Interferometric Transmon Readout with Two-Mode Squeezed Light, Part 3

SU(1,1) interferometers have been explored as an avenue for improving readout fidelity for a given photon-budget in many different problems, from gravitational wave detection using interferometers, atomic clocks, and recently - quantum state readout. Standard SU(1,1) interferometers rely on photon number measurements at the outputs, however, in superconducting circuits, implementing photon number measurement for readout is inherently challenging. To overcome this limitation, we propose a novel scheme that leverages a non-linear amplifier to efficiently execute the experiment using only coherent homodyne measurements. We view the non-linear amplifier as a generalized quantum processor, or a quantum reservoir computer, embedded directly in the measurement chain. The dynamical response of the non-linear amplifier can be harnessed to effectively compute the statistical properties of the state, which can encode state information. This modification enables the successful implementation of the SU(1,1) interferometer for readout using only homodyne measurements, offering valuable insights and advancements in quantum information processing using superconducting circuits.