Supercurrent reversal in quantum dots

TL;DR

This paper investigates supercurrent behavior in semiconductor nanowire quantum dots, revealing that adding a single electron spin can reverse the supercurrent's sign, with implications for quantum device control.

Contribution

It demonstrates supercurrent reversal in quantum dots by single-electron spin addition, highlighting the role of quantum coherence and orbital states in supercurrent sign control.

Findings

Supercurrent can reverse sign with a single electron spin addition.

Supercurrent sign depends on quantum dot orbital states.

Coherent tunneling enables supercurrent despite Coulomb blockade.

Abstract

When two superconductors become electrically connected by a weak link a zero-resistance supercurrent can flow. This supercurrent is carried by Cooper pairs of electrons with a combined charge of twice the elementary charge, e. The 2e charge quantum is clearly visible in the height of Shapiro steps in Josephson junctions under microwave irradiation and in the magnetic flux periodicity of h/2e in superconducting quantum interference devices. Several different materials have been used to weakly couple superconductors, such as tunnel barriers, normal metals, or semiconductors. Here, we study supercurrents through a quantum dot created in a semiconductor nanowire by local electrostatic gating. Due to strong Coulomb interaction, electrons only tunnel one-by-one through the discrete energy levels of the quantum dot. This nevertheless can yield a supercurrent when subsequent tunnel events are coherent. These quantum coherent tunnelling processes can result in either a positive or a negative supercurrent, i.e. in a normal or a pi-junction, respectively. We demonstrate that the supercurrent reverses sign by adding a single electron spin to the quantum dot. When excited states of the quantum dot are involved in transport, the supercurrent sign also depends on the character of the orbital wavefunctions.