Quantum interference [1] is a cornerstone phenomenon in quantum mechanics, arising when the probability amplitudes associated with different quantum pathways combine, leading to patterns of constructive or destructive interference. Coherent control [2] leverages this phenomenon by using external optical or electromagnetic pulses to steer the outcome of quantum interference. Through careful adjustment of the phase, amplitude, and frequency of these external pulses, it is possible to selectively enhance or suppress specific quantum transitions. This selective manipulation enables researchers to direct quantum pathways toward desired outcomes, making coherent control a powerful tool for exploring and utilizing quantum systems.
Quantum interference [1] is a cornerstone phenomenon in quantum mechanics, arising when the probability amplitudes associated with different quantum pathways combine, leading to patterns of constructive or destructive interference. Coherent control [2] leverages this phenomenon by using external optical or electromagnetic pulses to steer the outcome of quantum interference. Through careful adjustment of the phase, amplitude, and frequency of these external pulses, it is possible to selectively enhance or suppress specific quantum transitions. This selective manipulation enables researchers to direct quantum pathways toward desired outcomes, making coherent control a powerful tool for exploring and utilizing quantum systems.
Laser light has been used as a probe of atoms, molecules, and solids since the invention of the laser. The use of laser light in a more active role, to modify and process surfaces, and initiate chemical reactions, followed shortly thereafter. But usually it is the intensity and the directionality of the laser light that is employed, not necessarily its coherence, and not particularly the fact that it has a well-defined phase. 'Coherence control' can be broadly understood as the set of processes whereby light modifies matter in a way that is critically dependent on the incident light beams possessing well-defined phases. While in a laser matter is manipulated to produce light of the desired properties, in coherent control light is manipulated -- in particular, its phase and intensity is adjusted -- to produce a material response of the desired type. Of the various coherent control processes that are currently being investigated, some involve a transition in the material medium from an initial state to a final state by two or more possible processes. With each of these is associated a quantum mechanical amplitude, and hence the probability for the transition can show interference effects between the two amplitudes, just as in the familiar two-slit interference experiment the probability for the electron to be observed at a given position involves a probability that is the square of the sum of two amplitudes. In quantum interference control (QUIC), the relative phase of the two amplitudes is adjusted by adjusting the relative phase of two polarizations of a single beam, or the relative phase of two beams at different frequencies. It is this particular type of coherent control that is of interest in this communication.
Quantum mechanics, though a probabilistic theory, gives a 'deterministic' answer to the question of how the present determines the future. In essence, in order to predict future probabilities, we need to (numerically) propagate the time-dependent Schrödinger equation from the present to the future. It is interesting to note that classical mechanics of macroscopic bodies, though reputed to be a deterministic theory, does not allow, due to chaos (which unfortunately is more prevalent than integrability), such clear insights into the future. In contrast, small (e.g., atomic, molecular and photonic) systems which are best understood using the tools of quantum mechanics, do not suffer from chaos, rendering the prediction of the probability-distributions of future events possible. The field of quantum control deals with an important modification of this task, namely, it asks: given a wave function in the present, what dynamics, i.e. what Hamiltonian, guarantees a desired outcome or 'objective' in the future? In practice one may achieve this goal of modifying and finding the desired Hamiltonian by introducing external fields, e.g. laser light. It is then possible to reach the objective in a 'trial-and-error' fashion, performed either numerically or in the laboratory. We can guess or build a Hamiltonian, do an experiment, or propagate the initial wave function to the future, compare the result with the desirable objective, and correct the guess for the Hamiltonian until satisfactory agreement with the objective is reached. A systematic way of executing this procedure is the sub-field called 'optimal control'. The trial-and-error method is often very time consuming and rarely provides mechanistic insight. There are situations where analytical solutions exist, rendering the control strategies more transparent. This is especially so when one can identify quantum interferences as the heart of quantum control, the essence of the field called 'coherent control'. The experience accumulated so far by many groups is that quantum interferences are the mechanism underlying most successful control scenarios and control experiments. The identification of the control mechanism is especially facile when the time-dependence of the Hamiltonian merely serves to prepare a state which then evolves in the absence of external fields. Analytical theories also exist when the explicit time dependence of the Hamiltonian can be treated adiabatically. Under these circumstances one can identify the quantum interfering pathways that lead to control, allowing for the simultaneous exploration of entire 'landscapes' (and not just a single 'desirable' objective) resulting from varying all the experimental 'knobs' at our disposal, e.g., relative- and envelope-phases, intensities, polarizations, and pulse-sequencing. Such 'pulse-shaping' knobs are now available down to the sub-femtosecond regime in broad spectral regions ranging from the UV to the IR. We expect that comprehensive investigations of the quantum interference required to control dynamical processes will result in a more detailed understanding of the dynamics themselves. Together with experimental progress---the stage is now set for many exciting applications in fundamental and applied sciences by 'tailoring' light--matter interactions essentially at will.
Abstract Quantum interference plays a central role in the realm of quantum particles, revealing their wave‐like nature and probabilistic behavior. It relies on the concept of superposition, where the probability amplitudes of different processes that contribute to the given phenomenon interfere with each other. When combined, their phases can interfere either constructively or destructively. Quantum interference manifests in three distinct forms: optical interference, arising from the interaction of light waves and forming the basis for technologies such as lasers and optical filters; interference via atoms, which involves manipulating atomic states to control light–matter interactions and enables techniques like Stimulated Raman Adiabatic Passage and Electromagnetically Induced Transparency in quantum information processing; and interference of atoms with themselves, in which the matter waves of single atoms interfere with each other, as employed in precision measurements such as atom interferometry–a crucial tool for applications in quantum mechanics and precision navigation. These diverse forms of quantum interference have profound implications for numerous scientific disciplines, demonstrating that interference can involve particles beyond just photons.
Selective resonant excitations can be realized with a shaped ultrashort pulse in an optically dense collection of multilevel individual atoms, where the near dipole-dipole interactions induce intrinsic frequency shifts of the atomic resonance. A proper transform-limited pulse and modulation of excitation pulses can be controlled to establish constructive or destructive quantum interference between different excitation pathways towards a desired selective resonant excitation, even driven by pulses with broadband spectral components.
Quantum state collapse is a fundamental process in quantum mechanics, describing the irreversible transition from superposition to a definite state. This phenomenon, while seemingly random, is intimately linked to the observer and the very nature of reality. This paper explores a novel quantum state collapse protocol utilizing feedback loops, aiming to dynamically adjust collapse probabilities based on the observed system states. The core mechanism involves analyzing the collapsing process itself, adjusting the collapse probability to maintain a desired state or achieve a specific outcome, offering a potential pathway to controlled manipulation of quantum collapse. Current collapse protocols often rely on reactive observation, hindering precise control. This research proposes a fundamentally different approach, leveraging feedback to guide the collapse process towards a predetermined state or outcome, potentially offering a more insightful and controllable mechanism for quantum phenomena. The paper details the algorithm's architecture, demonstrates its effectiveness through simulated examples, and discusses future research directions.