UC Berkeley

Superconducting quantum bits (qubits) are a leading candidate towards realizing a processor that is able to outperform existing classical computers at select tasks. One particular qubit architecture, the transmon, boasts long coherence times compared to required gate lengths and flexibility with respect to circuit couplings as these 'artificial atoms' are photolithographically defined. These attributes allow qubits to be leveraged for other tasks as well — such as efficiently detecting itinerant microwave photons. Such a device may prove useful for remote entanglement protocols, quantum optics experiments in the microwave regime, and as a detector for low-mass dark matter.

Through a blend of circuit quantum electrodynamics (cQED) and waveguide quantum electrodynamics (wQED) theories, high detector performance was achieved even when the photon arrival time was not known. Utilizing an ensemble (N = 4) of identical transmon absorbers, photon mediated interactions along a common waveguide enable absorbed itinerant photons to be trapped for longer than the inverse of the bandwidth of an individual qubit. This extended trapping time allows for sufficient integration of the dispersively shifted readout resonator to resolve deviations from vacuum with high fidelity.