This paper investigates the feasibility of using sequences of unitary pulses for achieving coherent-control of quantum dynamical phenomena, including quantum control of electron tunnelling in archetype model systems and control of decoherence in a quantum dot system coupled to a thermal bath. The proposed dynamical decoupling scenario is based on the repetitive application of 2π pulses, affecting the interference phenomena between wave-packet components. The pulses affect the overall relaxation dynamics without collapsing the coherent-quantum evolution of the system. It is shown that both bound-to-bound state tunnelling and bound-to-continuum tunnelling processes can be inhibited and eventually halted by sufficiently frequent pulse fields that exchange energy with the system but do not affect the potential energy tunnelling barriers. Furthermore, the proposed quantum-control scenario is demonstrated as applied to manipulating the electronic quantum dynamics in a quantum dot coupled to a free standing quasi two-dimensional phonon cavity. The reported results are therefore particularly relevant to the understanding of coherent optical manipulation of electronic excitations in semiconductor devices where performance is limited by quantum tunnelling and decoherence.
This paper investigates the feasibility of using sequences of unitary pulses for achieving coherent-control of quantum dynamical phenomena, including quantum control of electron tunnelling in archetype model systems and control of decoherence in a quantum dot system coupled to a thermal bath. The proposed dynamical decoupling scenario is based on the repetitive application of 2π pulses, affecting the interference phenomena between wave-packet components. The pulses affect the overall relaxation dynamics without collapsing the coherent-quantum evolution of the system. It is shown that both bound-to-bound state tunnelling and bound-to-continuum tunnelling processes can be inhibited and eventually halted by sufficiently frequent pulse fields that exchange energy with the system but do not affect the potential energy tunnelling barriers. Furthermore, the proposed quantum-control scenario is demonstrated as applied to manipulating the electronic quantum dynamics in a quantum dot coupled to a free standing quasi two-dimensional phonon cavity. The reported results are therefore particularly relevant to the understanding of coherent optical manipulation of electronic excitations in semiconductor devices where performance is limited by quantum tunnelling and decoherence.
The feasibility of achieving coherent control of tunnelling dynamics, associated with electronic excitations in functionalized semiconductor nanostructures, is investigated. The coherent control scenario is based on the application of frequent stochastic unitary pulses, affecting the interference phenomena between electronic wave-packet components and consequently the overall electronic tunnelling dynamics without collapsing the coherent quantum evolution of the system. It is shown that tunnelling can be inhibited and eventually halted by sufficiently frequent pulse fields that exchange energy with the system but do not affect the potential energy tunnelling barriers. Further, the proposed stochastic quantum-control scenario is demonstrated more generally as applied to an archetype model, a particle tunnelling in a quartic double-well potential.
A general coherent control scenario to suppress or accelerate tunneling of quantum states decaying into a continuum is investigated. The method is based on deterministic, or stochastic, sequences of unitary pulses that affect the underlying interference phenomena responsible for quantum dynamics, without inducing decoherence, or collapsing the coherent evolution of the system. The influence of control sequences on the ensuing quantum dynamics is analyzed by using perturbation theory to first order in the control pulse fields and compared to dynamical decoupling protocols and to sequences of pulses that collapse the coherent evolution and induce quantum Zeno (QZE) or quantum anti-Zeno effects (AZE). The analysis reveals a subtle interplay between coherent and incoherent phenomena and demonstrates that dynamics analogous to the evolution due to QZE or AZE can be generated from stochastic sequences of unitary pulses when averaged over all possible realizations.
Coherent-optical-control schemes exploit the coherence of laser pulses to change the phases of interfering dynamical pathways and manipulate dynamical processes. These active control methods are closely related to dynamical decoupling techniques, popularized in the field of quantum information. Inspired by nuclear magnetic resonance spectroscopy, dynamical decoupling methods apply sequences of unitary operations to modify the interference phenomena responsible for the system dynamics thus also belonging to the general class of coherent-control techniques. This article reviews related developments in the fields of coherent optical control and dynamical decoupling, emphasizing the control of tunneling and decoherence in general model systems. Considering recent experimental breakthroughs in the demonstration of active control of a variety of systems, we anticipate that the reviewed coherent-control scenarios and dynamical-decoupling methods should raise significant experimental interest.
