75 research outputs found
Bell correlations at finite temperature
We show that spin systems with infinite-range interactions can violate at
thermal equilibrium a multipartite Bell inequality, up to a finite critical
temperature . Our framework can be applied to a wide class of spin systems
and Bell inequalities, to study whether nonlocality occurs naturally in quantum
many-body systems close to the ground state. Moreover, we also show that the
low-energy spectrum of the Bell operator associated to such systems can be well
approximated by the one of a quantum harmonic oscillator, and that
spin-squeezed states are optimal in displaying Bell correlations for such Bell
inequalities.Comment: 9 pages (7 + Appendix), 2 figures. Version accepted for publication
in Quantu
Split spin-squeezed Bose-Einstein Condensates
We investigate and model the behaviour of split spin-squeezed Bose-Einstein
condensates (BECs) system. In such a system, a spin-polarized BEC is first
squeezed using a interaction, then are split into two separate
clouds. After the split, we consider that the particle number in each cloud
collapses to a fixed number. We show that this procedure is equivalent to
applying an interaction corresponding to squeezing each cloud individually plus
an entangling operation. We analyse the system's entanglement properties and
show that it can be detected using correlation-based entanglement criteria. The
nature of the states are illustrated by Wigner functions and have the form of a
correlated squeezed state. The conditional Wigner function shows high degrees
of non-classicality for dimensionless squeezing times beyond ,
where is the number of particles per BEC.Comment: 11 pages, 7 figure
Number-phase entanglement and Einstein-Podolsky-Rosen steering
We use the uncertainty relation between the operators associated to the total
number of particles and to the relative phase of two bosonic modes to construct
entanglement and Einstein-Podolsky-Rosen steering criteria. These can be tested
experimentally in a variety of systems, such as optical fields, Bose-Einstein
condensates or mechanical oscillators. While known entanglement criteria
involving the phase observable typically require to perform interference
measurements by recombining the two systems, our criteria can be tested through
local measurements at two spatially distinct positions, to investigate the
nonlocal nature of quantum correlations. We present simple examples where our
criteria are violated, and show their robustness to noise. Apart from being
useful for state characterization, they might find application in quantum
information protocols, for example based on number-phase teleportation.Comment: Comments are welcome
Bounding the Set of Classical Correlations of a Many-Body System
We present a method to certify the presence of Bell correlations in experimentally observed statistics, and to obtain new Bell inequalities. Our approach is based on relaxing the conditions defining the set of correlations obeying a local hidden variable model, yielding a convergent hierarchy of semidefinite programs (SDP's). Because the size of these SDP's is independent of the number of parties involved, this technique allows us to characterize correlations in many-body systems. As an example, we illustrate our method with the experimental data presented in Science 352, 441 (2016)
Time-energy uncertainty relation for quantum events
Textbook quantum mechanics treats time as a classical parameter and not as a quantum observable with an associated Hermitian operator. For this reason, to make sense of usual time-energy uncertainty relations such as ΔtΔE≳ℏ, the term Δt must be interpreted as a time interval and not as a time measurement uncertainty due to quantum noise. However, quantum clocks allow for a measurement of the “time at which an event happens” by conditioning the system's evolution on an additional quantum degree of freedom. Within this framework we derive here two uncertainty relations that relate the uncertainty in the quantum measurement of the time at which a quantum event happens on a system to its energy uncertainty
Entanglement of Local Hidden States
Steering criteria are conditions whose violation excludes the possibility of describing the observed measurement statistics with local hidden state (LHS) models. When the available data do not allow to exclude arbitrary LHS models, it may still be possible to exclude LHS models with a specific separability structure. Here, we derive experimentally feasible criteria that put quantitative bounds on the multipartite entanglement of LHS. Our results reveal that separable states may contain hidden entanglement that can be unlocked by measurements on another system, even if no steering between the two systems is possible
Normalized Gaussian Path Integrals
Path integrals play a crucial role in describing the dynamics of physical
systems subject to classical or quantum noise. In fact, when correctly
normalized, they express the probability of transition between two states of
the system. In this work, we show a consistent approach to solve conditional
and unconditional Euclidean (Wiener) Gaussian path integrals that allow us to
compute transition probabilities in the semi-classical approximation from the
solutions of a system of linear differential equations. Our method is
particularly useful for investigating Fokker-Planck dynamics, and the physics
of string-like objects such as polymers. To give some examples, we derive the
time evolution of the d-dimensional Ornstein-Uhlenbeck process, and of the Van
der Pol oscillator driven by white noise. Moreover, we compute the end-to-end
transition probability for a charged string at thermal equilibrium, when an
external field is applied.Comment: 15 pages (7 of appendices), one figur
Does a large quantum Fisher information imply Bell correlations?
The quantum Fisher information (QFI) of certain multipartite entangled
quantum states is larger than what is reachable by separable states, providing
a metrological advantage. Are these nonclassical correlations strong enough to
potentially violate a Bell inequality? Here, we present evidence from two
examples. First, we discuss a Bell inequality designed for spin-squeezed states
which is violated only by quantum states with a large QFI. Second, we relax a
well-known lower bound on the QFI to find the Mermin Bell inequality as a
special case. However, a fully general link between QFI and Bell correlations
is still open.Comment: 4 pages, minor edit
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