Noise Reduction in Qubit Readout with a Two-Mode Squeezed Interferometer

Abstract

Fault-tolerant quantum information processing with flawed qubits and gates requires highly efficient, quantum nondemolition qubit readout. In superconducting circuits, qubit readout using coherent light with fidelity above 99% has been achieved by using quantum limited parametric amplifiers such as the Josephson parametric converter (JPC). However, further improvement of such measurement faces many challenges, including the vacuum fluctuation of the coherent light that limits measurement fidelity. In this work we demonstrate dispersive qubit readout with a two-mode squeezed light interferometer formed by two JPCs. The first JPC generates two-mode squeezed vacuum at its output, which is coherently recombined by the second JPC after one branch is phase shifted and displaced by its interaction with the qubit-cavity system on that arm of the interferometer. We observe a 31% improvement in the power signal-to-noise ratio (SNR) of projective readout after replacing vacuum fluctuation in the readout cavity with two-mode squeezed vacuum containing on average one photon. This demonstrates a more efficient way of improving SNR than by increasing readout power. Furthermore, we investigate the quantum properties of the two-mode squeezed light in the system through weak measurement. Surprisingly, we find that tuning the interferometer to be less projective increases the measurement efficiency relative to the optimum setting for projective measurement. These enhancements may enable remote entanglement with lower efficiency components in a system with qubits in both arms of the interferometer.