2,305 research outputs found
Real-Time Simulation of Large Open Quantum Spin Systems driven by Measurements
We consider a large quantum system with spins whose dynamics is
driven entirely by measurements of the total spin of spin pairs. This gives
rise to a dissipative coupling to the environment. When one averages over the
measurement results, the corresponding real-time path integral does not suffer
from a sign problem. Using an efficient cluster algorithm, we study the
real-time evolution of a 2-d Heisenberg antiferromagnet, which is driven to a
disordered phase, either by sporadic measurements or by continuous monitoring
described by Lindblad evolution.Comment: 5 pages, 7 figure
SO(3) "Nuclear Physics" with ultracold Gases
An ab initio calculation of nuclear physics from Quantum Chromodynamics
(QCD), the fundamental SU(3) gauge theory of the strong interaction, remains an
outstanding challenge. Here, we discuss the emergence of key elements of
nuclear physics using an SO(3) lattice gauge theory as a toy model for QCD. We
show that this model is accessible to state-of-the-art quantum simulation
experiments with ultracold atoms in an optical lattice. First, we demonstrate
that our model shares characteristic many-body features with QCD, such as the
spontaneous breakdown of chiral symmetry, its restoration at finite baryon
density, as well as the existence of few-body bound states. Then we show that
in the one-dimensional case, the dynamics in the gauge invariant sector can be
encoded as a spin S=3/2 Heisenberg model, i.e., as quantum magnetism, which has
a natural realization with bosonic mixtures in optical lattices, and thus sheds
light on the connection between non-Abelian gauge theories and quantum
magnetism.Comment: 34 pages, 9 figure
Atomic Quantum Simulation of U(N) and SU(N) Non-Abelian Lattice Gauge Theories
Using ultracold alkaline-earth atoms in optical lattices, we construct a
quantum simulator for U(N) and SU(N) lattice gauge theories with fermionic
matter based on quantum link models. These systems share qualitative features
with QCD, including chiral symmetry breaking and restoration at non-zero
temperature or baryon density. Unlike classical simulations, a quantum
simulator does not suffer from sign problems and can address the corresponding
chiral dynamics in real time.Comment: 12 pages, 5 figures. Main text plus one basic introduction to the
topic and one supplementary material on implementation. Final versio
Finite-Volume Energy Spectrum, Fractionalized Strings, and Low-Energy Effective Field Theory for the Quantum Dimer Model on the Square Lattice
We present detailed analytic calculations of finite-volume energy spectra,
mean field theory, as well as a systematic low-energy effective field theory
for the square lattice quantum dimer model. The analytic considerations explain
why a string connecting two external static charges in the confining columnar
phase fractionalizes into eight distinct strands with electric flux
. An emergent approximate spontaneously broken symmetry
gives rise to a pseudo-Goldstone boson. Remarkably, this soft phonon-like
excitation, which is massless at the Rokhsar-Kivelson (RK) point, exists far
beyond this point. The Goldstone physics is captured by a systematic low-energy
effective field theory. We determine its low-energy parameters by matching the
analytic effective field theory with exact diagonalization results and Monte
Carlo data. This confirms that the model exists in the columnar (and not in a
plaquette or mixed) phase all the way to the RK point.Comment: 35 pages, 16 figure
Two-dimensional Lattice Gauge Theories with Superconducting Quantum Circuits
A quantum simulator of U(1) lattice gauge theories can be implemented with
superconducting circuits. This allows the investigation of confined and
deconfined phases in quantum link models, and of valence bond solid and spin
liquid phases in quantum dimer models. Fractionalized confining strings and the
real-time dynamics of quantum phase transitions are accessible as well. Here we
show how state-of-the-art superconducting technology allows us to simulate
these phenomena in relatively small circuit lattices. By exploiting the strong
non-linear couplings between quantized excitations emerging when
superconducting qubits are coupled, we show how to engineer gauge invariant
Hamiltonians, including ring-exchange and four-body Ising interactions. We
demonstrate that, despite decoherence and disorder effects, minimal circuit
instances allow us to investigate properties such as the dynamics of electric
flux strings, signaling confinement in gauge invariant field theories. The
