143 research outputs found
Nonlocality threshold for entanglement under general dephasing evolutions: A case study
Determining relationships between different types of quantum correlations in
open composite quantum systems is important since it enables the exploitation
of a type by knowing the amount of another type. We here review, by giving a
formal demonstration, a closed formula of the Bell function, witnessing
nonlocality, as a function of the concurrence, quantifying entanglement, valid
for a system of two noninteracting qubits initially prepared in extended
Werner-like states undergoing any local pure-dephasing evolution. This formula
allows for finding nonlocality thresholds for the concurrence depending only on
the purity of the initial state. We then utilize these thresholds in a
paradigmatic system where the two qubits are locally affected by a quantum
environment with an Ohmic class spectrum. We show that steady entanglement can
be achieved and provide the lower bound of initial state purity such that this
stationary entanglement is above the nonlocality threshold thus guaranteeing
the maintenance of nonlocal correlations.Comment: 7 pages, 4 figures. Revised versio
Quantum entanglement of identical particles by standard information-theoretic notions
Quantum entanglement of identical particles is essential in quantum
information theory. Yet, its correct determination remains an open issue
hindering the general understanding and exploitation of many-particle systems.
Operator-based methods have been developed that attempt to overcome the issue.
We introduce a state-based method which, as second quantization, does not label
identical particles and presents conceptual and technical advances compared to
the previous ones. It establishes the quantitative role played by arbitrary
wave function overlaps, local measurements and particle nature (bosons or
fermions) in assessing entanglement by notions commonly used in quantum
information theory for distinguishable particles, like partial trace. Our
approach furthermore shows that bringing identical particles into the same
spatial location functions as an entangling gate, providing fundamental
theoretical support to recent experimental observations with ultracold atoms.
These results pave the way to set and interpret experiments for utilizing
quantum correlations in realistic scenarios where overlap of particles can
count, as in Bose-Einstein condensates, quantum dots and biological molecular
aggregates.Comment: 6+3 pages, 3 Figures. Stories on: Physics World
(http://physicsworld.com/cws/article/news/2016/feb/12/theorists-disentangle-particle-identity);
Phys.org
(http://phys.org/news/2016-02-entanglement-identical-particles-doesnt-textbook.html).
Invited article on 2Physics.com, presenting key developments in physics
(http://www.2physics.com/2016/03/a-new-approach-to-quantum-entanglement.html
Switching quantum memory on and off
Modifying the Markovian (memoryless) or non-Markovian (memory-keeping) nature of the environment-induced evolution of an open quantum system is crucial in quantum information theory, because it is linked to quantum memory control. A recent work (Brito and Werlang 2015 New J. Phys. 17 072001) shows that such a goal can be achieved without operating on unaccessible environmental features. In fact, transitions between Markovian and non-Markovian regimes of a qubit dynamics can be induced on demand if the qubit is coupled to a controlled auxiliary system. This is a step towards the improvement of quantum devices, aiming at exploiting dynamical memory effects by an external control
Validating and controlling quantum enhancement against noise by the motion of a qubit
Experimental validation and control of quantum traits for an open quantum system are important for any quantum information purpose. We consider a traveling atom qubit as a quantum memory with adjustable velocity inside a leaky cavity, adopting a quantum witness as a figure of merit for quantumness assessment. We show that this model constitutes an inherent physical instance where the quantum witness does not work properly if not suitably optimized. We then supply the optimal intermediate blind measurements which make the quantum witness a faithful tester of quantum coherence. We thus find that larger velocities protect quantumness against noise, leading to a lifetime extension of hybrid qubit-photon entanglement and to higher phase estimation precision. Control of qubit motion thus reveals itself as a quantum enhancer
Coherence and entanglement dynamics of vibrating qubits
We investigate the dynamics of coherence and entanglement of vibrating
qubits. Firstly, we consider a single trapped ion qubit inside a perfect cavity
and successively we use it to construct a bipartite system made of two of such
subsystems, taken identical and noninteracting. As a general result, we find
that qubit vibration can lead to prolonging initial coherence in both
single-qubit and two-qubit system. However, despite of this coherence
preservation, we show that the decay of the entanglement between the two qubits
is sped up by the vibrational motion of the qubits. Furthermore, we highlight
how the dynamics of photon-phonon correlations between cavity mode and
vibrational mode, which may serve as a further useful resource stored in the
single-qubit system, is strongly affected by the initial state of the qubit.
