Andreev Quantum Dots for Spin Manipulation

We investigate the feasibility of manipulating individual spin in a superconducting junction where Bogoliubov quasiparticles can be trapped in discrete Andreev levels. We call this system an Andreev quantum dot (AQD) to be contrasted with a common semiconductor quantum dot. We show that the AQD can be brought into a spin-$1/2$ state. The coupling between the spin and superconducting current facilitates manipulation and measurement of this state. We demonstrate that one can operate two inductively coupled AQDs as a XOR gate; this enables quantum computing applications.

Figure 1

(a) Excited states of an Andreev quantum dot in a given transport channel. (b) The energies of AQD states corresponding to a given transport level versus phase difference. The lower and upper curves correspond to ground state and excited singlet, respectively. The middle curves correspond to two spin-$1/2$ excited states, spin splitting being due to spin-orbit interaction.

Figure 2

The interaction between AQD qubits is organized by means of variable inductive coupling. This allows for further arraying. The circuit shown can be used as an XOR gate.