Longitudinal readout of the transverse component of a Qubit using a SNAIL coupler*

Fast and QND readout of qubits is essential for most quantum information experiments. Traditional superconducting qubit readout is based on dispersive coupling between the qubit and a readout resonator. The usual dispersive readout scheme does not allow us to measure the transverse component, which is important for error-correction, without a pre-rotation. Furthermore, longitudinal readout theoretically allows us to measure a qubit more rapidly. In this talk, we will present our work on implementing longitudinal QND readout of transverse qubit component using parametric processes involving a Superconducting Nonlinear Asymmetric Inductive eLement (SNAIL). One can generate three-wave mixing based parametric gain and conversion interaction between a SNAIL and qubit by driving the SNAIL at the difference and sum of the qubit and SNAIL frequencies respectively.  Matching the rates of these two processes leads to an effective longitudinal coupling between SNAIL and the qubit’s transverse component. This interaction creates a coherent state in the SNAIL whose phase depends on the qubit’s transverse component, creating a longitudinal readout of the qubit in the XY plane, with the axis of measurement controlled by the phase of the parametric drives. We will present experimental results on generating this interaction and collecting the SNAIL output signal to readout the x component of a transmon qubit. Additionally, we discuss our efforts to improve the measurement fidelity by decreasing the SNAIL self-Kerr and by adding a cavity to the measurement scheme.