Abstract
Macroscopic quantum tunneling is a fundamental phenomenon of quantum mechanics related to the actively debated topic of quantum-to-classical transition. The ability to realize macroscopic quantum tunneling affects implementation of qubit-based quantum computing schemes and their protection against decoherence. Decoherence in qubits can be reduced by means of topological protection, e.g., by exploiting various parity effects. In particular, paired phase slips can provide such protection for superconducting qubits. Here, we report on the direct observation of quantum paired phase slips in thin-wire superconducting loops. We show that in addition to conventional single phase slips that change the superconducting order parameter phase by , there are quantum transitions that change the phase by . Quantum paired phase slips represent a synchronized occurrence of two macroscopic quantum tunneling events, i.e., cotunneling. We demonstrate the existence of a remarkable regime in which paired phase slips are exponentially more probable than single ones.
- Received 30 July 2014
DOI:https://doi.org/10.1103/PhysRevX.5.021023
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Published by the American Physical Society
Popular Summary
In a quasi-one-dimensional system, phase slips are fluctuations in the superconducting order parameter that drive a system’s segments into a normal state for a short period of time. Phase slips, which were first proposed by Little in 1967, play a crucial role in the physics of thin nanowires where their proliferation leads to the destruction of superconductivity. Phase slips also change the winding number, i.e., the net number of vortices that have entered into a closed superconducting loop. Phase slips are stochastic in nature because they are induced by thermal fluctuations or by quantum tunneling. A quantum phase slip constitutes a prominent example of macroscopic quantum tunneling since it involves a large number of microscopic degrees of freedom. Here, we experimentally demonstrate the pairing of quantum phase slips, a process that preserves the parity of the number of vortices that have entered our superconducting loop. The difference between quantum and classical in our context is that quantum phase slip pairs represent a simultaneous and quantum-coherent entrance of two fluxons; classical pairs represent a sequential entrance of two fluxons where the first phase slip causes the second one.
It has been recently suggested that paired phase slips can be used to build topologically protected qubits, which show increased resistance to decoherence and may play key roles in the quantum computers of the future. We use microwave measurement techniques at 0.06–3 K to study the fluxoid states of a device composed of a superconducting loop that includes two homogeneous nanowires. We focus on transitions between different fluxoid states that occur through phase slips in the nanowires. We identify a regime for which paired phase slips are exponentially more likely than single unpaired phase slips. In this regime our thin-wire superconducting loop acts as a parity-conserving element that can be utilized for the topological protection of qubits, provided that the overall rate of quantum phase slips can be made sufficiently high. We argue that in the quantum tunneling regime the paired phase slips are not composed of sequential single phase slips but rather they occur via macroscopic quantum tunneling events that proceed in pairs, i.e., by means of “quantum cotunneling.”
We anticipate that our approach will stimulate new experiments on building parity-protected qubits based on superconducting nanowires.