate.We find that a particular two-qubit gate operates with high fidelity in the presence of moderate loss and give a simple iteration scheme to simplify the effects of loss on the qubits.We then attempt to reduce these effects by preparing the bus in more elaborate state, however no improvements are observed.We then apply the qubus scheme to the probabilistic generation of cluster states and develop an entangling gate working with high success probability.This allows us to produce cluster states far more efficiently than other proposals.Investigating new methods to analyze the performance of different generation strategies constitutes the second part of this set of results.We begin by making the large flow approximation, used in queuing theory, to obtain the optimal strategy in a regime with large resources.After what we take the other more familiar limit of single cluster growth and introduce absorbing Markov chains as a key mathematical tool.Finally we look at the transmission of composite quantum systems via a single higher dimensional bus.We provide generalized protocols and interactions guaranteeing a full transfer of the information from one composite system to another.These protocols can also serve information processing tasks, as useful logical operations can be applied to the data as it is transfered.We notice lastly that the qubus scheme constitutes a potential physical realization.
The effective Heisenberg interaction is generated by weakly interacting qubits with two strongly coupled chains. The system of two coupled chains can be regarded as a quantum bus. Universal quantum computation is enabled by restricting the bus to its ground state. The effective interaction is primarily determined by the energy spectrum of the bus. The interactions alternate between antiferromagnetic and ferromagnetic ones. The long-range interaction can also be attained in coupled chains with infinite size. The quantum gate operations with high precision can be realized when the quantum fluctuations are taken into account.
The generation and exploitation of entangled states has become one of the most important technological challenges of the twenty-first century. In order to reliably generate multi-particle entangled states, one needs controllable two (or more) qubit interactions, and there have been many proposals for suitable systems. We have recently begun investigating adiabatic transfer protocols (Greentree et al. 2004), which are distinguished from conventional, nonadiabatic approaches by their robustness and relative insensitivity to noise. Focussing on the phosphorus in silicon paradigm we have shown how such transport leads to new quasi-2D architectures (Hollenberg et al. 2006), and surprising particle coupling schemes based on operator measurements (Devitt et al. 2005; Greentree et al. 2006), which in turn may lead to efficient mechanisms for syndrome identification in quantum error correction. Here we describe the realisation of these operator measurements more fully based on adiabatic passage of a control particle, highlighting the applicability of our protocols by showing how they can improve the scalability of other implementations. Introduction The pursuit of devices which can exploit quantum coherence is widely seen as one of the most important and challenging tasks facing twenty-first Century Technology. Defined by the use of coherent quantum systems (at their simplest: qubits, two-state systems) such devices enable a range of tasks, from quantum key distribution via single qubits (Bennett and Brassard 1984) or entangled states (Ekert 1991), to protocols for quantum teleportation (Bennett et al. 1993), efficient quantum simulation (Feynmann 1982; Abrams and Lloyd 1997) and factorisation (Shor 1994). At the heart of most such protocols is the exploitation of entanglement, where qubits that have been entangled, in some sense feel an interaction even if space-like separated. In fact, entanglement is a ubiquitous feature of quantum systems: the task for quantum information devices is to generate controlled and isolated entanglement, so that only the desired qubits are entangled with each other, and deleterious entanglement to the environment avoided. Broadly speaking, entanglement can be generated in one of two ways, either by direct interactions between qubits, or by collective measurement of two (or more) particles in particular bases that are, by their very nature, entangled. Such projective measurements are called operator measurements, and a flexible mechanism to realise them forms the basis of this paper. Operator measurements are intimately linked to the concept of quantum error correction (QEC) (Nielsen and Chuang 2000), and as QEC is an extremely important task for quantum computing, it is possible