1,077 research outputs found
Increase of entanglement by local PT -symmetric operations
Entanglement plays a central role in the field of quantum information
science. It is well known that the degree of entanglement cannot be increased
under local operations. Here, we show that the concurrence of a bipartite
entangled state can be increased under the local PT -symmetric operation. This
violates the property of entanglement monotonicity. We also use the Bell-CHSH
and steering inequalities to explore this phenomenon.Comment: 6 pages, 5 figures, to appear in PRA (2014
Thermodynamic description of non-Markovian information flux of nonequilibrium open quantum systems
One of the fundamental issues in the field of open quantum systems is the
classification and quantification of non-Markovianity. In the contest of
quantity-based measures of non-Markovianity, the intuition of non-Markovianity
in terms of information backflow is widely discussed. However, it is not easy
to characterize the information flux for a given system state and show its
connection to non-Markovianity. Here, by using the concepts from thermodynamics
and information theory, we discuss a potential definition of information flux
of an open quantum system, valid for static environments. We present a simple
protocol to show how a system attempts to share information with its
environment and how it builds up system-environment correlations. We also show
that the information returned from the correlations characterizes the
non-Markovianity and a hierarchy of indivisibility of the system dynamics.Comment: 12 pages, 5 figure
Hierarchy of Non-Markovianity and -divisibility phase diagram of Quantum Processes in Open Systems
In recent years, much effort has been devoted to the construction of a proper
measure of quantum non-Markovianity. However, those proposed measures are shown
to be at variance with different situations. In this work, we utilize the
theory of -positive maps to generalize a hierarchy of -divisibility and
develop a powerful tool, called -divisibility phase diagram, which can
provide a further insight into the nature of quantum non-Markovianity. By
exploring the phase diagram with several paradigms, we can explain the origin
of the discrepancy between two frequently used measures and find the condition
under which the two measures coincide with each other.Comment: 5 pages, 4 figure
Witnessing Quantum Coherence: from solid-state to biological systems
Quantum coherence is one of the primary non-classical features of quantum
systems. While protocols such as the Leggett-Garg inequality (LGI) and quantum
tomography can be used to test for the existence of quantum coherence and
dynamics in a given system, unambiguously detecting inherent "quantumness"
still faces serious obstacles in terms of experimental feasibility and
efficiency, particularly in complex systems. Here we introduce two "quantum
witnesses" to efficiently verify quantum coherence and dynamics in the time
domain, without the expense and burden of non-invasive measurements or full
tomographic processes. Using several physical examples, including quantum
transport in solid-state nanostructures and in biological organisms, we show
that these quantum witnesses are robust and have a much finer resolution in
their detection window than the LGI has. These robust quantum indicators may
assist in reducing the experimental overhead in unambiguously verifying quantum
coherence in complex systems.Comment: 10 pages, 6 figures, journal pape
Surface plasmons in a metal nanowire coupled to colloidal quantum dots: Scattering properties and quantum entanglement
We investigate coherent single surface-plasmon transport in a metal nanowire
strongly coupled to two colloidal quantum dots. Analytical expressions are
obtained for the transmission and reflection coefficients by solving the
corresponding eigenvalue equation. Remote entanglement of the wave functions of
the two quantum dots can be created if the inter-dot distance is equal to a
multiple half-wavelength of the surface plasmon. Furthermore, by applying
classical laser pulses to the quantum dots, the entangled states can be stored
in metastable states which are decoupled from the surface plasmons.Comment: 9 pages, 5 figure
Entanglement swapping and testing quantum steering into the past via collective decay
We propose a scheme to realize entanglement swapping via superradiance,
entangling two distant cavities without a direct interaction. The successful
Bell-state-measurement outcomes are performed naturally by the electromagnetic
reservoir, and we show how, using a quantum trajectory method, the non-local
properties of the state obtained after the swapping procedure can be verified
by the steering inequality. Furthermore, we discuss how the unsuccessful
measurement outcomes can be used in an experiment of delayed-choice
entanglement swapping. An extension of testing the quantum steering inequality
with the observers at three different times is also consideredComment: 6 pages, 5 figures; accepted in Phys. Rev. A (2013
Collectively induced exceptional points of quantum emitters coupled to nanoparticle surface plasmons
Exceptional points, resulting from non-Hermitian degeneracies, have the
potential to enhance the capabilities of quantum sensing. Thus, finding
exceptional points in different quantum systems is vital for developing such
future sensing devices. Taking advantage of the enhanced light-matter
interactions in a confined volume on a metal nanoparticle surface, here we
theoretically demonstrate the existence of exceptional points in a system
consisting of quantum emitters coupled to a metal nanoparticle of subwavelength
scale. By using an analytical quantum electrodynamics approach, exceptional
points are manifested as a result of a strong coupling effect and observable in
a drastic splitting of originally coalescent eigenenergies. Furthermore, we
show that exceptional points can also occur when a number of quantum emitters
is collectively coupled to the dipole mode of localized surface plasmons. Such
a quantum collective effect not only relaxes the strong-coupling requirement
for an individual emitter, but also results in a more stable generation of the
exceptional points. Furthermore, we point out that the exceptional points can
be explicitly revealed in the power spectra. A generalized signal-to-noise
ratio, accounting for both the frequency splitting in the power spectrum and
the system's dissipation, shows clearly that a collection of quantum emitters
coupled to a nanoparticle provides a better performance of detecting
exceptional points, compared to that of a single quantum emitter
A temporal steering inequality
Quantum steering is the ability to remotely prepare different quantum states
by using entangled pairs as a resource. Very recently, the concept of steering
has been quantified with the use of inequalities, leading to substantial
applications in quantum information and communication science. Here, we
highlight that there exists a natural temporal analogue of the steering
inequality when considering measurements on a single object at different times.
We give non-trivial operational meaning to violations of this temporal
inequality by showing that it is connected to the security bound in the BB84
protocol and thus may have applications in quantum communication.Comment: 8 pages, 5 figures, to appear in PR
Security of modified Ping-Pong protocol in noisy and lossy channel
The "Ping-Pong" (PP) protocol is a two-way quantum key protocol based on
entanglement. In this protocol, Bob prepares one maximally entangled pair of
qubits, and sends one qubit to Alice. Then, Alice performs some necessary
operations on this qubit and sends it back to Bob. Although this protocol was
proposed in 2002, its security in the noisy and lossy channel has not been
proven. In this report, we add a simple and experimentally feasible
modification to the original PP protocol, and prove the security of this
modified PP protocol against collective attacks when the noisy and lossy
channel is taken into account. Simulation results show that our protocol is
practical.Comment: 7 pages, 2 figures, published in scientific report
More randomness from a prepare-and-measure scenario with independent devices
How to generate genuine quantum randomness from untrusted devices is an
important problem in quantum information processing. Inspired by previous work
on a self-testing quantum random number generator [T. Lunghi et al., Phys. Rev.
Lett. 114, 150501 (2015)], we present a method to generate quantum randomness
from a prepare-and-measure scenario with independent devices. In existing
protocols, the quantum randomness depends only on a witness value (e.g.,
Clauser-Horne-Shimony-Holt value), which is calculated with the observed
probabilities. Differently, here all the observed probabilities are directly
used to calculate the min-entropy in our method. Through numerical simulation,
we find that the min-entropy of our proposed scheme is higher than that in the
previous work when a typical untrusted Bennett-Brassard 1984 (BB84) setup is
used. Consequently, thanks to the proposed method, more genuine quantum random
numbers may be obtained than before.Comment: 8 pages, 3 figure
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