424 research outputs found
Optimal simulation of two-qubit Hamiltonians using general local operations
We consider the simulation of the dynamics of one nonlocal Hamiltonian by
another, allowing arbitrary local resources but no entanglement nor classical
communication. We characterize notions of simulation, and proceed to focus on
deterministic simulation involving one copy of the system. More specifically,
two otherwise isolated systems and interact by a nonlocal Hamiltonian
. We consider the achievable space of Hamiltonians such
that the evolution can be simulated by the interaction
interspersed with local operations. For any dimensions of and , and any
nonlocal Hamiltonians and , there exists a scale factor such that
for all times the evolution can be simulated by acting for
time interspersed with local operations. For 2-qubit Hamiltonians and
, we calculate the optimal and give protocols achieving it. The optimal
protocols do not require local ancillas, and can be understood geometrically in
terms of a polyhedron defined by a partial order on the set of 2-qubit
Hamiltonians.Comment: (1) References to related work, (2) protocol to simulate one
two-qudit Hamiltonian with another, and (3) other related results added. Some
proofs are simplifie
Measuring Polynomial Invariants of Multi-Party Quantum States
We present networks for directly estimating the polynomial invariants of
multi-party quantum states under local transformations. The structure of these
networks is closely related to the structure of the invariants themselves and
this lends a physical interpretation to these otherwise abstract mathematical
quantities. Specifically, our networks estimate the invariants under local
unitary (LU) transformations and under stochastic local operations and
classical communication (SLOCC). Our networks can estimate the LU invariants
for multi-party states, where each party can have a Hilbert space of arbitrary
dimension and the SLOCC invariants for multi-qubit states. We analyze the
statistical efficiency of our networks compared to methods based on estimating
the state coefficients and calculating the invariants.Comment: 8 pages, 4 figures, RevTex4, v2 references update
Optimal Entanglement Generation from Quantum Operations
We consider how much entanglement can be produced by a non-local two-qubit
unitary operation, - the entangling capacity of . For a single
application of , with no ancillas, we find the entangling capacity and
show that it generally helps to act with on an entangled state.
Allowing ancillas, we present numerical results from which we can conclude,
quite generally, that allowing initial entanglement typically increases the
optimal capacity in this case as well. Next, we show that allowing collective
processing does not increase the entangling capacity if initial entanglement is
allowed.Comment: v1.0 15 pages, 3 figures, written in revtex4. v2.0 References
updated. Submitted to Phys. Rev. A v3.0 16 pages, 4 figures. Expanded
explanation in section 3A, figures corrected and made clearer. Definition of
entangling capacity in section 4 made explicit. Other minor typos correcte
Enhancement of the Binding Energy of Charged Excitons in Disordered Quantum Wires
Negatively and positively charged excitons are identified in the
spatially-resolved photoluminescence spectra of quantum wires. We demonstrate
that charged excitons are weakly localized in disordered quantum wires. As a
consequence, the enhancement of the "binding energy" of a charged exciton is
caused, for a significant part, by the recoil energy transferred to the
remaining charged carrier during its radiative recombination. We discover that
the Coulomb correlation energy is not the sole origin of the "binding energy",
in contrast to charged excitons confined in quantum dots.Comment: 4 Fig
Measurements on the reality of the wavefunction
Quantum mechanics is an outstandingly successful description of nature,
underpinning fields from biology through chemistry to physics. At its heart is
the quantum wavefunction, the central tool for describing quantum systems. Yet
it is still unclear what the wavefunction actually is: does it merely represent
our limited knowledge of a system, or is it an element of reality? Recent no-go
theorems argued that if there was any underlying reality to start with, the
wavefunction must be real. However, that conclusion relied on debatable
assumptions, without which a partial knowledge interpretation can be maintained
to some extent. A different approach is to impose bounds on the degree to which
knowledge interpretations can explain quantum phenomena, such as why we cannot
perfectly distinguish non-orthogonal quantum states. Here we experimentally
test this approach with single photons. We find that no knowledge
interpretation can fully explain the indistinguishability of non-orthogonal
quantum states in three and four dimensions. Assuming that some underlying
reality exists, our results strengthen the view that the entire wavefunction
should be real. The only alternative is to adopt more unorthodox concepts such
as backwards-in-time causation, or to completely abandon any notion of
objective reality.Comment: 7 pages, 4 figure
Lower bounds on the complexity of simulating quantum gates
We give a simple proof of a formula for the minimal time required to simulate
a two-qubit unitary operation using a fixed two-qubit Hamiltonian together with
fast local unitaries. We also note that a related lower bound holds for
arbitrary n-qubit gates.Comment: 6 page
Towards a Formulation of Quantum Theory as a Causally Neutral Theory of Bayesian Inference
Quantum theory can be viewed as a generalization of classical probability
theory, but the analogy as it has been developed so far is not complete.
