Showing papers on "Quantum published in 1978"

Passive states and KMS states for general quantum systems

Abstract: We characterize equilibrium states of quantum systems by a condition of passivity suggested by the second principle of thermodynamics. Ground states and β-KMS states for all inverse temperatures β≧0 are completely passive. We prove that these states are the only completely passive ones. For the special case of states describing pure phases, assuming the passivity we reproduce the results of Haag et al.

Optical communication with two-photon coherent states--Part I: Quantum-state propagation and quantum-noise

Abstract: Recent theoretical work has shown that novel quantum states, called two-photon coherent states (TCS), have significant potential for improving free-space optical communications. The first part of a three-part study of the communication theory of TCS radiation is presented. The issues of quantum-field propagation and optimum quantum-state generation are addressed. In particular, the quantum analog of the classical paraxial diffraction theory for quasimonochromatic scalar waves is developed. This result, which describes the propagation of arbitrary quantum states as a boundary-value problem suitable for communication system analysis, is used to treat a number of quantum transmitter optimization problems. It is shown that, under near-field propagation conditions, a TCS transmitter maximizes field-measurement signal-to-noise ratio among all transmitter quantum states; the performance of the TCS system exceeds that for a conventional (coherent state) transmitter by a factor of N_{s} + 1 , where N_{s} is the average number of signal photons (transmitter energy constraint). Under far-field propagation conditions, it is shown that use of a TCS local oscillator in the receiver can, in principle, attenuate field-measurement quantum noise by a factor equal to the diffraction loss of the channel, if appropriate spatial mode mixing can be achieved. These communcation results are derived by assuming that field-quadrature quantum measurement is performed. In part II of this study, photoemissive reception of TCS radiation will be considered; it will be shown therein that homodyne detection of TCS fields can realize the field-quadrature signal - to-noise ratio performance of part I. In part III, the relationships between photoemissive detection and general quantum measurements will be explored. In particular, a synthesis procedure will be obtained for realizing all the measurements described by arbitrary TCS.

Unperformed experiments have no results

Abstract: This paper discusses correlations between the results of measurements performed on physical systems which are widely separated, but have interacted in the past. It is shown that quantum correlations are stronger than classical correlations. This property leads to the following paradox, known as Bell’s theorem: Let us assume that the outcome of an experiment performed on one of the systems is independent of the choice of the experiment performed on the other. Now, let us try to imagine the results of alternative measurements, which could have been performed on the same systems instead of the actual measurements. Then there is no way of contriving these hypothetical results so that they will satisfy all the quantum correlations with the results of the actual measurements. However, the weaker classical correlations can be satisfied.

Fourier transform multiple quantum nuclear magnetic resonance

Abstract: The excitation and detection of multiple quantum transitions in systems of coupled spins offers, among other advantages, an increase in resolution over single quantum n.m.r. since the number of lines decreases as the order of the transition increases. This paper reviews the motivation for detecting multiple quantum transitions by a Fourier transform experiment and describes an experimental approach to high resolution multiple quantum spectra in dipolar systems along with results on some protonated liquid crystal systems. A simple operator formalism for the essential features of the time development is presented and some applications in progress are discussed.

Black-Body Theory and the Quantum Discontinuity, 1894-1912

Abstract: "A masterly assessment of the way the idea of quanta of radiation became part of 20th-century physics The book not only deals with a topic of importance and interest to all scientists, but is also a polished literary work, described (accurately) by one of its original reviewers as a scientific detective story"-John Gribbin, "New Scientist" "Every scientist should have this book"-Paul Davies, "New Scientist"

Quantum nondemolition measurements of harmonic oscillators

Abstract: The complex amplitude X1+iX2≡(x+ip / mω)e^(iωt) of a harmonic oscillator is constant in the absence of driving forces. Although the uncertainty principle forbids precise measurements of X1 and X2 simultaneously (ΔX1ΔX2>~ℏ / 2mω), X1 alone can be measured precisely and continuously ("quantum nondemolition measurement"). Examples are given of measuring systems that do this job. Such systems might play a crucial role in gravitational-wave detection and elsewhere.

Quantum Coherence Effects and the Second Law of Thermodynamics

Abstract: Negative energy densities and fluxes due to quantum coherence effects in quantum field theories are discussed. Such negative energy fluxes seeming­ly lead to a breakdown of the second law of thermodynamics. It is argued that such a breakdown will not in fact occur if a negative energy flux F is constrained by an inequality of the form │ F │≲ T -2 where T is a character­istic time over which the negative energy flux occurs. Particular models in which negative energy fluxes occur are examined, and it is found that such an inequality is indeed satisfied. Quantum violations of the dominant energy condition, which requires that the local energy density be greater than or equal to the magnitude of the pressure are also discussed briefly.