experimental realization of these models in larger superconducting circuits
could address open questions beyond current computational capability.Comment: Published versio
Antiferromagnetically coupled CoFeB/Ru/CoFeB trilayers
This work reports on the magnetic interlayer coupling between two amorphous
CoFeB layers, separated by a thin Ru spacer. We observe an antiferromagnetic
coupling which oscillates as a function of the Ru thickness x, with the second
antiferromagnetic maximum found for x=1.0 to 1.1 nm. We have studied the
switching of a CoFeB/Ru/CoFeB trilayer for a Ru thickness of 1.1 nm and found
that the coercivity depends on the net magnetic moment, i.e. the thickness
difference of the two CoFeB layers. The antiferromagnetic coupling is almost
independent on the annealing temperatures up to 300 degree C while an annealing
at 350 degree C reduces the coupling and increases the coercivity, indicating
the onset of crystallization. Used as a soft electrode in a magnetic tunnel
junction, a high tunneling magnetoresistance of about 50%, a well defined
plateau and a rectangular switching behavior is achieved.Comment: 3 pages, 3 figure
Random field spin models beyond one loop: a mechanism for decreasing the lower critical dimension
The functional RG for the random field and random anisotropy O(N)
sigma-models is studied to two loop. The ferromagnetic/disordered (F/D)
transition fixed point is found to next order in d=4+epsilon for N > N_c
(N_c=2.8347408 for random field, N_c=9.44121 for random anisotropy). For N <
N_c the lower critical dimension plunges below d=4: we find two fixed points,
one describing the quasi-ordered phase, the other is novel and describes the
F/D transition. The lower critical dimension can be obtained in an
(N_c-N)-expansion. The theory is also analyzed at large N and a glassy regime
is found.Comment: 4 pages, 5 figure
Stability and distortions of liquid crystal order in a cell with a heterogeneous substrate
We study stability and distortions of liquid crystal nematic order in a cell
with a random heterogeneous substrate. Modeling this system as a bulk xy model
with quenched disorder confined to a surface, we find that nematic order is
marginally unstable to such surface pinning. We compute the length scale beyond
which nematic distortions become large and calculate orientational correlation
functions using the functional renormalization-group and matching methods,
finding universal logarithmic and double-logarithmic distortions in two and
three dimensions, respectively. We extend these results to a finite-thickness
liquid crystal cell with a second homogeneous substrate, detailing crossovers
as a function of random pinning strength and cell thickness. We conclude with
analysis of experimental signatures of these distortions in a conventional
crossed-polarizer-analyzer light microscopy.Comment: 27 pages, 15 figures, Published in PRE, with minor typos correcte
Atomic Quantum Simulation of Dynamical Gauge Fields coupled to Fermionic Matter: From String Breaking to Evolution after a Quench
Using a Fermi-Bose mixture of ultra-cold atoms in an optical lattice, we
construct a quantum simulator for a U(1) gauge theory coupled to fermionic
matter. The construction is based on quantum links which realize continuous
gauge symmetry with discrete quantum variables. At low energies, quantum link
models with staggered fermions emerge from a Hubbard-type model which can be
quantum simulated. This allows us to investigate string breaking as well as the
real-time evolution after a quench in gauge theories, which are inaccessible to
classical simulation methods.Comment: 14 pages, 5 figures. Main text plus one general supplementary
material and one basic introduction to the topic. Published versio
Systematic Low-Energy Effective Field Theory for Electron-Doped Antiferromagnets
In contrast to hole-doped systems which have hole pockets centered at , in lightly electron-doped antiferromagnets
the charged quasiparticles reside in momentum space pockets centered at
or . This has important consequences for
the corresponding low-energy effective field theory of magnons and electrons
which is constructed in this paper. In particular, in contrast to the
hole-doped case, the magnon-mediated forces between two electrons depend on the
total momentum of the pair. For the one-magnon exchange
potential between two electrons at distance is proportional to ,
while in the hole case it has a dependence. The effective theory
predicts that spiral phases are absent in electron-doped antiferromagnets.Comment: 25 pages, 7 figure
- …