2,275 research outputs found
"Hot Entanglement"? -- A Nonequilibrium Quantum Field Theory Scrutiny
The possibility of maintaining entanglement in a quantum system at finite,
even high, temperatures -- the so-called `hot entanglement' -- has obvious
practical interest, but also requires closer theoretical scrutiny. Since
quantum entanglement in a system evolves in time and is continuously subjected
to environmental degradation, a nonequilibrium description by way of open
quantum systems is called for. To identify the key issues and the contributing
factors that may permit `hot entanglement' to exist, or the lack thereof, we
carry out a model study of two spatially-separated, coupled oscillators in a
shared bath depicted by a finite-temperature scalar field. From the Langevin
equations we derived for the normal modes and the entanglement measure
constructed from the covariance matrix we examine the interplay between direct
coupling, field-induced interaction and finite separation on the structure of
late-time entanglement. We show that the coupling between oscillators plays a
crucial role in sustaining entanglement at intermediate temperatures and over
finite separations. In contrast, the field-induced interaction between the
oscillators which is a non-Markovian effect, becomes very ineffective at high
temperature. We determine the critical temperature above which entanglement
disappears to be bounded in the leading order by the inverse frequency of the
center-of-mass mode of the reduced oscillator system, a result not unexpected,
which rules out hot entanglement in such settings.Comment: 13 pages, 2 figure
Quantum Entanglement at High Temperatures? II. Bosonic Systems in Nonequilibrium Steady State
This is the second of a series of three papers examining how viable it is for
entanglement to be sustained at high temperatures for quantum systems in
thermal equilibrium (Case A), in nonequilibrium (Case B) and in nonequilibrium
steady state conditions (Case C). The system we analyze here consists of two
coupled quantum harmonic oscillators each interacting with its own bath
described by a scalar field, set at temperatures . For
\textit{constant bilinear inter-oscillator coupling} studied here (Case C1)
owing to the Gaussian nature, the problem can be solved exactly at arbitrary
temperatures even for strong coupling. We find that the valid entanglement
criterion in general is not a function of the bath temperature difference, in
contrast to thermal transport in the same NESS setting [1]. Thus lowering the
temperature of one of the thermal baths does not necessarily help to safeguard
the entanglement between the oscillators. Indeed, quantum entanglement will
disappear if any one of the thermal baths has a temperature higher than the
critical temperature . With the Langevin equations derived we give a full
display of how entanglement dynamics in this system depends on ,
, the inter-oscillator coupling and the system-bath coupling strengths. For
weak oscillator-bath coupling the critical temperature is about the order
of the inverse oscillator frequency, but for strong oscillator-bath coupling it
will depend on the bath cutoff frequency. We conclude that in most realistic
circumstances, for bosonic systems in NESS with constant bilinear coupling,
`hot entanglement' is largely a fiction. In Paper III we will examine the case
(C2) of \textit{time-dependent driven coupling } which contains the parametric
pumping type described in [2] wherein entanglement was first shown to sustain
at high temperatures.Comment: 47 pages, 9 figure
Generation of spiral bevel gears with conjugate tooth surfaces and tooth contact analysis
A new method for generation of spiral bevel gears is proposed. The main features of this method are as follows: (1) the gear tooth surfaces are conjugated and can transform rotation with zero transmission errors; (2) the tooth bearing contact is localized; (3) the center of the instantaneous contact ellipse moves in a plane that has a fixed orientation; (4) the contact normal performs in the process of meshing a parallel motion; (5) the motion of the contact ellipse provides improved conditions of lubrication; and (6) the gears can be manufactured by use of Gleason's equipment
Man-in-the-control-loop simulation of manipulators
A method to achieve man-in-the-control-loop simulation is presented. Emerging real-time dynamics simulation suggests a potential for creating an interactive design workstation with a human operator in the control loop. The recursive formulation for multibody dynamics simulation is studied to determine requirements for man-in-the-control-loop simulation. High speed computer graphics techniques provides realistic visual cues for the simulator. Backhoe and robot arm simulations are implemented to demonstrate the capability of man-in-the-control-loop simulation
Decoherence and Recoherence in Model Quantum Systems
We discuss the various manifestations of quantum decoherence in the forms of
dephasing, entanglement with the environment, and revelation of "which-path"
information. As a specific example, we consider an electron interference
experiment. The coupling of the coherent electrons to the quantized
electromagnetic field illustrates all of these versions of decoherence. This
decoherence has two equivalent interpretations, in terms of photon emission or
in terms of Aharonov-Bohm phase fluctuations. We consider the case when the
coherent electrons are coupled to photons in a squeezed vacuum state. The
time-averaged result is increased decoherence. However, if only electrons which
are emitted during selected periods are counted, the decoherence can be
suppressed below the level for the photon vacuum. This is the phenomenon of
recoherence. This effect is closely related to the quantum violations of the
weak energy condition, and is restricted by similar inequalities. We give some
estimates of the magnitude of the recoherence effect and discuss prospects for
observing it in an electron interferometry experiment.Comment: 8 pages, 3 figures, talk presented at the 7th Friedmann Seminar, Joao
Pessoa, Brazil, July 200
Generation of spiral bevel gears with zero kinematical errors and computer aided tooth contact analysis
Kinematic errors in spiral bevel gears are a major source of noise and vibrations in transmissions. A method for the generation of Gleason's spiral bevel gears which provides conjugated gear tooth surfaces and an improved bearing contact was developed. A computer program for the simulation of meshing, misalignment, and bearing contact was written
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