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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.

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.

This article presents a theory for a quantum cloning machine able to produce several copies of the state of a particle at atomic or sub-atomic scale, or quantum state. It could have implications for quantum information processing methods used, for example, in message encryption systems.

Quantum cloning is difficult because quantum mechanics laws only allow for an approximate copy-not an exact copy-of an original quantum state to be made, as measuring such a state prior to its cloning would alter it. The present work shows that it is theoretically possible to create four approximate copies of an initial quantum state, in a process called asymmetric cloning. The authors have extended previous work that was limited to quantum cloning providing only two or three copies of the original state. One key challenge was that the quality of the approximate copy decreases as the number of copies increases. It appears possible to optimise the quality of the cloned copies, thus yielding four good approximations of the initial quantum state. The present quantum cloning machine is shown to have the advantage of being universal and therefore able to work with any quantum state, ranging from a photon to an atom.

Asymetric quantum cloning has applications in analysing the security of messages encryption systems, based on shared secret quantum keys. Two people will know whether their communication is secure by analysing the quality of each copy of their secret key. Any third party trying to gain knowledge of that key would be detected as measuring it would disturb the state of that key.