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Physical Review A

Decoherence-induced strongly sub-Poissonian nonlocality

Qing Yang Xu

Published Oct 3, 2023• 1 citation
Publisher

Abstract

Quantum decoherence tends to bring about the emergence of the classical from the quantum world through the interaction of a quantum system with the environment, which is the main impediment to the realization of quantum information processing. Counterintuitively, we propose that the decoherence enables the creation of a bright source of strongly sub-Poissonian nonlocality. The absence of the decoherence makes such a source impossible. Decoherence is devised to be adjustable and manipulable on the basis of the dressed-atom approach and combination mode technique. The strong sub-Poissonianity and robust Bell violation are originated from the large squeezing parameter of the reservoir pertaining to the controllable decoherence within the framework of the reservoir theory.

DOI
10.1103/physreva.108.043701

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Journal of Physics B Atomic Molecular and Optical Physics•Apr 18, 2007•2 citations
Quantum decoherence tends to bring about the emergence of the classical from the quantum world through the interaction of a quantum system with the environment, which is the main impediment to the realization of quantum information processing. Counterintuitively, we propose that the decoherence enables the creation of a bright source of strongly sub-Poissonian nonlocality. The absence of the decoherence makes such a source impossible. Decoherence is devised to be adjustable and manipulable on the basis of the dressed-atom approach and combination mode technique. The strong sub-Poissonianity and robust Bell violation are originated from the large squeezing parameter of the reservoir pertaining to the controllable decoherence within the framework of the reservoir theory.
It is generally accepted that the role of quantum decoherence is to spoil the quantum superpositions and to yield the emergence of the classical behaviors from quantum mechanics. Contrary to the common knowledge of decoherence, we propose a scheme to generate a steady-state bright source of strongly sub-Poissonian squeezed light using the environmentally induced decoherence. The disappearance of the process of decoherence disables the generation of the squeezed state of the combination modes, while the presence of the decoherence results in the strongly nonclassical light source at the sub-shot-noise level and even near-perfect sub-Poissonian squeezed state of light. This excludes the use of the paradigm that the good insulation of a quantum system from its surrounding environment is highly desirable to explore its quantum property and fuel the growth of quantum information science. The interactions with the atom make no contribution to the preparation of squeezed light. These counterintuitive results are originated from the intrinsic quantum reservoir in the dressed-atom combination mode picture, which may open the door to exploring high levels of squeezing induced by the decoherence in the stationary regime.
Abstract The decoherence process has a tendency to yield the evolution of a pure state into a mixed one and to cause the quantum-to-classical transition by the coupling of a system of interest to the reservoir with infinitely many degrees of freedom. This is the major obstacle to the implementation of quantum computation and hence the realization of quantum computers. We propose a scheme to create unconditionally sub-Poissonian entangled and steerable states of the collective cavity field modes by use of the dissipation process. Based on the suitable choice of combination modes, the scheme uses the inherent, efficient and controllable two-mode squeezed vacuum reservoir coupled to the combination modes of concern rather than the original cavity modes in the two-level quantum beat laser. The decoherence is shown to pull the collective modes into the sub-Poissonian entangled and steerable states in the stationary regime, while the job of the dissipation of the individual cavity fields is to give rise to the degradation of the bipartite entanglement of the two individual modes and to inhibit the occurrence of the quantum steering from one cavity mode to the other. In particular for the case that the external driving field is close to the exact resonance with the atom, the collective fields are eventually prepared asymptotically in the stationary Einstein–Podolsky–Rosen state, while the two individual cavity modes are pulled into the vacuum states and reach steady state. The disappearance of the decoherence disables the nonclassical states of the collective modes, while the ignorance of the dissipation process of the cavity field modes guarantees the generation of the entanglement between the pair of individual modes. The decoherence-induced formation of a nonclassical source is ascribed to the four-wave mixing process together with the intrinsic amplitude and phase locking.
Decoherence tends to induce the quantum-to-classical transition, which leads to a crucial obstacle in the realization of reliable quantum information processing. Counterintuitively, we propose that the decoherence due to phase decay brings about the switch from photon correlation to anti-correlation. Stronger decoherence also gives rise to an enhancement of the transition from photon correlation to anti-correlation. This breaks the conventional correlation of strong decoherence with fast decorrelation.
Abstract Recent advancements in strong single-photon optomechanical coupling also demand a deeper understanding of environmental interactions in this regime. In this regime the standard Lindblad master equation, which is derived in the eigenbasis of the bare optical and mechanical modes, misassigns the dephasing rate. We therefore use the dressed-state master equation, which is formulated in the eigenbasis of the strongly coupled photon–phonon Hamiltonian, the dressed states of the system. This work investigates the impact of squeezed vacuum and thermal reservoirs on the decoherence of cavity photon Fock states in the strong coupling regime. We demonstrate that decoherence can be effectively controlled by tuning reservoir parameters, with the control mediated through a cavity dephasing term that becomes significant at high temperatures. The findings presented provide critical insights into reservoir engineering for precise control of quantum decoherence, advancing the understanding of strongly coupled optomechanical systems in engineered environments.
