Light Zeeman coherence

DOPPLER NARROWING AND COLLISION-INDUCED ZEEMAN COHERENCE IN FOUR-WAVE LIGHT MIXING... 

During the past year we have replaced the Coherent Model 599-03 by the Coherent Model 599-21. With both dye lasers actively stabilized in frequency, the instrumental resolution could be increased to 1 or 2 MHz. Although the system has not yet operated according to specifications, we have obtained new quantitative data on the linewidth of the central components. It is the purpose of this paper to present these new data. They demonstrate the phenomena of colli-sional narrowing of residual Doppler broadening in four-wave light mixing, as discussed in section 2, and of collision-induced Zeeman coherences, treated in section 3. 

Zeeman coherences in degenerate four-wave light mixing were explicitly considered previously by Steel, Lam and McFarlane. ... 

The major advantage of ultracold trapped hydrogen is that one may be able to achieve a coherence time comparable with the natural lifetime, 122 ms. As described in H-l, [16], The magnetic trapping fields can be reduced to a level where the residual Zeeman shift of the transition is on the order of the natural linewidth of 1.3 Hz. The light-induced shift and the photoionization rate can be reduced to the same level. 

A good introduction to electro- and magneto-optical effects can be found in the book by Harvey on Coherent Light [158]. The main effects and the relationship between them are indicated in table 4.1. Many atoms are readily produced as vapour columns, using standard laboratory methods [159]. The natural mode in which to conduct experiments on unperturbed free atoms is therefore in transmission. As table 4.1 emphasises (the reason is given below), the Faraday effect contains equivalent information to the Zeeman effect in transmission. Actually, what Harvey calls the Zeeman effect in transmission is usually referred to as the inverse Zeeman effect [160], to distinguish it from the Zeeman effect observed in emission.5... 

In this case, the levels with m = l are populated. As long as the Zeeman splitting is smaller than the homogeneous width of the Zeeman levels (e.g., the natural linewidth Aco = 1 /r), both components are excited coherently (even with monochromatic light ). The wave function of the excited state is represented by a linear combination xj/ = axjfa + bxj/b of the two wavefunc-tions of the Zeeman sublevels m = l. The fluorescence is nonisotropic. 

Fig. 2.31a-c. Coherent excitation of Zeeman sublevels with m = 1 (a) by linear polarized light with E B (b). The fluorescence is a superposition of and light (c)... 

Excitation by visible or UV light may also create a coherent superposition of Zeeman sublevels. As an example, we consider the transition of... 

Because of the special properties of the exponential function the light decays with the same time constant r as the population decay. The light decay can be followed by a fast detector connected to fast, time-resolving electronics. If the excited state has a substructure, e.g. because of the Zeeman effect or hyperfine structure, and an abrupt, coherent excitation is made, oscillations (quantum beats) in the light intensity will be recorded. The oscillation frequencies correspond to the energy level separations and can be used for structure determinations. We will first discuss the generation of short optical pulses and measurement techniques for fast optical transients. 

Excitation by visible or UV light may also create a coherent superposition of Zeeman sublevels. As an example, we consider the transition 6 6 Pi of the Hg atom at Z = 253.7nm (Fig.2.34). In a magnetic field B= 0, 0, Bz, the upper level 6 Pi splits into three Zeeman sub-levels with magnetic quantum numbers = 0, 1. Excitation with linear polarized light (E B) only populates the level mj = 0. The fluorescence emitted from this Zeeman level is also linearly polarized. 

These experiments are in fact entirely analagous to the Hanle effect or zero-field level-crossing experiments involving excited atoms discussed in Chapter 15. The coherent polarization of the pumping light referred to the quantization axis Oz in Fig.17.12 prepares the atoms in a coherent superposition of ground-state Zeeman sub-levels. The ensemble density matrix now has finite off-diagonal elements... 

Fig.17.13. Production of oscillating coherence by transverse optical pumping with modulated light. When the pumping light is modulated at the Larmor frequency coherence may be generated between the ground state Zeeman sub-levels. The coherence causes a change in the mean intensity of the transmitted light. (After Bell and Bloom (1961).)...

Quantum beats and hyperfine structure. In sections 15.8 and 16.7 we described experiments in which the resonance fluorescence from excited atoms was observed to be modulated at frequencies in the range 0 S - 10 MHz, corresponding to the separation of the Zeeman sub-levels of the excited atoms in low magnetic fields. These light beats arise because the atoms were prepared in a coherent superposition of at least two excited states,... 

---