Coherent microwave-to-optical conversion of a frequency qubit

Building a long-range network of superconducting quantum computers has been an ongoing challenge owing to complications associated with transduction of quantum information between microwave and optical domains. Coherent conversion of a qubit encoded in microwave photons into an optical state has remained an outstanding challenge. Here, we encode a qubit into a coherent state of two microwave frequency levels—generating a frequency qubit. These qubits are then efficiently converted to surface acoustic wave (SAW) cavity phonons using an interdigitated transducer. The multimode nature of the SAW cavity allows enhanced interaction between both microwave frequencies and a quantum dot located at the center of the cavity. The single photon emitting capability of the quantum dot and its resonance fluorescence with sub-natural linewidth ensures coherent transduction into quantum light. We report a computational state transfer with a fidelity of 0.90±0.07. Additionally, we show coherent state transfer by measuring the phase between the two optical frequency modes with an electro-optic phase modulator, yielding a visibility of 0.70±0.07. Coherent transduction of qubits is a key step towards transmission of quantum information encoded in superconducting qubits via optical fiber networks.