171 research outputs found
Effect of disorder on transport properties in a tight-binding model for lead halide perovskites
The hybrid organic-inorganic lead halide perovskite materials have emerged as
remarkable materials for photovoltaic applications. Their strengths include
good electric transport properties in spite of the disorder inherent in them.
Motivated by this observation, we analyze the effects of disorder on the energy
eigenstates of a tight-binding model of these materials. In particular, we
analyze the spatial extension of the energy eigenstates, which is quantified by
the inverse participation ratio. This parameter exhibits a tendency, and
possibly a phase transition, to localization as the on-site energy disorder
strength is increased. However, we argue that the disorder in the lead halide
perovskites corresponds to a point in the regime of highly delocalized states.
Our results also suggest that the electronic states of mixed-halide materials
tend to be more localized than those of pure materials, which suggests a weaker
tendency to form extended bonding states in the mixed-halide materials and is
therefore not favourable for halide mixing.Comment: 24 pages (preprint), 11 figure
Interference between a large number of independent Bose-Einstein condensates
We study theoretically the interference patterns produced by the overlap of
an array of Bose-Einstein condensates that have no phase coherence among them.
We show that density-density correlations at different quasimomenta, which play
an important role in two-condensate interference, become negligible for large
, where is the number of overlapping condensates. In order to understand
the physics of this phenomenon, it is sufficient to consider the periodicity of
the lattice and the statistical probability distribution of a random-walk
problem. The average visibility of such interference patterns decreases as
for large .Comment: 9 pages, 2 figure
Approximate joint measurement of qubit observables through an Arthur-Kelly type model
We consider joint measurement of two and three unsharp qubit observables
through an Arthur-Kelly type joint measurement model for qubits. We investigate
the effect of initial state of the detectors on the unsharpness of the
measurement as well as the post-measurement state of the system. Particular
emphasis is given on a physical understanding of the POVM to PVM transition in
the model and entanglement between system and detectors.Two approaches for
characterizing the unsharpness of the measurement and the resulting measurement
uncertainty relations are considered.The corresponding measures of unsharpness
are connected for the case where both the measurements are equally unsharp. The
connection between the POVM elements and symmetries of the underlying
Hamiltonian of the measurement interaction is made explicit and used to perform
joint measurement in arbitrary directions. Finally in the case of three
observables we derive a necessary condition for the approximate joint
measurement and use it show the relative freedom available when the observables
are non-orthogonal.Comment: 22 pages; Late
Bell's experiment with intra- and inter-pair entanglement: Single-particle mode entanglement as a case study
Theoretical considerations of Bell-inequality experiments usually assume
identically prepared and independent pairs of particles. Here we consider pairs
that exhibit both intra- and inter-pair entanglement. The pairs are taken from
a large many-body system where all the pairs are generally entangled with each
other. Using an explicit example based on single mode entanglement and an
ancillary Bose-Einstein condensate, we show that the Bell-inequality violation
in such systems can display statistical properties that are remarkably
different from those obtained using identically prepared, independent pairs. In
particular, one can have probabilistic violation of Bell's inequalities in
which a finite fraction of all the runs result in violation, even though there
could be no violation when averaging over all the runs. Whether or not a
particular run of results will end up being local realistically explainable is
"decided" by a sequence of quantum (random) outcomes.Comment: 7 pages (two column), 5 figure
Efficient quantum algorithm for preparing molecular-system-like states on a quantum computer
We present an efficient quantum algorithm for preparing a pure state on a
quantum computer, where the quantum state corresponds to that of a molecular
system with a given number of electrons occupying a given number of
spin orbitals. Each spin orbital is mapped to a qubit: the states and
of the qubit represent, respectively, whether the spin orbital is
occupied by an electron or not. To prepare a general state in the full Hilbert
space of qubits, which is of dimension %, controlled-NOT
gates are needed, i.e., the number of gates scales \emph{exponentially} with
the number of qubits. We make use of the fact that the state to be prepared
lies in a smaller Hilbert space, and we find an algorithm that requires at most
gates, i.e., scales \emph{polynomially} with the number
of qubits , provided . The algorithm is simulated numerically for
the cases of the hydrogen molecule and the water molecule. The numerical
simulations show that when additional symmetries of the system are considered,
the number of gates to prepare the state can be drastically reduced, in the
examples considered in this paper, by several orders of magnitude, from the
above estimate.Comment: 11 pages, 8 figures, errors are corrected, Journal information adde
Quantum two-level systems in Josephson junctions as naturally formed qubits
The two-level systems (TLSs) naturally occurring in Josephson junctions
constitute a major obstacle for the operation of superconducting phase qubits.
Since these TLSs can possess remarkably long decoherence times, we show that
such TLSs can themselves be used as qubits, allowing for a well controlled
initialization, universal sets of quantum gates, and readout. Thus, a single
current-biased Josephson junction (CBJJ) can be considered as a multiqubit
register. It can be coupled to other CBJJs to allow the application of quantum
gates to an arbitrary pair of qubits in the system. Our results indicate an
alternative way to realize superconducting quantum information processing.Comment: Reference adde
Weak and strong measurement of a qubit using a switching-based detector
We analyze the operation of a switching-based detector that probes a qubit's
observable that does not commute with the qubit's Hamiltonian, leading to a
nontrivial interplay between the measurement and free-qubit dynamics. In order
to obtain analytic results and develop intuitive understanding of the different
possible regimes of operation, we use a theoretical model where the detector is
a quantum two-level system that is constantly monitored by a macroscopic
system. We analyze how to interpret the outcome of the measurement and how the
state of the qubit evolves while it is being measured. We find that the answers
to the above questions depend on the relation between the different parameters
in the problem. In addition to the traditional strong-measurement regime, we
identify a number of regimes associated with weak qubit-detector coupling. An
incoherent detector whose switching time is measurable with high accuracy can
provide high-fidelity information, but the measurement basis is determined only
upon switching of the detector. An incoherent detector whose switching time can
be known only with low accuracy provides a measurement in the qubit's energy
eigenbasis with reduced measurement fidelity. A coherent detector measures the
qubit in its energy eigenbasis and, under certain conditions, can provide
high-fidelity information.Comment: 20 pages (two-column), 6 figure
Functionals and the Quantum Master Equation
The quantum master equation is usually formulated in terms of functionals of
the components of mappings from a space-time manifold M into a
finite-dimensional vector space. The master equation is the sum of two terms
one of which is the anti-bracket (odd Poisson bracket) of functionals and the
other is the Laplacian of a functional. Both of these terms seem to depend on
the fact that the mappings on which the functionals act are vector-valued. It
turns out that neither this Laplacian nor the anti-bracket is well-defined for
sections of an arbitrary vector bundle. We show that if the functionals are
permitted to have their values in an appropriate graded tensor algebra whose
factors are the dual of the space of smooth functions on M, then both the
anti-bracket and the Laplace operator can be invariantly defined. Additionally,
one obtains a new anti-bracket for ordinary functionals.Comment: 21 pages, Late
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