Linear Spin-Zero Quantum Fields in External Gravitational and Scalar Fields I. A One Particle Structure for the Stationary Case

Abstract: . We give mathematically rigorous results on the quantization of thecovariant Klein Gordon field with an external stationary scalar interaction in astationary curved space-time.We show how, following Segal, Weinless etc., the problem reduces tofinding a "one particle structure" for the corresponding classical system.Our main result is an existence theorem for such a one-particle structure fora precisely specified class of stationary space-times. Byproducts of ourapproach are:1) A discussion of when a given "equal-lime" surface in a given stationaryspace-time is Cauchy.2) A modification and extension of the methods of Chernoff [3] forproving the essential self-adjointness of certain partial differential operators. §0. Introduction In this series of papers, we discuss the quantization of the equation V)φ = Q, (0.1)— the covariant Klein Gordon equation in a fixed curved space-time (.,//, g μv ) and interacting with a fixed external scalar field V. (We shall always take (.,//,

The use of multiple quantum transitions for relaxation studies in coupled spin systems

Abstract: The utility of multiple quantum transitions is discussed with regard to the elucidation of relaxation mechanisms in coupled spin systems. The information contained in the decay rates of multiple quantum coherence is analysed in terms of the Redfield formalism, and experimental schemes suitable for their measurement are presented. As an illustration, external paramagnetic relaxation in a two-spin system is investigated, based on measured multiple and single quantum relaxation rates.

An Operational Approach to Quantum Probability

Abstract: In the two preceding papers ‘Completeness of Quantum Logic’ (CQL) and ‘Quantum Logical Calculi and Lattice Structures’ (QLC) an operational approach to formal quantum logic was developed. Beginning with a pragmatic definition of quantum mechanical propositions by means of material dialogs a formal dialog-game was introduced for establishing formally true propositions. It was shown in CQL that the formal dialog-game can be replaced by a calculus T eff of effective (intuitionistic) quantum logic which is complete and consistent with respect to the dialogic procedure. In QLC we showed that T eff is equivalent to a propositional calculus Q eff Since the calculus Q eff is a model for a certain lattice structure, called quasi-implicative lattice (L eff), the connection between quantum logic and the quantum theoretical formalism is provided. L qi is a weaker algebraic structure than the orthomodular lattice of the subspaces of a Hilbert space L q which can be interpreted as the pro-positional calculus of value-definite quantum logic. This establishes a quantum logical interpretation of L q.

Note on the quantum recurrence theorem

Abstract: An essential step in the proof of the quantum recurrence theorem is shown to follow from the Poincar\'e recurrence theorem of classical mechanics.

A classical/semiclassical theory for the interaction of infrared radiation with molecular systems

Abstract: A classical model for the interaction of molecular systems with infrared radiation is presented. It differs from the usual ’’semiclassical theory of radiation and matter’’ in that the molecular system is treated by classical rather than quantum mechanics, and the radiation field is described, as in Jaynes’ neoclassical theory, as classical (mechanical) harmonic oscillators rather than as a classical field (i.e., via Maxwell’s equations). The classical Hamiltonian for the composite system—molecules, radiation, and their interaction—is thus that of a completely mechanical system, and its classical dynamics is determined by computing the classical trajectories of the system. Quantum mechanical interference and tunneling effects can be built into the description within the framework of the classical S‐matrix theory. Even within the strictly classical limit of the model, it is shown that all dynamical effects in the interaction of radiation and matter are obtained; in the perturbative limit, for example, it is...

Widths and configuration mixings of two-electron systems below the N = 2 threshold

Abstract: Width calculations have been carried out for all doubly excited states of two-electron systems with orbital angular momentum L

"Jarring" of a quantum system and the corresponding stimulated transitions

Abstract: A systematic theory of sudden perturbations is derived for quantum systems whose states are described both by wave functions (a pure ensemble) and by a quantum density operator (a mixed ensemble). A perturbation series is written in powers of the parameter ωτ, which is small when the perturbation is "sudden"; ω is the typical eigenvalue of the unperturbed system; and τ is the characteristic collision time. When the perturbation (t), taken at different times, commutes with itself, the theory yields a compact analytic expression for the probabilities for stimulated transitions for any value of Vτ /. The results of many cross-section calculations for atomic collision processes are discussed from a common standpoint: the processes are treated as "jarring" processes which stimulate transitions in the quantum system. If a momentum δp is rapidly transferred to the system in a collision, regardless of the physical nature of the "jarring," the probabilities for the stimulated transitions are governed by the parameter N ~δpδR/ where δR is a measure of the uncertainty in the coordinates which is due to the relatively slow motions in the unperturbed system.

