34 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
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
Frustration, Entanglement, and Correlations in Quantum Many Body Systems
We derive an exact lower bound to a universal measure of frustration in
degenerate ground states of quantum many-body systems. The bound results in the
sum of two contributions: entanglement and classical correlations arising from
local measurements. We show that average frustration properties are completely
determined by the behavior of the maximally mixed ground state. We identify
sufficient conditions for a quantum spin system to saturate the bound, and for
models with twofold degeneracy we prove that average and local frustration
coincide.Comment: 9 pages, 1 figur
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
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
Statistical mechanics of multipartite entanglement
We characterize the multipartite entanglement of a system of n qubits in
terms of the distribution function of the bipartite purity over all balanced
bipartitions. We search for those (maximally multipartite entangled) states
whose purity is minimum for all bipartitions and recast this optimization
problem into a problem of statistical mechanics.Comment: final versio
Classical Statistical Mechanics Approach to Multipartite Entanglement
We characterize the multipartite entanglement of a system of n qubits in
terms of the distribution function of the bipartite purity over balanced
bipartitions. We search for maximally multipartite entangled states, whose
average purity is minimal, and recast this optimization problem into a problem
of statistical mechanics, by introducing a cost function, a fictitious
temperature and a partition function. By investigating the high-temperature
expansion, we obtain the first three moments of the distribution. We find that
the problem exhibits frustration.Comment: 38 pages, 10 figures, published versio
Multipartite Entanglement and Frustration
Some features of the global entanglement of a composed quantum system can be
quantified in terms of the purity of a balanced bipartition, made up of half of
its subsystems. For the given bipartition, purity can always be minimized by
taking a suitable (pure) state. When many bipartitions are considered, the
requirement that purity be minimal for all bipartitions can engender conflicts
and frustration arises. This unearths an interesting link between frustration
and multipartite entanglement, defined as the average purity over all
(balanced) bipartitions.Comment: 15 pages, 7 figure
Entanglement robustness and geometry in systems of identical particles
The robustness properties of bipartite entanglement in systems of N bosons
distributed in M different modes are analyzed using a definition of
separability based on commuting algebras of observables, a natural choice when
dealing with identical particles. Within this framework, expressions for the
robustness and generalized robustness of entanglement can be explicitly given
for large classes of boson states: their entanglement content results in
general much more stable than that of distinguishable particles states. Using
these results, the geometrical structure of the space of N boson states can be
explicitly addressed.Comment: 20 pages, LaTe