共查询到20条相似文献,搜索用时 263 毫秒
1.
Deepak Kumar 《Pramana》1998,51(5):567-575
The problem of measurement in Quantum Mechanics will be briefly reviewed. Since the measurement process involves a macroscopic
apparatus, the attention is focussed on the dynamics of a pointer-like variable of the apparatus when it interacts with a
quantum system. It is argued that since the measurement process requires an apparent collapse of the wave function in a certain
basis, and collapse is an irreversible process, understanding of irreversibility in a quantum macroscopic system is crucial.
The chief characteristics of an apparatus that are important in understanding measurement process are (a) its closely spaced
energy levels and (b) its interaction with environment. The coupling with the environment drives the density matrix of the
apparatus to diagonal form, but to have persistent correlations between system and apparatus states, it seems necessary to
have a pointer variable that has a classical limit 相似文献
2.
A dynamical model for the collapse of the wave function in a quantum measurement process is proposed by considering the interaction of a quantum system (spin -1/2) with a macroscopic quantum apparatus interacting with an environment in a dissipative manner. The dissipative interaction leads to decoherence in the superposition states of the apparatus, making its behaviour classical in the sense that the density matrix becomes diagonal with time. Since the apparatus is also interacting with the system, the probabilities of the diagonal density matrix are determined by the state vector of the system. We consider a Stern-Gerlach type model, where a spin-1/2 particle is in an inhomogeneous magnetic field, the whole set up being in contact with a large environment. Here we find that the density matrix of the combined system and apparatus becomes diagonal and the momentum of the particle becomes correlated with a spin operator, selected by the choice of the system-apparatus interaction. This allows for a measurement of spin via a momentum measurement on the particle with associated probabilities in accordance with quantum principles. 相似文献
3.
In this paper the macroscopic quantum state of
Bose-Einstein condensates in optical lattices is studied by solving the
periodic Gross-Pitaevskii equation in one-dimensional geometry. It is shown
that an exact solution seen to be a travelling wave of excited macroscopic
quantum states resultes in a persistent atom current, which can be controlled
by adjusting of the barrier height of the optical periodic potential. A
critical condition to generate the travelling wave is demonstrated and we
moreover propose a practical experiment to realize the persistent atom
current in a toroidal atom waveguide. 相似文献
4.
We revisit the question of how a definite phase between Bose-Einstein
condensates can spontaneously appear under the effect of measurements.
We first consider a system that is the juxtaposition of two subsystems in Fock
states with high populations, and assume that successive individual position
measurements are performed. Initially, the relative phase is totally
undefined, and no interference effect takes place in the first position
measurement. But, while successive measurements are accumulated, the relative
phase becomes better and better defined, and a clear interference pattern
emerges. It turns out that all observed results can be interpreted in terms of
a pre-existing, but totally unknown, relative phase, which remains exactly
constant during the experiment.
We then generalize the results to more condensates. We also consider other
initial quantum states than pure Fock states, and distinguish between
intrinsic phase of a quantum state and phase induced by
measurements. Finally, we examine the case of multiple condensates of spin
states. We discuss a curious quantum effect, where the measurement of the
spin angular momentum of a small number of particles can induce a big angular
momentum in a much larger assembly of particles, even at an arbitrary
distance. This spin observable can be macroscopic, analogous to the pointer
of a measurement apparatus, which illustrates the non-locality of standard
quantum mechanics with particular clarity. The effect can be described as the
teleportation at arbitrary distances of the continuous classical result of a
local experiment. The EPR argument, transposed to this case, takes a
particularly convincing form since it does not involve incompatible
measurements and deals only with macroscopic variables. 相似文献
5.
L.S. Schulman 《Physics letters. A》1984,102(9):396-400
Measurements that give definite results can arise by ordinary quantum evolution from special initial states of the measuring apparatus and the measured system. We demonstrate the existence of such states (relating that existence to the presence of invariant tori in corresponding classical systems) and consider the hypothesis that for all actual measurements the apparatus is in such “special” states. 相似文献
6.
