8,915 research outputs found

### AB effect and Aharonov-Susskind charge non-superselection

We consider a particle in a coherent superposition of states with different
electric charge moving in the vicinity of a magnetic flux. Formally, it should
acquire a (gauge-dependent) AB relative phase between the charge states, even
for an incomplete loop. If measureable, such a geometric, rather than
topological, AB-phase would seem to break gauge invariance. Wick, Wightman and
Wigner argued that since (global) charge-dependent phase transformations are
physically unobservable, charge state superpositions are unphysical (`charge
superselection rule'). This would resolve the apparent paradox in a trivial
way. However, Aharonov and Susskind disputed this superselection rule: they
distinguished between such global charge-dependent transformations, and
transformations of the relative inter-charge phases of two particles, and
showed that the latter \emph{could} in principle be observable! Finally, the
paradox again disappears once we considers the `calibration' of the phase
measured by the Aharonov-Susskind phase detectors, as well as the phase of the
particle at its initial point. It turns out that such a detector can only
distinguish between the relative phases of two paths if their (oriented)
difference forms a loop around the flux

### Correlated Equilibria of Classical Strategic Games with Quantum Signals

Correlated equilibria are sometimes more efficient than the Nash equilibria
of a game without signals. We investigate whether the availability of quantum
signals in the context of a classical strategic game may allow the players to
achieve even better efficiency than in any correlated equilibrium with
classical signals, and find the answer to be positive.Comment: 8 pages, LaTe

### Zeno effect and ergodicity in finite-time quantum measurements

We demonstrate that an attempt to measure a non-local in time quantity, such
as the time average \la A\ra_T of a dynamical variable $A$, by separating
Feynman paths into ever narrower exclusive classes traps the system in
eigensubspaces of the corresponding operator \a. Conversely, in a long
measurement of \la A\ra_T to a finite accuracy, the system explores its
Hilbert space and is driven to a universal steady state in which von Neumann
ensemble average of \a coincides with \la A\ra_T. Both effects are
conveniently analysed in terms of singularities and critical points of the
corresponding amplitude distribution and the Zeno-like behaviour is shown to be
a consequence of conservation of probability

### Unexpected reemergence of von Neumann theorem

Is is shown here that the "simple test of quantumness for a single system" of
arXiv:0704.1962 (for a recent experimental realization see arXiv:0804.1646) has
exactly the same relation to the discussion of to the problem of describing the
quantum system via a classical probabilistic scheme (that is in terms of hidden
variables, or within a realistic theory) as the von Neumann theorem (1932). The
latter one was shown by Bell (1966) to stem from an assumption that the hidden
variable values for a sum of two non-commuting observables (which is an
observable too) have to be, for each individual system, equal to sums of
eigenvalues of the two operators. One cannot find a physical justification for
such an assumption to hold for non-commeasurable variables. On the positive
side. the criterion may be useful in rejecting models which are based on
stochastic classical fields. Nevertheless the example used by the Authors has a
classical optical realization

### Pseudo-Hermitian Quantum Mechanics with Unbounded Metric Operators

We extend the formulation of pseudo-Hermitian quantum mechanics to
eta-pseudo-Hermitian Hamiltonian operators H with an unbounded metric operator
eta. In particular, we give the details of the construction of the physical
Hilbert space, observables, and equivalent Hermitian Hamiltonian for the case
that H has a real and discrete spectrum and its eigenvectors belong to the
domain of eta and consequently its positive square root.Comment: 8 pages, accepted for publication in Phil. Trans. R. Soc.

### No-cloning theorem in thermofield dynamics

We discuss the relation between the no-cloning theorem from quantum
information and the doubling procedure used in the formalism of thermofield
dynamics (TFD). We also discuss how to apply the no-cloning theorem in the
context of thermofield states defined in TFD. Consequences associated to mixed
states, von Neumann entropy and thermofield vacuum are also addressed.Comment: 16 pages, 3 figure

### Definition and evolution of quantum cellular automata with two qubits per cell

Studies of quantum computer implementations suggest cellular quantum computer
architectures. These architectures can simulate the evolution of quantum
cellular automata, which can possibly simulate both quantum and classical
physical systems and processes. It is however known that except for the trivial
case, unitary evolution of one-dimensional homogeneous quantum cellular
automata with one qubit per cell is not possible. Quantum cellular automata
that comprise two qubits per cell are defined and their evolution is studied
using a quantum computer simulator. The evolution is unitary and its linearity
manifests itself as a periodic structure in the probability distribution
patterns.Comment: 13 pages, 4 figure

### Strong entanglement causes low gate fidelity in inaccurate one-way quantum computation

We study how entanglement among the register qubits affects the gate fidelity
in the one-way quantum computation if a measurement is inaccurate. We derive an
inequality which shows that the mean gate fidelity is upper bounded by a
decreasing function of the magnitude of the error of the measurement and the
amount of the entanglement between the measured qubit and other register
qubits. The consequence of this inequality is that, for a given amount of
entanglement, which is theoretically calculated once the algorithm is fixed, we
can estimate from this inequality how small the magnitude of the error should
be in order not to make the gate fidelity below a threshold, which is specified
by a technical requirement in a particular experimental setup or by the
threshold theorem of the fault-tolerant quantum computation.Comment: 4 pages, 3 figure

### Does inflation targeting matter?

This paper studies the inflation and interest rate performances since the late 1970s for six former highinflation countries that adopted inflation targeting (IT) in the early 1990â€™s. Using Germany, Switzerland and the US for comparison, we look at various aspects of central bank performance in a pre-IT period (1978-92) and a post-IT period (1993-01). The results of all types of evidence considered uniformly lead to the general conclusion that IT has proven a useful strategy for reducing the level and volatility of inflation. However, IT central banks did not outperform the central banks used as reference cases during the second period. We then present an event study of monetary policy comparing inflation and interest rate developments after the 1978 and the 1998 oil price shocks. Here we find that IT central banks realized significantly larger gains in credibility than the central banks in the reference group. . This result corroborates the conclusion that IT is a useful framework for communicating a monetary policy strategy aiming at low inflation rates. --

### A general hidden variable model for the two-qubits system

We generalize Bell's hidden variable model describing the singlet state of a
two-qubits system by extending it to arbitrary states and observables. As in
the original work, we assume a uniform, state-independent probability
distribution for the hidden variables which are identified with the unit
vectors of a 3-dimensional real space. By slightly modifying our model, we
provide also a minimal hidden variable description of the two-qubits system,
relying on a single hidden variable. We discuss the main features and the
implications of the model.Comment: 4 pages, submitted for publicatio

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