Date: 18 January 2017
Time: 15:00 to 16:00
Location: EC Stoner 7.73
Speaker: Brendon Lovett (St Andrews)
Optically active nanostructures sharing a common electromagnetic environment experience field-mediated interactions, which may in turn influence how they absorb and emit light. Interestingly, this opens possibilities for engineering their dissipative behaviour through quantum interference.
Re-emission of absorbed photons is an important factor in the Shockley Queisser limit on the efficiency of conventional photovoltaic devices. Quantum interference between two optical dipoles enables states enjoying dark-state protection, where optical excitations are stored until their energy has been converted into a more useful longer-lived form. Going beyond a previous proposal for idealised highly symmetric systems , I here show that this concept equally applies to wide classes of non-identical organic molecular dimers whose properties were extracted from a quantum chemistry database. Rather surprisingly, our analysis shows that these messier asymmetric dimers can even outperform the idealised case under realistic constraints .
Beyond dimers, ring-like systems subject to additional dissipation from a condensed matter environment leads to another effect featuring excited states that cannot decay optically but which are ready to absorb further photons. This optical ‘ratcheting’ could serve as a buffer for a stream of incident photons arriving with a random distribution of arrival times. I will show how rings exploiting such ratchet states can be particularly effective in a regime where charge separation times are slow, as is the case in biological systems .
Time permitting, I will go on to discuss the problem of decoherence of several qubits coupled to a common environment more generally. In particular, I will demonstrate that coherence properties of some parts of a system can actually improve as temperature is increased .
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 K. D. B. Higgins, B. W. Lovett and E. M. Gauger, arXiv:1504.05849 (2015).
 H. M. Cammack, P. Kirton, J. Keeling and B. W. Lovett, arXiv:1609.04965 (2016)