These results provide new insights about the ability of systems made of moving
qubits in maintaining quantum resources compared to systems of stationary
qubits.Comment: 7 pages, 5 figures. Prepared for the Virtual Special Issue (VSI) on
Quantum Correlations, in the journal Optics Communications
Comparison of non-Markovianity criteria in a qubit system under random external fields
We give the map representing the evolution of a qubit under the action of
non-dissipative random external fields. From this map we construct the
corresponding master equation that in turn allows us to phenomenologically
introduce population damping of the qubit system. We then compare, in this
system, the time-regions when non-Markovianity is present on the basis of
different criteria both for the non-dissipative and dissipative case. We show
that the adopted criteria agree both in the non-dissipative case and in the
presence of population damping.Comment: 8 pages, 1 figure. Some changes made. In press on Physica Scripta T
(special issue
Protecting entanglement by adjusting the velocities of moving qubits inside non-Markovian environments
Efficient entanglement preservation in open quantum systems is a crucial
scope towards a reliable exploitation of quantum resources. We address this
issue by studying how two-qubit entanglement dynamically behaves when two atom
qubits move inside two separated identical cavities. The moving qubits
independently interact with their respective cavity. As a main general result,
we find that under resonant qubit-cavity interaction the initial entanglement
between two moving qubits remains closer to its initial value as time passes
compared to the case of stationary qubits. In particular, we show that the
initial entanglement can be strongly protected from decay by suitably adjusting
the velocities of the qubits according to the non-Markovian features of the
cavities. Our results supply a further way of preserving quantum correlations
against noise with a natural implementation in cavity-QED scenarios and are
straightforwardly extendable to many qubits for scalability.Comment: To be published in Laser Physics Letter
Cavity-based architecture to preserve quantum coherence and entanglement
Quantum technology relies on the utilization of resources, like quantum
coherence and entanglement, which allow quantum information and computation
processing. This achievement is however jeopardized by the detrimental effects
of the environment surrounding any quantum system, so that finding strategies
to protect quantum resources is essential. Non-Markovian and structured
environments are useful tools to this aim. Here we show how a simple
environmental architecture made of two coupled lossy cavities enables a switch
between Markovian and non-Markovian regimes for the dynamics of a qubit
embedded in one of the cavity. Furthermore, qubit coherence can be indefinitely
preserved if the cavity without qubit is perfect. We then focus on entanglement
control of two independent qubits locally subject to such an engineered
environment and discuss its feasibility in the framework of circuit quantum
electrodynamics. With up-to-date experimental parameters, we show that our
architecture allows entanglement lifetimes orders of magnitude longer than the
spontaneous lifetime without local cavity couplings. This cavity-based
architecture is straightforwardly extendable to many qubits for scalability.Comment: 12 pages, 9 figures, 1 table. To appear on Nature Scientific Report
Efficient generation of -photon generalized binomial states in a cavity
Extending a previous result on the generation of two-photon generalized
binomial field states, here we propose an efficient scheme to generate with
high-fidelity, in a single-mode high-Q cavity, N-photon generalized binomial
states with a maximum number of photons N>2. Besides their interest for
classical-quantum border investigations, we discuss the applicative usage of
these states in realizing universal quantum computation, describing in
particular a scheme that performs a controlled-NOT gate by dispersive
interaction with a control atom. We finally analyze the feasibility of the
proposed schemes, showing that they appear to be within the current
experimental capabilities.Comment: 8 pages, 2 figure
Harnessing non-Markovian quantum memory by environmental coupling
Controlling the non-Markovian dynamics of open quantum systems is essential
in quantum information technology since it plays a crucial role in preserving
quantum memory. Albeit in many realistic scenarios the quantum system can
simultaneously interact with composite environments, this condition remains
little understood, particularly regarding the effect of the coupling between
environmental parts. We analyze the non-Markovian behavior of a qubit
interacting at the same time with two coupled single-mode cavities which in
turn dissipate into memoryless or memory-keeping reservoirs. We show that
increasing the control parameter, that is the two-mode coupling, allows for
triggering and enhancing a non-Markovian dynamics for the qubit starting from a
Markovian one in absence of coupling. Surprisingly, if the qubit dynamics is
non-Markovian for zero control parameter, increasing the latter enables
multiple transitions from non-Markovian to Markovian regimes. These results
hold independently on the nature of the reservoirs. This work highlights that
suitably engineering the coupling between parts of a compound environment can
efficiently harness the quantum memory, stored in a qubit, based on
non-Markovianity.Comment: 8 pages, 5 figures. To appear in Phys. Rev.
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