263 research outputs found
Dynamics of the entanglement spectrum in spin chains
We study the dynamics of the entanglement spectrum, that is the time
evolution of the eigenvalues of the reduced density matrices after a
bipartition of a one-dimensional spin chain. Starting from the ground state of
an initial Hamiltonian, the state of the system is evolved in time with a new
Hamiltonian. We consider both instantaneous and quasi adiabatic quenches of the
system Hamiltonian across a quantum phase transition. We analyse the Ising
model that can be exactly solved and the XXZ for which we employ the
time-dependent density matrix renormalisation group algorithm. Our results show
once more a connection between the Schmidt gap, i.e. the difference of the two
largest eigenvalues of the reduced density matrix and order parameters, in this
case the spontaneous magnetisation.Comment: 16 pages, 8 figures, comments are welcome! Version published in JSTAT
special issue on "Quantum Entanglement In Condensed Matter Physics
Andreev tunneling into a one-dimensional Josephson junction array
In this letter we consider Andreev tunneling between a normal metal and a one
dimensional Josephson junction array with finite-range Coulomb energy. The
characteristics strongly deviate from the classical linear Andreev
current. We show that the non linear conductance possesses interesting scaling
behavior when the chain undergoes a T=0 superconductor-insulator transition of
Kosterlitz-Thouless-Berezinskii type. When the chain has quasi-long range
order, the low lying excitation are gapless and the curves are power-law
(the linear relation is recovered when charging energy can be disregarded).
When the chain is in the insulating phase the Andreev current is blocked at a
threshold which is proportional to the inverse correlation length in the chain
(much lower than the Coulomb gap) and which vanishes at the transition point.Comment: 8 pages LATEX, 3 figures available upon reques
Coherent response of a low T_c Josephson junction to an ultrafast laser pulse
By irradiating with a single ultrafast laser pulse a superconducting
electrode of a Josephson junction it is possible to drive the quasiparticles
(qp's) distribution strongly out of equilibrium. The behavior of the Josephson
device can, thus, be modified on a fast time scale, shorter than the qp's
relaxation time. This could be very useful, in that it allows fast control of
Josephson charge qubits and, in general, of all Josephson devices. If the
energy released to the top layer contact of the junction is of the order
of , the coherence is not degradated, because the perturbation is
very fast. Within the framework of the quasiclassical Keldysh Green's function
theory, we find that the order parameter of decreases. We study the
perturbed dynamics of the junction, when the current bias is close to the
critical current, by integrating numerically its classical equation of motion.
The optical ultrafast pulse can produce switchings of the junction from the
Josephson state to the voltage state. The switches can be controlled by tuning
the laser light intensity and the pulse duration of the Josephson junction.Comment: 17 pages, 5 figure
Long-range Heisenberg models in quasi-periodically driven crystals of trapped ions
We introduce a theoretical scheme for the analog quantum simulation of
long-range XYZ models using current trapped-ion technology. In order to achieve
fully-tunable Heisenberg-type interactions, our proposal requires a
state-dependent dipole force along a single vibrational axis, together with a
combination of standard resonant and detuned carrier drivings. We discuss how
this quantum simulator could explore the effect of long-range interactions on
the phase diagram by combining an adiabatic protocol with the quasi-periodic
drivings and test the validity of our scheme numerically. At the isotropic
Heisenberg point, we show that the long-range Hamiltonian can be mapped onto a
non-linear sigma model with a topological term that is responsible for its
low-energy properties, and we benchmark our predictions with
Matrix-Product-State numerical simulations.Comment: closer to published versio
Thermal transport driven by charge imbalance in graphene in magnetic field, close to the charge neutrality point at low temperature: Non local resistance
Graphene grown epitaxially on SiC, close to the charge neutrality point
(CNP), in an orthogonal magnetic field shows an ambipolar behavior of the
transverse resistance accompanied by a puzzling longitudinal magnetoresistance.
