Trapping states quantum interference

A.J. Gonzalez Martinez, J.R. Crespo L6pez-Urrutia, J. Braun, G. Brenner, H. Bruhns, A. Lapierre, et al., State-selective quantum interference observed in the recombination of highly charged Hg75+"78+ mercury ions in an electron beam ion trap, Phys. Rev. Lett. 94 (20) (2005) 203201. 

The effect of quantum interference on spontaneous emission in atomic and molecular systems is the generation of superposition states that can be manipulated, to reduce the interaction with the environment, by adjusting the polarizations of the transition dipole moments, or the amplitudes and phases of the external driving fields. With a suitable choice of parameters, the superposition states can decay with controlled and significantly reduced rates. This modification can lead to subnatural linewidths in the fluorescence and absorption spectra [5,10]. Furthermore, as will be shown in this review, the superposition states can even be decoupled from the environment and the population can be trapped in these states without decaying to the lower levels. These states, known as dark or trapped states, were predicted in many configurations of multilevel systems [11], as well as in multiatom systems [12],... 

Another area of interest in quantum interference effects, which has been studied extensively, is the response of a V-type three-level atom to a coherent laser field directly coupled to the decaying transitions. This was studied by Cardimona et al. [36], who found that the system can be driven into a trapping state in which quantum interference prevents any fluorescence from the excited levels, regardless of the intensity of the driving laser. Similar predictions have been reported by Zhou and Swain [5], who have shown that ultrasharp spectral lines can be predicted in the fluorescence spectrum when the dipole moments of the atomic transitions are nearly parallel and the fluorescence can be completely quenched when the dipole moments are exactly parallel. 

Traps and recombination centers which depend on purity, crystal defects and preparation, can exert an influence, and electrode contacts, carrier injections, and other factors can interfere with measurements. Yet there is no doubt that the photoconductive gain (quantum yield) G can be reproduced by different methods. As in the case of dark conductivity, the photoconductivity properties are related to the electronic and structural behavior of pure and doped organic compounds, also those in the polycrystalline state. 

Figure 5 shows the ratio of trapped to ionized population for several peak intensities and three pulse widths. This ratio is close to zero for peak intensities ( peai) less than I s, as expected. For > Ires the ratio grows, and can be as much as one half. The trapped population is found mostly in states with / > 4 and quantum numbers > 7. Longer pulse widths result in more excitation and ionization, but a lower overall trapped/ionized ratio. Eventually, as the peak intensity is raised and the channel closing occurs earlier in the pulse, the ratio decreases since most of the ionization then occurs non-resonantly. The structure in the curves may be indicative of interference between resonant excitation on the rising and falling edges of the pulse, a subject we will return to in the next section. 

Since the eigenfunction n) is not in the Hermitian domain of the Hamiltonian the definition of the inner product that we should use should be questioned. If we will keep the usual definition of the scalar product in quantum mechanics the coefficients an in Eq. 33 will get real positive values only (as well as a(e) in Eq. 32) and the possibility of interference among different resonance states which leads to the trapping of an electron due to the molecular vibrations will be eliminated. As was mentioned before the generalized definition of the inner product (.... ..) rather than the usual scalar product has to be used since the Hamiltonian is... 

The above considerations deal with information in a disembodied form. If one actually wants to make a quantum computer, there are all sorts of fabrication, interaction, decoherence, and interference considerations. This is a very rich area of experimental science, and many different avenues have been attempted. Nuclear magnetic resonance, ion traps, molecular vibrational states, and solid-state implementations have all been used in attempts to produce actual quantum computers. 

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