Quantum interference systems

To realize an automatic evaluation system, it would be desirable to also suppress geometrically caused signals as well, so that only the actual defect signals are obtained. Several approaches have already been made which are also to be implemented as part of a SQUID research project (SQUID = Super Conducting Quantum Interference Device). 

Solomon GC, Andrews DQ, Goldsmith RH, Hansen T, Wasielewski MR, Van Duyne RP, Ratner MA (2008) Quantum interference in acyclic systems conductance of cross-conjugated molecules. J Am Chem Soc 130(51) 17301—17308... 

In previous papers [10,11] we have formulated a procedure for splitting the ground-state energy of a multifermionic system into an averaged, structure-less part, E, and a residual, shell-structure part, 8E. The latter originates from quantum interference effects of the one-particle motion in the confining potential [12] and has the form of a shell-correction expansion 5E = It was also shown [11] that the first-order corrective term,... 

The theory has been generalized by us to finite temperatures and to qubits driven by an arbitrary time-dependent field, which may cause the failure of the rotating-wave approximation (RWA) [11]. It has also been extended to the analysis of multilevel systems, where quantum interference between the levels may either inhibit or accelerate the decay [19]. 

As discussed by M. Shapiro and R Brumer in the book Quantum Control of Molecular Processes, there are two general control strategies that can be applied to harness and direct molecular dynamics optimal control and coherent control. The optimal control schemes aim to find a sef of external field parameters that conspire - through quantum interferences or by incoherent addition - to yield the best possible outcome for a specific, desired evolution of a quantum system. Coherent control relies on interferences, constructive or destructive, that prohibit or enhance certain reaction pathways. Both of these control strategies meet with challenges when applied to molecular collisions. 

In summary, we have experimentally demonstrated laser control of a branching photochemical reactions using quantum interference phenomena. In addition we have overcome two major experimental obstacles to the general implementation of optical control of reactions (a) we have achieved control using incoherently related light sources, and (b) we have affected control in a bulk, thermally equilibrated, system. 

The current view is that certain forms of decoherence can cause the Ioss ( quantum interference in just such a way that the system then obeys cla mechanics [158]. This view does not obviate the possibility that classical meet is, in fact, the limit of quantum mechanics when ft —> 0 (i.e, when the system <... 

The optimization procedure yields a set of coefficients a,-. Of considerable interiasff is the question of whether these coefficients merely define a new vector that is simply a vector in a rotated coordinate system. If so, this would indicate that the optimqifl solution corresponds to a simple classical reorientation of the di atomic-moleciilee angular momentum vector. Examination of the optimal results [238] indicate " this is not the case. That is, control is the result of quantum interference effects ... 

All of the quantum control scenarios involve a host of laser and system parameters. To obtain maximal control in any scenario necessitates a means of tuning the system and laser parameters to optimally achieve the desired objective. This topic, optimal control, is introduced and discussed in Chapters 4 and 13. The role of quantum interference effects in optimal control are discussed as well, providing a uniform picture of control via optimal pulse shaping and coherent control. 

Recent years have also witnessed exciting developments in the active control of unimolecular reactions [30,31]. Reactants can be prepared and their evolution interfered with on very short time scales, and coherent hght sources can be used to imprint information on molecular systems so as to produce more or less of specified products. Because a well-controlled unimolecular reaction is highly nonstatistical and presents an excellent example in which any statistical theory of the reaction dynamics would terribly fail, it is instmctive to comment on how to view the vast control possibihties, on the one hand, and various statistical theories of reaction rate, on the other hand. Note first that a controlled unimolecular reaction, most often subject to one or more external fields and manipulated within a very short time scale, undergoes nonequilibrium processes and is therefore not expected to be describable by any unimolecular reaction rate theory that assumes the existence of an equilibrium distribution of the internal energy of the molecule. Second, strong deviations Ifom statistical behavior in an uncontrolled unimolecular reaction can imply the existence of order in chaos and thus more possibilities for inexpensive active control of product formation. Third, most control scenarios rely on quantum interference effects that are neglected in classical reaction rate theory. Clearly, then, studies of controlled reaction dynamics and studies of statistical reaction rate theory complement each other. 

Baranger, H.U., Jalabert, R.A. and Stone, A.D. (1993b). Tiransmission through ballistic cavities chaos and quantum interference, in Quantum Dynamics of Chaotic Systems, eds. J.-M. Yuan, D. H. Feng and G. M. Zaslavsky (Gordon and Breach, Amsterdam). 

Experimental realization of a quantum computer requires isolated quantum systems that act as the quantum bits (qubits), and the presence of controlled unitary interactions between the qubits. As pointed out by many authors [97-99], if the qubits are not sufficiently isolated from outside influences, decoherences can destroy the quantum interferences that actually form the computation. 

As a consequence of the collective motion of the neutral system across the homogeneous magnetic field, a motional Stark term with a constant electric field arises. This Stark term inherently couples the center of mass and internal degrees of freedom and hence any change of the internal dynamics leaves its fingerprints on the dynamics of the center of mass. In particular the transition from regularity to chaos in the classical dynamics of the internal motion is accompanied in the center of mass motion by a transition from bounded oscillations to an unbounded diffusional motion. Since these observations are based on classical dynamics, it is a priori not clear whether the observed classical diffusion will survive quantization. From both the theoretical as well as experimental point of view a challenging question is therefore whether quantum interference effects will lead to a suppression of the diffusional motion, i.e. to quantum localization, or not. 

Spin-orbit(SO) coupling is an important mechanism that influences the electron spin state [1], In low-dimensional structures Rashba SO interaction comes into play by introducing a potential to destroy the symmetry of space inversion in an arbitrary spatial direction [2-6], Then, based on the properties of Rashba effect, one can realize the controlling and manipulation of the spin in mesoscopic systems by external fields. Recently, Rashba interaction has been applied to some QD systems [6-8]. With the application of Rashba SO coupling to multi-QD structures, some interesting spin-dependent electron transport phenomena arise [7]. In this work, we study the electron transport properties in a three-terminal Aharonov-Bohm (AB) interferometer where the Rashba interaction is taken into account locally to a QD. It is found that Rashba interaction changes the quantum interference in a substantial way. 

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