We propose a quantum measurement model where the environment is taken to be a squeezed reservoir. This kind of meter-environment coupling has a dramatic influence on the decoherence rate. Depending on the phase and the squeezing parameter the decoherence time can be enhanced or reduced arbitrarily. We derive the Wigner function for the system-meter state, and discuss its nonclassical features. Further, we test quantum nonlocality in phase space on the basis of coincidence measurements of the entangled system-meter scheme, and investigate the effect of a highly reduced decoherence rate. Surprisingly, in sharp contrast to popular notion, violations of the Bell inequality can be observed even when the meter has reached a macroscopic state.
Theory and experiment have not fully resolved the apparent dichotomy , which has agonized physics for the past eighty years: on the one hand, the description of microsystems by quantum mechanics and, on the other, the description of macrosystems by classical dynamics or statistical mechanics. Derivations of the time-irreversible Liouville equation for an open quantum system, based on projecting out its environment, have narrowed the gap between the quantum and classical descriptions. Yet our `classical' intuition continues to be confronted by quantum-mechanical results like the Einstein--Podolsky--Rosen paradox that challenges the classical notion of locality, or the quantum Zeno effect which suggests that the isolation of a system is not the only way to preserve its quantum state. There are two key concepts in any discussion of such issues. The first, which is responsible for the most salient nonclassical properties, is entanglement , that is partial or complete correlation or, more generally, inseparability of the elements comprising a quantum ensemble . Even after their interaction has ceased, this inseparability, originating from their past interaction, can affect the state of one element when another element is subject to a nonunitary action, such as its measurement, tracing- out, or thermalization. The second key concept is decoherence of open quantum systems, which is the consequence of their entanglement with their environment, a `meter' or a thermal `reservoir', followed by the tracing-out of the latter. Despite new insights into entanglement and decoherence, there are still no complete, unequivocal answers to the fundamental questions of the transition from quantal to classical behaviour: how do irreversibility and classicality emerge from unitarity as systems and their environments become increasingly complex ? At what stage does system--meter entanglement give rise to a classical readout of the meter? Is there an upper limit on the size or complexity of systems displaying entanglement? Major developments have opened new vistas into controlled entanglement , which is the resource of quantum information processing. However, these developments have mainly focused on ensembles of simple two- and three-level systems that are thoroughly isolated from their environments. Treatments of coherence in quantum computing have mostly assumed that only a single or a few the elements of the quantum ensemble may simultaneously undergo an uncontrolled intervention---a quantum error. Decoherence-control protocols for more general types of errors are still incomplete. In order to resolve the outstanding issues of the quantum--classical transition, and study the control of entanglement and decoherence without the foregoing restrictions, we must venture into the domain of Quantum Complex Systems (QUACS), either consisting of a large number of inseparable elements or having many coupled degrees of freedom. Our conviction is that fundamental understanding and manipulation of entanglement within QUACS or their entanglement with the environment or a meter, call for the creation of a new conceptual framework or paradigm, that would encompass phenomena common to cold atoms in laser fields, large molecules, Josephson junctions, quantum gases and solids, with the view of employing these systems for quantum information processing and computing. Progress within this paradigm should allow us to answer the questions: does entanglement play an essential role in the evolution of large collections of complex systems? What are the size and complexity limits of systems and ensembles still controllable by an external intervention? What are the most appropriate decoherence protection schemes and control algorithms? This special issue addresses the challenge of understanding in depth and manipulating the basic quantum properties of optical, atomic, molecular and condensed-matter QUACS, and large ensembles thereof. In view of the interdisciplinary character of this issue, we find it expedient to look at the presented articles not only according to the physical objects they describe, but also from a unifying standpoint. Several unifying themes may be discerned: memory effects in dephasing and relaxation; protection of quantum information by its distribution throughout the ensemble; control of decoherence and entanglement by time-dependent interventions or measurements; manifestations of entanglement in scattering off statistical ensembles; measurment-induced dynamics. The concerted discussion of the topics outlined above should help us advance the new paradigm that addresses our abilities to diagnose and manipulate the entangled states of complex quantum objects and their robustness against decoherence. These abilities are required for quantum information (QI) applications in matter--wave interferometry in molecular, semiconducting or superconducting systems. On the fundamental level, this book may help establish the notion of dynamical information exchange between quantum systems and chart in detail the route from unitarity to classicality. Further developments within the outlined paradigm should yield novel, advantageous QI processing schemes and high-sensitivity interferometers, owing to decoherence suppression and effective control of many degrees of freedom. In the long run, these strategies may prove to be the first step towards the (hitherto unattempted) use of entanglement in nanotechnology, metrology and chemistry, with potentially remarkable novel applications.