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International Conference on Quantum Information

Qubus Computation and its applications to Hybrid Quantum Repeaters

Kae Nemoto

Published Jan 1, 2008• 0 citations
Publisher

Abstract

Qubus computation is a type of quantum information processing (QIP) where computational qubits couple through quantum bus (qubus). Qubus computation are flexible in terms of physical systems, properties and scales. We explore the hybrid nature and distributed nature of qubus computation, which can be considered to be important and essential feature for scalable QIP. As its application, we investigate the characteristics of a quantum repeater protocol based on the quantum bus (qubus) approach for long-distance quantum communication.

DOI
10.1364/icqi.2008.qwd3

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Qubus computation is a type of quantum information processing (QIP) where computational qubits couple through quantum bus (qubus). Qubus computation are flexible in terms of physical systems, properties and scales. We explore the hybrid nature and distributed nature of qubus computation, which can be considered to be important and essential feature for scalable QIP. As its application, we investigate the characteristics of a quantum repeater protocol based on the quantum bus (qubus) approach for long-distance quantum communication.
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.
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.
Quantum networks play a pivotal role in quantum information science, which not only provide a secure communication platform for remote access to quantum computers but also serve as the strategic core for achieving large-scale quantum information processing, forming the foundational infrastructure for the future global-scale quantum internet. Quantum teleportation, which enables the transmission of unknown quantum states over long distances by employing quantum entanglement together with classical communication, is essential for the distribution of quantum resources in the construction of the global-scale quantum internet. To realize a global-scale quantum internet, quantum repeater protocols represent one of the most promising approaches for enabling quantum communication between any nodes. This concise review presents representative experimental demonstrations of quantum teleportation for constructing quantum networks across different physical platforms. Along this trajectory, the review discusses current challenges, open issues, and future perspectives toward scalable and practical quantum internet.
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.
We propose a hybrid quantum computing scheme where qubit degrees of freedom for computation are combined with quantum continuous variables for communication. In particular, universal two-qubit gates can be implemented deterministically through qubit-qubit communication, mediated by a continuous-variable bus mode (``qubus''), without direct interaction between the qubits and without any measurement of the qubus. The key ingredients are controlled rotations of the qubus and unconditional qubus displacements. The controlled rotations are realizable through typical atom-light interactions in quantum optics. For such interactions, our scheme is universal and works in any regime, including the limits of weak and strong nonlinearities.