Fermion-parity qubit in a proximitized double quantum dot

TL;DR

This paper proposes a fermion-parity qubit in a double quantum dot system embedded in a Josephson junction, which is robust against charge noise and fluctuations, enabling stable quantum information encoding and manipulation.

Contribution

It introduces a novel fermion-parity qubit design in proximitized quantum dots with noise protection at the sweet spot, including methods for initialization, readout, and gate operations.

Findings

Qubit states have zero charge dipole moment at the sweet spot.

Protection against charge noise and inter-dot tunneling fluctuations.

Feasible protocols for qubit initialization, readout, and gate operations.

Abstract

Bound states in quantum dots coupled to superconductors can be in a coherent superposition of states with different electron number but with the same fermion parity. Electrostatic gating can tune this superposition to a sweet spot, where the quantum dot has the same mean electric charge independent of its electron-number parity. Here, we propose to encode quantum information in the local fermion parity of two tunnel-coupled quantum dots embedded in a Josephson junction. At the sweet spot, the qubit states have zero charge dipole moment. This protects the qubit from dephasing due to charge noise acting on the potential of each dot, as well as fluctuations of the (weak) inter-dot tunneling. At weak inter-dot tunneling, relaxation is suppressed because of disjoint qubit states. On the other hand, for strong inter-dot tunneling the system is protected against noise affecting each quantum dot separately (energy level noise, dot-superconductor tunneling fluctuations, and hyperfine interactions). Finally, we describe initialization and readout as well as single-qubit and two-qubit gates by pulsing gate voltages.