When physicists make measurements on photons, the process disturbs these quantum-scale systems. Although the minimum disturbance would allow making multiple measurements and achieve approximately the same result, most real measurements cause greater disturbance than this ideal minimum. Within the EU-funded project ONDEQUAM (Optimal non-demolition quantum measurements), physicists have demonstrated a new way to make 'ideal' measurements. Quantum non-demolition measurements allow the detection of single particles repeatedly without destroying them. Compared to the conventional scheme, the physicists showed that their adaptive method was 45 % faster in measuring the number of photons inside a superconducting ultra-high-finesse cavity. This is of particular importance when applied in the case of a decaying photon field. To perform non-demolition measurements of the photon number, the ONDEQUAM team used a Ramsey interferometer to detect the phase shift that atoms experience while passing through the cavity. The amount of information each probe atom extracts depends on the phase shift and the orientation of the interferometer. A so-called forward-backward analysis of the photon number is then carried out to improve the efficiency of quantum non-demolition measurements. By evaluating a posteriori data corresponding to the future of the current state, physicists were able to significantly reduce the noise level. The ONDEQUAM method lifts ambiguities in the photon number that are present in standard non-demolition measurements of photons as a result of the periodic readout of the probe atoms' phases. Moreover, the time resolution of photon measurements is highly improved. Achieving quantum non-demolition measurements of photons, although challenging, could lead to the development of new quantum information technologies. If feedback of the quantum systems could also be implemented, physicists could imagine using these systems for quantum simulations and quantum computations.
Quantum non-demolition measurements, photons, ONDEQUAM, Ramsey interferometer, quantum information