  Optical communication is the standard for high-bandwidth information transfer
in today's digital age. The increasing demand for bandwidth has led to the
maturation of coherent transceivers that use phase- and amplitude-modulated
optical signals to encode more bits of information per transmitted pulse. Such
encoding schemes achieve higher information density, but also require more
complicated receivers to discriminate the signaling states. In fact, achieving
the ultimate limit of optical communication capacity, especially in the low
light regime, requires coherent joint detection of multiple pulses. Despite
their superiority, such joint detection receivers are not in widespread use
because of the difficulty of constructing them in the optical domain. In this
work we describe how optomechanical transduction of phase information from
coherent optical pulses to superconducting qubit states followed by the
execution of trained short-depth variational quantum circuits can perform joint
detection of communication codewords with error probabilities that surpass all
classical, individual pulse detection receivers. Importantly, we utilize a
model of optomechanical transduction that captures non-idealities such as
thermal noise and loss in order to understand the transduction performance
necessary to achieve a quantum advantage with such a scheme. We also execute
the trained variational circuits on an IBM-Q device with the modeled transduced
states as input to demonstrate that a quantum advantage is possible even with
current levels of quantum computing hardware noise.
