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arXiv:cond-mat/0509236 · Physical Review B

Superconducting qubits can be coupled and addressed as trapped ions

Yu-xi Liu, L. F. Wei, J. R. Johansson, Jaw-Shen Tsai, Franco Nori

Published Oct 31, 2007• 62 citations
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Abstract

Exploiting the intrinsic nonlinearity of superconducting Josephson junctions, we propose a scalable circuit with superconducting qubits (SCQs) which is very similar to the successful one now being used for trapped ions. The SCQs are coupled to the ``vibrational'' mode provided by a superconducting $LC$ circuit or its equivalent (e.g., a superconducting quantum interference device). Both single-qubit rotations and qubit-$LC$-circuit couplings and/or decouplings can be controlled by the frequencies of the time-dependent magnetic fluxes. The circuit is scalable since the qubit-qubit interactions, mediated by the $LC$ circuit, can be selectively performed, and the information transfer can be realized in a controllable way.

DOI
10.1103/physrevb.76.144518

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Exploiting the intrinsic nonlinearity of superconducting Josephson junctions, we propose a scalable circuit with superconducting qubits (SCQs) which is very similar to the successful one now being used for trapped ions. The SCQs are coupled to the ``vibrational'' mode provided by a superconducting LCLCLC circuit or its equivalent (e.g., a superconducting quantum interference device). Both single-qubit rotations and qubit-LCLCLC-circuit couplings and/or decouplings can be controlled by the frequencies of the time-dependent magnetic fluxes. The circuit is scalable since the qubit-qubit interactions, mediated by the LCLCLC circuit, can be selectively performed, and the information transfer can be realized in a controllable way.
Superconducting circuits and trapped ions are promising architectures for quantum information processing. However, the natural frequencies for controlling these systems---radio-frequency ion control and microwave domain superconducting qubit control---make direct Hamiltonian interactions between them weak. In this paper we describe a technique for coupling a trapped ion's motion to the fundamental mode of a superconducting circuit, by applying to the circuit a carefully modulated external magnetic flux. In conjunction with a nonlinear element (Josephson junction), this gives the circuit an effective time-dependent inductance. We then show how to tune the external flux to generate a resonant coupling between the circuit and ion's motional mode and discuss the limitations of this approach compared to using a time-dependent capacitance.
After a brief review on superconducting qubits, we overview our recent studies on controllable couplings in superconducting flux qubits via a variable frequency (ac) magnetic flux. In particular: (i) we describe how to use a variable frequency magnetic flux to control the coupling between two inductively coupled flux qubits, (ii) Based on this approach, using a variable frequency controlled coupling, we present a proposal on how to achieve scalable quantum computing circuits with flux qubits. We also explain why superconducting qubits can be coupled and addressed as trapped ions, as well as describe how to utilize dressed states to couple and decouple qubits and the data bus.
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We propose an effective scheme for manipulating quantum information stored in a superconducting nanocircuit. The Josephson qubits are coupled via their separate interactions with an information bus, a large current-biased Josephson junction treated as an oscillator with adjustable frequency. The bus is sequentially coupled to only one qubit at a time. Distant Josephson qubits without any direct interaction can be indirectly coupled with each other by independently interacting with the bus sequentially, via exciting/deexciting vibrational quanta in the bus. This is a superconducting analog of the successful ion trap experiments on quantum computing. Our approach differs from previous schemes that simultaneously coupled two qubits to the bus, as opposed to their sequential coupling considered here. The significant quantum logic gates can be realized by using these tunable and selective couplings. The decoherence properties of the proposed quantum system are analyzed within the Bloch-Redfield formalism. Numerical estimations of certain important experimental parameters are provided.
Superconducting quantum circuits based on Josephson junctions have made rapid progress in demonstrating quantum behavior and scalability.However, the future prospects ultimately depend upon the intrinsic coherence of Josephson junctions, and whether superconducting qubits can be adequately isolated from their environment.We introduce a new architecture for superconducting quantum circuits employing a three-dimensional resonator that suppresses qubit decoherence while maintaining sufficient coupling to the control signal.With the new architecture, we demonstrate that Josephson junction qubits are highly coherent, with T 2 $ 10 to 20 s without the use of spin echo, and highly stable, showing no evidence for 1=f critical current noise.These results suggest that the overall quality of Josephson junctions in these qubits will allow error rates of a few 10 À4 , approaching the error correction threshold.