2,042 research outputs found
On a Time Symmetric Formulation of Quantum Mechanics
We explore further the suggestion to describe a pre- and post-selected system
by a two-state, which is determined by two conditions. Starting with a formal
definition of a two-state Hilbert space and basic operations, we systematically
recast the basics of quantum mechanics - dynamics, observables, and measurement
theory - in terms of two-states as the elementary quantities. We find a simple
and suggestive formulation, that ``unifies'' two complementary observables:
probabilistic observables and non-probabilistic `weak' observables.
Probabilities are relevant for measurements in the `strong coupling regime'.
They are given by the absolute square of a two-amplitude (a projection of a
two-state). Non-probabilistic observables are observed in sufficiently `weak'
measurements, and are given by linear combinations of the two-amplitude. As a
sub-class they include the `weak values' of hermitian operators. We show that
in the intermediate regime, one may observe a mixing of probabilities and weak
values. A consequence of the suggested formalism and measurement theory, is
that the problem of non-locality and Lorentz non-covariance, of the usual
prescription with a `reduction', may be eliminated. We exemplify this point for
the EPR experiment and for a system under successive observations.Comment: LaTex, 44 pages, 4 figures included. Figure captions and related text
in sections 3.1, 4.2 are revised. A paragraph in pages 9-10 about non-generic
two-states is clarified. Footnotes adde
Paradoxes of the Aharonov-Bohm and the Aharonov-Casher effects
For a believer in locality of Nature, the Aharonov-Bohm effect and the
Aharonov-Casher effect are paradoxes. I discuss these and other Aharonov's
paradoxes and propose a local explanation of these effects. If the solenoid in
the Aharonov-Bohm effect is treated quantum mechanically, the effect can be
explained via local interaction between the field of the electron and the
solenoid. I argue that the core of the Aharonov-Bohm and the Aharonov-Casher
effects is that of quantum entanglement: the quantum wave function describes
all systems together.Comment: To be published in Yakir Aharonov 80th birthday Festschrif
Observing the evolution of a quantum system that does not evolve
This article deals with the problem of gathering information on the time
evolution of a single metastable quantum system whose evolution is impeded by
the quantum Zeno effect. It has been found it is in principle possible to
obtain some information on the time evolution and, depending on the specific
system, even to measure its average decay rate, even if the system does not
undergo any evolution at all.Comment: Two over three PRA referees didn't like the old title... And no more
quantum circuits in the new versio
Fast Universal Quantum Computation with Railroad-switch Local Hamiltonians
We present two universal models of quantum computation with a
time-independent, frustration-free Hamiltonian. The first construction uses
3-local (qubit) projectors, and the second one requires only 2-local
qubit-qutrit projectors. We build on Feynman's Hamiltonian computer idea and
use a railroad-switch type clock register. The resources required to simulate a
quantum circuit with L gates in this model are O(L) small-dimensional quantum
systems (qubits or qutrits), a time-independent Hamiltonian composed of O(L)
local, constant norm, projector terms, the possibility to prepare computational
basis product states, a running time O(L log^2 L), and the possibility to
measure a few qubits in the computational basis. Our models also give a
simplified proof of the universality of 3-local Adiabatic Quantum Computation.Comment: Added references to work by de Falco et al., and realized that
Feynman's '85 paper already contained the idea of a switch in i
Variance Control in Weak Value Measurement Pointers
The variance of an arbitrary pointer observable is considered for the general
case that a complex weak value is measured using a complex valued pointer
state. For the typical cases where the pointer observable is either its
position or momentum, the associated expressions for the pointer's variance
after the measurement contain a term proportional to the product of the weak
value's imaginary part with the rate of change of the third central moment of
position relative to the initial pointer state just prior to the time of the
measurement interaction when position is the observable - or with the initial
pointer state's third central moment of momentum when momentum is the
observable. These terms provide a means for controlling pointer position and
momentum variance and identify control conditions which - when satisfied - can
yield variances that are smaller after the measurement than they were before
the measurement. Measurement sensitivities which are useful for estimating weak
value measurement accuracies are also briefly discussed.Comment: submitted to Phys Rev
Weak Measurement of the Arrival Times of Single Photons and Pairs of Entangled Photons
In this paper we propose a setup for the weak measurement of photon arrival
time. It is found that the weak values of this arrival time can lie far away
from the expectation value, and in principle also in regions forbidden by
special relativity. We discuss in brief the implications of these results as
well as their reconciliation with the principle of causality. Furthermore, an
analysis of the weak arrival times of a pair of photons in a Bell state shows
that these weak arrival times are correlated.Comment: 4 pages, 1 figur
PR-box correlations have no classical limit
One of Yakir Aharonov's endlessly captivating physics ideas is the conjecture
that two axioms, namely relativistic causality ("no superluminal signalling")
and nonlocality, so nearly contradict each other that a unique theory - quantum
mechanics - reconciles them. But superquantum (or "PR-box") correlations imply
that quantum mechanics is not the most nonlocal theory (in the sense of
nonlocal correlations) consistent with relativistic causality. Let us consider
supplementing these two axioms with a minimal third axiom: there exists a
classical limit in which macroscopic observables commute. That is, just as
quantum mechanics has a classical limit, so must any generalization of quantum
mechanics. In this classical limit, PR-box correlations violate relativistic
causality. Generalized to all stronger-than-quantum bipartite correlations,
this result is a derivation of Tsirelson's bound without assuming quantum
mechanics.Comment: for a video of this talk at the Aharonov-80 Conference in 2012 at
Chapman University, see quantum.chapman.edu/talk-10, published in Quantum
Theory: A Two-Time Success Story (Yakir Aharonov Festschrift), eds. D. C.
Struppa and J. M. Tollaksen (New York: Springer), 2013, pp. 205-21
Nonlocal Aspects of a Quantum Wave
Various aspects of nonlocality of a quantum wave are discussed. In
particular, the question of the possibility of extracting information about the
relative phase in a quantum wave is analyzed. It is argued that there is a
profound difference in the nonlocal properties of the quantum wave between
fermion and boson particles. The phase of the boson quantum state can be found
from correlations between results of measurements in separate regions. These
correlations are identical to the Einstein-Podolsky-Rosen (EPR) correlations
between two entangled systems. An ensemble of results of measurements performed
on fermion quantum waves does not exhibit the EPR correlations and the relative
phase of fermion quantum waves cannot be found from these results. The
existence of a physical variable (the relative phase) which cannot be measured
locally is the nonlocality aspect of the quantum wave of a fermion.Comment: 12 page
Relational Reality in Relativistic Quantum Mechanics
Up to now it has been impossible to find a realistic interpretation for the
reduction process in relativistic quantum mechanics. The basic problem is the
dependence of the states on the frame within which collapse takes place. A
suitable use of the causal structure of the devices involved in the measurement
process allows us to introduce a covariant notion for the collapse of quantum
states.Comment: 4 pages, final version accepted for publication in Phys. Lett.
Weak values of a quantum observable and the cross-Wigner distribution
We study the weak values of a quantum observable from the point of view of
the Wigner formalism. The main actor is here the cross-Wigner transform of two
functions, which is in disguise the cross-ambiguity function familiar from
radar theory and time-frequency analysis. It allows us to express weak values
using a complex probability distribution. We suggest that our approach seems to
confirm that the weak value of an observable is, as conjectured by several
authors, due to the interference of two wavefunctions, one coming from the
past, and the other from the future.Comment: Submitted for publicatio
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