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arXiv:0905.2242 · Journal of the Physical Society of Japan

Fluxon-Based Generation of Graph States in Josephson Qubits

Toshiyuki Fujii, Masaru Fukunaga, Noriyuki Hatakenaka

Published Oct 13, 2009• 0 citations
arXivHTMLPDF

Abstract

Graph states are a special kind of multiparticle entangled state with great potential for applications in quantum information technologies, especially in measurement-based quantum computers. These states cause significant reductions of the number of qubits needed for a given computation, leading to shorter execution time. Here we propose a simple scheme for generating such graph states by using special gate operations, i.e., control-phase and swap gate operations, inherent in superconducting quantum nanocircuits.

DOI
10.1143/jpsj.78.103801

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Graph states are a special kind of multiparticle entangled state with great potential for applications in quantum information technologies, especially in measurement-based quantum computers. These states cause significant reductions of the number of qubits needed for a given computation, leading to shorter execution time. Here we propose a simple scheme for generating such graph states by using special gate operations, i.e., control-phase and swap gate operations, inherent in superconducting quantum nanocircuits.
We propose an improved architecture for the physical implementation of two-qubit operations between arbitrary pairs of qubits by current biasing of coupled Josephson junctions. This improvement is necessary for the realization of a functional and scalable quantum computer. The proposed architecture is within the reach of current experimental technology. Secondly, we investigate a simple scheme for generating the graph states via the improved architecture. Since the controlled-phase gate between arbitrary pairs of qubits can be implemented easily, we can generate any multi-qubit graph states corresponding to various shape graphs, which meet the expectations of various quantum information processing schemes. It is a simple, scalable and feasible scheme for the generation of various graph states based on the current-controlled superconducting charge qubit network.
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We present a scheme for rapidly entangling matter qubits in order to create graph states for one-way quantum computing. The qubits can be simple 3-level systems in separate cavities. Coupling involves only local fields and a static (unswitched) linear optics network with detectors that need not be photon number resolving. Fusion of graph state sections occurs with, in principle, zero probability of damaging the nascent graph state. We avoid the finite thresholds of other schemes by operating on two entangled pairs, so that each generates exactly one photon. We do not require the relatively slow single qubit local flips to be applied during the growth phase: growth of the graph state can then become a purely optical process. The scheme naturally generates graph states with vertices of high degree and so is easily able to construct minimal graph states, with consequent resource savings. The most efficient approach will be to create new graph state edges even as qubits elsewhere are measured, in a 'just in time' approach. An error analysis indicates that the scheme is robust against imperfections in the apparatus.
The technologies of Josephson-junction-based qubits have been progressing rapidly, ever since its first demonstration by a superconducting charge qubit1. A variety of systems have been implemented with remarkable progress in coherence time and read-out schemes. Although the current level of this solid-state device is still not as advanced as that of the most advanced microscopic-system-based qubits, these developments, together with the potential scalability, have renewed its position as a strong candidate as a building block for the quantum computer. Recently, coherent oscillation and microwave spectroscopy in capacitively-coupled superconducting qubits have been reported. The next challenging step toward quantum computation is a realization of logic gates. Here we demonstrate a conditional gate operation using a pair of coupled superconducting charge qubits. Using a pulse technique, we prepare different input states and show that they can be transformed by controlled-NOT (C-NOT) gate operation in the amplitude of the states. Although the phase evolution during the gate operation is still to be clarified, the present results are a major step toward the realization of a universal solid-state quantum gate.