237 research outputs found
Optimal control of a leaking qubit
Physical implementations of quantum bits can contain coherent transitions to
energetically close non-qubit states. In particular, for anharmonic oscillator
systems such as the superconducting phase qubit and the transmon a two-level
approximation is insufficient. We apply optimal control theory to the envelope
of a resonant Rabi pulse in a qubit in the presence of a single, weakly
off-resonant leakage level. The gate error of a spin flip operation reduces by
orders of magnitude compared to simple pulse shapes. Near-perfect gates can be
achieved for any pulse duration longer than an intrinsic limit given by the
nonlinearity. The pulses can be understood as composite sequences that refocus
the leakage transition. We also discuss ways to improve the pulse shapes.Comment: 4 pages, 2 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
Non-Markovian Quantum Jumps in Excitonic Energy Transfer
We utilize the novel non-Markovian quantum jump (NMQJ) approach to
stochastically simulate exciton dynamics derived from a time-convolutionless
master equation. For relevant parameters and time scales, the time-dependent,
oscillatory decoherence rates can have negative regions, a signature of
non-Markovian behavior and of the revival of coherences. This can lead to
non-Markovian population beatings for a dimer system at room temperature. We
show that strong exciton-phonon coupling to low frequency modes can
considerably modify transport properties. We observe increased exciton
transport, which can be seen as an extension of recent environment-assisted
quantum transport (ENAQT) concepts to the non-Markovian regime. Within the NMQJ
method, the Fenna-Matthew-Olson protein is investigated as a prototype for
larger photosynthetic complexes.Comment: 9 pages, 4 figures, submitted to Journal of Chemical Physic
Symmetry-enhanced supertransfer of delocalized quantum states
Coherent hopping of excitation rely on quantum coherence over physically
extended states. In this work, we consider simple models to examine the effect
of symmetries of delocalized multi-excitation states on the dynamical
timescales, including hopping rates, radiative decay, and environmental
interactions. While the decoherence (pure dephasing) rate of an extended state
over N sites is comparable to that of a non-extended state, superradiance leads
to a factor of N enhancement in decay and absorption rates. In addition to
superradiance, we illustrate how the multi-excitonic states exhibit
`supertransfer' in the far-field regime: hopping from a symmetrized state over
N sites to a symmetrized state over M sites at a rate proportional to MN. We
argue that such symmetries could play an operational role in physical systems
based on the competition between symmetry-enhanced interactions and localized
inhomogeneities and environmental interactions that destroy symmetry. As an
example, we propose that supertransfer and coherent hopping play a role in
recent observations of anomolously long diffusion lengths in nano-engineered
assembly of light-harvesting complexes.Comment: 6 page
Stochastic exclusion processes versus coherent transport
Stochastic exclusion processes play an integral role in the physics of
non-equilibrium statistical mechanics. These models are Markovian processes,
described by a classical master equation. In this paper a quantum mechanical
version of a stochastic hopping process in one dimension is formulated in terms
of a quantum master equation. This allows the investigation of coherent and
stochastic evolution in the same formal framework. The focus lies on the
non-equilibrium steady state. Two stochastic model systems are considered, the
totally asymmetric exclusion process and the fully symmetric exclusion process.
The steady state transport properties of these models is compared to the case
with additional coherent evolution, generated by the -Hamiltonian
Bichromatic Driving of a Solid State Cavity QED System
The bichromatic driving of a solid state cavity quantum electrodynamics
system is used to probe cavity dressed state transitions and observe coherent
interaction between the system and the light field. We theoretically
demonstrate the higher order cavity-dressed states, supersplitting, and AC
stark shift in a solid state system comprised of a quantum dot strongly coupled
to a photonic crystal cavity for on- and far off-resonant cases. For the
off-resonant case, phonons mediate off-resonant coupling between the quantum
dot and the photonic resonator, a phenomenon unique to solid state cavity
quantum electrodynamics.Comment: 8 pages 6 figure
An efficient method to calculate excitation energy transfer in light harvesting systems. Application to the FMO complex
A master equation, derived from the non-Markovian quantum state diffusion
(NMQSD), is used to calculate excitation energy transfer in the photosynthetic
Fenna-Matthews-Olson (FMO) pigment-protein complex at various temperatures.
This approach allows us to treat spectral densities that contain explicitly the
coupling to internal vibrational modes of the chromophores. Moreover, the
method is very efficient, with the result that the transfer dynamics can be
calculated within about one minute on a standard PC, making systematic
investigations w.r.t. parameter variations tractable. After demonstrating that
our approach is able to reproduce the results of the numerically exact
hierarchical equations of motion (HEOM) approach, we show how the inclusion of
vibrational modes influences the transfer
Optimal number of pigments in photosynthetic complexes
We study excitation energy transfer in a simple model of photosynthetic
complex. The model, described by Lindblad equation, consists of pigments
interacting via dipole-dipole interaction. Overlapping of pigments induces an
on-site energy disorder, providing a mechanism for blocking the excitation
transfer. Based on the average efficiency as well as robustness of random
configurations of pigments, we calculate the optimal number of pigments that
should be enclosed in a pigment-protein complex of a given size. The results
suggest that a large fraction of pigment configurations are efficient as well
as robust if the number of pigments is properly chosen. We compare optimal
results of the model to the structure of pigment-protein complexes as found in
nature, finding good agreement.Comment: 20 pages, 7 figures; v2.: new appendix, published versio
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