Planck Spectroscopy and the Quantum-Mechanics of Microwave Beam splitters

Beam splitters occupy a central role in quantum optical architectures. We present an experimental study on the quantum-mechanical behaviour of microwave beam splitters based on a cross-correlation heterodyne detector. This method allows us to measure the covariance matrix of thermal states at the beam splitter input ports. Each matrix element represents a Planck distribution as a function of both temperature and frequency. Such a Planck spectroscopy makes possible to set an experimental limit to the measurement of vacuum fluctuations and unveils the partition noise properties of microwave beam splitters.