Josephson Tunneling and Quantum Mechanical Phase

Michael Martin Nieto

By using properly defined sine and cosine phase variables, a model is proposed which yields a rigorous quantum-mechanical derivation of Josephson tunneling with exact current eigenstates. The model predicts that for dc tunneling the coherent fluids on both sides of the junction must be in the same state, and the "phase is quantized." This prediction can in principle be investigated experimentally, and may afford a fundamental test of quantum mechanics.

A. J. Leggett

Marion Matters, W. J. Elion, Johan E. Mooij

We study the interplay of controlled quantum-mechanical charge and phase fluctuations by measuring their influence on the supercurrent through a small superconducting grain. The grain is connected to leads by two small Josephson tunnel junctions. The phase fluctuations are controlled by the magnetic flux threading a SQUID that connects the grain to a large superconductor. The charge fluctuations are modulated by a voltage on a gate capacitor. As a function of gate voltage we observe a broadening of the Coulomb resonance of the supercurrent for decreasing phase fluctuations, in agreement with theoretical predictions.

Physikalisch-Technische Bundesanstalt

H. Lübbig

Junpeng Hou, Xi-Wang Luo, Kuei Sun +5 more

The Josephson effect is a prominent phenomenon of quantum supercurrents that has been widely studied in superconductors and superfluids. Typical Josephson junctions consist of two real-space superconductors (superfluids) coupled through a weak tunneling barrier. Here we propose a momentum-space Josephson junction in a spin-orbit coupled Bose-Einstein condensate, where states with two different momenta are coupled through Raman-assisted tunneling. We show that Josephson currents can be induced not only by applying the equivalent of "voltages," but also by tuning tunneling phases. Such tunneling-phase-driven Josephson junctions in momentum space are characterized through both full mean field analysis and a concise two-level model, demonstrating the important role of interactions between atoms. Our scheme provides a platform for experimentally realizing momentum-space Josephson junctions and exploring their applications in quantum-mechanical circuits.