Metrology near exceptional points from superconducting circuits with loss*

The underlying mechanism of sensors, amplifiers, and metrological devices is a strong response to small perturbations. Such strong responses are expected for classical systems governed by non-Hermitian Hamiltonians. We use post-selection to create an effective non-Hermitian Hamiltonian for a superconducting quantum circuit and measure the sensitivity of the circuit to a coherent drive for different system parameters. We find that the quantum Fischer information about the drive amplitude diverges at the exceptional point, indicating enhanced sensitivity. However, enhanced sensitivity is achieved at the cost of additional experimental data due to post-selection statistics. With this experiment, we highlight the interplay of dissipation, dephasing, and loss near exceptional points of non-Hermitian open quantum systems and, we observe the role of quantum measurement backaction for quantum sensing.