A general coherent control scenario to suppress, or accelerate, tunneling of quantum states decaying into a continuum, is investigated. The method is based on deterministic, or stochastic, sequences of unitary pulses that affect the underlying interference phenomena responsible for quantum dynamics, without inducing decoherence, or collapsing the coherent evolution of the system. The influence of control sequences on the ensuing quantum dynamics is analyzed by using perturbation theory to first order in the control pulse fields and compared to dynamical decoupling (DD) protocols and sequences of pulses that collapse the coherent evolution and induce quantum Zeno (QZE) or quantum anti-Zeno effects (AZE). The analysis reveals a subtle interplay between coherent and incoherent phenomena and demonstrating that dynamics analogous to evolution due to QZE or AZE can be generated from stochastic sequences of unitary pulses when averaged over all possible realizations.
A model of semiconductor quantum dots coupled to phonons and laser light, which is relevant in the limit of strong electronic confinement, is investigated. For an arbitrary sequence of excitations by ultrafast pulses, analytical solutions are obtained for all pertinent density-matrix elements. The results are nonperturbative with respect to the coupling to both phonons and laser light. The derivation depends neither on a particular choice of electronic wave functions nor on a particular wave-vector dependence of the phonon interaction or phonon dispersion. All components of the density matrix exhibit oscillatory dependences on the pulse areas of the exciting pulses analogously to the Rabi oscillations known from simple two-level systems. Three typical applications of the general results are worked out: (a) the influence of phonons on four-wave-mixing spectra, (b) the coherent control of electronic dot occupations in the presence of phonons, and (c) the dynamics of phonon correlations including the formation of a stable polaron spatially localized in the dot and the emission of a pulsed phonon wave packet that leaves the dot.
Any real system, no matter how well isolated, unavoidably interacts with the environment. This interaction makes the evolution of a quantum system nonunitary and destroys the coherence of quantum superpositions. This process, known as decoherence, is widely regarded as the most important and fundamental obstacle to the practical realization of quantum information processing. On the other hand, the objectives of quantum control are predicated on the ability to manipulate the system coherently over a period of time. Quantum control has been applied successfully to several experimental systems and coupling such a controlled system to an environment makes the study of coherence loss possible. The standard model of decoherence is formulated in terms of a system, defined by the observables that can be measured, and a reservoir, de¯ned by those degrees of freedom that are not accessed by observation. The vibrational and rotational degree of freedom of diatomic molecules form an experimental model of such a system and bath 1. The nature of the ro-vibrational coupling does not allow a Decoherence Free Subspace, instead we try to identify a Decoherence Resistant Subspace using closed loop control. We set up a learning control loop experiment to monitor the coherence in a vibrational wavepacket, consisting of about 6 vibrational states of the A(1Σ+g ) electronic state, excited by a shaped, ultrashort optical pulse, and then to determine the pulse shape that yielded the longest coherence lifetime. The coherence of the vibrational wavepacket was monitored by measuring the fluorescence quantum beats from the outer turning point of the wave packet's trajectory. The closed loop controller altered the shape of the driving optical pulse to maximize the the depth of modulation of the quantum beats at a prescribed time after excitation, within the constraints of the laser system. The outcome of this closed loop control experiment shows an improvement of the beat decay constant of about 60%, as shown in Fig. (1), a result which occurs for several di®erent pulse shapes. Our experiments show that a number of interesting decoherence-resistant excitations exist. The nature of the dephasing was examined theoretically. Using measured optical pulse shapes, we simulated numerically the molecular wavepacket in the limit of perturbation theory. The experiment and simulations show that there are di®erent mechanisms contributing to the decoherence resistant excitation. The shaped pulse reduces the °uctuations in the wave packet spatial extent, which in turn changes the system reservoir coupling. Other components of the optimal pulse are a sequence of pulses that creates constructive interference of the vibrational wavepacket at the desired time, and chirp which compensates for the anharmonicity of the adiabatic potential. The experimental results show general agreement with numerical simulations. We conclude that wavepackets resistant to dephasing do exist,and that learning algorithms provide an effcient means for finding them.