Researchers have long known of an intriguing theoretical parallel to the Josephson effect in which insulator and superconductor are reversed: rather than electric charges jumping from one superconducting layer to another across an insulating layer, magnetic flux quanta jump from one insulator to another across a superconducting layer (Figure 1). Quantum tunneling of electrons in the Josephson junction is replaced in this parallel by the coherent "slip" of the phase, a quantum variable that, in superconducting circuits, plays a dual role to that of electric charge.



Coherent quantum phase slip (CQPS), as this phenomenon is known, has long been limited to theory-but no more. In a paper in Nature, Oleg Astafiev and colleagues at the RIKEN Advanced Science Institute (ASI) and NEC Smart Energy Research Laboratories report on the first direct observation of CQPS in a narrow superconducting wire of indium-oxide (InOx). The wire is inserted into a larger superconducting loop to form a new device called a phase-slip qubit, with the superconducting layer (the thin wire) sandwiched between insulating layers of empty space (Figure 2).



By tuning the magnetic flux penetrating this loop while scanning microwave frequencies, the researchers detected a band gap in the energy curves for the two flux states of the system (Figure 3), just as theory predicts. This gap is a result of quantum mechanics, which prevents the two states from occupying the same energy level, forcing them to tunnel across the superconducting layer-and through a quantum phase-slip in the narrow wire-to avoid it. While demonstrating conclusively the existence of CQPS, the successful experiment also ushers in a novel class of devices that exploit the unique functionality of quantum phase-slip to forge a new path in superconducting electronics.



For more information, please contact:



Tsai Jaw-Shen

Macroscopic Quantum Coherence Team

RIKEN Advanced Science Institute

Tel: +81-(0)29-850-1161 / Fax: +81-(0)29-850-2624



Global Relations Office

RIKEN

Tel: +81-(0)48-462-1225 / Fax: +81-(0)48-463-3687

Email: koho@riken.jp



Reach us on Twitter: @rikenresearch



Reference



O. V. Astafiev, L. B. Ioffe, S. Kafanov, Yu. A. Pashkin, K. Yu. Arutyunov, D. Shahar, O. Cohen, & J. S. Tsai. "Coherent quantum phase slip." Nature, 2012, DOI: 10.1038/nature10930





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