Physicists Model Andreev Qubit Readout Without Ancillae at up to 99.85% Accuracy

Physicists from the Beijing Institute of Technology and the University of São Paulo reported a method to read Andreev quantum dot qubits without auxiliary ancilla qubits in Physical review. B. doi.orgPhysical review. B./Physical review. Bdoi.orgOpen source ↗. The theoretical protocol uses the macroscopic oscillatory supercurrent produced by the quantum dot Josephson junction itself to measure arbitrary superposition states. According to the team's numerical simulations, the readout achieves accuracies of 99.52%, 99.81%, and 99.85%, depending on the number of measurement cycles doi.orgPhysical review. B./Physical review. Bdoi.orgOpen source ↗.

Existing quantum nondemolition protocols typically transfer quantum information to helper ancilla systems, which requires extra steps and repeated reinitializations. In the proposed setup, an indium arsenide nanowire forms a quantum dot coupled to superconductors in a closed circuit. The interference between the many-body eigenstates of the Andreev qubit creates an alternating-current-like supercurrent without an applied external voltage doi.orgPhysical review. B./Physical review. Bdoi.orgOpen source ↗.

Intrinsic Current Oscillations

The authors demonstrated that information about the qubit is directly encoded into the time-dependent behavior of this macroscopic current doi.orgPhysical review. B./Physical review. Bdoi.orgOpen source ↗. The constant component of the supercurrent and the oscillation amplitude define the relative probabilities of states 0 and 1 in the superposition. Meanwhile, the initial phase of the time-dependent oscillation reveals the relative quantum phase between the two eigenstates doi.orgPhysical review. B./Physical review. Bdoi.orgOpen source ↗.

Magnetic Detection via SQUID

To perform measurements with minimal backaction, the proposal employs an external superconducting quantum interference device placed near the circuit. The sensor monitors the weak magnetic field generated by the oscillating supercurrent through inductive coupling doi.orgPhysical review. B./Physical review. Bdoi.orgOpen source ↗. Because this coupling remains faint, the backaction on the quantum dot remains low enough to approach the nondestructive measurement limit under standard noise conditions doi.orgPhysical review. B./Physical review. Bdoi.orgOpen source ↗.

The authors noted that the protocol relies on established components and realistic experimental parameters, requiring no auxiliary qubits or repetitive system reinitializations doi.orgPhysical review. B./Physical review. Bdoi.orgOpen source ↗.