We consider the description of quantum noise within the framework of the standard Copenhagen interpretation of quantum mechanics applied to a composite system environment setting. Averaging over the environmental degrees of freedom leads to a stochastic quantum dynamics, described by equations complying with the constraints arising from the statistical structure of quantum mechanics. Simple examples are considered in the framework of open system dynamics described within a master equation approach, pointing in particular to the appearance of the phenomenon of decoherence and to the relevance of quantum correlation functions of the environment in the determination of the action of quantum noise.
We consider the description of quantum noise within the framework of the standard Copenhagen interpretation of quantum mechanics applied to a composite system environment setting. Averaging over the environmental degrees of freedom leads to a stochastic quantum dynamics, described by equations complying with the constraints arising from the statistical structure of quantum mechanics. Simple examples are considered in the framework of open system dynamics described within a master equation approach, pointing in particular to the appearance of the phenomenon of decoherence and to the relevance of quantum correlation functions of the environment in the determination of the action of quantum noise.
We provide an alternative derivation of a master equation formalism for open quantum systems and apply it to illustrative examples. Using this formalism, we show that generally there can be interference between non-interacting baths on a system's reduced dynamics, confirming results in Phys. Rev. A 89, 042117 (2014). However, we also find that, up to second order, such interference vanishes as long as the baths are initially statistically independent, thereby identifying a satisfiable condition for non-interference. We use a two-dimensional series expansion method to develop a master equation formalism that deals with the effects of both system-environment interaction and classical noise on a system's dynamics in a unified manner. We apply this formalism to the example of a Zeeman-splitted atom in a stochastic B-field interacting with an optical cavity and find that the atom's total decoherence rate is, up to second order, a simple sum of the decoherence rate due to the atom-cavity interaction alone and that due to the stochastic B-field alone. We also develop a formalism for the dynamics of the (marginal) probability density of open classical systems in a similar fashion as the master equation formalism for open quantum systems.
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.
Open quantum systems describe quantum systems that interact unavoidably with their environments. These notes introduce the physical origins of decoherence and dissipation in realistic settings, developing the theoretical framework of reduced dynamics and quantum master equations within the connection to thermodynamics in the quantum regime. Additionally, a set of relevant exercises are proposed to reinforce the acquisition of knowledge. The text equips readers with the tools to model and analyze open quantum dynamics.
Quantum decoherence is a proposed mechanism for the emergence of classical physics from the quantum mechanics. It has been developed extensively in recent years, but is sufficiently technically complicated to discourage widespread understanding. In this paper we provide a gentle introduction to quantum decoherence. We introduce state operators and their density matrix representations to describe composite systems, such as an experiment and its environment. We illustrate how the loss of information about a subsystem can cause a quantum system to appear classical. We first analyze a discrete example of phase randomization, then a Bell state, and finally a continuous system. In the final case we provide an accessible derivation of a major early result of decoherence theory, the master equation of quantum Brownian motion. We conclude by applying the master equation to the decoherence of a simple harmonic oscillator, with results reminiscent of our earlier discrete examples.
Open quantum systems and their dynamics are usually studied in terms of reduced density matrices. This approach allows a nonsymmetric description, which is useful when one of the two subsystems is to be considered an environment, at the expense of a loss of information, as tracing out the environmental degrees of freedom is an irreversible process. This has a series of consequences which can be severe. In this work we present an alternative description, which is still nonsymmetric but yet exact: It is based on a parametric representation of composite systems, as obtained by introducing environmental coherent states, such that the principal system get to be described by a set of pure states parametrically dependent on environmental variables. The representation allows one to relate properties which typically arise in studying systems with parametrically dependent Hamiltonians, such as the emergence of geometrical phases, with features which specifically characterize open quantum systems, such as decoherence and entanglement generation.
Quantum information science is a rapidly growing research area for that it provides new insights to the fundamentals of quantum mechanics and offers a platform for the architecture of novel quantum technologies. The successfulness and practicality of this important scientific field rely on the controllability of a quantum system subject to a realistic noisy environment. The environment always leads to unintended dynamics of the system, and thus destroys its coherence and limits its applications. It is therefore important to understand these decoherence mechanisms from first principles, in order to minimize, or even remove, its adverse effect on the quantum system. The study of this open quantum system problem is usually based on some effective paradigms, where the environment is assumed to be "large", such that it can affect the quantum system without any back action. However, due to the demand for a high precision in quantum computation, such an approximate framework becomes questionable. We provide a new theoretical approach to treat this type of open quantum system problem, including the correlated dynamics between the system and the environment, by using a diagrammatic technique in the same spirit as the Keldysh non-equilibrium Green's function. In this formalism, both the environment and the photonic control are quantized. The dynamics of the system can be evaluated accurately for a time scale of small decoherence, but arbitrary quantum control, relevant to the need for quantum technologies. This offers a way of precise quantum noise calculations. We find how fundamental quantum correlations between the quantum control and quantum environment can arise, and are missing in the existing Master equation approximations. On the other hand, the study of the environment not only provides a better understanding of the decoherence, it also allows applicable designs of quantum operations between different qubit systems. In particular, we engineer a new protocol to entangle two qubits at a distance by projection measurements of their environments, the resonance fluorescence photons. We find exceptional improvements on the probability of success and the rate of entanglement based on the multiphoton environment approach, in comparison with the existing single photon entanglement scheme