Single-Photon Subtraction in Circuit QED

Single-photon subtraction (SPS) is a quantum control primitive that transfers a single excitation from a bosonic oscillator to a coupled two-level system, furnishing a means to coherently test the occupancy predicate ???>?0. We investigate how SPS might be implemented in circuit quantum electrodynamics (cQED), a relatively mature technology with native oscillators and controllable two-level systems. In cQED, application of linear microwave drives provides universal control of a coupled resonator-qubit system. As such, an SPS control protocol could in principle be implemented via optimal control techniques; however, we are given no promises about the protocol speed/fidelity tradeoff or its interpretability. We propose principled methods to deterministically and coherently transfer a single microwave photon from a superconducting resonator to a transmon qubit, and compare these with known techniques in the optical domain. We discuss steps to implement SPS with existing cQED technology, as well as applications to NISQ quantum algorithms.