1. Laboratoire Kastler Brossel, Département de Physique de l'Ecole Normale Supérieure, 24 rue Lhomond, F-75231, Paris Cedex 05, France

Correspondence to: S. Haroche¹ Correspondence and requests for materials should be addressed to S.H. (Email: haroche@physique.ens.fr).

Abstract

Light detection is usually a destructive process, in that detectors annihilate photons and convert them into electrical signals, making it impossible to see a single photon twice. But this limitation is not fundamental—quantum non-demolition strategies^{1, 2, 3} permit repeated measurements of physically observable quantities, yielding identical results. For example, quantum non-demolition measurements of light intensity have been demonstrated^{4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14}, suggesting possibilities for detecting weak forces and gravitational waves³. But such experiments, based on nonlinear optics, are sensitive only to macroscopic photon fluxes. The non-destructive measurement of a single photon requires an extremely strong matter–radiation coupling; this can be realized in cavity quantum electrodynamics¹⁵, where the strength of the interaction between an atom and a photon can overwhelm all dissipative couplings to the environment. Here we report a cavity quantum electrodynamics experiment in which we detect a single photon non-destructively. We use atomic interferometry to measure the phase shift in an atomic wavefunction, caused by a cycle of photon absorption and emission. Our method amounts to a restricted quantum non-demolition measurement which can be applied only to states containing one or zero photons. It may lead to quantum logic gates¹⁶ based on cavity quantum electrodynamics, and multi-atom entanglement¹⁷.

1. Laboratoire Kastler Brossel, Département de Physique de l'Ecole Normale Supérieure, 24 rue Lhomond, F-75231, Paris Cedex 05, France

Correspondence to: S. Haroche¹ Correspondence and requests for materials should be addressed to S.H. (Email: haroche@physique.ens.fr).