Generation and efficient measurement of single photons from fixed-frequency superconducting qubits
William Kindel, M. D. Schroer, Konrad W. Lehnert · Physical Review A · 2016
We demonstrate and evaluate an on-demand source of single itinerant microwave photons. Photons are generated using a highly coherent, fixed-frequency qubit-cavity system, and a protocol where the microwave control field is far detuned from the photon emission frequency. By using a Josephson parametric amplifier (JPA), we perform efficient single-quadrature detection of the state emerging from the cavity. We characterize the imperfections of the photon generation and detection, including detection inefficiency and state infidelity caused by measurement back-action over a range of JPA gains from 17 to 33 dB. We observe that both detection efficiency and undesirable back-action increase with JPA gain. We find that the density matrix has its maximum single-photon component ${\ensuremath{\rho}}_{11}=0.36\ifmmode\pm\else\textpm\fi{}0.01$ at 29 dB JPA gain. At this gain, back-action of the JPA creates cavity photon number fluctuations that we model as a thermal distribution with an average photon number $\overline{n}=0.041\ifmmode\pm\else\textpm\fi{}0.003$.