A Direct Coupling Coherent Quantum Observer for a Qubit, including Observer Measurements
For quantum control and estimation, this provides a method for coherent quantum observers that do not disturb the observed variable, with a Kalman filter formulation.
This paper designs a direct coupling coherent quantum observer for a qubit plant using a quantum harmonic oscillator with homodyne detection, achieving non-demolition measurement and convergence of the observer output to the plant variable. A minimum variance unbiased estimator (Kalman filter) is derived for linear quantum systems and applied.
This paper proposes a direct coupling coherent quantum observer for a quantum plant which consists of a two level quantum system. The quantum observer, which is a quantum harmonic oscillator, includes homodyne detection measurements. It is shown that the observer can be designed so that it does not affect the quantum variable of interest in the quantum plant and that measured output converges in a given sense to the plant variable of interest. Also, the plant variable of interest-observer system can be described by a set of linear quantum stochastic differential equations. A minimum variance unbiased estimator form of the Kalman filter is derived for linear quantum systems and applied to the direct coupled coherent quantum observer.