Coherent light, definition

By definition, quantum control relies upon the unique quantum properties of light and matter, principally the wavelilce nature of both. As such, maintenance of the phase information contained in both the matter and light is central to the success of the control scenarios. Chapter 5 deals with decoherence, that is, the loss of phase information due to the influence of the external environment in reducing the system coherence. Methods of countering decoherence are also discussed. 

The phenomenon of optical interference is commonly describable in completely classical terms, in which optical fields are represented by classical waves. Classical and quantum theories of optical interference readily explain the presence of an interference pattern, but there are interference effects that distinguish the quantum (photon) nature of light from the wave nature. In this section, we present elementary concepts and definitions of both the classical and quantum theories of optical interference and illustrate the role of optical coherence. 

Whilst the above is perfectly adequate for the description of processes observed with continuous-wave (cw) input, proper representation of the optical response to pulsed laser radiation requires one further modification to the theory. It is commonly thought difficult to represent pulses of light using quantum field theory indeed, it is impossible if a number state basis is employed. However by expressing the radiation as a product of coherent states with a definite phase relationship, it is relatively simple to construct a wavepacket to model pulsed laser radiation [39]. The physical basis for this approach is that pulses necessarily have a finite linewidth and therefore in fact entail a large number of radiation modes, so that for the pump radiation, it is appropriate to construct a coherent superposition... 

This chapter provides an introduction to different spectroscopic techniques that are based either on the coherent excitation of atoms and molecules or on the coherent superposition of light scattered by molecules and small particles. The coherent excitation establishes definite phase relations between the amplitudes of the atomic or molecular wave functions this, in turn, determines the total amplitudes of the emitted, scattered, or absorbed radiation. 

T2 transvers relaxation time) The value Tz/Tj- = 5 has been assumed, where T(- is the correlation time (reciprocal spectral width) of light sources. The coherence parameter P represents the extent of random phase distribution, and the dispersion parameter W represents the degree of regular phase-modulation due to material dispersion (see text for definition). The cross relaxation effect has been neglected. 

The zero-temperature phases can be observed using Bragg scattering with optical light, which allows for probing the crystalline phase. The detection of vortices can be used as a definitive signature of superfluidity. We notice that the two-dimensional (quasi) condensate involves a fraction of the total density only, and therefore we expect only small coherence peaks in a time-of-flight experiment. 

In light scattering spectroscopy we exploit the two important properties of laser light sources monochromaticity and coherence. A good laser is monochromatic to approximately 1 MHz and coherent i.e. displaying a definite phase relation in space over the relevant macroscopic sample dimensions. Quantitatively these properties of laser light are reflected in the small deviation of the frequency w and the wave vector k from their respective mean values. 

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