Performing cat-basis quantum logic on modulated coherent state pulses using a photonic NISQ-era processor*

Universal quantum-logic manipulation of modulated quantum light underlies several quantum-enhanced classical communications and sensing applications. In quantum-limited optical communications with binary phase shift keyed (BPSK) coherent states, attaining quantum-enhanced superadditive capacity hinges on the ability to perform quantum operations in the approximate qubit subspace spanned by cat states associated with the BPSK states. Such logic capability would enable the implementation of structured quantum joint detection receivers, e.g., based on the recently proposed belief propagation with quantum messages (BPQM) algorithm that can in principle discriminate codeword blocks of coherent state pulses quantum optimally. We consider alternative approaches to realizing photonic cat basis logic using linear optics, ancilla cat states, and photon number and homodyne measurements that have been previously proposed. We analyze the gate fidelities as a function of single-mode squeezing required in Gaussian boson sampling-type circuits that we use to generate the ancilla cat states involved in the gates and report thresholds on the squeezing levels necessary to retain quantum enhancement in discriminating communication codewords of small example codes using the BPQM receiver.