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Published online: 4 February 2009 | doi:10.1038/nchina.2009.18

Quantum optics: Josephson effect — the light version

Felix Cheung

Introduction

© (2009) APS

In superconductivity, the Josephson effect describes the tunnelling of electron pairs through a thin insulating barrier separating two superconductors. The phenomenon is central to the operation of superconducting quantum circuits, which are used in quantum computing, and superconducting quantum interference devices, which are used to measure extremely small magnetic fields. Wuming Liu and co-workers at the Chinese Academy of Sciences in Beijing¹ have now experimentally created photonic Josephson effects.

The researchers built an optical system (see image) consisting of two optical cavities (the equivalent of two superconductors) linked by an intercavity tunnelling amplitude (the equivalent of an insulating barrier). The cavities were loaded with ultracold two-level atoms for generating entangled photon pairs (the equivalent of electron pairs) on demand.

By moving atoms in one of the cavities at a constant velocity, the researchers observed a population imbalance (more atoms in the upper level than the lower); a constant photonic current flowed between the two cavities — the photonic version of the so-called direct-current Josephson effect. Moreover, by moving atoms at a pace faster than a certain critical velocity, the researchers observed the oscillations of population difference between the two cavities — the photonic version of the so-called alternating-current Josephson effect.

The findings are expected to help scientists explore new phenomena in quantum optics and realize future quantum devices.

The authors of this work are from:
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, China; Department of Physics, Capital Normal University, Beijing, China; Department of Physics, Oklahoma State University, Stillwater, Oklahoma, USA.

Reference

1. Ji, A. C., Sun, Q., Xie, X. C. & Liu., W. M. Josephson effect for photons in two weakly linked microcavities. Phy. Rev. Lett. 102, 023602 (2009).  | Article |