Photodetection is usually a rather brutal process, in which the photons are destroyed, their energy being converted into an electrical or chemical signal. Photons die while delivering their message! This destruction is not a requirement of quantum mechanics, which allows for transparent photodetectors, recording the presence of a photon without absorbing it. We have realized such an ideal photon counting for a microwave field stored in a high-Q cavity, repeatedly interrogated by non-resonant circular Rydberg atoms acting as transparent probes.
We have observed the quantum jumps of light, revealing the birth and death of individual photons. The analysis of the jump statistics provides a detailed insight into field relaxation. Quantum Zeno effect occurs when repeated QND measurements compete with the creation of a coherent field by a classical source.
Through QND measurements on an ensemble of cavity fields prepared in the same state, we fully reconstruct this state and its Wigner function. We apply this method to a gallery of states, including non-classical Fock and `Schrödinger cat' states, with negativities in their Wigner function. We observe in real time the decoherence of a Schrödinger cat. This quantum state reconstruction opens fascinating perspectives for detailed non-classical state studies and investigations of decoherence processes.