Article

# Phase-slip flux qubits

08/2005; DOI:doi:10.1088/1367-2630/7/1/219
Source: arXiv

ABSTRACT In thin superconducting wires, phase-slip by thermal activation near the critical temperature is a well-known effect. It has recently become clear that phase-slip by quantum tunnelling through the energy barrier can also have a significant rate at low temperatures. In this paper it is suggested that quantum phase-slip can be used to realize a superconducting quantum bit without Josephson junctions. A loop containing a nanofabricated very thin wire is biased with an externally applied magnetic flux of half a flux quantum, resulting in two states with opposite circulating current and equal energy. Quantum phase-slip should provide coherent coupling between these two macroscopic states. Numbers are given for a wire of amorphous niobium-silicon that can be fabricated with advanced electron beam lithography. Comment: Submitted to New Journal of Physics, special issue solid state quantum information

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ABSTRACT: Phase slips are topological fluctuations that carry the superconducting order-parameter field between distinct current-carrying states. Owing to these phase slips, superconducting nanowires acquire electrical resistance. In such wires, it is well known that at higher temperatures phase slips occur through the process of thermal barrier-crossing by the order-parameter field. At low temperatures, the general expectation is that phase slips should proceed through quantum tunnelling events, which are known as quantum phase slips. However, resistive measurements have produced evidence both for and against the occurrence of quantum phase slips. Here, we report evidence for the observation of individual quantum phase-slip events in homogeneous ultranarrow wires at high bias currents. We accomplish this through measurements of the distribution of switching currents for which the width exhibits a rather counter-intuitive, monotonic increase with decreasing temperature. Importantly, measurements show that in nanowires with larger critical currents, quantum fluctuations dominate thermal fluctuations up to higher temperatures.
Nature Physics 05/2009; 5(7):503-508. · 18.97 Impact Factor
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##### Article:Phase-slip oscillator: few-photon non-linearities
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ABSTRACT: Non-linear effects on driven oscillations are important in many fields of physics, ranging from applied mechanics to optics. They are instrumental for quantum applications. A limitation is that the non-linearities known up to now are featureless functions of the number of photons N in the oscillator. Here we show that the non-linearities found in an oscillator where superconducting inductance is subject to coherent phase-slips, are more interesting. They oscillate as a function of number of photons N with a period of the order of square root of N, which is the spread of the coherent state. We prove that such non-linearities result in multiple metastable states encompassing few photons and study oscillatory dependence of various responses of the resonator. The experimental realization of our proposal can deliver an unambiguous verification of coherent quantum phase-slips.
12/2009;
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##### Article:Flux-charge duality and topological quantum phase fluctuations in quasi-one-dimensional superconductors
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ABSTRACT: It has long been thought that superconductivity breaks down even at zero temperature in lower-dimensional systems due to enhanced topological quantum phase fluctuations. In quasi-1D wires, these fluctuations are described in terms of "quantum phase-slip" (QPS): tunneling of the superconducting order parameter for the wire between states differing by $\pm2\pi$ in their relative phase between the wire's ends. Many deviations from conventional bulk superconducting behavior have been observed in ultra-narrow superconducting nanowires over the last several decades which have been identified with QPS, and at least some of the observations are consistent with existing theories. However, other observations in many cases point to contradictory conclusions or cannot be explained by these theories, such that a unified understanding of the nature of quantum phase slip and its relationship to the various observations has yet to be achieved. In this paper we present a new model for QPS which takes as its starting point an idea originally postulated by Mooij and Nazarov [Nature Physics {\bf 2}, 169 (2006)]: that \textit{flux-charge duality}, a classical symmetry of Maxwell's equations, can be used to relate QPS to the well-known effect of Josephson tunneling of Cooper pairs. Our model provides an alternative, and qualitatively different, conceptual basis for QPS and the phenomena which arise from it in experiments, and it appears to permit for the first time a unified understanding of observations across several different types of experiments and materials systems.
01/2012;

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### Keywords

amorphous niobium-silicon

electron beam lithography

energy barrier

equal energy

externally applied magnetic flux

New Journal

significant rate

special issue solid state quantum information

superconducting quantum bit

thin superconducting wires

two macroscopic states

well-known effect