LAUSR

One of the hallmarks of quantum mechanics is the impossibility of simultaneous measurement of non-commuting observables with projective measurements. This, however, can be circumvented by using continuous quantum measurements. Here we investigate the temporal correlations of the output signals of detectors continuously and simultaneously measuring the qubit observables σz and σz cosφ + σx sinφ, for various angles φ. Using the quantum Bayesian formalism, we obtain analytical expressions for the correlators, which we find to be in good agreement with those obtained from experimentally measured output signals. The agreement is particularly good for cross-correlators, even at times shorter than the cavity modes decay time. We further discuss how the correlators can be applied for parameter estimation, and use them to infer a small residual qubit Hamiltonian arising from calibration inaccuracy in the experimental data. Our work opens up new possibilities to perform quantum metrology based on temporal correlations of measured data.Quantum measurement: Incompatible observables make sensitive measurementsContinuous quantum measurements of incompatible observables can be used to measure and monitor changes in a quantum system’s properties. It is well known the values of non-commuting ‘incompatible’ observables, such as position and momentum, cannot be measured at the same time due to the uncertainty principle. However, weak measurements that give partial knowledge can be performed, resulting in non-trivial quantum measurement back action. Juan Atalaya and colleagues from the University of California in Riverside and Berkeley have theoretically analysed how a qubit undergoing continuous measurement of incompatible observables evolves. Deriving formulae for the measurements and their cross-correlations allowed the authors to identify how the observed outcomes depend on system parameters. With this information, continuous quantum measurement may allow tracking of slow changes in the parameters, which are common in many experimental systems.