  Quantum annealing (QA) is a way to solve combinational optimization problems.
Kerr nonlinear parametric oscillators (KPOs) are promising devices for
implementing QA. When we solve the combinational optimization problems using
KPOs, it is necessary to precisely control the photon number of the KPOs. Here,
we propose a feasible method to estimate the photon number of the KPO. We
consider coupling an ancillary qubit to the KPO and show that spectroscopic
measurements on the ancillary qubit provide information on the photon number of
the KPO.
