Letter

Nature 449, 579-583 (4 October 2007) | doi:10.1038/nature06186; Received 4 May 2007; Accepted 15 August 2007; Corrected 5 October 2007

The a.c. and d.c. Josephson effects in a Bose–Einstein condensate

S. Levy1, E. Lahoud1, I. Shomroni1 & J. Steinhauer1

  1. Department of Physics, Technion—Israel Institute of Technology, Technion City, Haifa 32000, Israel

Correspondence to: J. Steinhauer1 Correspondence and requests for materials should be addressed to J.S. (Email: jeffs@physics.technion.ac.il).

The alternating- and direct-current (a.c. and d.c.) Josephson effects were first discovered in a system of two superconductors, the macroscopic wavefunctions of which are weakly coupled via a tunnelling barrier1, 2. In the a.c. Josephson effect1, 2, 3, 4, 5, 6, 7, a constant chemical potential difference (voltage) is applied, which causes an oscillating current to flow through the barrier. Because the frequency is proportional to the chemical potential difference only, the a.c. Josephson effect serves as a voltage standard2. In the d.c. Josephson effect, a small constant current is applied, resulting in a constant supercurrent flowing through the barrier4, 5, 8. In a sense, the particles do not 'feel' the presence of the tall tunnelling barrier, and flow freely through it with no driving potential. Bose–Einstein condensates should also support Josephson effects9; however, while plasma oscillations have been seen10 in a single Bose–Einstein condensate Josephson junction, the a.c. Josephson effect remains elusive. Here we observe the a.c. and d.c. Josephson effects in a single Bose–Einstein condensate Josephson junction. The d.c. Josephson effect has been observed previously only in superconducting systems11; in our study, it is evident when we measure the chemical potential–current relation of the Bose–Einstein condensate Josephson junction4, 11. Our system constitutes a trapped-atom interferometer12, 13 with continuous readout14, which operates on the basis of the a.c. Josephson effect. In addition, the measured chemical potential–current relation shows that the device is suitable for use as an analogue of the superconducting quantum interference device, which would sense rotation2, 15.

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