2,323 research outputs found
Quantum criticality of the sub-Ohmic spin-boson model within displaced Fock states
The spin-boson model is analytically studied using displaced Fock states
(DFS) without discretization of the continuum bath. In the orthogonal displaced
Fock basis, the ground-state wavefunction can be systematically improved in a
controllable way. Interestingly, the zeroth-order DFS reproduces exactly the
well known Silbey-Harris results. In the framework of the second-order DFS, the
magnetization and the entanglement entropy are exactly calculated. It is found
that the magnetic critical exponent is converged to in the whole
sub-Ohmic bath regime , compared with that by the exactly solvable
generalized Silbey-Harris ansatz. It is strongly suggested that the system with
sub-Ohmic bath is always above its upper critical dimension, in sharp contrast
with the previous findings. This is the first evidence of the violation of the
quantum-classical Mapping for .Comment: 8 pages, 4 figure
Improved Silbey-Harris polaron ansatz for the spin-boson model
In this paper, the well-known Silbey-Harris (SH) polaron ansatz for the
spin-boson model is improved by adding orthogonal displaced Fock states. The
obtained results for the ground state in all baths converge very quickly within
finite displaced Fock states and corresponding SH results are corrected
considerably. Especially for the sub-Ohmic spin-boson model, the converging
results are obtained for 0 < s < 1/2 in the fourth-order correction and very
accurate critical coupling strengths of the quantum phase transition are
achieved. Converging magnetization in the biased spin-boson model is also
arrived at. Since the present improved SH ansatz can yield very accurate, even
almost exact results, it should have wide applications and extensions in
various spin-boson model and related fields.Comment: 9 pages, 6 figures. arXiv admin note: substantial text overlap with
arXiv:1410.099
Concise analytic solutions to the quantum Rabi model with two arbitrary qubits
Using extended coherent states, an analytical exact study has been carried
out for the quantum Rabi model (QRM) with two arbitrary qubits in a very
concise way. The -functions with determinants are generally
derived. For the same coupling constants, the simplest -function, resembling
that in the one-qubit QRM, can be obtained. Zeros of the -function yield the
whole regular spectrum. The exceptional eigenvalues, which do not belong to the
zeros of the function, are obtained in the closed form. The Dark states in
the case of the same coupling can be detected clearly in a continued-fraction
technique. The present concise solution is conceptually clear and practically
feasible to the general two-qubit QRM and therefore has many applications.Comment: 13 pages, 3 figure
Exact solvability, non-integrability, and genuine multipartite entanglement dynamics of the Dicke model
In this paper, the finite size Dicke model of arbitrary number of qubits is
solved analytically in an unified way within extended coherent states. For the
or Dicke models ( is an integer), the -function, which is
only an energy dependent determinant, is derived in a transparent
manner. The regular spectrum is completely and uniquely given by stable zeros
of the -function. The closed-form exceptional eigenvalues are also derived.
The level distribution controlled by the pole structure of the -functions
suggests non-integrability for model at any finite coupling in the sense
of recent criterion in literature. A preliminary application to the exact
dynamics of genuine multipartite entanglement in the finite Dicke model is
presented using the obtained exact solutions.Comment: 18 pages, 5 figure
A better method to determine the stability region of an L-stable implicit-explicit Runge-Kutta scheme
We propose a better method to determine the stability region of an L-stable
implicit-explicit Runge-Kutta scheme. This method always provides the correct
result, while other methods sometimes give wrong result. It is useful in the
analysis for implicit-explicit Runge-Kutta schemes.
Keywords: Stability region, Implicit-explicit (IMEX) schem
Quantum phase transitions in the spin-boson model without the counterrotating terms
We study the spin-boson model without the counterrotating terms by a
numerically exact method based on variational matrix product states.
Surprisingly, the second-order quantum phase transition (QPT) is observed for
the sub-Ohmic bath in the rotating-wave approximations. Moreover, first-order
QPTs can also appear before the critical points. With the decrease of the bath
exponents, these first-order QPTs disappear successively, while the
second-order QPT remains robust. The second-order QPT is further confirmed by
multi-coherent-states variational studies, while the first-order QPT is
corroborated with the exact diagonalization in the truncated Hilbert space.