《Physica A》1988,153(1):97-113
The aim of quantum mechanics is to explain macroscopic, objectively recorded phenomena. Microscopic objects are measured by enabling them to interact with a macroscopic measuring apparatus prepared in a metastable state. Macroscopic objects, such as cats, are not above the laws of quantum mechanics, but owing to their enormously dense level spectrum other aspects than single eigenvalues and eigenfunctions are prominent. These aspects can be described in classical terms, such as probabilities instead of probability amplitudes. The measuring act is fully described by the Schrödinger equation for object system and apparatus together. The collapse of the wave function is a consequence rather than an additional postulate. A model is constructed to demonstrate these statements. It also appears that the entropies of the object system and the apparatus increase by the same amount, namely the entropy difference between the metastable initial state and the stable final state of the apparatus. 相似文献
7.
Izuru Fujiwara 《Foundations of Physics》1972,2(2-3):83-110
The central problem in the quantum theory of measurement, how to describe the process of state reduction in terms of the quantum mechanical formalism, is solved on the basis of the relativity of quantal states, which implies that once the apparatus is detected in a well-defined state, the object state must reduce to a corresponding one. This is a process termed by Schrödinger disentanglement. Here, it is essential to observe that Renninger's negative result does constitute an actual measurement process. From this point of view, Heisenberg's interpretation of his microscope experiment and the Einstein-Podolsky-Rosen arguments are reinvestigated. Satisfactory discussions are given to various experimental situations, such as the Stern-Gerlach-type experiment, successive measurements, macroscopic measurements, and Schrödinger's cat. Finally it is proposed to regard a state vector in quantum mechanics as an irreducible physical construct, in Margenau's sense, that is not further analyzable both mathematically and conceptually. 相似文献
8.
Blagowest A. Nikolov 《Foundations of Physics》1996,26(2):257-269
A simplified derivation of the Gudder-Hemion quantum probability formula is proposed. Defining configurations as the classical (q, p) deterministic states and generalized action as the (quantum) generating function of a canonical transformation, we obtain the usual quantization rules (for arbitrary polynomial quantities) and derive the Schrödinger wave equation on the same grounds. This approach suggests a statistical interpretation of the wave function in terms of the classical canonical transformations. 相似文献
9.
L. Bass 《Foundations of Physics》1975,5(1):159-172
The reduction of a quantum mechanical wave function by the entry of a datum into the consciousness of an observer is used, in a semirealistic neurochemical model, to bring about excitation of a nerve cell in that observer's central nervous system. It is suggested that mind can induce muscular movements by choosing to note data originating from specialized elements of the nervous system. Only the freedom to note or not to note a relevant datum is postulated for the observer's mind; the consequences of either choice are deterministic on the neural scale of events, so that quantum indeterminacy is consistent with physiological determinacy. The proposed mind-body coupling depends on the possibility of the biological evolution of a macroscopic device which has strikingly different neural correlates of its pure and mixed quantum states, respectively. An example of such a device is outlined in terms of components which are familiar from existing nervous systems. 相似文献
10.
We propose two schemes for quantum secure direct communication (QSDC) and deterministic secure quantum communication (DSQC) over collective dephasing noisy channel. In our schemes, four special two-qubit states are used as the quantum channel. Since
these states are unchanged through the collective dephasing noisy
channel, the effect of the channel noise can be perfectly overcome.
Simultaneously, the security against some usual attacks can be
ensured by utilizing the various checking procedures. Furthermore,
these two schemes are feasible with present-day technique. 相似文献
11.
With the two forms of the quantum entanglement control, the quantum entanglement swapping and preservation are demonstrated in a three-qubit nuclear magnetic resonance quantum computer. The pseudopure state is prepared to represent the quantum entangled states through macroscopic signals. Entanglement swapping is directly realized by a swap operation. By controlling the interactions between the system and its environment,we can preserve an initial entangled state for a longer time. The experimental results are in agreement with the experiment. 相似文献
12.
13.
