Exploiting Kerr cross nonlinearity in circuit quantum electrodynamics for nondemolition measurements

We propose a scheme for dispersive readout of stored energy in one mode of a nonlinear-superconducting microwave ring resonator by detection of the frequency shift of a second mode coupled to the first via a Kerr nonlinearity. Symmetry is used to enhance the device responsivity while minimizing self-nonlinearity of each mode. Assessment of the signal to noise ratio indicates that the scheme will function at the single-photon level, allowing quantum nondemolition measurement of the photon-number state of one mode. Experimental data from a simplified version of the device demonstrating the principle of operation are presented. © 2010 The American Physical Society.