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arXiv:1501.07712 · Physical Review A

Scalable quantum computation architecture using always-on Ising interactions via quantum feedforward

Takahiko Satoh, Yuichiro Matsuzaki, Kosuke Kakuyanagi, William J. Munro, Kouichi Semba, Hiroshi Yamaguchi, Shiro Saito

Published May 28, 2015• 3 citations
arXivHTMLPDF

Abstract

Here, we propose a way to control the interaction between qubits with always-on Ising interaction. Unlike the standard method to change the interaction strength with unitary operations, we fully make use of nonunitary properties of projective measurements so that we can effectively turn the interaction on or off via feedforward. Our scheme is useful for generating two- or three-dimensional cluster states that are universal resources for fault-tolerant quantum computation with this scheme, and it provides an alternative way to realize a scalable quantum processor.

DOI
10.1103/physreva.91.052329

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Here, we propose a way to control the interaction between qubits with always-on Ising interaction. Unlike the standard method to change the interaction strength with unitary operations, we fully make use of nonunitary properties of projective measurements so that we can effectively turn the interaction on or off via feedforward. Our scheme is useful for generating two- or three-dimensional cluster states that are universal resources for fault-tolerant quantum computation with this scheme, and it provides an alternative way to realize a scalable quantum processor.
Recently, fault tolerant quantum computation such as surface coding and topological quantum computation has been paid attention to due to high threshold against computational errors compared with that of conventional gate-model quantum computation.These schemes use a two or three-dimensional cluster state as universal resources for quantum computation.We can generate a cluster state by performing controlled-phase gates on an initial state prepared in |+⟩ state, where the control-phase gates are realized by nearest neighbor Ising type interactions.There are many proposal to realize Ising type interaction such as ultracold atoms in an optical lattice, superconducting charge qubits, superconducting spin qubits, resonator wave guides, nitrogen-vacancy centers, and quantum dots.Among many candidates, we especially discuss superconducting flux qubits.Superconducting flux qubits have a strong anharmonicity, so that we can perform high-speed single-qubit rotation with excellent fidelity.Usually, implementation of high-fidelity two-qubit gates is the hardest part for the realization of quantum computation.Two qubit gates require in-situ turn on/off the interaction between qubits by the external control apparatus.Since imperfection of the interaction control tends to induce spatially correlated errors between qubits, sophisticated technology is required to suppress such error rate below the threshold of fault tolerant quantum computation.Up to now, the generation of a large cluster state using solid-state qubit has not been demonstrated in a scalable manner yet.One of the main obstacles is the requirement for independent control of two-qubit gates that are necessary for the scalable quantum information processing.For a superconducting flux qubit, existing method uses applied magnetic field to control the interaction.It is known that the application of magnetic field to local area is difficult.Thus, it is hard to implement local control of the interaction without cross-talk problem where unwanted errors occurs between qubits.Firstly, we propose a novel way to control a number of two-qubit gates independently without using the on/off switching of locally applied magnetic field.Specifically, we assume the superconducting flux qubits that are inductively coupled with each other via always-on Ising interaction.Unlike the previous method to change the interaction strength with unitary operations, we fully make use of non-unitary properties of projective measurements so that we can effectively turn on/off the interaction via quantum feedforward.Also, we show how to generate a two or three-dimensional cluster state that are universal resource for fault tolerant quantum computation with constant stepsize operations.The on/off switching of locally applied magnetic field is not necessary to First of all, I would like to thank my adviser Professor Hiroshi Imai for his valuable comments and continuous support during the last five years.
Quantum computation can be performed by encoding logical qubits into the states of two or more physical qubits, and control of effective exchange interactions and possibly a global magnetic field. This "encoded universality" paradigm offers potential simplifications in quantum computer design since it does away with the need to control physical qubits individually. Here we show how encoded universality schemes can be combined with fault-tolerant quantum error correction, thus establishing the scalability of such schemes.
We propose a measurement-based model for fault-tolerant quantum computation that can be realized with one-dimensional cluster states and fusion measurements only; basic resources that are readily available with scalable photonic hardware. Our simulations demonstrate high thresholds compared with other measurement-based models realized with basic entangled resources and 2-qubit fusion measurements. Its high tolerance to noise indicates that our practical construction offers a promising route to scalable quantum computing with quantum emitters and linear-optical elements.
We present a detailed description of an architecture for fault-tolerant quantum computation, which is based on the cluster model of encoded qubits. In this cluster-based architecture, concatenated computation is implemented in a quite different way from the usual circuit-based architecture where physical gates are recursively replaced by logical gates with error-correction gadgets. Instead, some relevant cluster states, say fundamental clusters, are recursively constructed through verification and postselection in advance for the higher-level one-way computation, which namely provides error-precorrection of gate operations. A suitable code such as the Steane seven-qubit code is adopted for transversal operations. This concatenated construction of verified fundamental clusters has a simple transversal structure of logical errors, and achieves a high noise threshold  3~3% 3 for computation by using appropriate verification procedures. Since the postselection is localized within each fundamental cluster with the help of deterministic bare controlled-ZZZ gates without verification, divergence of resources is restrained, which reconciles postselection with scalability.
Photonics is the platform of choice to build a modular, easy-to-network quantum computer operating at room temperature. However, no concrete architecture has been presented so far that exploits both the advantages of qubits encoded into states of light and the modern tools for their generation. Here we propose such a design for a scalable fault-tolerant photonic quantum computer informed by the latest developments in theory and technology. Central to our architecture is the generation and manipulation of three-dimensional resource states comprising both bosonic qubits and squeezed vacuum states. The proposal exploits state-of-the-art procedures for the non-deterministic generation of bosonic qubits combined with the strengths of continuous-variable quantum computation, namely the implementation of Clifford gates using easy-to-generate squeezed states. Moreover, the architecture is based on two-dimensional integrated photonic chips used to produce a qubit cluster state in one temporal and two spatial dimensions. By reducing the experimental challenges as compared to existing architectures and by enabling room-temperature quantum computation, our design opens the door to scalable fabrication and operation, which may allow photonics to leap-frog other platforms on the path to a quantum computer with millions of qubits.
Photonics is the platform of choice to build a modular, easy-to-network quantum computer operating at room temperature. However, no concrete architecture has been presented so far that exploits both the advantages of qubits encoded into states of light and the modern tools for their generation. Here we propose such a design for a scalable fault-tolerant photonic quantum computer informed by the latest developments in theory and technology. Central to our architecture is the generation and manipulation of three-dimensional resource states comprising both bosonic qubits and squeezed vacuum states. The proposal exploits state-of-the-art procedures for the non-deterministic generation of bosonic qubits combined with the strengths of continuous-variable quantum computation, namely the implementation of Clifford gates using easy-to-generate squeezed states. Moreover, the architecture is based on two-dimensional integrated photonic chips used to produce a qubit cluster state in one temporal and two spatial dimensions. By reducing the experimental challenges as compared to existing architectures and by enabling room-temperature quantum computation, our design opens the door to scalable fabrication and operation, which may allow photonics to leap-frog other platforms on the path to a quantum computer with millions of qubits.