An article was published in Nature Nanotechnology this week about quantum interference in molecular charge transport. Two of its authors were the Groningen chemists Prof. Kees Hummelen and Dr Hennie Valkenier, who received her PhD with honours, with Hummelen as supervisor. Hummelen is the driving force behind the department of Organic Chemistry, which instigated this research a few years ago. ‘Now things really get interesting, with this article being published’, he says. ‘The idea that we developed – and we’re just chemists – has now found strong experimental and theoretical support from physicists in Leiden and Denmark with whom we are collaborating.'
In 2006, Dr Marleen van der Veen gained her PhD, supervised by Hummelen, based on an entirely theoretical thesis. This was somewhat remarkable as organic chemists are usually always found in the lab, attempting to synthesize new materials. Van der Veen had to restrict herself to deciding upon the different routes that an electron can take as a result of electricity conduction through complex molecules with many ring structures and double bonds – molecules that do not in fact exist. If such molecules could be synthesized, the aim would then be to use them as digitally integrated circuits for the molecular quantum computer of the future.
Valkenier’s article in Nature Nanotechnology means that Van der Veen’s work has become somewhat less theoretical and that the quantum computer with logical circuits the size of a single molecule has now drawn nearer. Van der Veen would very much have liked to have synthesized these herself, but this proved impossible as the molecules she described in her thesis are extremely difficult to fabricate. However, in the years between her thesis being published and the current Nature article, Valkenier has set to work as a member of Hummelen’s team.
Valkenier managed to synthesize a number of series of molecules and to test the basic principles with them. She did this together with physics groups capable of measuring the charge transport of a few molecules in various ways. Hummelen: ‘Using this material, just last year the Stratingh Institute for Chemistry – in collaboration with two members of our team, Prof. Ryan Chieci and Davide Fracasso – found very strong evidence for quantum interference. The article in Nature now really furnishes convincing evidence.’
The molecules in question are string-shaped – the most simple ones in Van der Veen’s thesis – where in principle molecules move from one end A to the other end B in linear fashion. The secret is that there is also one molecule among them where the electron at a certain point arrives at a junction where it – simply put – can choose whether to turn right or left.
What happens then resembles what happens in a well-known physics experiment where light passes through two closely adjacent narrow slits. Hummelen: ‘Behind the slits interference occurs; the light waves are either in phase, and amplify each other, or they are not, and then cancel each other out. Something similar happens with electron waves that reunite on the molecule’s single through route after having been split up earlier.’
The consequence of interference is that electrical conduction suddenly drops sharply at certain voltages. This happens in a way that is characteristic and hence easily recognizable. Normally, when conduction drops due to part of the molecule conducting poorly, this results in a normal convex curve when the charge transport is measured. In the case of interference, conduction drops sharply and the resulting pattern when measured is V-shaped.
Physicists were very sceptical of the chemist’s topological theory in the past, says Hummelen. ‘The criticism of Van der Veen’s research boiled down to their claiming that the world – in any case the molecular world – was not as simple as we molecular “design chemists” tended to believe.’ After new research in collaboration with theoretical physicists from the Technical University of Denmark and experimental physicists from Leiden University, the scepticism will no doubt quickly pass.
Hummelen, however, is cautious about claiming that the result is revolutionary or ground-breaking. Quantum interference has been already been proven, albeit at much lower temperatures and in much larger structures than molecules. But now that it works at room temperature, chances have increased that it will someday be applicable as a switch in the quantum computer. ‘And if it ever proves possible to do so in the more complex molecules that we thought up together with Marleen van der Veen, it will be possible to make a logical switch with a single molecule, where we now need a series of switching elements. I think that then we will have reached the boundaries of computer miniaturization – smaller than a molecule should prove impossible.’
See also the NWO press release Quantum mechanical switching at room temperature
Press release University Leiden: Switching molecules on and off
Contact: Prof. J.C. Hummelen
Article: Observation of quantum interference in molecular charge transport, Constant M. Guédon, Hennie Valkenier, Troels Markussen, Kristian S. Thygesen, Jan C. Hummelen en Sense Jan van der Molen. Nature Nanotechnology, online: 25 March 2012. DOI:10.1038/NNANO.2012.37
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