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Journal of Physics Condensed Matter

An energy relaxation tolerant approach to quantum entanglement, information transfer, and gates with superconducting-quantum-interference-device qubits in cavity QED

Chui‐Ping Yang, Shih‐I Chu, Siyuan Han

Published Mar 12, 2004• 17 citations
Publisher

Abstract

A scheme is proposed for realizing quantum entanglement, information transfer, CNOT gates, and SWAP gates with superconducting-quantum-interference-device (SQUID) qubits in cavity QED. In the scheme, the two logical states of a qubit are the two lowest levels of the SQUID. An intermediate level of the SQUID is utilized to facilitate coherent control and manipulation of quantum states of the qubits. The method presented here does not involve a real excitation of the intermediate levels during the operations. Thus, decoherence due to energy relaxation of intermediate levels is minimized. In addition, the present method does not require the adjustment of the SQUID level spacings, which simplifies the operation significantly.

DOI
10.1088/0953-8984/16/12/001

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We present a scheme to achieve maximally entangled states, controlled phase-shift gate, and SWAP gate for two superconducting-quantum-interference-device (SQUID) qubits, by placing SQUIDs in a microwave cavity. We also show how to transfer quantum information from one SQUID qubit to another. In this scheme, no transfer of quantum information between the SQUIDs and the cavity is required, the cavity field is only virtually excited and thus the requirement on the quality factor of the cavity is greatly relaxed.
An alternative approach is proposed to realize an n -qubit Toffoli gate with superconducting quantum-interference devices (SQUIDs) in cavity quantum electrodynamics (QED). In the proposal, we represent two logical gates of a qubit with the two lowest levels of a SQUID while a higher-energy intermediate level of each SQUID is utilized for the gate manipulation. During the operating process, because the cavity field is always in vacuum state, the requirement on the cavity is greatly loosened and there is no transfer of quantum information between the cavity and SQUIDs.
An alternative scheme is proposed for generating the Greenberg–Horne–Zeilinger (GHZ) and W types of the entangled states with multiple superconducting quantum-interference device (SQUID) qubits in a single-mode microwave cavity field. In this scheme, there is no transfer of quantum information between the SQUIDs and the cavity, the cavity is always in the vacuum and thus the requirement on the quality of cavity is greatly loosened. In addition, during the process of the generation of the W entangled state, the present method does not involve a real excitation of intermediate levels. Thus, decoherence due to energy relaxation of intermediate levels is minimized.
We propose an approach to realize an nnn-qubit controlled-UUU gate with superconducting quantum interference devices (SQUIDs) in cavity QED. In this approach, the two lowest levels of a SQUID represent the two logical states of a qubit while a higher-energy intermediate level serves the gate manipulation. Our method operates essentially by creating a single photon through one of the control SQUIDs and then performing an arbitrary unitary transformation on the target SQUID with the assistance of the cavity photon. In addition, we show that the method can be applied to implement an nnn-qubit controlled-UUU gate with atomic qubits in cavity QED.