74 research outputs found
A Simple Scheme to Entangle Distant Qubits from a Mixed State via an Entanglement Mediator
A simple scheme to prepare an entanglement between two separated qubits from
a given mixed state is proposed. A single qubit (entanglement mediator) is
repeatedly made to interact locally and consecutively with the two qubits
through rotating-wave couplings and is then measured. It is shown that we need
to repeat this kind of process only three times to establish a maximally
entangled state directly from an arbitrary initial mixed state, with no need to
prepare the state of the qubits in advance or to rearrange the setup step by
step. Furthermore, the maximum yield realizable with this scheme is compatible
with the maximum entanglement, provided that the coupling constants are
properly tuned.Comment: 9 pages, 3 figures; the version accepted for publication (with the
new title
Time development of a wave packet and the time delay
A one-dimensional scattering problem off a -shaped potential is
solved analytically and the time development of a wave packet is derived from
the time-dependent Schr\"odinger equation. The exact and explicit expression of
the scattered wave packet supplies us with interesting information about the
"time delay" by potential scattering in the asymptotic region. It is
demonstrated that a wave packet scattered by a spin-flipping potential can give
us quite a different value for the delay times from that obtained without
spin-degrees of freedom.Comment: 13 pages, plain TeX, 2 eps figures, tar+gzip+uuencode
Stochastic Quantization of Bottomless Systems: Stationary quantities in a diffusive process
By making use of the Langevin equation with a kernel, it was shown that the
Feynman measure exp(-S) can be realized in a restricted sense in a diffusive
stochastic process, which diverges and has no equilibrium, for bottomless
systems. In this paper, the dependence on the initial conditions and the
temporal behavior are analyzed for 0-dim bottomless systems. Furthermore, it is
shown that it is possible to find stationary quantities.Comment: LaTeX2e, 10 pages with 4 eps figures, to be published in Prog. Theor.
Phys. 102; revised page layou
Temporal behavior of quantum mechanical systems
The temporal behavior of quantum mechanical systems is reviewed. We study the
so-called quantum Zeno effect, that arises from the quadratic short-time
behavior, and the analytic properties of the ``survival" amplitude. It is shown
that the exponential behavior is due to the presence of a simple pole in the
second Riemannian sheet, while the contribution of the branch point yields a
power behavior for the amplitude. The exponential decay form is cancelled at
short times and dominated at very long times by the branch-point contributions,
which give a Gaussian behavior for the former and a power behavior for the
latter. In order to realize the exponential law in quantum theory, it is
essential to take into account a certain kind of macroscopic nature of the
total system. Some attempts at extracting the exponential decay law from
quantum theory, aiming at the master equation, are briefly reviewed, including
van Hove's pioneering work and his well-known ``" limit. We clarify
these general arguments by introducing and studying a solvable dynamical model.
Some implications for the quantum measurement problem are also discussed, in
particular in connection with dissipation.Comment: 48 pages, LaTeX, uuencoded file with 7 figures include
Understanding the quantum Zeno effect
The quantum Zeno effect consists in the hindrance of the evolution of a
quantum system that is very frequently monitored and found to be in its initial
state at every single measurement. On the basis of the correct formula for the
survival probability, i.e. the probability of finding the system in its initial
state at every single measurement, we critically analyze a recent proposal and
experimental test, that make use of an oscillating system.Comment: 9 pages, LaTeX, including 1 epsfigure, tar+gzip+uuencoded to appear
in Phys. Lett.
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