Quantum decoherence can be viewed as the mechanism responsible for the quantum-to-classical transition as the initially prepared quantum state interacts with its environment in an irreversible manner. One of the most common mechanisms responsible for the macroscopically observed decoherence involves collisions of an atom or molecule, initially prepared in a coherent superposition of states, with gas particles. In this work, a coherent superposition of quantum internal states of NO molecules is prepared by the interaction between the molecule with both a static and a radiofrequency electric field. Subsequently, NO + Ar collision decoherence experiments are investigated by measuring the loss of coherence as a function of the number of collisions. Data analysis using a model based on the interaction potential of the collisional partners allowed to unravel the molecular mechanism responsible for the loss of coherence in the prepared NO quantum superposition of internal states. The relevance of the present work relies on several aspects. On the one hand, the use of radio-waves introduces a new way for the production of coherent beams. On the other hand, the employed methodology could be useful in investigating the Stereodynamics of chemical reactions with coherent reagents.
Quantum decoherence can be viewed as the mechanism responsible for the quantum-to-classical transition as the initially prepared quantum state interacts with its environment in an irreversible manner. One of the most common mechanisms responsible for the macroscopically observed decoherence involves collisions of an atom or molecule, initially prepared in a coherent superposition of states, with gas particles. In this work, a coherent superposition of quantum internal states of NO molecules is prepared by the interaction between the molecule with both a static and a radiofrequency electric field. Subsequently, NO + Ar collision decoherence experiments are investigated by measuring the loss of coherence as a function of the number of collisions. Data analysis using a model based on the interaction potential of the collisional partners allowed to unravel the molecular mechanism responsible for the loss of coherence in the prepared NO quantum superposition of internal states. The relevance of the present work relies on several aspects. On the one hand, the use of radio-waves introduces a new way for the production of coherent beams. On the other hand, the employed methodology could be useful in investigating the Stereodynamics of chemical reactions with coherent reagents.
In modern physics, quantum decoherence, a subject still under debate, is viewed as the mechanism responsible for the quantum -to-classical transition as the initially prepared quantum state interacts with its environment in an irreversible manner. As expected, one of the most common mechanisms responsible of the macroscopically observed decoherence involves collisions of an atom or molecule, initially prepared in a coherent superposition of states, with gas particles. In this work, a coherent superposition of quantum internal states of NO molecules is prepared by the interaction between the molecule with both a static and a radiofrequency electric field. Subsequently, NO+Ar collision decoherence experiments, are investigated by measuring the loss of coherence as a function of the number of collisions. Data analysis in the light of the interaction potential of the collisional partners allowed us to unravel the molecular mechanisms responsible for the loss of coherence in the prepared NO quantum superposition of internal states. The relevance of the present work relies on several aspects. On the one hand, the use of radio-waves introduces a new way for the production of coherent beams. On the other hand, the employed methodology, when satisfactorily applied to more collision systems, could be useful in designing experiments to reduce the environmental decoherence rate to levels necessary for quantum information processes.
We present here a physico-mathematical picture for explaining the unexpectedly large decoherence cross-section (almost 10 times larger than its gas-kinematic cross-section) recently observed by Ureña and coworkers in their scattering experiment involving a coherent NO beam with Ar gas. The present topological picture consists of a stereographic projection and the cusp catastrophe theory of Thom, and we find that this model enables us to clarify the origin of the collisional decoherence. From the view of the stereographic projection, we can naturally introduce the wave property originating from the singular point at the “North pole” on the circumference S1 coordinate corresponding to a critical point for the collisional decoherence (condition 1). This picture also predicts the sudden changes of wave-phase collapse due to network interaction in the many-body system (condition 2). Thus it is hoped that the model proposed by Ureña et al. based on the dipole-induced dipole interaction in the NO + Ar system could be modified through this picture by including interactions with many Ar atoms in the environment. One way to fill the gap between the single-pair interaction picture and the multiple interaction one would be to employ theoretical calculations by use of the density matrix theory with and without adding the second Ar atom to the NO–Ar system. The cusp catastrophe theory reinforces the necessity of some cooperative network interaction between the coherent NO molecule and many neighboring Ar atoms and provides a qualitative scenario in which the whole system leads to a sudden change of the collisional decoherence of NO as a function of the control parameters (a, b). At this stage, the present physico-mathematical picture cannot give any specific values of the decoherence distance by the theory itself, but it clearly provides us a new topological concept for clarifying the origin of collisional decoherence which is strongly connected with the complexity of the system. Thus it gives us a global guide map toward further clarification of the collisional decoherence phenomenon with the aid of more sophisticated quantum mechanical calculations in the future.