Extension to the Ohmic bath is also performed, and many first-order QPTs appear
successively in a wide coupling regime, in contrast to previous findings. The
previous pictures for many physical phenomena for the spin-boson model in the
rotating-wave approximation have to be modified at least at the strong
coupling.Comment: 10 pages, 10 figure
Zeno effect of the open quantum system in the presence of 1/f noise
We study the quantum Zeno effect (QZE) and quantum anti-Zeno effect (QAZE) in
a two-level system(TLS) interacting with an environment owning 1/f noise. Using
a numerically exact method based on the thermo field dynamics(TFD) theory and
the matrix product states(MPS), we obtain exact evolutions of the TLS and
bath(environment) under repetitive measurements at both zero and finite
temperatures. At zero temperature, we observe a novel transition from a pure
QZE in the short time scale to a QZE-QAZE crossover in the long time scale, by
considering the measurement induced non-Markvoian effect. At finite
temperature, we exploit that the thermal fluctuation suppresses the decay of
the survival probability in the short time scale, whereas it enhances the decay
in the long time scale.Comment: 9 pages, 6 figure
Rich phase diagram of quantum phases in the anisotropic subohmic spin-boson model
We study the anisotropic spin-boson model (SBM) with the subohmic bath by a
numerically exact method based on variational matrix product states. A rich
phase diagram is found in the anisotropy-coupling strength plane by calculating
several observables. There are three distinct quantum phases: a delocalized
phase with even parity (phase I), a delocalized phase with odd parity (phase
II), and a localized phase with broken symmetry (phase III), which
intersect at a quantum tricritical point. The competition between those phases
would give overall picture of the phase diagram. For small power of the
spectral function of the bosonic bath, the quantum phase transition (QPT) from
phase I to III with mean-field critical behavior is present, similar to the
isotropic SBM. The novel phase diagram full with three different phases can be
found at large power of the spectral function: For highly anisotropic case, the
system experiences the QPTs from phase I to II via 1st-order, and then to the
phase III via 2nd-order with the increase of the coupling strength. For low
anisotropic case, the system only experiences the continuous QPT from phase I
to phase III with the non-mean-field critical exponents. Very interestingly, at
the moderate anisotropy, the system would display the continuous QPTs for
several times but with the same critical exponents. This unusual reentrance to
the same localized phase is discovered in the light-matter interacting systems.
The present study on the anisotropic SBM could open an avenue to the rich
quantum criticality.Comment: 9 pages, 6 figure
Cosmological model-independent test of CDM with two-point diagnostic by the observational Hubble parameter data
Aiming at exploring the nature of dark energy (DE), we use forty-three
observational Hubble parameter data (OHD) in the redshift range to make a cosmological model-independent test of the
CDM model with two-point diagnostic. In
CDM model, with equation of state (EoS) , two-point diagnostic
relation is tenable, where is the
present matter density parameter, and is the Hubble parameter divided by
100 . We utilize two methods: the weighted mean and
median statistics to bin the OHD to increase the signal-to-noise ratio of the
measurements. The binning methods turn out to be promising and considered to be
robust. By applying the two-point diagnostic to the binned data, we find that
although the best-fit values of fluctuate as the continuous redshift
intervals change, on average, they are continuous with being constant within 1
confidence interval. Therefore, we conclude that the CDM
model cannot be ruled out.Comment: 14 pages, 7 figures. arXiv admin note: text overlap with
arXiv:1507.0251
Quantum Zeno effect in the multimode quantum Rabi model
We study the quantum Zeno effect (QZE) and quantum anti-Zeno effect (QAZE) of
the multimode quantum Rabi model(MQRM). We derive an analytic expression for
the decay rate of the survival probability where cavity modes are initially
prepared as thermal equilibrium states. A crossover from QZE to QAZE is
observed due to the energy backflow induced by high frequency cavity modes. In
addition, we apply a numerically exact method based on the thermofield
dynamics(TFD) theory and the matrix product states(MPS) to study the effect of
squeezing of the cavity modes on the QZE of the MQRM. The influence of the
squeezing angle, squeezing strength and temperature on the decay rate of the
survival probability are discussed.Comment: 10 pages, 5 figure
- …