Superconducting Cavity-Qubit Architectures II

In continuous-variable quantum computation, information is encoded in bosonic modes, which, due to their large Hilbert space, allow for hardware-efficient quantum error correction. These systems also benefit from homodyne and heterodyne detection techniques for measuring the quadratures of the field. However, these techniques cannot be straightforwardly extended to the measurement of the stationary modes such as those confined in superconducting resonators and microwave cavities. Here, we address this limitation by implementing the qubitdyne protocol as proposed in [1]. The protocol utilizes repeated measurements on a transmon qubit coupled to a bosonic mode to accurately reconstruct the measurement statistics of homodyne and heterodyne detection. We perform a sequence of partial-swap using the |g,n+1> → |f,n> transition between the qubit and the bosonic mode, followed by qubit measurement in the σ_x basis. The qubit is reset to |g> state for reuse, and the sequence is repeated to track the trajectory of the mode. By measuring a large ensemble of these trajectories, the full probability distribution of the state can be determined. These measurements are sufficient to perform quantum state verification, efficient boson sampling and demonstration of verifiable quantum advantage [2].