Vladimir Manucharyan, Harvard University Society of Fellows
Abstract
We present tunneling spectroscopy of individual quasiparticle bound states in an Al/InAs/Al mesoscopic Josephson junction. The device is broken superconducting Al loop bridged by a short section of a crystalline semiconducting InAs nanowire. The loop is pierced with a magnetic flux to control the superconducting phase difference across the junction; the electron density in the nanowire can be gate-tuned down to a full depletion; spectroscopy is performed using a separate normal metal lead.
At a large electron density in the nanowire, the spectrum shows a phase-tunable minigap, characteristic of the superconducting proximity effect in diffusive metals. Reducing the density, we resolve a discrete sub-gap resonance. This resonance corresponds to an Andreev bound state of a Bogolyubov quasiparticle trapped in the junction region. We argue that the corresponding BCS-like ground state of the nanowire consists of only a single Cooper pair "leaked" into InAs from the Al leads.
Our experiment reveals how superconductivity can be proximity-induced in systems containing only a few electronic states, and may find applications in quantum computing with individual Bogolyubov quasiparticles and in proximity-engineering of topological superconductivity.