QUROPE Quantum Information Processing and Communication in Europe

Quantum states and measurements exhibit wave-like - continuous, or particle-like - discrete, character. Hybrid discrete-continuous quantum optical systems are key to investigating fundamental quantum phenomena such as the violation of local realism by Einstein-Podolsky-Rosen states, measuring non-classical correlations of radiation fields and superpositions of macroscopic states, and form essential resources for quantum-enhanced applications and high- efficient optical telecommunications. Realizing the full potential of these hybrid systems requires quantum optical measurements sensitive to complementary observables such as field quadrature amplitude and photon number. However, a thorough understanding of the practical performance of an optical detector interpolating between these two regions is absent. Here, we report the implementation of full quantum detector tomography, thus enabling the characterization of the simultaneous wave and photon-number sensitivities of quantum detectors. This tomography yields the largest parametrization to-date, requiring the development of novel theoretical tools. Our results reveal the role of coherent superpositions in quantum measurements and demonstrate the tunability of hybrid quantum optical detectors.