When injecting a transverse current at one end of the Hall bar, a sizeable non
local transverse magnetoresistance is measured at low temperature. While Zeeman
spin effect seems not to be able to justify these phenomena, some dissipation
involving edge states at the boundaries could explain the order of magnitude of
the non local transverse magnetoresistance, but not the asymmetry when the
orientation of the orthogonal magnetic field is reversed. As a possible
contribution to the explanation of the measured non local magnetoresistance
which is odd in the magnetic field, we derive a hydrodynamic approach to
transport in this system, which involves particle and hole Dirac carriers, in
the form of charge and energy currents. We find that thermal diffusion can take
place on a large distance scale, thanks to long recombination times, provided a
non insulating bulk of the Hall bar is assumed, as recent models seem to
suggest in order to explain the appearance of the longitudinal resistance. In
presence of the local source, some leakage of carriers from the edges generates
an imbalance of carriers of opposite sign, which are separated in space by the
magnetic field and diffuse along the Hall bar generating a non local transverse
voltage.Comment: 25 pages, 12 figure
Boundary quantum critical phenomena with entanglement renormalization
We extend the formalism of entanglement renormalization to the study of
boundary critical phenomena. The multi-scale entanglement renormalization
ansatz (MERA), in its scale invariant version, offers a very compact
approximation to quantum critical ground states. Here we show that, by adding a
boundary to the scale invariant MERA, an accurate approximation to the critical
ground state of an infinite chain with a boundary is obtained, from which one
can extract boundary scaling operators and their scaling dimensions. Our
construction, valid for arbitrary critical systems, produces an effective chain
with explicit separation of energy scales that relates to Wilson's RG
formulation of the Kondo problem. We test the approach by studying the quantum
critical Ising model with free and fixed boundary conditions.Comment: 8 pages, 12 figures, for a related work see arXiv:0912.289
Simulation of two-dimensional quantum systems using a tree tensor network that exploits the entropic area law
This work explores the use of a tree tensor network ansatz to simulate the
ground state of a local Hamiltonian on a two-dimensional lattice. By exploiting
the entropic area law, the tree tensor network ansatz seems to produce
quasi-exact results in systems with sizes well beyond the reach of exact
diagonalisation techniques. We describe an algorithm to approximate the ground
state of a local Hamiltonian on a L times L lattice with the topology of a
torus. Accurate results are obtained for L={4,6,8}, whereas approximate results
are obtained for larger lattices. As an application of the approach, we analyse
the scaling of the ground state entanglement entropy at the quantum critical
point of the model. We confirm the presence of a positive additive constant to
the area law for half a torus. We also find a logarithmic additive correction
to the entropic area law for a square block. The single copy entanglement for
half a torus reveals similar corrections to the area law with a further term
proportional to 1/L.Comment: Major rewrite, new version published in Phys. Rev. B with highly
improved numerical results for the scaling of the entropies and several new
sections. The manuscript has now 19 pages and 30 Figure
Matrix product states for critical spin chains: finite size scaling versus finite entanglement scaling
We investigate the use of matrix product states (MPS) to approximate ground
states of critical quantum spin chains with periodic boundary conditions (PBC).
We identify two regimes in the (N,D) parameter plane, where N is the size of
the spin chain and D is the dimension of the MPS matrices. In the first regime
MPS can be used to perform finite size scaling (FSS). In the complementary
regime the MPS simulations show instead the clear signature of finite
entanglement scaling (FES). In the thermodynamic limit (or large N limit), only
MPS in the FSS regime maintain a finite overlap with the exact ground state.
This observation has implications on how to correctly perform FSS with MPS, as
well as on the performance of recent MPS algorithms for systems with PBC. It
also gives clear evidence that critical models can actually be simulated very
well with MPS by using the right scaling relations; in the appendix, we give an
alternative derivation of the result of Pollmann et al. [Phys. Rev. Lett. 102,
255701 (2009)] relating the bond dimension of the MPS to an effective
correlation length.Comment: 18 pages, 13 figure
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