6,104 research outputs found
On localization and position operators in Moebius-covariant theories
Some years ago it was shown that, in some cases, a notion of locality can
arise from the group of symmetry enjoyed by the theory, thus in an intrinsic
way. In particular, when Moebius covariance is present, it is possible to
associate some particular transformations to the Tomita Takesaki modular
operator and conjugation of a specific interval of an abstract circle. In this
context we propose a way to define an operator representing the coordinate
conjugated with the modular transformations. Remarkably this coordinate turns
out to be compatible with the abstract notion of locality. Finally a concrete
example concerning a quantum particle on a line is also given.Comment: 19 pages, UTM 705, version to appear in RM
Supercurrent-phase relationship of a Nb/InAs(2DES)/Nb Josephson junction in overlapping geometry
Superconductor/normal conductor/superconductor (SNS) Josephson junctions with
highly transparent interfaces are predicted to show significant deviations from
sinusoidal supercurrent-phase relationships (CPR) at low temperatures. We
investigate experimentally the CPR of a ballistic Nb/InAs(2DES)/Nb junction in
the temperature range from 1.3 K to 9 K using a modified Rifkin-Deaver method.
The CPR is obtained from the inductance of the phase-biased junction. Transport
measurements complement the investigation. At low temperatures, substantial
deviations of the CPR from conventional tunnel-junction behavior have been
observed. A theoretical model yielding good agreement to the data is presented.Comment: RevTex4, 4 pages including 3 figure
Relativistic Quantum Mechanics and Relativistic Entanglement in the Rest-Frame Instant Form of Dynamics
A new formulation of relativistic quantum mechanics is proposed in the
framework of the rest-frame instant form of dynamics with its instantaneous
Wigner 3-spaces and with its description of the particle world-lines by means
of derived non-canonical predictive coordinates. In it we quantize the frozen
Jacobi data of the non-local 4-center of mass and the Wigner-covariant relative
variables in an abstract (frame-independent) internal space whose existence is
implied by Wigner-covariance. The formalism takes care of the properties of
both relativistic bound states and scattering ones. There is a natural solution
to the \textit{relativistic localization problem}. The non-relativistic limit
leads to standard quantum mechanics but with a frozen Hamilton-Jacobi
description of the center of mass. Due to the \textit{non-locality} of the
Poincar\'e generators the resulting theory of relativistic entanglement is both
\textit{kinematically non-local and spatially non-separable}: these properties,
absent in the non-relativistic limit, throw a different light on the
interpretation of the non-relativistic quantum non-locality and of its impact
on foundational problems.Comment: 73 pages, includes revision
Unsharp Quantum Reality
The positive operator (valued) measures (POMs) allow one to generalize the notion of observable beyond the traditional one based on projection valued measures (PVMs). Here, we argue that this generalized conception of observable enables a consistent notion of unsharp reality and with it an adequate concept of joint properties. A sharp or unsharp property manifests itself as an element of sharp or unsharp reality by its tendency to become actual or to actualize a specific measurement outcome. This actualization tendency-or potentiality-of a property is quantified by the associated quantum probability. The resulting single-case interpretation of probability as a degree of reality will be explained in detail and its role in addressing the tensions between quantum and classical accounts of the physical world will be elucidated. It will be shown that potentiality can be viewed as a causal agency that evolves in a well-defined way
Self-adjoint Lyapunov variables, temporal ordering and irreversible representations of Schroedinger evolution
In non relativistic quantum mechanics time enters as a parameter in the
Schroedinger equation. However, there are various situations where the need
arises to view time as a dynamical variable. In this paper we consider the
dynamical role of time through the construction of a Lyapunov variable - i.e.,
a self-adjoint quantum observable whose expectation value varies monotonically
as time increases. It is shown, in a constructive way, that a certain class of
models admit a Lyapunov variable and that the existence of a Lyapunov variable
implies the existence of a transformation mapping the original quantum
mechanical problem to an equivalent irreversible representation. In addition,
it is proved that in the irreversible representation there exists a natural
time ordering observable splitting the Hilbert space at each t>0 into past and
future subspaces.Comment: Accepted for publication in JMP. Supercedes arXiv:0710.3604.
Discussion expanded to include the case of Hamiltonians with an infinitely
degenerate spectru
Inhibition of spontaneous emission in Fermi gases
Fermi inhibition is a quantum statistical analogue for the inhibition of
spontaneous emission by an excited atom in a cavity. This is achieved when the
relevant motional states are already occupied by a cloud of cold atoms in the
internal ground state. We exhibit non-trivial effects at finite temperature and
in anisotropic traps, and briefly consider a possible experimental realization.Comment: 4 pages with 3 figure
At what time does a quantum experiment have a result?
This paper provides a general method for defining a generalized quantum
observable (or POVM) that supplies properly normalized conditional
probabilities for the time of occurrence (i.e., of detection). This method
treats the time of occurrence as a probabilistic variable whose value is to be
determined by experiment and predicted by the Born rule. This avoids the
problematic assumption that a question about the time at which an event occurs
must be answered through instantaneous measurements of a projector by an
observer, common to both Rovelli (1998) and Oppenheim et al. (2000). I also
address the interpretation of experiments purporting to demonstrate the quantum
Zeno effect, used by Oppenheim et al. (2000) to justify an inherent uncertainty
for measurements of times.Comment: To appear in proceedings of 2015 ETH Zurich Workshop on Time in
Physic
Spectroscopy of Neon for the Advanced Undergraduate Laboratory
We describe a spectroscopy experiment, suitable for upper-division laboratory courses, that investigates saturated absorption spectroscopy and polarization spectroscopy in a neon discharge. Both experiments use nearly identical components, allowing students to explore both techniques in a single apparatus. Furthermore, because the wavelength of the laser is in the visible part of the spectrum (640ânm), the experiment is well-suited for students with limited experience in optical alignment. The labs nicely complement a course in atomic or plasma physics, provide students with the opportunity to gain important technical skills in the area of optics and lasers, and can provide an introduction to radio-frequency electronics
Wannier functions of elliptic one-gap potentials
Wannier functions of the one dimensional Schroedinger equation with elliptic
one gap potentials are explicitly constructed. Properties of these functions
are analytically and numerically investigated. In particular we derive an
expression for the amplitude of the Wannier function in the origin, a power
series expansion valid in the vicinity of the origin and an asymptotic
expansion characterizing the decay of the Wannier function at large distances.
Using these results we construct an approximate analytical expression of the
Wannier function which is valid in the whole spatial domain and is in good
agreement with numerical results.Comment: 24 pages, 5 figure
Trap Loss in a Dual-Species Rb-Ar* Magneto-Optical Trap
We have investigated trap loss in a dual-species magneto-optical trap (MOT) comprised of 85Rb and metastable 40Ar. We measure the trap loss rate coefficients for each species due to the presence of the other as a function of trap light intensity. We clearly identify both Penning ionization of Rb by Ar* and associative ionization to form the molecular ion RbAr+ as two of the trap loss channels. We have also measured the trap loss rate coefficient for the Ar* MOT alone and observe production of Ar+ and Ar2+ ions
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