242 research outputs found
Surface code fidelity at finite temperatures
We study the dependence of the fidelity of the surface code in the presence
of a single finite-temperature massless bosonic environment after a quantum
error correction cycle. The three standard types of environment are considered:
super-Ohmic, Ohmic, and sub-Ohmic. Our results show that, for regimes relevant
to current experiments, quantum error correction works well even in the
presence of environment-induced, long-range inter-qubit interactions. A
threshold always exists at finite temperatures, although its temperature
dependence is very sensitive to the type of environment. For the super-Ohmic
case, the critical coupling constant separating high- from low-fidelity
decreases with increasing temperature. For both Ohmic and super-Ohmic cases,
the dependence of the critical coupling on temperature is weak. In all cases,
the critical coupling is determined by microscopic parameters of the
environment. For the sub-Ohmic case, it also depends strongly on the duration
of the QEC cycle.Comment: 13 pages, 6 figure
Surface Code Threshold in the Presence of Correlated Errors
We study the fidelity of the surface code in the presence of correlated
errors induced by the coupling of physical qubits to a bosonic environment. By
mapping the time evolution of the system after one quantum error correction
cycle onto a statistical spin model, we show that the existence of an error
threshold is related to the appearance of an order-disorder phase transition in
the statistical model in the thermodynamic limit. This allows us to relate the
error threshold to bath parameters and to the spatial range of the correlated
errors.Comment: 5 pages, 2 figure
Adiabatic Charge Pumping through Quantum Dots in the Coulomb Blockade Regime
We investigate the influence of the Coulomb interaction on the adiabatic
pumping current through quantum dots. Using nonequilibrium Green's functions
techniques, we derive a general expression for the current based on the
instantaneous Green's function of the dot. We apply this formula to study the
dependence of the charge pumped per cycle on the time-dependent pumping
potentials. The possibility of charge quantization in the presence of a finite
Coulomb repulsion energy is investigated in the light of recent experiments.Comment: 11 pages, 10 figure
Chaos in one-dimensional lattices under intense laser fields
A model is investigated where a monochromatic, spatially homogeneous laser
field interacts with an electron in a one-dimensional periodic lattice. The
classical Hamiltonian is presented and the technique of stroboscopic maps is
used to study the dynamical behavior of the model. The electron motion is found
to be completely regular only for small field amplitudes, developing a larger
chaotic region as the amplitude increases. The quantum counterpart of the
classical Hamiltonian is derived. Exact numerical diagonalizations show the
existence of universal, random-matrix fluctuations in the electronic energy
bands dressed by the laser field. A detailed analysis of the classical phase
space is compatible with the statistical spectral analysis of the quantum
model. The application of this model to describe transport and optical
absorption in semiconductor superlattices submitted to intense infrared laser
radiation is proposed.Comment: 9 pages, RevTex 3.0, EPSF (6 figures), to appear in Europhys. J.
