595 research outputs found
Renyi entropies as a measure of the complexity of counting problems
Counting problems such as determining how many bit strings satisfy a given
Boolean logic formula are notoriously hard. In many cases, even getting an
approximate count is difficult. Here we propose that entanglement, a common
concept in quantum information theory, may serve as a telltale of the
difficulty of counting exactly or approximately. We quantify entanglement by
using Renyi entropies S(q), which we define by bipartitioning the logic
variables of a generic satisfiability problem. We conjecture that
S(q\rightarrow 0) provides information about the difficulty of counting
solutions exactly, while S(q>0) indicates the possibility of doing an efficient
approximate counting. We test this conjecture by employing a matrix computing
scheme to numerically solve #2SAT problems for a large number of uniformly
distributed instances. We find that all Renyi entropies scale linearly with the
number of variables in the case of the #2SAT problem; this is consistent with
the fact that neither exact nor approximate efficient algorithms are known for
this problem. However, for the negated (disjunctive) form of the problem,
S(q\rightarrow 0) scales linearly while S(q>0) tends to zero when the number of
variables is large. These results are consistent with the existence of fully
polynomial-time randomized approximate algorithms for counting solutions of
disjunctive normal forms and suggests that efficient algorithms for the
conjunctive normal form may not exist.Comment: 13 pages, 4 figure
Numerical evaluation of the fidelity error threshold for the surface code
We study how the resilience of the surface code is affected by the coupling
to a non-Markovian environment at zero temperature. The qubits in the surface
code experience an effective dynamics due to the coupling to the environment
that induces correlations among them. The range of the effective induced
qubit-qubit interaction depends on parameters related to the environment and
the duration of the quantum error correction cycle. We show numerically that
different interaction ranges set different intrinsic bounds on the fidelity of
the code. These bounds are unrelated to the error thresholds based on
stochastic error models. We introduce a definition of stabilizers based on
logical operators that allows us to efficiently implement a Metropolis
algorithm to determine upper bounds to the fidelity error threshold
A microscopic formulation of dynamical spin injection in ferromagnetic-nonmagnetic heterostructures
We develop a microscopic formulation of dynamical spin injection in
heterostructure comprising nonmagnetic metals in contact with ferromagnets. The
spin pumping current is expressed in terms of Green's function of the
nonmagnetic metal attached to the ferromagnet where a precessing magnetization
is induced. The formulation allows for the inclusion of spin-orbit coupling and
disorder. The Green's functions involved in the expression for the current are
expressed in real-space lattice coordinates and can thus be efficiently
computed using recursive methods.Comment: 18 pages, 6 figure
Virtual Parallel Computing and a Search Algorithm using Matrix Product States
We propose a form of parallel computing on classical computers that is based
on matrix product states. The virtual parallelization is accomplished by
representing bits with matrices and by evolving these matrices from an initial
product state that encodes multiple inputs. Matrix evolution follows from the
sequential application of gates, as in a logical circuit. The action by
classical probabilistic one-bit and deterministic two-bit gates such as NAND
are implemented in terms of matrix operations and, as opposed to quantum
computing, it is possible to copy bits. We present a way to explore this method
of computation to solve search problems and count the number of solutions. We
argue that if the classical computational cost of testing solutions (witnesses)
requires less than local two-bit gates acting on bits, the search
problem can be fully solved in subexponential time. Therefore, for this
restricted type of search problem, the virtual parallelization scheme is faster
than Grover's quantum algorithmComment: 4 pages, 1 figure (published version
Disorder and Electronic Transport in Graphene
In this review, we provide an account of the recent progress in understanding
electronic transport in disordered graphene systems. Starting from a
theoretical description that emphasizes the role played by band structure
properties and lattice symmetries, we describe the nature of disorder in these
systems and its relation to transport properties. While the focus is primarily
on theoretical and conceptual aspects, connections to experiments are also
included. Issues such as short versus long-range disorder, localization (strong
and weak), the carrier density dependence of the conductivity, and conductance
fluctuations are considered and some open problems are pointed out.Comment: 18 pages, 5 figures, Topical Revie
The recursive Green's function method for graphene
We describe how to apply the recursive Green's function method to the
computation of electronic transport properties of graphene sheets and
nanoribbons in the linear response regime. This method allows for an amenable
