There is currently in developing practical quantum information processors for applications such as quantum simulation and computation. These machines require a host of technologies to initialize, manipulate, and readout quantum bits with high fidelity. In this talk, I will present the use of third order nonlinearities and parametric drives to address two of these issues: enhancing the readout fidelity of transmon qubit with two-mode squeezed readout, and creating an engineered bath to change the relaxation rate and steady-state population of a transmon qubit. 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). In the first part of the talk, I will demonstrate a new scheme to measure a transmon qubit/cavity system with an unbalanced two-mode squeezed light interferometer formed from two JPCs. The first amplifier generates two-mode squeezed vacuum at its output, which is coherently recombined by the second amplifier after one branch is shifted and displaced by the transmon's state after it interacts with the qubit/cavity system on one arm of the interferometer. We have observed a 44% improvement in power Signal-to-Noise Ratio (SNR) of projective readout compared to that of coherent light readout in the same system. To investigate the quantum properties of the two-mode squeezed light in the system, I also studied weak measurement and found, surprisingly, that tuning the interferometer to be as unprojective as possible was associated with an increase in the quantum efficiency of our readout relative to the optimum setting for projective measurement. In the second part of my talk, I will introduce the implementation of a controllable thermal bath for a transmon qubit by parametrically coupling it to a lossy Superconducting Nonlinear Asymmetric Inductive eLement (SNAIL). The carefully chosen parametric processes together with the dissipative SNAIL allow us to control the effective “heating” and “cooling” processes of the system, which leads to an engineerable bath that is seen by the qubit. The equilibrium temperature of the qubit and the rate it relaxes to its steady state is therefore controlled by the parametric drives. I will conclude with a discussion of the outlook for both experiments, and for parametric controls more generally.
Please contact Stephen Bullwinkel (firstname.lastname@example.org) for Zoom link.