The light emitted by a laser, in the visible range of the spectrum, or by a cell phone, in the microwave range, is classically described as a wave. Yet we know that light can also be described as a set of particles: the photons. In order to reveal this granularity, one can use a photocounter.If most of these detectors absorb the energy of light, preventing any further manipulation, photons can also be counted without being destroyed using ancillary objects that are sensitive to their number.
By probing a superconducting circuit in a cavity, the Quantum Electronics group managed to determine whether or not the cavity hosts a given number of microwave photons. This method had already been used to measure the probability to get a photon number when the cavity is driven by a classical wave.
The experiment becomes counter-intuitive if that measure is repeated frequently. By asking continuously the question «are there 4 photons in the cavity», the researchers of the group have shown that it is possible to prevent the cavity to ever reach 4 photons provided that the initial answer is no! This is the quantum Zeno dynamics of light. Under that constraint, the field dynamics is deeply modified. By irradiating the cavity on resonance, the intra-cavity field is not a classical wave anymore. It becomes a superposition between two waves of opposite phases instead of a single one, which is called a Schrödinger cat of light. This new control technique of the quantum properties of light could apply to the development of quantum computers by protecting qubits from decoherence. These results were published in Science in May 2015.
Quantum dynamics of an electromagnetic mode that cannot contain N photons
L. Bretheau, P. Campagne-Ibarcq, E. Flurin, F. Mallet, and B. Huard
Science 15 May 2015: 776-779.