Volterra filters for quantum estimation and detection

The implementation of optimal statistical inference protocols for high-dimensional quantum systems is often computationally expensive. To avoid the difficulties associated with optimal techniques, here I propose an alternative approach to quantum estimation and detection based on Volterra filters. Volterra filters have a clear hierarchy of computational complexities and performances, depend only on finite-order correlation functions, and are applicable to systems with no simple Markovian model. These features make Volterra filters appealing alternatives to optimal nonlinear protocols for the inference and control of complex quantum systems. Applications of the first-order Volterra filter to continuous-time quantum filtering, the derivation of a Heisenberg-picture uncertainty relation, quantum state tomography, and qubit readout are discussed.Comment: v2: added more in-depth discussions, more references, and a qubit readout example with two new figures. Improved presentation; v3: extended and publishe

Continuous Quantum Hypothesis Testing

I propose a general quantum hypothesis testing theory that enables one to test hypotheses about any aspect of a physical system, including its dynamics, based on a series of observations. For example, the hypotheses can be about the presence of a weak classical signal continuously coupled to a quantum sensor, or about competing quantum or classical models of the dynamics of a system. This generalization makes the theory useful for quantum detection and experimental tests of quantum mechanics in general. In the case of continuous measurements, the theory is significantly simplified to produce compact formulae for the likelihood ratio, the central quantity in statistical hypothesis testing. The likelihood ratio can then be computed efficiently in many cases of interest. Two potential applications of the theory, namely quantum detection of a classical stochastic waveform and test of harmonic-oscillator energy quantization, are discussed.Comment: v1: first draft; v2: updated and submitted; v3: accepted by PRL, 5 pages, 1 figure, and Supplementary Materia