255 research outputs found
Time-Reversal Symmetry and Universal Conductance Fluctuations in a Driven Two-Level System
In the presence of time-reversal symmetry, quantum interference gives strong
corrections to the electric conductivity of disordered systems. The
self-interference of an electron wavefunction traveling time-reversed paths
leads to effects such as weak localization and universal conductance
fluctuations. Here, we investigate the effects of broken time-reversal symmetry
in a driven artificial two-level system. Using a superconducting flux qubit, we
implement scattering events as multiple Landau-Zener transitions by driving the
qubit periodically back and forth through an avoided crossing. Interference
between different qubit trajectories give rise to a speckle pattern in the
qubit transition rate, similar to the interference patterns created when
coherent light is scattered off a disordered potential. Since the scattering
events are imposed by the driving protocol, we can control the time-reversal
symmetry of the system by making the drive waveform symmetric or asymmetric in
time. We find that the fluctuations of the transition rate exhibit a sharp peak
when the drive is time-symmetric, similar to universal conductance fluctuations
in electronic transport through mesoscopic systems
On the Complexity of Case-Based Planning
We analyze the computational complexity of problems related to case-based
planning: planning when a plan for a similar instance is known, and planning
from a library of plans. We prove that planning from a single case has the same
complexity than generative planning (i.e., planning "from scratch"); using an
extended definition of cases, complexity is reduced if the domain stored in the
case is similar to the one to search plans for. Planning from a library of
cases is shown to have the same complexity. In both cases, the complexity of
planning remains, in the worst case, PSPACE-complete
Characterization and benchmarking of a phase-sensitive two-qubit gate using direct digital synthesis
We implement an iSWAP gate with two transmon qubits using a flux-tunable
coupler. Precise control of the relative phase of the qubit-control pulses and
the parametric-coupler drive is achieved with a multi-channel instrument called
Presto using direct digital synthesis (DDS), a promising technique for scaling
up quantum systems. We describe the process of tuning and benchmarking the
iSWAP gate, where the relative phase of the pulses is controlled via software.
We perform the iSWAP gate in 290 ns, validate it with quantum-state tomography,
and measure 2\% error with interleaved randomized benchmarking
Compilability of Abduction
Abduction is one of the most important forms of reasoning; it has been
successfully applied to several practical problems such as diagnosis. In this
paper we investigate whether the computational complexity of abduction can be
reduced by an appropriate use of preprocessing. This is motivated by the fact
that part of the data of the problem (namely, the set of all possible
assumptions and the theory relating assumptions and manifestations) are often
known before the rest of the problem. In this paper, we show some complexity
results about abduction when compilation is allowed
Density-functional Study of Small Molecules within the Krieger-Li-Iafrate Approximation
We report density-functional studies of several small molecules (, and ) within the Krieger-Li-Iafrate (KLI)
approximation to the exact Kohn-Sham local exchange potential, using a
three-dimensional real-space finite-difference pseudopotential method. It is
found that exchange-only KLI leads to markedly improved eigenvalue spectra
compared to those obtained within the standard local-density approximation
(LDA), the generalized gradient approximation (GGA), and the Hartree-Fock (HF)
method. For structural properties, exchange-only KLI results are close to the
corresponding HF values. We find that the addition of LDA or GGA correlation
energy functionals to the KLI exact exchange energy functional does not lead to
systematic improvements.Comment: 16 pages including 1 fugure, to be published in Phys. Rev. A Nov. 1
'9
Driven dynamics and rotary echo of a qubit tunably coupled to a harmonic oscillator
We have investigated the driven dynamics of a superconducting flux qubit that
is tunably coupled to a microwave resonator. We find that the qubit experiences
an oscillating field mediated by off-resonant driving of the resonator, leading
to strong modifications of the qubit Rabi frequency. This opens an additional
noise channel, and we find that low-frequency noise in the coupling parameter
causes a reduction of the coherence time during driven evolution. The noise can
be mitigated with the rotary-echo pulse sequence, which, for driven systems, is
analogous to the Hahn-echo sequence
One-way multigrid method in electronic structure calculations
We propose a simple and efficient one-way multigrid method for
self-consistent electronic structure calculations based on iterative
diagonalization. Total energy calculations are performed on several different
levels of grids starting from the coarsest grid, with wave functions
transferred to each finer level. The only changes compared to a single grid
calculation are interpolation and orthonormalization steps outside the original
total energy calculation and required only for transferring between grids. This
feature results in a minimal amount of code change, and enables us to employ a
sophisticated interpolation method and noninteger ratio of grid spacings.
Calculations employing a preconditioned conjugate gradient method are presented
for two examples, a quantum dot and a charged molecular system. Use of three
grid levels with grid spacings 2h, 1.5h, and h decreases the computer time by
about a factor of 5 compared to single level calculations.Comment: 10 pages, 2 figures, to appear in Phys. Rev. B, Rapid Communication
Theoretical Study of Cubic Structures Based on Fullerene Carbon Clusters: CC and (C
We study a new hypothetical form of solid carbon \csc, with a unit cell which
is composed of the \cs \ fullerene cluster and an additional single carbon atom
arranged in the zincblende structure. Using {\it ab initio} calculations, we
show that this new form of solid carbon has lower energy than hyperdiamond, the
recently proposed form composed of \cs \ units in the diamond structure. To
understand the bonding character of of these cluster-based solids, we analyze
the electronic structure of \csc \ and of hyperdiamond and compare them to the
electronic states of crystalline cubic diamond.Comment: 15 pages, latex, no figure
Sensing remote nuclear spins
Sensing single nuclear spins is a central challenge in magnetic resonance
based imaging techniques. Although different methods and especially diamond
defect based sensing and imaging techniques in principle have shown sufficient
sensitivity, signals from single nuclear spins are usually too weak to be
distinguished from background noise. Here, we present the detection and
identification of remote single C-13 nuclear spins embedded in nuclear spin
baths surrounding a single electron spins of a nitrogen-vacancy centre in
diamond. With dynamical decoupling control of the centre electron spin, the
weak magnetic field ~10 nT from a single nuclear spin located ~3 nm from the
centre with hyperfine coupling as weak as ~500 Hz is amplified and detected.
The quantum nature of the coupling is confirmed and precise position and the
vector components of the nuclear field are determined. Given the distance over
which nuclear magnetic fields can be detected the technique marks a firm step
towards imaging, detecting and controlling nuclear spin species external to the
diamond sensor
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