The classical path approximation in time‐dependent quantum collision theory

Abstract: Time‐dependent treatments of molecular collisions have frequently employed the combination of quantum equations for the internal degrees of freedom (i.e., vibration, rotation) with a classical description of the translational motion. In this paper, it is shown how energy‐conserving classical path equations may be derived from first principles, thereby obtaining quantum correction terms of arbitrary order. These expressions have interesting implications for the question of how the classical limit is approached. If the translational degree of freedom is assumed to be described by a very narrow wave packet, the correction term to first order in h/ is purely imaginary and therefore non‐Hermitian. These techniques can be generalized to a classical treatment of other degrees of freedom, and the case of rotation in an atom–vibrotor collision is explicitly considered. For both the translational and rotational examples, the full series of corrections takes on an interesting and suggestive exponential form. The failure of classical path methods to satisfy microscopic reversibility is ascribed to a difficulty with the boundary conditions for the translational motion.

Hot electron quantum magneto-transport

Abstract: A critical review is given of recent experimental and quantum transport theoretical studies of hot electrons in quantizing magnetic fields. Particular attention is paid to new effective temperature models and to numerical solutions to exact quantum kinetic equations. The latter are discussed within a unified framework based on resolvent analyses of the Liouville equation. Special emphasis is given to carrier-carrier scattering, non-parabolicity, resonance phenomena and non-heating non-linear departures from Ohm's law. The optical determination of quantum distributions and the application of Fourier transform and harmonic detection techniques to hot carrier magneto-resonance spectroscopy is discussed with emphasis on magnetophonon and magnetic-impurity phenomena.

Temporal coherence in multiphoton absorption. Far off‐resonance intermediate states

Abstract: We develop a consistent formalism for the treatment of temporal atomic or molecular coherence in multiphoton absorption. The formalism is fully quantum mechanical under the assumption that the exciting laser fields are well described by coherent states. We make use of the language and methodology of resonance physics to the extent possible, but deliberately avoid the rotating‐wave approximation, and do not restrict the allowed atomic states to be finite in number or the electric field strengths to be small in magnitude. For compactness in the presentation only electric dipole transitions are considered. As all of the transitions, both stimulated and spontaneous, are due to the activation of quantum mechanical dipoles, it is most efficient to construct the formalism so that it emphasizes the role of dipole operators, and we do this. The one strong restriction imposed here, but avoided in a following paper, is to consider only multiphoton transitions that have one resonance between initial and final states and no intermediate resonance. We describe, in effect, the time dependences associated with the early multiphoton absorption calculations of Bebb and Gold.

Quantum rate theory for a symmetric double‐well potential

Abstract: A quantum rate theory is presented for a symmetric double‐well potential which is defined by a piecewise quadratic function. The theory is based on stationary states which are decomposed into‐ and left‐moving states. The flux and transmission coefficients for the latter are found in terms of parabolic cylinder functions and are thermally averaged. Close analogies between the quantum and classical formulations are found when an appropriate phase space representation is used. The theory shows good agreement with experimental results for the diffusion of hydrogen and deuterium in niobium, but the agreement is poorer for the same process with the host metal vanadium and the theory does not predict the observed anomalous isotope effect for this process in palladium.

Preparation and decay of unstable molecular states undergoing memory effects: Excitation by coherent electromagnetic pulses of arbitrary strength

Abstract: It is shown that the preparation and decay of non-markovian dissipative systems can be handled without any restriction on both excitation intensity and memory strength. Any perturbative approximation in the treatment of non-markovian interaction is avoided. The theory is explicity related to physical models relevant to the problem of molecular radiationless decay. In this respect, it is shown that a quantitative evaluation of the “radiative state” time evolution along the lines of the recent paper by Rhodes can easily be performed whatever the strength of external interaction. It is emphasized that no quantum beat phenomenon may be interpreted in an unambiguous way in the absence of any information on the dissipation coupling. As a new effect of coherent excitation on the nature of the excited state, it is shown that increasing light intensity may allow the molecule to be prepared in the “radiative state” without using § excitations. It is stressed, furthermore, that the excitation of a discrete set of states by long duration pulses has the effect of invalidating the replacement of a complex molecular system by a more simple one, which in turn is to be regarded as the basic idea of the theory. The above physical circumstance seems to be the unique one under which the present approach is no longer applicable. Evidence for this statement is provided by a quantitative simulation of hypothetical experiments of the same kind as the recent ones by Zewail et al.

Quantum Statistics of Homodyne and Heterodyne Detection

Abstract: There are three basic techniques that may be employed to detect optical radiation: photon detection, homodyne detection, and heterodyne detection (see Fig. 1). From the photocounting theory of Kelley and Kleiner [1], one can readily show that photon detection, in the limit of unity quantum efficiency, can be interpreted as the quantum measurement of the total photon number operator N for any quantum state of a quasi-monochromatic input field. No similar quantum description is as yet available for homodyne or heterodyne detection. In this paper, we shall derive the abstract quantum description of homodyne detection. Because heterodyne detection involves the more complicated concept of simultaneous measurement of non-commuting observables, its quantum description will only be briefly discussed.