Microwave Photon Detection Using an Impedance-Matched $\Lambda$ System

We demonstrate microwave photon detection using an impedance-matched $\Lambda$ system consisting of dressed states in a circuit QED system~$[1, 2]$. When a microwave photon resonant with the $\Lambda$ system is input, it deterministically induces a Raman transition in the system and excites the qubit, enabling its applications to the single photon detection by reading out a qubit state before its relaxation. The resonant microwave pulses with an average photon number of $\sim 0.1$ are applied to the $\Lambda$ system. The qubit state is read out by using a parametric phase-locked oscillator, which enables a fast, single-shot, and non-destructive readout~$[3]$. The photon detection efficiency of $\sim 70\%$ has been achieved. The loss of the efficiency is mainly attributed to the relaxation of the qubit state due to short $T_1$.\\ \\ $[1]$ K. Koshino {\it et al}., Phys. Rev. Lett. {\bf 111}, 153606 (2013).\\ $[2]$ K. Inomata {\it et al}., Phys. Rev. Lett. {\bf 113}, 063604 (2014).\\ $[3]$ Z.~R. Lin {\it et al}., Nat. Commun. {\bf 5}, 4480 (2014).\\