inclusion of several disorder mechanisms at the microscopic level, as well as
inhomogeneous gating, finite temperature, and, to some extend, dephasing. We
present algorithms for computing the conductance, density of states, and
current densities for armchair and zigzag atomic edge alignments. Several
numerical results are presented to illustrate the usefulness of the method.Comment: 26 pages, 15 figures; submitted to Journal of Computational
Electronics (special issue on graphene
The single-electron transport in a three-ion magnetic molecule modulated by a transverse field
We study single-electron transport in a three-ion molecule with strong
uniaxial anisotropy and in the presence of a transverse magnetic field. Two
magnetic ions are connected to each other through a third, nonmagnetic ion. The
magnetic ions are coupled to ideal metallic leads and a back gate voltage is
applied to the molecule, forming a field-effect transistor. The microscopic
Hamiltonian describing this system includes inter-ion hopping, on-site
repulsions, and magnetic anisotropies. For a range of values of the parameters
of the Hamiltonian, we obtain an energy spectrum similar to that of
single-molecule magnets in the giant-spin approximation where the two states
with maximum spin projection along the uniaxial anisotropy axis are well
separated from other states. In addition, upon applying an external in-plane
magnetic field, the energy gap between the ground and first excited states of
the molecule oscillates, going to zero at certain special values of the field,
in analogy to the diabolical points resulting from Berry phase interference in
the giant spin model. Thus, our microscopic model provides the same
phenomenological behavior expected from the giant spin model of a
single-molecule magnet but with direct access to the internal structure of the
molecule, thus making it more appropriate for realistic electronic transport
studies. To illustrate this point, the nonlinear electronic transport in the
sequential tunneling regime is evaluated for values of the field near these
degeneracy points. We show that the existence of these points has a clear
signature in the I-V characteristics of the molecule, most notably the
modulation of excitation lines in the differential conductance.Comment: 10 pages, 13 figure
Fidelity of the surface code in the presence of a bosonic bath
We study the resilience of the surface code to decoherence caused by the
presence of a bosonic bath. This approach allows us to go beyond the standard
stochastic error model commonly used to quantify decoherence and error
threshold probabilities in this system. The full quantum mechanical system-bath
dynamics is computed exactly over one quantum error correction cycle. Since all
physical qubits interact with the bath, space-time correlations between errors
are taken into account. We compute the fidelity of the surface code as a
function of the quantum error correction time. The calculation allows us to map
the problem onto an Ising-like statistical spin model with two-body
interactions and a fictitious temperature which is related to the inverse bath
coupling constant. The model departs from the usual Ising model in the sense
that interactions can be long ranged and can involve complex exchange
couplings; in addition, the number of allowed configurations is restricted by
the syndrome extraction. Using analytical estimates and numerical calculations,
we argue that, in the limit of an infinite number of physical qubits, the spin
model sustain a phase transition which can be associated to the existence of an
error threshold in the surface code. An estimate of the transition point is
given for the case of nearest-neighbor interactions.Comment: 15 pages, 5 figure
Emergent irreversibility and entanglement spectrum statistics
We study the problem of irreversibility when the dynamical evolution of a
many-body system is described by a stochastic quantum circuit. Such evolution
is more general than a Hamiltonian one, and since energy levels are not well
defined, the well-established connection between the statistical fluctuations
of the energy spectrum and irreversibility cannot be made. We show that the
entanglement spectrum provides a more general connection. Irreversibility is
marked by a failure of a disentangling algorithm and is preceded by the
appearance of Wigner-Dyson statistical fluctuations in the entanglement
spectrum. This analysis can be done at the wave-function level and offers an
alternative route to study quantum chaos and quantum integrability.Comment: updated to published versio
Phonon Decoherence of a Double Quantum Dot Charge Qubit
We study decoherence of a quantum dot charge qubit due to coupling to
piezoelectric acoustic phonons in the Born-Markov approximation. After
including appropriate form factors, we find that phonon decoherence rates are
one to two orders of magnitude weaker than was previously predicted. We
calculate the dependence of the Q-factor on lattice temperature, quantum dot
size, and interdot coupling. Our results suggest that mechanisms other than
phonon decoherence play a more significant role in current experimental setups.Comment: RevTex, 7 pages, 5 figures. v2: appendix added, more details
provided. Accepted for publication in PR
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