Michael Hatridge – Qubit measurement with two-mode squeezed light

Qubit measurement with two-mode squeezed light

Michael Hatridge, University of Pittsburgh
January 25, 2019, Physics Research Building Rm. 1080 (Smith Lecture Hall, note different room!), 12:30-1:30 PM


High fidelity qubit measurement is essential for scalable, fault-tolerant quantum computing. In superconducting circuits, qubit readout with fidelity above 99% has been achieved by using a quantum-limited parametric amplifier such as the Josephson Parametric Converter (JPC) as the first stage amplifier. However, the Signal-to-Noise Ratio (SNR) of such readout is fundamentally limited by quantum fluctuations in the coherent readout pulse. Alternatively, readout with squeezed light can be used to reduce fluctuation along certain quadratures and thus improve the SNR. In this talk, we demonstrate a readout scheme with two-mode squeezed light both produced and amplified by JPCs in an interferometer unbalanced by a transmon qubit/cavity. This configuration has been predicted to improve the SNR compared to readout with both coherent states and single-mode squeezed light.  We have demonstrated a ~50% improvement in SNR compared to coherent state readout, and find that the system actually works best when we deliberately break the path for signals in the system, so that only the fluctuations passing through it interfere.  We’ll also discuss the prospects for placing qubits on both arms of the interferometer and performing measurements which generate remote entanglement between them.