64 research outputs found
Weak Value in Wave Function of Detector
A simple formula to read out the weak value from the wave function of the
measuring device after the postselection with the initial Gaussian profile is
proposed. We apply this formula for the weak value to the classical experiment
of the realization of the weak measurement by the optical polarization and
obtain the weak value for any pre- and post-selections. This formula
automatically includes the interference effect which is necessary to yields the
weak value as an outcome of the weak measurement.Comment: 3 pages, no figures, Published in Journal of the Physical Society of
Japa
Building all Time Evolutions with Rotationally Invariant Hamiltonians
All elementary Hamiltonians in nature are expected to be invariant under
rotation. Despite this restriction, we usually assume that any arbitrary
measurement or unitary time evolution can be implemented on a physical system,
an assumption whose validity is not obvious. We introduce two different schemes
by which any arbitrary unitary time evolution and measurement can be
implemented with desired accuracy by using rotationally invariant Hamiltonians
that act on the given system and two ancillary systems serving as reference
frames. These frames specify the z and x directions and are independent of the
desired time evolution. We also investigate the effects of quantum fluctuations
that inevitably arise due to usage of a finite system as a reference frame and
estimate how fast these fluctuations tend to zero when the size of the
reference frame tends to infinity. Moreover we prove that for a general
symmetry any symmetric quantum operations can be implemented just by using
symmetric interactions and ancillas in the symmetric states.Comment: 26 pages, 5 figures; V2 published version (Typos corrected, Figures
changed, more discussion about metric
Experimental Observation of Quantum Correlations in Modular Variables
We experimentally detect entanglement in modular position and momentum
variables of photon pairs which have passed through -slit apertures. We
first employ an entanglement criteria recently proposed in [Phys. Rev. Lett.
{\bf 106}, 210501 (2011)], using variances of the modular variables. We then
propose an entanglement witness for modular variables based on the Shannon
entropy, and test it experimentally. Finally, we derive criteria for
Einstein-Podolsky-Rosen-Steering correlations using variances and entropy
functions. In both cases, the entropic criteria are more successful at
identifying quantum correlations in our data.Comment: 7 pages, 4 figures, comments welcom
Complementarity and quantum walks
We show that quantum walks interpolate between a coherent `wave walk' and a
random walk depending on how strongly the walker's coin state is measured;
i.e., the quantum walk exhibits the quintessentially quantum property of
complementarity, which is manifested as a trade-off between knowledge of which
path the walker takes vs the sharpness of the interference pattern. A physical
implementation of a quantum walk (the quantum quincunx) should thus have an
identifiable walker and the capacity to demonstrate the interpolation between
wave walk and random walk depending on the strength of measurement.Comment: 7 pages, RevTex, 2 figures; v2 adds references; v3 updated to
incorporate feedback and updated references; v4 substantially expanded to
clarify presentatio
Action principle formulation for motion of extended bodies in General Relativity
We present an action principle formulation for the study of motion of an
extended body in General Relativity in the limit of weak gravitational field.
This gives the classical equations of motion for multipole moments of arbitrary
order coupling to the gravitational field. In particular, a new force due to
the octupole moment is obtained. The action also yields the gravitationally
induced phase shifts in quantum interference experiments due to the coupling of
all multipole moments.Comment: Revised version derives Octupole moment force. Some clarifications
and a reference added. To appear in Phys. Rev.
Realistic interpretation of a superposition state does not imply a mixture
Contrary to previous claims, it is shown that, for an ensemble of either
single-particle systems or multi-particle systems, the realistic interpretation
of a superposition state that mathematically describes the ensemble does not
imply that the ensemble is a mixture. Therefore it cannot be argued that the
realistic interpretation is wrong on the basis that some predictions derived
from the mixture are different from the corresponding predictions derived from
the superposition state
Quantum random walks in optical lattices
We propose an experimental realization of discrete quantum random walks using
neutral atoms trapped in optical lattices. The random walk is taking place in
position space and experimental implementation with present day technology
--even using existing set-ups-- seems feasible. We analyze the influence of
possible imperfections in the experiment and investigate the transition from a
quantum random walk to the classical random walk for increasing errors and
decoherence.Comment: 8 pages, 4 figure
Bohmian description of a decaying quantum system
We present a Bohmian description of a decaying quantum system. A particle is
initially confined in a region around the origin which is surrounded by a
repulsive potential barrier. The particle leaks out in time tunneling through
the barrier. We determine Bohm trajectories with which we can visualize various
features of the decaying system.Comment: 14 pages, 5 figure
Spin dependent observable effect for free particles using the arrival time distribution
The mean arrival time of free particles is computed using the quantum
probability current. This is uniquely determined in the non-relativistic limit
of Dirac equation, although the Schroedinger probability current has an
inherent non-uniqueness. Since the Dirac probability current involves a
spin-dependent term, an arrival time distribution based on the probability
current shows an observable spin-dependent effect, even for free particles.
This arises essentially from relativistic quantum dynamics, but persists even
in the non-relativistic regime.Comment: 5 Latex pages, 2.eps figures; discussions sharpened and references
added; accepted for publication in Physical Review
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