Photon blockade in circuit quantum electrodynamics

Strong photon-photon interactions arise in a cavity strongly coupled to an atom or qubit, resulting in blockaded transmission[1].~ In such a system, the resonant frequency of the cavity shifts with the presence of a single photon due to the strong number-dependent nature of the cavity nonlinearity.~ Here, we investigate the photon blockade regime in superconducting circuits with integrated transmon qubits.~ To maximize the nonlinear effects, both the cavity $Q$ and qubit-cavity coupling are made extremely large by design, with $Q$ exceeding 100,000.~ Cavity transmission is characterized using a microwave generator with a controllable output bandwidth.~ Measurements of transmitted power and spectra versus incident center frequency and bandwidth are presented.~ [1] K.M. Birnbaum et al., Nature, 436, 87 (2005).