45 research outputs found
Experimental determination of multipartite entanglement with incomplete information
Multipartite entanglement is very poorly understood despite all the
theoretical and experimental advances of the last decades. Preparation,
manipulation and identification of this resource is crucial for both practical
and fundamental reasons. However, the difficulty in the practical manipulation
and the complexity of the data generated by measurements on these systems
increase rapidly with the number of parties. Therefore, we would like to
experimentally address the problem of how much information about multipartite
entanglement we can access with incomplete measurements. In particular, it was
shown that some types of pure multipartite entangled states can be witnessed
without measuring the correlations [M. Walter et al., Science 340, 1205 (2013)]
between parties, which is strongly demanding experimentally. We explore this
method using an optical setup that permits the preparation and the complete
tomographic reconstruction of many inequivalent classes of three- and
four-partite entangled states, and compare complete versus incomplete
information. We show that the method is useful in practice, even for non-pure
states or non ideal measurement conditions.Comment: 12 pages, 7 figures. Close to published versio
On the quantumness of correlations in nuclear magnetic resonance
Nuclear Magnetic Resonance (NMR) was successfully employed to test several
protocols and ideas in Quantum Information Science. In most of these
implementations the existence of entanglement was ruled out. This fact
introduced concerns and questions about the quantum nature of such bench tests.
In this article we address some issues related to the non-classical aspects of
NMR systems. We discuss some experiments where the quantum aspects of this
system are supported by quantum correlations of separable states. Such
quantumness, beyond the entanglement-separability paradigm, is revealed via a
departure between the quantum and the classical versions of information theory.
In this scenario, the concept of quantum discord seems to play an important
role. We also present an experimental implementation of an analogous of the
single-photon Mach-Zehnder interferometer employing two nuclear spins to encode
the interferometric paths. This experiment illustrate how non-classical
correlations of separable states may be used to simulate quantum dynamics. The
results obtained are completely equivalent to the optical scenario, where
entanglement (between two field modes) may be present
The irreversibility of relativistic time-dilation
The fluctuation relations, which characterize irreversible processes in
Nature, are among the most important results in non-equilibrium physics. In
short, these relations say that it is exponentially unlikely for us to observe
a time-reversed process and, thus, establish the thermodynamic arrow of time
pointing from low to high entropy. On the other hand, fundamental physical
theories are invariant under time-reversal symmetry. Although in Newtonian and
quantum physics the emergence of irreversible processes, as well as fluctuation
relations, is relatively well understood, many problems arise when relativity
enters the game. In this work, by considering a specific class of spacetimes,
we explore the question of how the time-dilation effect enters into the
fluctuation relations. We conclude that a positive entropy production emerges
as a consequence of both the special relativistic and the gravitational
(enclosed in the equivalence principle) time-dilation effects.Comment: 7 page
Experimental investigation of linear-optics-based quantum target detection
The development of new techniques to improve measurements is crucial for all
sciences. By employing quantum systems as sensors to probe some physical
property of interest allows the application of quantum resources, such as
coherent superpositions and quantum correlations, to increase measurement
precision. Here we experimentally investigate a scheme for quantum target
detection based on linear optical measurment devices, when the object is
immersed in unpolarized background light. By comparing the quantum
(polarization-entangled photon pairs) and the classical (separable polarization
states), we found that the quantum strategy provides us an improvement over the
classical one in our experiment when the signal to noise ratio is greater than
1/40, or about 16dB of noise. This is in constrast to quantum target detection
considering non-linear optical detection schemes, which have shown resilience
to extreme amounts of noise. A theoretical model is developed which shows that,
in this linear-optics context, the quantum strategy suffers from the
contribution of multiple background photons. This effect does not appear in our
classical scheme. By improving the two-photon detection electronics, it should
be possible to achieve a polarization-based quantum advantage for a signal to
noise ratio that is close to 1/400 for current technology.Comment: comments are welcome, submitted to PR
Sudden change in quantum and classical correlations and the Unruh effect
We use the Unruh effect to analyze the dynamics of classical and quantum
correlations for a two-qubit system when one of them is uniformly accelerated
for a finite amount of proper time. We show that the quantum correlation is
completely destroyed in the limit of infinite acceleration, while the classical
one remains nonzero. In particular, we show that such correlations exhibit the
so-called sudden-change behavior as a function of acceleration. Eventually, we
discuss how our results can be interpreted when the system lies in the vicinity
of the event horizon of a Schwarzschild black hole.Comment: Published versio
Classical and quantum correlations under decoherence
Recently some authors have pointed out that there exist nonclassical
correlations which are more general, and possibly more fundamental, than
entanglement. For these general quantum correlations and their classical
counterparts, under the action of decoherence, we identify three general types
of dynamics that include a peculiar sudden change in their decay rates. We show
that, under suitable conditions, the classical correlation is unaffected by
decoherence. Such dynamic behavior suggests an operational measure of both
classical and quantum correlations that can be computed without any
extremization procedure.Comment: Published versio
Quantum and classical thermal correlations in the XY spin-1/2 chain
We investigate pairwise quantum correlation as measured by the quantum
discord as well as its classical counterpart in the thermodynamic limit of
anisotropic XY spin-1/2 chains in a transverse magnetic field for both zero and
finite temperatures. Analytical expressions for both classical and quantum
correlations are obtained for spin pairs at any distance. In the case of zero
temperature, it is shown that the quantum discord for spin pairs farther than
second-neighbors is able to characterize a quantum phase transition, even
though pairwise entanglement is absent for such distances. For finite
temperatures, we show that quantum correlations can be increased with
temperature in the presence of a magnetic field. Moreover, in the XX limit, the
thermal quantum discord is found to be dominant over classical correlation
while the opposite scenario takes place for the transverse field Ising model
limit