Supercurrent reversal in quantum dots

This paper reports a study of supercurrents through a quantum dot created in a semiconductor nanowire by local electrostatic gating. Owing 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. When two superconductors are electrically connected by a weak link—such as a tunnel barrier—a zero-resistance supercurrent can flow 1 , 2 . This supercurrent is carried by Cooper pairs of electrons with a combined charge of twice the elementary charge, e . The 2 e charge quantum is clearly visible in the height of voltage steps in Josephson junctions under microwave irradiation, and in the magnetic flux periodicity of h /2 e (where h is Planck's constant) in superconducting quantum interference devices 2 . Here we study supercurrents through a quantum dot created in a semiconductor nanowire by local electrostatic gating. Owing 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 3 , 4 , 5 , 6 , 7 . These quantum coherent tunnelling processes can result in either a positive or a negative supercurrent, that is, in a normal or a π-junction 8 , 9 , 10 , 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.