Optimal quantum metrology using superconducting qubits

Among the many advantages of quantum technology, one of the most promising is the ability to measure with sensitivity beyond classical limits. Specifically, quantum metrology—where unknown physical parameters are measured with uniquely quantum resources—has applications in diverse fields including gravitational wave detection, biological sensing, and dark matter searches. Recent theory work has determined that, when searching for a small displacement signal, the optimal strategy is to prepare a probe in a Fock state and detect the displacement by counting photons. Here, we experimentally demonstrate this optimal metrological strategy using a superconducting cavity and qubit device to search for dark matter. We use a qubit to both prepare highly non classical Fock input states [1] and resolve individual photon numbers [2]. We demonstrate a 460 times detection enhancement relative to a conventional search strategy using a vacuum probe and ideal homodyne measurement.

[1] A. Agrawal, A. V. Dixit, et. al. arXiv:2305.03700 (2023)

[2] A. V. Dixit, A. Agrawal, et. al. Phys. Rev. Lett. 126, 141302 (2021)