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.
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.
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.
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. There is no requirement for direct interaction between the 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. In effect, the approach integrates together qubit degrees of freedom for computation with quantum continuous variables for communication and interaction.
Thesis chapter. The fragility of quantum information is a fundamental constraint faced by anyone trying to build a quantum computer. A truly useful and powerful quantum computer has to be a robust and scalable machine. In the case of many qubits which may interact with the environment and their neighbors, protection against decoherence becomes quite a challenging task. The scalability and decoherence issues are the main difficulties addressed by the distributed model of quantum computation. A distributed quantum computer consists of a large quantum network of distant nodes — stationary qubits which communicate via flying qubits. Quantum information can be transferred, stored, processed and retrieved in decoherence-free fashion by nodes of a quantum network realized by an atomic medium — an atomic quantum memory. Atomic quantum memories have been developed and demonstrated experimentally in recent years. With the help of linear optics and laser pulses, one is able to manipulate quantum information stored inside an atomic quantum memory by means of electromagnetically induced transparency and associated propagation phenomena. Any quantum computation or communication necessarily involves entanglement. Therefore, one must be able to entangle distant nodes of a distributed network. In this article, we focus on the probabilistic entanglement generation procedures such as well-known DLCZ protocol. We also demonstrate theoretically a scheme based on atomic ensembles and the dipole blockade mechanism for generation of inherently distributed quantum states so-called cluster states. In the protocol, atomic ensembles serve as single qubit systems. Hence, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultra-cold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single stepQ-qubit GHZ states with success probability psuccess ~ ηQ/2, where η is the combined detection and source efficiency. This is significantly more efficient than any known robust probabilistic entangling operation. The GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer.
We introduce an architecture for robust and scalable quantum computation using both stationary qubits (e.g., single photon sources made out of trapped atoms, molecules, ions, quantum dots, or defect centers in solids) and flying qubits (e.g., photons). Our scheme solves some of the most pressing problems in existing nonhybrid proposals, which include the difficulty of scaling conventional stationary qubit approaches, and the lack of practical means for storing single photons in linear optics setups. We combine elements of two previous proposals for distributed quantum computing, namely the efficient photon-loss tolerant build up of cluster states by Barrett and Kok [Phys. Rev. A 71, 060310(R) (2005)] with the idea of repeat-until-success (RUS) quantum computing by Lim et al. [Phys. Rev. Lett. 95, 030505 (2005)]. This idea can be used to perform eventually deterministic two qubit logic gates on spatially separated stationary qubits via photon pair measurements. Under nonideal conditions, where photon loss is a possibility, the resulting gates can still be used to build graph states for one-way quantum computing. In this paper, we describe the RUS method, present possible experimental realizations, and analyze the generation of graph states.
In this thesis I explore using the qubus for quantum computing. The qubus is an architecture of quantum computing, where a continuous variable ancilla is used to generate operations between matter qubits. I concentrate on using the qubus for two purposes - quantum simulation, and generating cluster states. Quantum simulation is the idea of using a quantum computer to simulate a quantum system. I focus on conducting a simulation of the BCS Hamiltonian. I demonstrate how to perform the necessary two qubit operations in a controlled fashion using the qubus. In particular I demonstrate an O(N3) saving over an implementation on an NMR computer, and a factor of 2 saving over a naıve technique. I also discuss how to perform the quantum Fourier transform on the qubus quantum computer. I show that it is possible to perform the quantum Fourier transform using just, 24⌊N/2⌋ + 7N − 6, this is an O(N) saving over a naıve method. In the second part of the thesis, I move on, and consider generating cluster states using the qubus. A cluster state, is a universal resource for one-way or measurement-based computation. In one-way computation, the pre-generated, entangled resource is used to perform calculations, which only require local corrections and measurement. I demonstrate that the qubus can generate cluster states deterministically, and in a relatively short time. I discuss several techniques of cluster state generation, one of which is optimal, given the physical architecture we are using. This can generate an n × m cluster in only 3nm − 2n − 2m + 4 operations. The alternative techniques look at generating a cluster using layers or columns, allowing it to be built dynamically, while the cluster is used to perform calculations. I then move on, and discuss problems with error accumulation in the generation process.