317 research outputs found
Entanglement in fermion systems and quantum metrology
Entanglement in fermion many-body systems is studied using a generalized
definition of separability based on partitions of the set of observables,
rather than on particle tensor products. In this way, the characterizing
properties of non-separable fermion states can be explicitly analyzed, allowing
a precise description of the geometric structure of the corresponding state
space. These results have direct applications in fermion quantum metrology:
sub-shot noise accuracy in parameter estimation can be obtained without the
need of a preliminary state entangling operation.Comment: 26 pages, LaTe
Entangling two unequal atoms through a common bath
The evolution of two, non-interacting two-level atoms immersed in a weakly
coupled bath can be described by a refined, time coarse grained Markovian
evolution, still preserving complete positivity. We find that this improved
reduced dynamics is able to entangle the two atoms even when their internal
frequencies are unequal, an effect which appears impossible in the standard
weak coupling limit approach. We study in detail this phenomenon for an
environment made of quantum fields.Comment: 18 pages, LaTe
Sub-shot-noise quantum metrology with entangled identical particles
The usual notion of separability has to be reconsidered when applied to
states describing identical particles. A definition of separability not related
to any a priori Hilbert space tensor product structure is needed: this can be
given in terms of commuting subalgebras of observables. Accordingly, the
results concerning the use of the quantum Fisher information in quantum
metrology are generalized and physically reinterpreted.Comment: 17 pages, LaTe
Squeezing Inequalities and Entanglement for Identical Particles
By identifying non-local effects in systems of identical Bosonic qubits
through correlations of their commuting observables, we show that entanglement
is not necessary to violate certain squeezing inequalities that hold for
distinguishable qubits and that spin squeezing may not be necessary to achieve
sub-shot noise accuracies in ultra-cold atom interferometry.Comment: 13 pages, LaTe
Entanglement and non-locality in quantum protocols with identical particles
We study the role of entanglement and non-locality in quantum protocols that make use of systems of identical particles. Unlike in the case of distinguishable particles, the notions of entanglement and non-locality for systems whose constituents cannot be distinguished and singly addressed are still debated. We clarify why the only approach that avoids incongruities and paradoxes is the one based on the second quantization formalism, whereby it is the entanglement of the modes that can be populated by the particles that really matters and not the particles themselves. Indeed, by means of a metrological and of a teleportation protocol, we show that inconsistencies arise in formulations that force entanglement and non-locality to be properties of the identical particles rather than of the modes they can occupy. The reason resides in the fact that orthogonal modes can always be addressed while identical particles cannot
Bipartite entanglement in systems of identical particles: the partial transposition criterion
We study bipartite entanglement in systems of N identical bosons distributed
in M different modes. For such systems, a definition of separability not
related to any a priori Hilbert space tensor product structure is needed and
can be given in terms of commuting subalgebras of observables. Using this
generalized notion of separability, we classify the states for which partial
transposition turns out to be a necessary and sufficient condition for
entanglement detection.Comment: LaTeX, 22 page
Exact dynamics of interacting qubits in a thermal environment: Results beyond the weak coupling limit
We demonstrate an exact mapping of a class of models of two interacting
qubits in thermal reservoirs to two separate spin-bath problems. Based on this
mapping, exact numerical simulations of the qubits dynamics can be performed,
beyond the weak system-bath coupling limit. Given the time evolution of the
system, we study, in a numerically exact way, the dynamics of entanglement
between pair of qubits immersed in boson thermal baths, showing a rich
phenomenology, including an intermediate oscillatory behavior, the entanglement
sudden birth, sudden death, and revival. We find that stationary entanglement
develops between the qubits due to their coupling to a thermal environment,
unlike the isolated qubits case in which the entanglement oscillates. We also
show that the occurrence of entanglement sudden death in this model depends on
the portion of the zero and double excitation states in the subsystem initial
state. In the long-time limit, analytic expressions are presented at weak
system-bath coupling, for a range of relevant qubit parameters
Multi-mode entanglement of N harmonic oscillators coupled to a non-Markovian reservoir
Multi-mode entanglement is investigated in the system composed of coupled
identical harmonic oscillators interacting with a common environment. We treat
the problem very general by working with the Hamiltonian without the
rotating-wave approximation and by considering the environment as a
non-Markovian reservoir to the oscillators. We invoke an -mode unitary
transformation of the position and momentum operators and find that in the
transformed basis the system is represented by a set of independent harmonic
oscillators with only one of them coupled to the environment. Working in the
Wigner representation of the density operator, we find that the covariance
matrix has a block diagonal form that it can be expressed in terms of multiples
of and matrices. This simple property allows to treat
the problem to some extend analytically. We illustrate the advantage of working
in the transformed basis on a simple example of three harmonic oscillators and
find that the entanglement can persists for long times due to presence of
constants of motion for the covariance matrix elements. We find that, in
contrast to what one could expect, a strong damping of the oscillators leads to
a better stationary entanglement than in the case of a weak damping.Comment: 21 pages, 4 figure
- …