1,502 research outputs found
Realistic and verifiable coherent control of excitonic states in a light harvesting complex
We explore the feasibility of coherent control of excitonic dynamics in light
harvesting complexes, analyzing the limits imposed by the open nature of these
quantum systems. We establish feasible targets for phase and phase/amplitude
control of the electronically excited state populations in the
Fenna-Mathews-Olson (FMO) complex and analyze the robustness of this control
with respect to orientational and energetic disorder, as well as decoherence
arising from coupling to the protein environment. We further present two
possible routes to verification of the control target, with simulations for the
FMO complex showing that steering of the excited state is experimentally
verifiable either by extending excitonic coherence or by producing novel states
in a pump-probe setup. Our results provide a first step toward coherent control
of these complex biological quantum systems in an ultrafast spectroscopy setup.Comment: 12 pages, 8 figure
Optimal control of a qubit coupled to a non-Markovian environment
A central challenge for implementing quantum computing in the solid state is
decoupling the qubits from the intrinsic noise of the material. We investigate
the implementation of quantum gates for a paradigmatic, non-Markovian model: A
single qubit coupled to a two-level system that is exposed to a heat bath. We
systematically search for optimal pulses using a generalization of the novel
open systems Gradient Ascent Pulse Engineering (GRAPE) algorithm. We show and
explain that next to the known optimal bias point of this model, there are
optimal shapes which refocus unwanted terms in the Hamiltonian. We study the
limitations of controls set by the decoherence properties. This can lead to a
significant improvement of quantum operations in hostile environments.Comment: 5 pages, 3 figures, improved pulse shape
Quantum control theory for coupled 2-electron dynamics in quantum dots
We investigate optimal control strategies for state to state transitions in a
model of a quantum dot molecule containing two active strongly interacting
electrons. The Schrodinger equation is solved nonperturbatively in conjunction
with several quantum control strategies. This results in optimized electric
pulses in the THz regime which can populate combinations of states with very
short transition times. The speedup compared to intuitively constructed pulses
is an order of magnitude. We furthermore make use of optimized pulse control in
the simulation of an experimental preparation of the molecular quantum dot
system. It is shown that exclusive population of certain excited states leads
to a complete suppression of spin dephasing, as was indicated in Nepstad et al.
[Phys. Rev. B 77, 125315 (2008)].Comment: 24 pages, 9 figure
Optimal Dynamical Decoupling Sequence for Ohmic Spectrum
We investigate the optimal dynamical decoupling sequence for a qubit coupled
to an ohmic environment. By analytically computing the derivatives of the
decoherence function, the optimal pulse locations are found to satisfy a set of
nonlinear equations which can be easily solved. These equations incorporates
the environment information such as high-energy (UV) cutoff frequency \omega_c,
giving a complete description of the decoupling process. The solutions explain
previous experimental and theoretical results of locally optimized dynamical
decoupling (LODD) sequence in high-frequency dominated environment, which were
obtained by purely numerical computation and experimental feedback. As shown in
numerical comparison, these solutions outperform the Uhrig dynamical decoupling
(UDD) sequence by one or more orders of magnitude in the ohmic case.Comment: 5 pages, 4 figures, to appear in Phys. Rev.
Optimal control for one-qubit quantum sensing
Quantum systems can be exquisite sensors thanks to their sensitivity to
external perturbations. This same characteristic also makes them fragile to
external noise. Quantum control can tackle the challenge of protecting quantum
sensors from environmental noise, while leaving their strong coupling to the
target field to be measured. As the compromise between these two conflicting
requirements does not always have an intuitive solution, optimal control based
on numerical search could prove very effective. Here we adapt optimal control
theory to the quantum sensing scenario, by introducing a cost function that,
unlike the usual fidelity of operation, correctly takes into account both the
unknown field to be measured and the environmental noise. We experimentally
implement this novel control paradigm using a Nitrogen Vacancy center in
diamond, finding improved sensitivity to a broad set of time varying fields.
The demonstrated robustness and efficiency of the numerical optimization, as
well as the sensitivity advantaged it bestows, will prove beneficial to many
quantum sensing applications
Application of Optimal Control to CPMG Refocusing Pulse Design
We apply optimal control theory (OCT) to the design of refocusing pulses
suitable for the CPMG sequence that are robust over a wide range of B0 and B1
offsets. We also introduce a model, based on recent progress in the analysis of
unitary dynamics in the field of quantum information processing (QIP), that
describes the multiple refocusing dynamics of the CPMG sequence as a dephasing
Pauli channel. This model provides a compact characterization of the
consequences and severity of residual pulse errors. We illustrate the methods
by considering a specific example of designing and analyzing broadband OCT
refocusing pulses of length 10 t180 that are constrained by the maximum
instantaneous pulse power. We show that with this refocusing pulse, the CPMG
sequence can refocus over 98% of magnetization for resonance offsets up to 3.2
times the maximum RF amplitude, even in the presence of +/- 10% RF
inhomogeneity.Comment: 23 pages, 10 figures; Revised and reformatted version with new title
and significant changes to Introduction and Conclusions section
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