5,558 research outputs found
Creation of Two-Particle Entanglement in Open Macroscopic Quantum Systems
We consider an open quantum system of N not directly interacting spins
(qubits) in contact with both local and collective thermal environments. The
qubit-environment interactions are energy conserving. We trace out the
variables of the thermal environments and N-2 qubits to obtain the
time-dependent reduced density matrix for two arbitrary qubits. We numerically
simulate the reduced dynamics and the creation of entanglement (concurrence) as
a function of the parameters of the thermal environments and the number of
qubits, N. Our results demonstrate that the two-qubit entanglement generally
decreases as N increases. We show analytically that in the limit N tending to
infinity, no entanglement can be created. This indicates that collective
thermal environments cannot create two-qubit entanglement when many qubits are
located within a region of the size of the environment coherence length. We
discuss possible applications of our approach to the development of a new
quantum characterization of noisy environments
Revisiting noninteracting string partition functions in Rindler space
We revisit non-interacting string partition functions in Rindler space by
summing over fields in the spectrum. In field theory, the total partition
function splits in a natural way in a piece that does not contain surface terms
and a piece consisting of solely the so-called edge states. For open strings,
we illustrate that surface contributions to the higher spin fields correspond
to open strings piercing the Rindler origin, unifying the higher spin surface
contributions in string language. For closed strings, we demonstrate that the
string partition function is not quite the same as the sum over the partition
functions of the fields in the spectrum: an infinite overcounting is present
for the latter. Next we study the partition functions obtained by excluding the
surface terms. Using recent results of JHEP 1505 (2015) 106, this construction,
first done by Emparan, can be put on much firmer ground. We generalize to type
II and heterotic superstrings and demonstrate modular invariance. All of these
exhibit an IR divergence that can be interpreted as a maximal acceleration
close to the black hole horizon. Ultimately, since these partition functions
are only part of the full story, divergences here should not be viewed as a
failure of string theory: maximal acceleration is a feature of a faulty
treatment of the higher spin fields in the string spectrum. We comment on the
relevance of this to Solodukhin's recent proposal. A possible link with the
firewall paradox is apparent.Comment: 33 pages, v2: added several clarifications including a section on the
difference between closed strings and the sum-of-fields approach, matches
published versio
Thermal entanglement in a triple quantum dot system
We present studies of thermal entanglement of a three-spin system in
triangular symmetry. Spin correlations are described within an effective
Heisenberg Hamiltonian, derived from the Hubbard Hamiltonian, with
super-exchange couplings modulated by an effective electric field. Additionally
a homogenous magnetic field is applied to completely break the degeneracy of
the system. We show that entanglement is generated in the subspace of doublet
states with different pairwise spin correlations for the ground and excited
states. At low temperatures thermal mixing between the doublets with the same
spin destroys entanglement, however one can observe its restoration at higher
temperatures due to the mixing of the states with an opposite spin orientation
or with quadruplets (unentangled states) always destroys entanglement. Pairwise
entanglement is quantified using concurrence for which analytical formulae are
derived in various thermal mixing scenarios. The electric field plays a
specific role -- it breaks the symmetry of the system and changes spin
correlations. Rotating the electric field can create maximally entangled qubit
pairs together with a separate spin (monogamy) that survives in a relatively
wide temperature range providing robust pairwise entanglement generation at
elevated temperatures.Comment: 9 pages, 5 figures, accepted in Eur. Phys. J.
Production and detection of three-qubit entanglement in the Fermi sea
Building on a previous proposal for the entanglement of electron-hole pairs
in the Fermi sea, we show how 3 qubits can be entangled without using
electron-electron interactions. As in the 2-qubit case, this electronic scheme
works even if the sources are in (local) thermal equilibrium -- in contrast to
the photonic analogue. The 3 qubits are represented by 4 edge-channel
excitations in the quantum Hall effect (2 hole excitations plus 2 electron
excitations with identical channel index). The entangler consists of an
adiabatic point contact flanked by a pair of tunneling point contacts. The
irreducible 3-qubit entanglement is characterized by the tangle, which is
expressed in terms of the transmission matrices of the tunneling point
contacts. The maximally entangled Greenberger-Horne-Zeilinger (GHZ) state is
obtained for channel-independent tunnel probabilities. We show how
low-frequency noise measurements can be used to determine an upper and lower
bound to the tangle. The bounds become tighter the closer the electron-hole
state is to the GHZ state.Comment: 8 pages including 4 figures; [2017: fixed broken postscript figures
Dynamic entanglement in oscillating molecules and potential biological implications
We demonstrate that entanglement can persistently recur in an oscillating
two-spin molecule that is coupled to a hot and noisy environment, in which no
static entanglement can survive. The system represents a non-equilibrium
quantum system which, driven through the oscillatory motion, is prevented from
reaching its (separable) thermal equilibrium state. Environmental noise,
together with the driven motion, plays a constructive role by periodically
resetting the system, even though it will destroy entanglement as usual. As a
building block, the present simple mechanism supports the perspective that
entanglement can exist also in systems which are exposed to a hot environment
and to high levels of de-coherence, which we expect e.g. for biological
systems. Our results furthermore suggest that entanglement plays a role in the
heat exchange between molecular machines and environment. Experimental
simulation of our model with trapped ions is within reach of the current
state-of-the-art quantum technologies.Comment: Extended version, including supplementary information. 9 pages, 8
figure
Robust Entanglement in Anti-ferromagnetic Heisenberg Chains by Single-spin Optimal Control
We demonstrate how near-perfect entanglement (in fact arbitrarily close to
maximal entanglement) can be generated between the end spins of an
anti-ferromagnetic isotropic Heisenberg chain of length , starting from the
ground state in the excitation subspace, by applying a magnetic field
along a given direction, acting on a single spin only. Temporally optimal
magnetic fields to generate a singlet pair between the two end spins of the
chain are calculated for chains up to length 20 using optimal control theory.
The optimal fields are shown to remain effective in various non-ideal
situations including thermal fluctuations, magnetic field leakage, random
system couplings and decoherence. Furthermore, the quality of the entanglement
generated can be substantially improved by taking these imperfections into
account in the optimization. In particular, the optimal pulse of a given
thermal initial state is also optimal for any other initial thermal state with
lower temperature.Comment: 10 pages, revte
Many-body localization: an introduction and selected topics
What happens in an isolated quantum system when both disorder and
interactions are present? Over the recent years, the picture of a
non-thermalizing phase of matter, the many-localized phase, has emerged as a
stable solution. We present a basic introduction to the topic of many-body
localization, using the simple example of a quantum spin chain which allows us
to illustrate several of the properties of this phase. We then briefly review
the current experimental research efforts probing this physics. The largest
part of this review is a selection of more specialized questions, some of which
are currently under active investigation. We conclude by summarizing the
connections between many-body localization and quantum simulations.Comment: Review article. 28 pages, 8 figures, Comptes Rendus Physique (2018
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