Controlling isolated quantum particles (2)

The lesson described the simple set-up for these microwave spectroscopy studies. Excited atoms passed through a cavity formed by two mirrors facing each other, in which the microwave was applied, before exiting the cavity and being detected by ionization. In the course of these experiments, it became apparent that the atoms, if initially carried into the excited level of a resonant transition with a cavity mode, spontaneously transitioned to the lower Rydberg level of the transition without the application of an external microwave. The system thus constituted a very special maser, with a very low threshold. Whereas ordinary masers can only oscillate with an amplifying medium containing billions of atoms, a few hundred deRydberg atoms were enough to ensure the emission of a microwave pulse into the cavity. This very low threshold was due to the very high electric dipole value of these atoms, and it became clear at that point that it would be possible to design a system in which the maser would have a threshold corresponding to a single atom present in the cavity. The effect could also be seen as an exaltation of the spontaneous emission of an atom induced by the presence of the cavity, an effect that had been predicted theoretically in 1946 by Edward Purcell, one of the fathers of magnetic resonance.

To observe this effect, however, the quality factor of the cavity had to be increased. A first step in this direction was to replace the mirrors, originally made of copper, with superconducting niobium. The threshold of the Rydberg atom maser was rapidly lowered until the spontaneous emission exaltation effect was observed for the first time in 1982. At the same time, D. Kleppner's group at MIT was observing the opposite effect, that of inhibiting the spontaneous emission of an atom in a cavity not supporting a mode at the frequency of the atomic transition. These experiments were the actual birth certificates of cavityelectrodynamics. In parallel with these experiments, the ENS group also carried out superradiance experiments, in which the emission in a resonant cavity of a large number of initially excited atoms was studied. The collective evolution of the atomic sample was analyzed in detail in a series of experiments constituting the first experimental study of the effect of symmetric coupling of a set of atoms to a field mode, an effect that had been described theoretically by R. Dicke in 1954.

From the observation of the exaltation effect of spontaneous cavity emission, the aim of the ENS group was to observe the regime of strong atom-cavity coupling where the photon emitted by the atom survives long enough to be subsequently absorbed, returning the atom to its excited state. The atom-cavity system must then evolve reversibly, oscillating between two states. The atom and the field then periodically exchange a quantum of energy, and are most often in an entangled state. To observe this so-called "vacuum Rabi oscillation", the quality factor of the cavities had to be increased by one to two orders of magnitude. This was first achieved by Herbert Walther's rival group in Munich, using closed cylindrical superconducting cavities. This group's experiments, which led to the realization and study of the Rydberg atom micromaser, are described in the lesson. The ENS group also adopted these cylindrical superconducting cavities for certain experiments, enabling them to operate a two-photon maser for the first time.

However, the lesson analyzed the reasons why these open cavities were unsuitable for the experiments planned by the ENS group: atoms having to pass through small holes were subjected to parasitic electric fields destroying state superpositions. In addition, the structure of the closed cavities made it impossible to apply static electric fields to the Rydberg atoms needed to manipulate them in certain experiments. The lesson therefore concluded with a description of the improvements the ENS group had made to the open superconducting cavities to increase their quality factor. These improvements were to lead to the realization in 2006 of a cavity in which photons can bounce between mirrors for more than a tenth of a second, allowing thousands of atoms passing through one by one to interact with the field before it dampens. Anticipating the history of the ENS group's experiments, the lesson concluded with a description of this exceptional cavity.