3,399 research outputs found
Dressing the chopped-random-basis optimization: a bandwidth-limited access to the trap-free landscape
In quantum optimal control theory the success of an optimization algorithm is
highly influenced by how the figure of merit to be optimized behaves as a
function of the control field, i.e. by the control landscape. Constraints on
the control field introduce local minima in the landscape --false traps-- which
might prevent an efficient solution of the optimal control problem. Rabitz et
al. [Science 303, 1998 (2004)] showed that local minima occur only rarely for
unconstrained optimization. Here, we extend this result to the case of
bandwidth-limited control pulses showing that in this case one can eliminate
the false traps arising from the constraint. Based on this theoretical
understanding, we modify the Chopped Random Basis (CRAB) optimal control
algorithm and show that this development exploits the advantages of both
(unconstrained) gradient algorithms and of truncated basis methods, allowing to
always follow the gradient of the unconstrained landscape by bandwidth-limited
control functions. We study the effects of additional constraints and show that
for reasonable constraints the convergence properties are still maintained.
Finally, we numerically show that this approach saturates the theoretical bound
on the minimal bandwidth of the control needed to optimally drive the system.Comment: 8 pages, 6 figure
Quantum optimal control within the rotating wave approximation
We study the interplay between rotating wave approximation and optimal
control. In particular, we show that for a wide class of optimal control
problems one can choose the control field such that the Hamiltonian becomes
time-independent under the rotating wave approximation. Thus, we show how to
recast the functional minimization defined by the optimal control problem into
a simpler multi-variable function minimization. We provide the analytic
solution to the state-to-state transfer of the paradigmatic two-level system
and to the more general star configuration of an -level system. We
demonstrate numerically the usefulness of this approach in the more general
class of connected acyclic -level systems with random spectra. Finally, we
use it to design a protocol to entangle Rydberg via constant laser pulses atoms
in an experimentally relevant range of parameters.Comment: 8 pages, 5 figure
Alignment of Continuous Auditory and Visual Distractor Stimuli Is Leading to an Increased Performance
Information across different senses can affect our behavior in both positive and
negative ways. Stimuli aligned with a target stimulus can lead to improved behavioral
performances, while competing, transient stimuli often negatively affect our task
performance. But what about subtle changes in task-irrelevant multisensory stimuli?
Within this experiment we tested the effect of the alignment of subtle auditory and
visual distractor stimuli on the performance of detection and discrimination tasks
respectively. Participants performed either a detection or a discrimination task on a
centrally presented Gabor patch, while being simultaneously subjected to a random
dot kinematogram, which alternated its color from green to red with a frequency of
7.5 Hz and a continuous tone, which was either a frequency modulated pure tone for the
audiovisual congruent and incongruent conditions or white noise for the visual control
condition. While the modulation frequency of the pure tone initially differed from the
modulation frequency of the random dot kinematogram, the modulation frequencies
of both stimuli could align after a variable delay, and we measured accuracy and
reaction times around the possible alignment time. We found increases in accuracy
for the audiovisual congruent condition suggesting subtle alignments of multisensory
background stimuli can increase performance on the current task
Audio-visual synchrony and feature-selective attention co-amplify early visual processing
Our brain relies on neural mechanisms of selective attention and converging sensory processing to efficiently cope with rich and unceasing multisensory inputs. One prominent assumption holds that audio-visual synchrony can act as a strong attractor for spatial attention. Here, we tested for a similar effect of audio-visual synchrony on feature-selective attention. We presented two superimposed Gabor patches that differed in colour and orientation. On each trial, participants were cued to selectively attend to one of the two patches. Over time, spatial frequencies of both patches varied sinusoidally at distinct rates (3.14 and 3.63 Hz), giving rise to pulse-like percepts. A simultaneously presented pure tone carried a frequency modulation at the pulse rate of one of the two visual stimuli to introduce audio-visual synchrony. Pulsed stimulation elicited distinct time-locked oscillatory electrophysiological brain responses. These steady-state responses were quantified in the spectral domain to examine individual stimulus processing under conditions of synchronous versus asynchronous tone presentation and when respective stimuli were attended versus unattended. We found that both, attending to the colour of a stimulus and its synchrony with the tone, enhanced its processing. Moreover, both gain effects combined linearly for attended in-sync stimuli. Our results suggest that audio-visual synchrony can attract attention to specific stimulus features when stimuli overlap in space
Optimal Phonon-to-Spin Mapping in a system of a trapped ion
We propose a protocol for measurement of the phonon number distribution of a
harmonic oscillator based on selective mapping to a discrete spin-1/2 degree of
freedom. We consider a system of a harmonically trapped ion, where a transition
between two long lived states can be driven with resolved motional sidebands.
The required unitary transforms are generated by amplitude-modulated
polychromatic radiation fields, where the time-domain ramps are obtained from
numerical optimization by application of the Chopped RAndom Basis (CRAB)
algorithm. We provide a detailed analysis of the scaling behavior of the
attainable fidelities and required times for the mapping transform with respect
to the size of the Hilbert space. As one application we show how the mapping
can be employed as a building block for experiments which require measurement
of the work distribution of a quantum process
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