  Fault-tolerant quantum information processing with flawed qubits and gates
requires highly efficient, quantum non-demolition (QND) qubit readout. 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). However, further improvement of
such measurement is fundamentally limited by the vacuum fluctuations of the
coherent light used for readout. In this work we 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
31% 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, we 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. These enhancements may enable remote
entanglement with lower efficiency components in a system with qubits in both
arms of the interferometer.