The present thesis deals with two main topics. First, I study in detail the interaction of a delocalized molecular matter-wave with a standing-wave laser grating. Due to absorption of photons in an optical grating, molecules can get excited and numerous internal photophysical processes are induced. For the theoretical description of the molecule-light interaction I take three relevant internal molecular processes into account: intersystem crossing, fluorescence and internal conversion. Furthermore, the fact that molecules can change their optical properties because of the photoexcitation is shown to have a measurable effect on matter-wave interferometry. The second topic of this thesis is about collisions between a polar molecule and non-polar environmental gas atoms, and their effects on the coherence of the center-of-mass state. Because the scattering potential depends on the molecular orientation the molecular rotational state must also be considered. Assuming a slow rotation of the molecule compared to the scattering time and that the molecular mass fairly exceeds the mass of a gas atom, I present a master equation of collisional decoherence, which depends on the molecular orientation. Its predictions for matter-wave interferometry are discussed both in the near and far field.
Based on the superposition principle, any two states of a quantum system may be coherently superposed to yield a novel state. Such a simple construction is at the heart of genuinely quantum phenomena such as interference of massive particles or quantum entanglement. Yet, these superpositions are susceptible to environmental influences, eventually leading to a complete disappearance of the system's quantum character. In principle, two distinct mechanisms responsible for this process of decoherence may be identified. In a classical decoherence setting, on the one hand, stochastic fluctuations of classical, ambient fields are the relevant source. This approach leads to a formulation in terms of stochastic Hamiltonians; the dynamics is unitary, yet stochastic. In a quantum decoherence scenario, on the other hand, the system is described in the language of open quantum systems. Here, the environmental degrees of freedom are to be treated quantum mechanically, too. The loss of coherence is then a direct consequence of growing correlations between system and environment. The purpose of the present thesis is to clarify the distinction between classical and quantum decoherence. It is known that there exist decoherence processes that are not reconcilable with the classical approach. We deem it desirable to have a simple, feasible model at hand of which it is known that it cannot be understood in terms of fluctuating fields. Indeed, we find such an example of true quantum decoherence. The calculation of the norm distance to the convex set of classical dynamics allows for a quantitative assessment of the results. In order to incorporate genuine irreversibility, we extend the original toy model by an additional bath. Here, the fragility of the true quantum nature of the dynamics under increasing coupling strength is evident. The geometric character of our findings offers remarkable insights into the geometry of the set of non-classical decoherence maps. We give a very intuitive geometrical measure---a volume---for the quantumness of dynamics. This enables us to identify the decoherence process of maximum quantumness, that is, having maximal distance to the convex set of dynamics consistent with the stochastic, classical approach. In addition, we observe a distinct correlation between the decoherence potential of a given dynamics and its achievable quantumness. In a last step, we study the notion of quantum decoherence in the context of a bipartite system which couples locally to the subsystems' respective environments. A simple argument shows that in the case of a separable environment the resulting dynamics is of classical nature. Based on a realistic experiment, we analyze the impact of entanglement between the local environments on the nature of the dynamics. Interestingly, despite the variety of entangled environmental states scrutinized, no single instance of true quantum decoherence is encountered. In part, the identification of the classical nature relies on numerical schemes. However, for a large class of dynamics, we are able to exclude analytically the true quantum nature.
In connection with the recent experiments on quantum measurement, such as the “which-way”detection using the Bragg scattering of cold atoms and quantum interference using a small molecular C 60 quantum system, we introduce the quantum measurement problem and its relevant fundamental concepts and ideas, such as quantum coherence, quantum decoherence and quantum entanglement. In order to understand the loss of quantum coherence for a quantum system due to its interaction with another system behaving classically or macroscopically, we discuss some quantum dynamical models to describe the quantum decoherence and quantum dissipation induced by couplings to the external environment or inner motions. In the classical or macroscopic limit, it is shown that the wave function of the total system formed by the quantum system plus another system interacting with it is factorized as an entangled state with correlation between the measured quantum states and the configurations of the external large system or the inner degrees of freedom. In association with a novel viewpoint about quantum measurement, which has been directly verified by recent experiments (e.g.,S. Durr et al., Nature,1998,33:359), it is shown that quantum entanglement is indeed responsible for the quantum decoherence, and thus it can be regarded as a “clean” quantum measurement when the large system behaves as a classical object. These arguments give us a possible solution to the “Schrdinger cat paradox” and to whether the coherent superposition of macroscopic states exists. These discussions are also applied to the quantum decoherence problems in quantum information including quantum computing and quantum communication.