Interferometric Transmon Readout with Two-Mode Squeezed Light, Part 2

In the second part of this presentation, we characterize the noise of interferometric, two-mode squeezed light (TMSL)-based qubit readout. Our system consists of two high-saturation power, single-mode amplifiers, the first generating a two-mode squeezed beam of light at frequencies above and below its center frequency, which then interact with a qubit-cavity system before being further squeezed/amplified by a second, downstream amplifier. The cavity interacts with only one sideband, and thus shifts the phase of one component of the TMSL transiting the interferometer. Because the noise in this system contains energy, it can carry information and thus (weakly) measure the qubit even in the absence of displacement drives. We demonstrate that the state-dependent noise can vary widely based on interferometer settings, for instance falling at least – 1.4 dB relative to un-squeezed vacuum. We use the QICK control firmware [Stefanazzi et. al. 2022] to ensure phase-stability of the amplifier pumps and will discuss its implementation to control this experiment. We will also discuss the prospects for adding nonlinearity to the downstream amplifier, allowing it to also serve as a reservoir computer to distinguish qubit states with high fidelity using only squeezed noise.