Field quantum state reconstruction

An ENS team [1] has been able to trap light in a « photon box » (a microwave cavity) for a time long enough to fully determine its quantum state and observe its evolution [2] . This state is a mathematical object which allows physicists to predict the statistics of all possible measurements performed on the trapped photons. It can in general be represented by a map of values in a “phase plane” which looks as a three dimension geographical map. Each of its points, defined by its distance to the plane origin and its direction, is associated to a value of the light field amplitude and phase. Usual radiation (emitted by heated bodies, by lasers or by a combination of such sources) is quite generally described by a landscape of positive peaks centred at the points corresponding to the most probable values of the field. Physicists are now able to generate also non-classical fields, whose features are much less intuitive. Their maps present oscillations exhibiting negative values in some areas of the phase plane. By probing the photon box with very sensitive atoms, the ENS team has reconstructed the maps of these strange states. Fields with well defined photon numbers have maps presenting concentric oscillations. So called Schrödinger cat states, which are quantum superpositions of classical states with different phases, are described by maps with two positive peaks corresponding to their classical components and, in between, a landscape of alternating positive ridges and negative valleys. By initially preparing these states in the box and recording their subsequent evolution, the ENS team has observed the progressive vanishing of the negative features. This reveals the fragility of the non-classical states which rapidly evolve into mundane classical states with positive maps. Being able to reconstruct in details the dynamics of this decoherence phenomenon, which is at the heart of the quantum to classical boundary, opens the way to the manipulation and control of decoherence. Procedures of quantum-feedback become possible, in which atoms will be used to maintain in real time the non-classical features of a light field, and thus preserve the quantum properties which are essential for the realization of quantum information operations with light.