Quantum Coherence with a Single Cooper Pair

TLDR

A metallic electrode connected by tunnel junctions to reservoirs possesses discrete charge states involving macroscopic numbers of electrons, analogous to photon number states, and normally decays irreversibly to lower‑energy states. The study aims to measure the Josephson effect involving only a single Cooper pair. The authors perform an experiment on a superconducting island coupled to superconducting reservoirs to detect this single‑Cooper‑pair Josephson effect. They find that the ground state of the superconducting island can be a coherent superposition of charge states differing by two electrons, i.e., a Cooper pair.

Abstract

A metallic electrode connected to electron reservoirs by tunnel junctions has a series of charge states corresponding to the number of excess electrons in the electrode. In contrast with the charge state of an atomic or molecular ion, the charge states of such an "island" involve a macroscopic number of conduction electrons of the island. Island charge states bear some resemblance with the photon number states of the cavity in cavity QED, the phase conjugate to the number of electrons being analogous to the phase of the field in the cavity. For a normal island, charge states decay irreversibly into charge states of lower energies. However, the ground state of a superconducting island connected to superconducting reservoirs can be a coherent superposition of charge states differing by two electrons (i.e. a Cooper pair). We describe an experiment in which this Josephson effect involving only one Cooper pair is measured.