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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We thank W. Ketterle, M. Segev, E. Altman, Y. Kafri, E. Akkermans, E. Polturak, B. Shapiro and R. Pugatch for readings of the manuscript. This work was supported by the Israel Science Foundation and the Russell Berrie Nanotechnology Institute.
The file contains Supplementary Figure 1, Supplementary Discussion and Supplementary Notes. The supplementary information gives additional details of the experimental system. Furthermore, the calibration of the experiment is discussed. Also, explicit definitions from the two-state model are given, as well as an approximate calculation of the conductance. We calculate the energy of an analogous superconducting circuit. The sensitivity of a BEC SQUID is given.
About this article
New Journal of Physics (2019)