that our schemes may prove extremely useful in future quantum computers (QC). Operator measurements are also compatible with the concept of ‘repeat-until-success’ quantum computing (Lim et al. 2005; Barrett and Kok 2005; Lim et al. 2006) which has potential advantages for QC in realistic settings. Our method for generating operator measurements is based on protocols to transfer particles adiabatically around a quantum network. There are now several different protocols for such spatial adiabatic passage, which are all extensions of the original STIRAP (STImulated Raman Adiabatic Passage) (Vitanov et al. 2001) protocol. Spatial adiabatic passage protocols can be mediated via electromagnetic pulses (Brandes et al. 2001; Siewert et al. 2006), or direct control of wavefunction overlap (Greentree et al. 2004; Eckert et al. 2004; Petrosyan 2006). All such schemes share a common theme, namely that a particular eigenstate, which is in the null space of the Hamiltonian, can be smoothly evolved as a function of time. Such protocols are therefore extremely robust, requiring little regard for timing provided that the adiabaticity criterion (Vitanov et al. 2001) is respected. For concreteness, we will review the MRAP protocol (Multiple Recipient Adiabatic Passage) introduced by us earlier (Devitt et al. 2005; Greentree et al. 2006), and show explicitly how it can be used to mediate operator measurements. We will then comment on some of the implications of this protocol for QC designs, especially when concepts such as defect-tolerance are introduced. Multiple-Recipient Adiabatic Passage Consider a network, defined by four quantum dots that share a single electron, and two attached qubits, as depicted in Fig. 1(a). Employing quantum communication terminology, we suppose that Alice wishes to send a particle (with information encoded in the electronic spin states) to either Bob 1 or Bob 2, or to some arbitrary superposition of the Bobs, Australian Institute of Physics 17th National Congress 2006 – Brisbane, 3-8 December 2006 RiverPhys Paper No. 474 using the network. For clarity we refer to the particle transferred through the network as the bus particle. We label the quantum dots, A (Alice), 1 B (Bob 1), 2 B (Bob 2), C (Central). Also, the Bobs have qubits 1 Q and 2 Q which will be used later. We further assume that each participant has control of the coupling of their quantum dot to the central dot, and we label this coupling Ωα for α = A, B1, B2. With all dots maintained at constant energy, the Hamiltonian for this four dot system (ignoring the spin degree of freedom) is ( ) ( ) ( ) . . 2 2 1 1 c h C B t C B t A C t H B B A + Ω + Ω + Ω = where the couplings are allowed to vary as a function of time (for example by local control of surface gates to modify the barrier potential). The null space of the Hamiltonian above is a two-dimensional space, spanned by the vectors
The last two decades have witnessed the rapid growth of quantum information processing as an active field of multidisciplinary science and technology. Tremendous theoretical and experimental progress has been made in quantum communication and quantum computation, and it seems clear that some of this progress will soon result in practical applications outside the laboratory. Quantum computation aims to build a quantum computer and to explore the origins and limits of quantum computing power. The original circuit model of a quantum computer, due to Deutsch, has a strong formal similarity with the circuit model of a classical computer. Both models depict a computation as processing via the wiring of a number of fiducial gates, each of which dynamically couples a small number of input (qu)bits in a controlled manner. In other words, unitary evolution is the basic mechanism of information processing in the circuit model of quantum computers. Starting with a given fixed state of many qubits, measurement based quantum computation (MBQC) processes information by applying a sequence of measurements to designated qubits in designated bases. This is remarkable, given the probabilistic nature of quantum measurement and the murky physical status of the dynamics of 'wavefunction collapse'. The final result of the computation is determined from the classical data of all the measurement outcomes. There exist two principle models: the teleportation based model and the 'one-way quantum computer' of 'cluster state computation'. The formalism of cluster states has proven to be a powerful way of describing the essential entanglement resources needed to perform quantum information processing. Unlike the circuit model, the MBQC models have no natural classical counterpart. There are a number of physical architectures for which measurements on the relevant systems are more easily