Peter D. Finch 《Foundations of Physics》1982,12(4):327-345
The trace formulation of quantum mechanical expectations is derived in a classical deterministic setting by averaging over an assembly of states. Interference of probabilities is discussed and its usual Hilbert space formulation is questioned. Nevertheless, it is shown that the observable predictions of quantum statics remain unchanged in the framework developed here. 相似文献
14.
15.
C.P. Sun X.F. Liu D.L. Zhou S.X. Yu 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,17(1):85-92
To account for the phenomenon of quantum decoherence of a macroscopic object, such as the localization and disappearance of
interference, we invoke the adiabatic quantum entanglement between its collective states (such as that of the center-of-mass
(CM)) and its inner states based on our recent investigation. Under the adiabatic limit where motion of the CM does not excite
the transition of inner states, it is shown that the wave function of the macroscopic object can be written as an entangled
state with correlation between adiabatic inner states and quasi-classical motion configurations of the CM. Since the adiabatic
inner states are factorized with respect to each component of the macroscopic object, this adiabatic separation can induce
the quantum decoherence. This observation thus provides us with a possible solution to the Schr?dinger cat paradox.
Received 24 October 2000 and Received in final form 8 March 2001 相似文献
16.
J. Gemmer G. Mahler 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,17(3):385-393
For a bi-partite quantum system defined in a finite dimensional Hilbert-space we investigate in what sense entanglement change
and interactions imply each other. For this purpose we introduce an entanglement-operator, which is then shown to represent
a non-conserved property for any bi-partite system and any type of interaction. This general relation does not exclude the
existence of special initial product states, for which the entanglement remains small over some period of time, despite interactions.
For this case we derive an approximation to the full Schr?dinger-equation, which allows the treatment of the composite systems
in terms of product states. The induced error is estimated. In this factorization-approximation one subsystem appears as an
effective potential for the other. A pertinent example is the Jaynes-Cummings model, which then reduces to the semi-classical
rotating wave approximation.
Received 8 June 2001 相似文献
17.
We propose a simple quantum network to detect multipartite entangled states of bosons and show how to implement this network for neutral atoms stored in an optical lattice. We investigate the special properties of cluster states, multipartite entangled states, and superpositions of distinct macroscopic quantum states that can be identified by the network. 相似文献
18.
We examine the dynamics of a wave packet that initially corresponds to a coherent state in the model of a quantum rotator
excited by a periodic sequence of kicks. This model is the main model of quantum chaos and allows for a transition from regular
behavior to chaotic in the classical limit. By doing a numerical experiment we study the generation of squeezed states in
quasiclassical conditions and in a time interval when quantum-classical correspondence is well-defined. We find that the degree
of squeezing depends on the degree of local instability in the system and increases with the Chirikov classical stochasticity
parameter. We also discuss the dependence of the degree of squeezing on the initial width of the packet, the problem of stability
and observability of squeezed states in the transition to quantum chaos, and the dynamics of disintegration of wave packets
in quantum chaos.
Zh. éksp. Teor. Fiz. 113, 111–127 (January 1998) 相似文献
19.
Roland Omnès 《Foundations of Physics》2013,43(11):1339-1368
When a quantum system is macroscopic and becomes entangled with a microscopic one, entanglement is not immediately total, but gradual and local. A study of this locality is the starting point of the present work and shows unexpected and detailed properties in the generation and propagation of entanglement between a measuring apparatus and a microscopic measured system. Of special importance is the propagation of entanglement in nonlinear waves with a finite velocity. When applied to the entanglement between a macroscopic system and its environment, this study yields also new results about the resulting disordered state. Finally, a mechanism of wave function collapse is proposed as an effect of perturbation in the growth of local entanglement between a measuring system and the measured one by waves of entanglement with the environment. 相似文献
20.
We study the robustness, against the leakage of bosons, of wave functions of interacting many bosons confined in a finite box by deriving and analyzing a general equation of motion for the reduced density operator. We identify a robust wave function that remains a pure state, whereas other wave functions, such as the Bogoliubov's ground state and the ground state with a fixed number of bosons, evolve into mixed states. Although these states all have the off-diagonal long-range order, and the same energy, we argue that only the robust state is realized as a macroscopic quantum state. 相似文献