Theoretical Analysis of the Reduction of Neel Temperature in La(CuZn(or Mg)O
Using Tyablikov's decoupling approximation, we calculate the initial
suppression rate of the Neel temperature, , in a quasi two-dimensional diluted Heisenberg antiferromagnet with
nonmagnetic impurities of concentration . In order to explain an
experimental fact that of the Zn-substitution is different
from of the Mg-substitution, we propose a model in which
impurity substitution reduces the intra-plane exchange couplings surrounding
impurities, as well as dilution of spin systems. The decrease of 12% in
exchange coupling constants by Zn substitution and decrease of 6% by Mg
substitution explain those two experimental results, when an appropriate value
of the interplane coupling is used.Comment: 2 pages, 3 figure
RKKY Interactions in Graphene: Dependence on Disorder and Gate Voltage
We report the dependence of Ruderman-Kittel-Kasuya-Yoshida\,(RKKY)
interaction on nonmagmetic disorder and gate voltage in grapheme. First the
semiclassical method is employed to reserve the expression for RKKY interaction
in clean graphene. Due to the pseudogap at Dirac point, the RKKY coupling in
undoped grapheme is found to be proportional to . Next, we investigate
how the RKKY interaction depends on nonmagnetic disorder strength and gate
voltage by studying numerically the Anderson tight-binding model on a honeycomb
lattice. We observe that the RKKY interaction along the armchair direction is
more robust to nonmagnetic disorder than in other directions. This effect can
be explained semiclassically: The presence of multiple shortest paths between
two lattice sites in the armchair directions is found to be responsible for the
reduceddisorder sensitivity. We also present the distribution of the RKKY
interaction for the zigzag and armchair directions. We identify three different
shapes of the distributions which are repeated periodically along the zigzag
direction, while only one kind, and more narrow distribution, is observed along
the armchair direction. Moreover, we find that the distribution of amplitudes
of the RKKY interaction crosses over from a non-Gaussian shape with very long
tails to a completely log-normal distribution when increasing the nonmagnetic
disorder strength. The width of the log-normal distribution is found to
linearly increase with the strength of disorder, in agreement with analytical
predictions. At finite gate voltage near the Dirac point, Friedel oscillation
appears in addition to the oscillation from the interference between two Dirac
points. This results in a beating pattern. We study how these beating patterns
are effected by the nonmagnetic disorder in doped graphene
The role of the disorder range and electronic energy in the graphene nanoribbons perfect transmission
Numerical calculations based on the recursive Green's functions method in the
tight-binding approximation are performed to calculate the dimensionless
conductance in disordered graphene nanoribbons with Gaussian scatterers.
The influence of the transition from short- to long-ranged disorder on is
studied as well as its effects on the formation of a perfectly conducting
channel. We also investigate the dependence of electronic energy on the
perfectly conducting channel. We propose and calculate a backscattering
estimative in order to establish the connection between the perfectly
conducting channel (with ) and the amount of intervalley scattering.Comment: 7 pages, 9 figures. To be published on Phys. Rev.
RKKY Interaction in Disordered Graphene
We investigate the effects of nonmagnetic disorder on the
Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction in graphene by studying
numerically the Anderson model with on-site and hopping disorder on a honeycomb
lattice at half filling. We evaluate the strength of the interaction as a
function of the distance R between two magnetic ions, as well as their lattice
positions and orientations. In the clean limit, we find that the strength of
the interaction decays as 1/R^3, with its sign and oscillation amplitude
showing strong anisotropy. With increasing on-site disorder, the mean amplitude
decreases exponentially at distances exceeding the elastic mean free path. At
smaller distances, however, the oscillation amplitude increases strongly and
its sign changes on the same sublattice for all directions but the armchair
direction. For random hopping disorder, no sign change is observed. No
significant changes to the geometrical average values of the RKKY interaction
are found at small distances, while exponential suppression is observed at
distances exceeding the localization length.Comment: 4+\epsilon\ pages, 5 figure
Kondo-Anderson Transitions
Dilute magnetic impurities in a disordered Fermi liquid are considered close
to the Anderson metal-insulator transition (AMIT). Critical Power law
correlations between electron wave functions at different energies in the
vicinity of the AMIT result in the formation of pseudogaps of the local density
of states. Magnetic impurities can remain unscreened at such sites. We
determine the density of the resulting free magnetic moments in the zero
temperature limit. While it is finite on the insulating side of the AMIT, it
vanishes at the AMIT, and decays with a power law as function of the distance
to the AMIT. Since the fluctuating spins of these free magnetic moments break
the time reversal symmetry of the conduction electrons, we find a shift of the
AMIT, and the appearance of a semimetal phase. The distribution function of the
Kondo temperature is derived at the AMIT, in the metallic phase and in
the insulator phase. This allows us to find the quantum phase diagram in an
external magnetic field and at finite temperature . We calculate the
resulting magnetic susceptibility, the specific heat, and the spin relaxation
rate as function of temperature. We find a phase diagram with finite
temperature transitions between insulator, critical semimetal, and metal
phases. These new types of phase transitions are caused by the interplay
between Kondo screening and Anderson localization, with the latter being
shifted by the appearance of the temperature-dependent spin-flip scattering
rate. Accordingly, we name them Kondo-Anderson transitions (KATs).Comment: 18 pages, 9 figure
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