Coherent phase control

Coherent control Control of the motion of a microscopic object by using the coherent properties of an electromagnetic held. Coherent phase control uses a pair of lasers with long pulse durations and a well-defined relative phase to excite the target by two independent paths. Wave packet control uses tailored ultrashort pulses to prepare a wave packet at a desired position at a given time. 

The underlying principle of coherent phase control is that the probability of an event occurring is given by the square of the sum of the quantum mechanical amplitudes... 

FIGURE 3 Illustration of coherent phase control by one- and three-photon excitation in the cases of (a) no intermediate resonances and (b) a quasi-bound state at the two-photon level that introduces a channel phase shift of 13- [Reproduced with permission from Fiss, J. A., Khachatrian, A., Truhins, K., Zhu, L., Gordon, R. J., and Seideman, T. (2000). Phys. Rev. Lett. 85, 2096.]... 

FIGURE 4 Theoretical calculation of the coherent phase control of the photodissociation of IBr by one- and three-photon excitation. [Reproduced with permission from Chan, C. K., Brumer, R, and Shapiro, M. (1991). J. Chem. Phys. 94,2688. Copyright American Institute of Physics.]... 

FIGURE 5 Apparatus used for coherent phase control. 

Polyatomic molecules provide a still richer environment for studying phase control, where coupling between different dissociation channels can occur. Indeed, one of the original motivations for studying coherent control was to develop a means for bond-selective chemistry [25]. The first example of bond-selective two-pathway interference is the dissociation of dimethyl-sulfide to yield either H or CH3 fragments [74]. The peak in Fig. 11 is indicative of a resonance embedded in an elastic continuum (case 4). 

Lim, S. H., Caster, A. G., and Leone, S. R. 2005. Single-pulse phase-control interferometric coherent anti-Stokes Raman scattering spectroscopy. Phys. Rev. A 72(4) 041803. 

Phase control provides an additional avenue for fine tuning of the interference patterns [34-36]. Interference of several coherent beams can be simulated numerically by summation of plane waves according to Eqs. 1 and 2. This is a helpful method for designing structures with desired architecture. 

We first survey in Section 4.2 the Kurizki-Shapiro-Brumer scheme [5-7] of phase-coherent photocurrent control and focus on its robustness to decoherence, relaxation, and quantum (Langevin) noise induced by the environment. We then... 

PHASE-COHERENT PHOTOCURRENT CONTROL THE KURIZKI-SHAPmO-BRUMER SCHEME... 

Interest in pyrazine is also reflected by past smdies in which coherent control (CC) [44] of the IC process in a four-mode pyrazine model has been demonstrated [28], In particular, it was shown [28] that the extent of phase control over the flow out of the S2 vibrational manifold depends on the participation of overlapping... 

Phase control of two-channel photo-ionization rates and coherent population trapping induced by four laser fields operating on an atomic system initially in its ground state 1), which proceeds via a pair of intermediate bound states, 2) and 3), to a manifold of structureless continua, has also... 

H. Ring, R.T. Carter, R. Huber, Creation and phase control of molecular coherences using pulsed magnetic fields. Laser Rhys. 9, 253 (1999)... 

Phase-dependent coherence and interference can be induced in a multi-level atomic system coupled by multiple laser fields. Two simple examples are presented here, a three-level A-type system coupled by four laser fields and a four-level double A-type system coupled also by four laser fields. The four laser fields induce the coherent nonlinear optical processes and open multiple transitions channels. The quantum interference among the multiple channels depends on the relative phase difference of the laser fields. Simple experiments show that constructive or destructive interference associated with multiple two-photon Raman channels in the two coherently coupled systems can be controlled by the relative phase of the laser fields. Rich spectral features exhibiting multiple transparency windows and absorption peaks are observed. The multicolor EIT-type system may be useful for a variety of application in coherent nonlinear optics and quantum optics such as manipulation of group velocities of multicolor, multiple light pulses, for optical switching at ultra-low light intensities, for precision spectroscopic measurements, and for phase control of the quantum state manipulation and quantum memory. 

One more trend in laser control is based on the use of the property of coherence of the laser light. To effect coherent laser control, it is necessary that not only the light, but also the atom (or molecule) should be in a coherent state during the interaction. For atoms in a beam or in a low-pressure gas, the phase relaxation time of their wave functions depends on spontaneous decay or on collisions and can be comparatively long (from 10 to 10 s). It was for precisely this reason that the main experiments on coherent interaction were conducted with atoms. These experiments led in the final analysis to the discovery of new effects, such as coherent population trapping (Arimondo 1996), electromagnetically induced transparency (Harris 1997), and the slow-light effect (Hau et al. 1999 Kash et al. 1999). 

The situation with molecules is much more involved for several reasons. First, for polyatomic molecules, the intramolecular relaxation processes that occur on a subpicosecond timescale are essential. It was for exactly this reason that the first successful experiments were conducted on the noncoherent laser control of polyatomic molecules with intermolecular selectivity. Second, the phase relaxation time in a condensed medium is also on a subpicosecond scale because of the interaction between the quantum system and its surroundings. Therefore, it was only the creation of relatively simple and inexpensive femtosecond lasers that made it possible to set about realizing the ideas of the coherent laser control of unimolecular processes (Tannor and Rice 1985 Brumer and Shapiro 1986 Judson and Rabitz 1992), particularly the... 

Let us emphasize that noncoherent laser control of molecules depends on differences in the absorption spectrum between different molecular species, for example isotopic molecules. Coherent laser control is distinguished by its nontrivial manipulation of the phase coherence of excited states in molecules that can be similar in their absorption spectra but different in their phase coherence properties (Brixner et al. 20016). 

The requirements for amplitude and phase control of RF pulses are similar to those in high-resolution NMR spectrometers, though the addition of phase coherent frequency switching can be an advantage for multislice fast spin-echo experiments. 

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