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arXiv:0707.1805 · Physical Review B

Diffusion and transfer of entanglement in an array of inductively coupled flux qubits

Rosanna Migliore, Kazuya Yuasa, M. Guccione, Hiromichi Nakazato, ANTONINO MESSINA

Published Aug 3, 2007• 6 citations
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Abstract

A theoretical scheme to generate multipartite entangled states in a Josephson planar-designed architecture is reported. This scheme improves the one published by Migliore et al. [Phys. Rev. B 74, 104503 (2006)] since it speeds up the generation of $W$ entangled states in an $M\ifmmode\times\else\texttimes\fi{}N$ array of inductively coupled Josephson flux qubits by reducing the number of necessary steps. In addition, the same protocol is shown to be able to transfer the $W$ state from one row to the other.

DOI
10.1103/physrevb.76.052501

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A theoretical scheme to generate multipartite entangled states in a Josephson planar-designed architecture is reported. This scheme improves the one published by Migliore et al. [Phys. Rev. B 74, 104503 (2006)] since it speeds up the generation of WWW entangled states in an M\ifmmode×\else\texttimes\fiNM\ifmmode\times\else\texttimes\fi{}NM\ifmmode×\else\texttimes\fiN array of inductively coupled Josephson flux qubits by reducing the number of necessary steps. In addition, the same protocol is shown to be able to transfer the WWW state from one row to the other.
We propose and analyze a scheme for the generation of multipartite entangled states in a system of inductively coupled Josephson flux qubits. The qubits have fixed eigenfrequencies during the whole process in order to minimize decoherence effects and their inductive coupling can be turned on and off at will by tuning an external control flux. Within this framework, we will show that a WWW state in a system of three or more qubits can be generated by exploiting the sequential one by one coupling of the qubits with one of them playing the role of an entanglement mediator.
We study a one-step approach to the fast generation of Greenberger-Horne-Zeilinger (GHZ) states in a circuit QED system with superconducting flux qubits. The GHZ state can be generated in about 10 ns, which is much shorter than the coherence time of flux qubits and comparable with the time of single-qubit operation. In our proposal a time-dependent microwave field is applied to a superconducting transmission line resonator (TLR) and displaces the resonator in a controlled manner, thus inducing indirect qubit-qubit coupling without residual entanglement between the qubits and the resonator. The design of a tunably coupled TLR circle array provides us with the potential for extending this one-step scheme to the case of many qubits coupled via several TLRs.
A theoretical scheme for the generation of maximally entangled states of two superconducting flux qubits via their sequential interaction with a monochromatic quantum field is presented. The coupling of the qubits with the quantized field can be tuned on and off resonance by modulating the effective Josephson energy of each qubit via an externally applied magnetic flux. The system operates in such a way as to transfer the entanglement from a bipartite field-qubit subsystem to the two qubits. This scheme is attractive in view of the implementation of practical quantum processing systems.
Following the scheme proposed by X. L. He et al. [Phys. Rev. B76 (2007) 024517], we investigate the generation of a maximally entangled state between the charge and flux qubits. It is shown that macroscopic Einstein–Podolsky–Rosen (EPR) pairs can be created and the entanglement can be long-lived. Since the large Josephson junction is only virtually excited, no quantum information will be transferred from the charge and flux qubits to the large Josephson junction. Based on the strong inductive coupling, a fast entanglement operation can be achieved.