Vol. 43 No.1 - Highlights
Energy transport for biomolecular networks (Vol. 43 No. 1)
image Probability density of the average energy transfer time τ as a function of the de-phasing rate γ. As indicated by the median (white line), for a typical configuration, τ is reduced by the de-phasing up to an optimal rate γ, where the transfer becomes most efficient. However, these noise-assisted transfer times are longer than the minimum transfer time achieved by an optimized configuration for vanishing de-phasing (minimum of dot-dashed line on the left).

Recent experimental demonstrations of long-lived quantum coherence in certain photosynthetic light-harvesting structures have launched a flurry of controversy over the role of coherence in biological function. An ongoing investigation into the astonishingly high-energy transport efficiency of these structures suggests that nature takes advantage of quantum coherent dynamics.

We inquire on the fundamental principles of quantum coherent energy transport in ensembles of spatially disordered molecular networks subjected to dephasing noise. De-phasing reduces the coherence between individual network nodes and has already been shown to assist transport substantially provided that quantum coherence is disadvantageous by reason of destructive interference, e.g. in the presence of disorder and quantum localization. In a statistical survey, we map the probability landscape of transport efficiency for the whole ensemble of disordered networks, in search of specially adapted molecular conformations that nature may select in order to facilitate energy transport: We thus find certain optimal molecular configurations that by virtue of constructive quantum interference yield the highest transport efficiencies in the absence of dephasing noise. Moreover, the transport efficiencies realized by these optimal configurations are systematically higher than the noise-assisted efficiencies mentioned above. As discussed in the article, this defines a clear incentive to select configurations for which quantum coherence can be harnessed.

The optimization topography of exciton transport
T. Scholak, T. Wellens and A. Buchleitner, EPL, 96, 10001 (2011)

Double ionization with absorbers (Vol. 43 No. 1)
image The system is initially described by a two-particle wave function (top). As a particle is ionized and subsequently hits the absorber (Τ), the remaining electron is described by a one-particle density matrix (middle), which may be thought of as an ensemble of several one-particle wave functions. As also the one-particle system may be absorbed, the vacuum state may also be populated (bottom).

In quantum dynamics, unbound systems, such as atoms being ionised, are typically very costly to describe numerically as their extension is not limited. This problem should be reduced if one could settle for a description of the remainder of the system and disregard the escaping particles. Removing the escaping particles may be achieved by introducing absorbers close to the boundary of the numerical grid. The problem is, however, that when such "interactions" are combined with the Schrödinger equation, all information about the system is lost as a particle is absorbed. Thus, if we wish to still describe the remaining particles, a generalization of the formalism is called for. As it turns out, this generalisation is provided by the Lindblad equation.

This generalised formalism has been applied to calculate two-photon double ionisation probabilities for a model helium atom exposed to laser fields. In the simulations, the remaining electron was reconstructed as the first electron was absorbed. Since there was a finite probability for also the second electron to hit the absorber at some point, the system could, with a certain probability, end up in the vacuum state, i.e. the state with no particles. As this probability was seen to converge, it was interpreted as the probability of double ionisation.

The validity of this approach was verified by comparing its predictions with those of a more conventional method applying a large numerical grid.

A master equation approach to double ionization of helium
S. Selstø, T. Birkeland, S. Kvaal, R. Nepstad and M. Førre, J. Phys. B: At. Mol. Opt. Phys. 44, 215003 (2011)

Entanglement or separability: factorisation of density matrix algebra (Vol. 43 No. 1)
image The separable quantum states form the blue double pyramid while the entangled states are located in the remaining tetrahedron cones

The present theoretical description of teleportation phenomena in sub-atomic scale physical systems proves that mathematical tools let free to choose how to separate out the constituting matter of a complex physical system by selectively analysing its so-called quantum state. That is the state in which the system is found when performing measurement, being either entangled or not. The state of entanglement corresponds to a complex physical system in a definite (pure) state, while its parts taken individually are not. This concept of entanglement used in quantum information theory applies when measurement in laboratory A (called Alice) depends on the definite measurement in laboratory B (called Bob), as both measurements are correlated. This phenomenon cannot be observed in larger-scale physical systems.

The findings show that the entanglement or separability of a quantum state -whether its sub-states are separable or not; i.e., whether Alice and Bob were able to find independent measurements - depends on the perspective used to assess its status.

A so-called density matrix is used to mathematically describe a quantum state. To assess this state's status, the matrix can be factorised in different ways, similar to the many ways a cake can be cut. The Vienna physicists have shown that by choosing a particular factorisation, it may lead to entanglement or separability; this can, however, only be done theoretically, as experimentally the factorisation is fixed by experimental conditions.

This is applied to model physical systems of quantum information including the theoretical study of teleportation, which is the transportation of a single quantum state. Other practical applications include gaining a better understanding of K-meson creation and decay in particle physics, and of the quantum Hall effect, where electric conductivity takes quantified values.

Entanglement or separability: the choice of how to factorise the algebra of a density matrix
W. Thirring, R.A. Bertlmanna, P. Köhler and H. Narnhofer, Eur. Phys. J. D, 64/2, 181 (2011)

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