513 research outputs found
Quantum Cellular Automata Pseudo-Random Maps
Quantum computation based on quantum cellular automata (QCA) can greatly
reduce the control and precision necessary for experimental implementations of
quantum information processing. A QCA system consists of a few species of
qubits in which all qubits of a species evolve in parallel. We show that, in
spite of its inherent constraints, a QCA system can be used to study complex
quantum dynamics. To this aim, we demonstrate scalable operations on a QCA
system that fulfill statistical criteria of randomness and explore which
criteria of randomness can be fulfilled by operators from various QCA
architectures. Other means of realizing random operators with only a few
independent operators are also discussed.Comment: 7 pages, 8 figures, submitted to PR
Hole-pair hopping in arrangements of hole-rich/hole-poor domains in a quantum antiferromagnet
We study the motion of holes in a doped quantum antiferromagnet in the
presence of arrangements of hole-rich and hole-poor domains such as the
stripe-phase in high- cuprates. When these structures form, it becomes
energetically favorable for single holes, pairs of holes or small bound-hole
clusters to hop from one hole-rich domain to another due to quantum
fluctuations. However, we find that at temperature of approximately 100 K, the
probability for bound hole-pair exchange between neighboring hole-rich regions
in the stripe phase, is one or two orders of magnitude larger than single-hole
or multi-hole droplet exchange. As a result holes in a given hole-rich domain
penetrate further into the antiferromagnetically aligned domains when they do
it in pairs. At temperature of about 100 K and below bound pairs of holes hop
from one hole-rich domain to another with high probability. Therefore our main
finding is that the presence of the antiferromagnetic hole-poor domains act as
a filter which selects, from the hole-rich domains (where holes form a
self-bound liquid), hole pairs which can be exchanged throughout the system.
This fluid of bound hole pairs can undergo a superfluid phase ordering at the
above mentioned temperature scale.Comment: Revtex, 6 two-column pages, 4 figure
Green's Function Monte Carlo for Lattice Fermions: Application to the t-J Model
We develop a general numerical method to study the zero temperature
properties of strongly correlated electron models on large lattices. The
technique, which resembles Green's Function Monte Carlo, projects the ground
state component from a trial wave function with no approximations. We use this
method to determine the phase diagram of the two-dimensional t-J model, using
the Maxwell construction to investigate electronic phase separation. The shell
effects of fermions on finite-sized periodic lattices are minimized by keeping
the number of electrons fixed at a closed-shell configuration and varying the
size of the lattice. Results obtained for various electron numbers
corresponding to different closed-shells indicate that the finite-size effects
in our calculation are small. For any value of interaction strength, we find
that there is always a value of the electron density above which the system can
lower its energy by forming a two-component phase separated state. Our results
are compared with other calculations on the t-J model. We find that the most
accurate results are consistent with phase separation at all interaction
strengths.Comment: 22 pages, 22 figure
Entanglement Generation of Nearly-Random Operators
We study the entanglement generation of operators whose statistical
properties approach those of random matrices but are restricted in some way.
These include interpolating ensemble matrices, where the interval of the
independent random parameters are restricted, pseudo-random operators, where
there are far fewer random parameters than required for random matrices, and
quantum chaotic evolution. Restricting randomness in different ways allows us
to probe connections between entanglement and randomness. We comment on which
properties affect entanglement generation and discuss ways of efficiently
producing random states on a quantum computer.Comment: 5 pages, 3 figures, partially supersedes quant-ph/040505
Quantum Fidelity Decay of Quasi-Integrable Systems
We show, via numerical simulations, that the fidelity decay behavior of
quasi-integrable systems is strongly dependent on the location of the initial
coherent state with respect to the underlying classical phase space. In
parallel to classical fidelity, the quantum fidelity generally exhibits
Gaussian decay when the perturbation affects the frequency of periodic phase
space orbits and power-law decay when the perturbation changes the shape of the
orbits. For both behaviors the decay rate also depends on initial state
location. The spectrum of the initial states in the eigenbasis of the system
reflects the different fidelity decay behaviors. In addition, states with
initial Gaussian decay exhibit a stage of exponential decay for strong
perturbations. This elicits a surprising phenomenon: a strong perturbation can
induce a higher fidelity than a weak perturbation of the same type.Comment: 11 pages, 11 figures, to be published Phys. Rev.
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