Quantum Nondemolition Charge Measurement of a Josephson Qubit

TL;DR

This paper proposes a quantum nondemolition measurement scheme for flux qubits that measures charge without disturbing the qubit's energy eigenstates, enabling more accurate and faster readouts.

Contribution

It introduces a measurement method where the charge measurement operator commutes with the qubit Hamiltonian, preserving the qubit state during readout.

Findings

Measurement operator commutes with the Hamiltonian in the reduced space

Enables nondemolition charge measurement of flux qubits

Potentially improves readout fidelity and speed

Abstract

In a qubit system, the measurement operator does not necessarily commute with the qubit Hamiltonian, so that the readout process demolishes (mixes) the qubit energy eigenstates. The readout time is therefore limited by such a mixing time and its fidelity will be reduced. A quantum nondemolition readout scheme is proposed in which the charge of a flux qubit is measured. The measurement operator is shown to commute with the qubit Hamiltonian in the reduced two-level Hilbert space, even though the Hamiltonian contains non-commuting charge and flux terms.