The One-Atom Maser

The divergence of this atom laser beam is very small and the brightness of such a very cold atomic beam source is larger by several orders of magnitude than that achievable with Zeeman slower (see Sect. 14.1.4). The brightness of the atom laser beam, defined as the integrated flux of atoms per source area divided by the velocity spread AvxAvyAvz is about 2 x 10 atoms (s m ) [14.126]. 

Very well collimated atomic beams with such a high brightness can be used for the investigation of scattering, chemical reactions at very low relative velocities and for surface scattering experiments. 

In Sect. 14.3 we discussed techniques to store and observe single ions in traps. We will now present some recently performed experiments that allow investigations of single atoms and their interaction with weak radiation fields in a microwave resonator [14.127]. The results of these experiments provide crucial tests of basic problems in quantum mechanics and quantum electrodynamics (often labeled cavity QED ). Most of these experiments were performed with alkali atoms. The experimental setup is shown in Fig, 14.48. 

It turns out that the spontaneous lifetime of the Rydberg levels is shortened if the cavity is tuned into resonance with the frequency coq of the atomic transition — 1). It is prolonged if no cavity mode matches a o 

Without a thermal radiation field in the resonantly tuned cavity, the populations N n, T) and N n — 1, T) of the Rydberg levels should be a periodic function of the transit time T = d/v, with a period 7r that corresponds to the Rabi oscillation period. The incoherent thermal radiation field causes induced emission and absorption with statistically distributed phases. This leads to a damping of the Rabi oscillation (Fig. 14.50). This effect can be proved experimentally if the atoms pass a velocity selector before they enter the resonator, which allows a continuous variation of the velocity and therefore of the transit time T = dlv. 

The gravitational constant G is from all physical constants the one with the largest experimental uncertainty. Therefore new measuring techniques have been invented to reach a higher accuracy. One of these techniques is based on atom interferometry. Its basic principle can be understood as follows The acceleration of atoms in two clouds in an atomic fountain is measured with stimulated Raman transitions (Fig. 9.71). Two heavy masses at two different positions are added (Fig. 9.72). In the first position one mass was located above the upper atom cloud and the other below the lower cloud. Then the masses were shifted into a position where one mass was below the upper cloud and the other above the lower cloud. The change of the atom acceleration for the two positions were determined [1276]. 

In Sect. 9.3 we discussed techniques to store and observe single ions in traps. We will now present some recently performed experiments that allow investigations of 

It turns out that the spontaneous lifetime of the Rydberg levels is shortened if the cavity is tuned into resonance with the frequency a o of the atomic transition n) n — 1). liis prolonged if no cavity mode matches coq [1297]. This effect, which had been predicted by quantum electrodynamics, can intuitively be understood as follows in the resonant case, that part of the thermal radiation field that is in the resonant cavity mode can contribute to stimulated emission in the transition n) n — 1), resulting in a shortening of the lifetime (Sect. 6.3). For the 

The one-atom maser can be used to investigate the statistical properties of non-classical light [1298, 1299]. If the cavity resonator is cooled down to T 0.5 K, the number of thermal photons becomes very small and can be neglected. The number of photons induced by the atomic fluorescence can be measured via the fluctuations in the number of atoms leaving the cavity in the lower level n — 1). It turns out that the statistical distribution does not follow Poisson statistics, as in the output of a laser with many photons per mode, but shows a sub-Poisson distribution with photon number fluctuations 70 % below the vacuum-state limit [1300]. In cavities with low losses, pure photon number states of the radiation field (Fock states) can be observed (Fig. 9.77) [1301], with photon lifetimes as high as 0.2 s At very low 

Because of its large transition dipole moment n ( ) the atom 

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Fig. 9.73 Schematic experimental setup for the one-atom maser with resonance cavity and state-selective detection of the Rydberg atoms [1294]...

Fig. 9.74 Experimental setup for experiments with the one-atom maser with three excitation lasers, cavity resonance microwave generation and a state-selective detection system...

G. Rempe, H. Walther, The one-atom maser and cavity quantum electrodynamics, in Methods of Laser Spectroscopy, ed. by Y. Prior, A. Ben-Reuven, M. Rosenbluth (Plenum, New York, 1986)... 

Fig. 14.49. (a) Level diagram of the maser transition in rubidium. The transition frequencies are given in MHz. (b) Measured ion signal, which is proportional to the number of Rydberg atoms in level n), as a function of the cavity resonance frequency. Maser operation of the one-atom maser manifests itself in a decrease in the number of atoms in level n) [14.130]... 

G. Rempe, M.O. Scully, H. Walther The one-atom maser and the generation of nonclassical light . In Proc. ICAP 12, Ann Arbor (1990)... 

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