performed than controlled unitary evolution. Moreover, it appears the MBQC models can offer novel computational benefits, especially regarding issues of fault-tolerance and parallelizability of algorithms. As a result, the theory of MBQC is now shaping the latest experimental proposals across the full spectrum of quantum information processing technologies. Naturally, understanding the scope of measurement based quantum computation is also yielding many insights into what is necessary (as opposed to merely sufficient) for us to harness the power of quantum computers. Quantum communication aims to offer efficient and secure ways of information exchange, directly based on quantum mechanics. Photons serve as natural information carriers, owing to their robustness against decoherence. A major problem for photonic communication systems is due to photon loss. It turns out that this difficulty can be overcome by the quantum repeater, a measurement-based technique. The basic idea is to divide the transmission channel into many segments and perform some quantum measurement on each segment. Additional measurement-based protocols and technologies have also been demonstrated to improve the quality of quantum communication, such as entanglement swapping and purification. The articles in this invited Focus Issue cover topics both in measurement-based quantum communication and computation, but the emphasis is given to quantum computation. A significant number of articles deal with the preparation of cluster states, the experimental realization of a one-way quantum computer, and the associated efficiency and benefits. We hope that you will find that some of these new results are of interest to you, and further hope that the issue will increase your interest in this interdisciplinary research field if you are a non-expert. Focus on Measurement-Based Quantum Information Processing Contents A proof-of-principle experiment of eliminating photon-loss errors in cluster states Wei-Bo Gao, Xiao-Qi Zhou, Jin Zhang, Tao Yang and Jian-Wei Pan Continuous variable multipartite entanglement and optical implementations of quantum communication networks Yimin Lian, Changde Xie and Kunchi Peng Generalized flow and determinism in measurement-based quantum computation Daniel E Browne, Elham Kashefi, Mehdi Mhalla and Simon Perdrix The entanglement of the four-photon cluster state C Schmid, N Kiesel, W Wieczorek and H Weinfurter Optical zeno gate: bounds for fault tolerant operation Patrick M Leung and Timothy C Ralph Towards experimental entanglement connection with atomic ensembles in the single excitation regime Julien Laurat, Chin-wen Chou, Hui Deng, Kyung Soo Choi, Daniel Felinto, Hugues de Riedmatten and H J Kimble Towards minimal resources of measurement-based quantum computation Simon Perdrix Experimental realization of a quantum game on a one-way quantum computer Robert Prevedel, Andre Stefanov, Philip Walther and Anton Zeilinger Fundamentals of universality in one-way quantum computation M Van den Nest, W Dur, A Miyake and H J Briegel Loss tolerant linear optical quantum memory by measurement-based quantum computing Michael Varnava, Daniel E Browne and Terry Rudolph Graph state generation with noisy mirror-inverting spin chains Stephen R Clark, Alexander Klein, Martin Bruderer and Dieter Jaksch One-way quantum computing in adecoherence-free subspace M S Tame, M Paternostro and M S Kim Cluster state preparation using gates operating at arbitrary success probabilities K Kieling, D Gross and J Eisert Topological fault-tolerance in cluster state quantum computation R Raussendorf, J Harrington and K Goyal Strategies for the preparation of large cluster states using non-deterministic gates Peter P Rohde and Sean D Barrett A repeat-until-success quantum computing scheme A Beige, Y L Lim and L C Kwek Efficient growth of complex graph states via imperfect path erasure Earl T Campbell, Joseph Fitzsimons, Simon C Benjamin and Pieter Kok Investigation of a single-photon source based on quantum interference T B Pittman, J D Franson and B C Jacobs Entanglement and local information access for graph states Damian Markham, Akimasa Miyake and Shashank Virmani The efficiencies of generating cluster states with weak nonlinearities Sebastien G R Louis, Kae Nemoto, W J Munro and T P Spiller A direct approach to fault-tolerance in measurement-based quantum computation via teleportation Marcus Silva, Vincent Danos, Elham Kashefi and Harold Ollivier Experimental ancilla-assisted qubit transmission against correlated noise T Yamamoto, R Nagase, J Shimamura, S K Ozdemir, M Koashi and N Imoto Terry Rudolph, Imperial College, London, UK Jian-Wei Pan, University of Science and Technology of China, Hefei, People's Republic of China and Universitat Heidelberg, Germany