804 research outputs found
Energetic Suppression of Decoherence in Exchange-Only Quantum Computation
Universal quantum computation requiring only the Heisenberg exchange
interaction and suppressing decoherence via an energy gap is presented. The
combination of an always-on exchange interaction between the three physical
qubits comprising the encoded qubit and a global magnetic field generates an
energy gap between the subspace of interest and all other states. This energy
gap suppresses decoherence. Always-on exchange couplings greatly simplify
hardware specifications and the implementation of inter-logical-qubit gates. A
controlled phase gate can be implemented using only three Heisenberg exchange
operations all of which can be performed simultaneously.Comment: 4 pages, 4 figure
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
Theory of the Reentrant Charge-Order Transition in the Manganites
A theoretical model for the reentrant charge-order transition in the
manganites is examined. This transition is studied with a purely electronic
model for the Mn electrons: the extended Hubbard model. The electron-phonon
coupling results in a large nearest-neighbor repulsion between electrons. Using
a finite-temperature Lanczos technique, the model is diagonalized on a 16-site
periodic cluster to calculate the temperature-dependent phase boundary between
the charge-ordered and homogeneous phases. A reentrant transition is found. The
results are discussed with respect to the specific topology of the 16-site
cluster.Comment: 3 pages, 2 ps figures included in text, submitted to the 8th
MMM-Intermag conferenc
A Scalable Architecture for Coherence-Preserving Qubits
We propose scalable architectures for the coherence-preserving qubits
introduced by Bacon, Brown, and Whaley [Phys. Rev. Lett. {\bf 87}, 247902
(2001)]. These architectures employ extra qubits providing additional degrees
of freedom to the system. We show that these extra degrees of freedom can be
used to counter errors in coupling strength within the coherence-preserving
qubit and to combat interactions with environmental qubits. The presented
architectures incorporate experimentally viable methods for inter-logical-qubit
coupling and can implement a controlled phase gate via three simultaneous
Heisenberg exchange operations. The extra qubits also provide flexibility in
the arrangement of the physical qubits. Specifically, all physical qubits of a
coherent-preserving qubit lattice can be placed in two spatial dimensions. Such
an arrangement allows for universal cluster state computation.Comment: 4 pages, 4 figure
Ferroelectricity in strained CaSrTiO from first principles
We present a density functional theory investigation of strained
CaSrTiO. We have determined the structure and polarization
for a number of arrangements of Ca and Sr in a 222 supercell.
The a and b lattice vectors are strained to match the lattice constants of the
rotated Si(001) face. To set the context for the CSTO study, we also include
simulations of the Si(001) constrained structures for CaTiO and SrTiO.
Our primary findings are that all CaSrTiO structures
examined except one are ferroelectric, exhibiting polarizations ranging from
0.08 C/m for the lowest energy configuration to about 0.26 C/m for the
higher energy configurations. We find that the configurations with larger
polarizations have lower c/a ratios. The net polarization of the cell is the
result of Ti-O ferroelectric displacements regulated by A-site cations.Comment: 13 pages, 4 figure
Multiple time scale blinking in InAs quantum dot single-photon sources
We use photon correlation measurements to study blinking in single,
epitaxially-grown self-assembled InAs quantum dots situated in circular Bragg
grating and microdisk cavities. The normalized second-order correlation
function g(2)(\tau) is studied across eleven orders of magnitude in time, and
shows signatures of blinking over timescales ranging from tens of nanoseconds
to tens of milliseconds. The g(2)(\tau) data is fit to a multi-level system
rate equation model that includes multiple non-radiating (dark) states, from
which radiative quantum yields significantly less than 1 are obtained. This
behavior is observed even in situations for which a direct histogramming
analysis of the emission time-trace data produces inconclusive results
Luttinger Liquid Instability in the One Dimensional t-J Model
We study the t-J model in one dimension by numerically projecting the true
ground state from a Luttinger liquid trial wave function. We find the model
exhibits Luttinger liquid behavior for most of the phase diagram in which
interaction strength and density are varied. However at small densities and
high interaction strengths a new phase with a gap to spin excitations and
enhanced superconducting correlations is found. We show this phase is a
Luther-Emery liquid and study its correlation functions.Comment: REVTEX, 11 pages. 4 Figures available on request from
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