Whereas the manner in which inferences are made in classical probability theory
is independent of the causal relation that holds between the conditioned
variable and the conditioning variable, in the conventional quantum formalism,
there is a significant difference between how one treats experiments involving
two systems at a single time and those involving a single system at two times.
In this article, we develop the formalism of quantum conditional states, which
provides a unified description of these two sorts of experiment. In addition,
concepts that are distinct in the conventional formalism become unified:
channels, sets of states, and positive operator valued measures are all seen to
be instances of conditional states; the action of a channel on a state,
ensemble averaging, the Born rule, the composition of channels, and
nonselective state-update rules are all seen to be instances of belief
propagation. Using a quantum generalization of Bayes' theorem and the
associated notion of Bayesian conditioning, we also show that the remote
steering of quantum states can be described within our formalism as a mere
updating of beliefs about one system given new information about another, and
retrodictive inferences can be expressed using the same belief propagation rule
as is used for predictive inferences. Finally, we show that previous arguments
for interpreting the projection postulate as a quantum generalization of
Bayesian conditioning are based on a misleading analogy and that it is best
understood as a combination of belief propagation (corresponding to the
nonselective state-update map) and conditioning on the measurement outcome.Comment: v1 43 pages, revTeX4. v2 42 pages, edited for clarity, added
references and corrected minor errors, submitted to Phys. Rev. A. v3 41
pages, improved figures, added two new figures, added extra explanation in
response to referee comments, minor rewrites for readability. v4 44 pages,
added "towards" to title, rewritten abstract, rewritten introduction with new
table
The NASA Exoplanet Archive: Data and Tools for Exoplanet Research
We describe the contents and functionality of the NASA Exoplanet Archive, a
database and tool set funded by NASA to support astronomers in the exoplanet
community. The current content of the database includes interactive tables
containing properties of all published exoplanets, Kepler planet candidates,
threshold-crossing events, data validation reports and target stellar
parameters, light curves from the Kepler and CoRoT missions and from several
ground-based surveys, and spectra and radial velocity measurements from the
literature. Tools provided to work with these data include a transit ephemeris
predictor, both for single planets and for observing locations, light curve
viewing and normalization utilities, and a periodogram and phased light curve
service. The archive can be accessed at
http://exoplanetarchive.ipac.caltech.edu.Comment: Accepted for publication in the Publications of the Astronomical
Society of the Pacific, 4 figure
Einstein, incompleteness, and the epistemic view of quantum states
Does the quantum state represent reality or our knowledge of reality? In
making this distinction precise, we are led to a novel classification of hidden
variable models of quantum theory. Indeed, representatives of each class can be
found among existing constructions for two-dimensional Hilbert spaces. Our
approach also provides a fruitful new perspective on arguments for the
nonlocality and incompleteness of quantum theory. Specifically, we show that
for models wherein the quantum state has the status of something real, the
failure of locality can be established through an argument considerably more
straightforward than Bell's theorem. The historical significance of this result
becomes evident when one recognizes that the same reasoning is present in
Einstein's preferred argument for incompleteness, which dates back to 1935.
This fact suggests that Einstein was seeking not just any completion of quantum
theory, but one wherein quantum states are solely representative of our
knowledge. Our hypothesis is supported by an analysis of Einstein's attempts to
clarify his views on quantum theory and the circumstance of his otherwise
puzzling abandonment of an even simpler argument for incompleteness from 1927.Comment: 18 pages, 8 figures, 1 recipe for cupcakes; comments welcom
- …