Quantum Diffraction at a Breath of Nothing

Physicists build stable diffraction structure in atomically thin graphene.
The quantum mechanical wave nature of matter is the basis for a number of modern technologies like high resolution electron microscopy, neutron-based studies on solid state materials or highly sensitive inertial sensors working with atoms. Prof. Markus Arndt group focused on how one can extend such technologies to large molecules and cluster.

In order to demonstrate the quantum mechanical nature of a massive object it has to be delocalized first >> achieved by Heisenberg’s uncertainty relation: If molecules are emitted from a point-like source, they start to ‘forget’ their position after a while and delocalize. If you place a grating into their way, they cannot know, not even in principle, through which slit they are flying. It is as if they traversed several slits at the same time. This results in a characteristic distribution of particles behind the grating, ie diffraction or interference pattern. It can only be understood if we take the particles’ quantum mechanical wave nature into account.

In a European collaboration (NANOQUESTFIT) + others demonstrated for the 1st time such gratings can be fabricated even from the thinnest conceivable membranes. They milled transmission masks into ultra-thin membranes of silicon nitride, biphenyl molecules or carbon with a focussed ion beam and analysed them with ultra-high resolution electron microscopy. The team succeeded in fabricating stable and sufficiently large gratings even in atomically thin single layer graphene.

A grand challenge was to reduce the material thickness and thus the molecular attractive interactions of these masks down to the ultimate limit while retaining a mechanically stable structure.”Given the gratings’ thickness of a millionth of a millimetre, the interaction time between the mask and the molecule is roughly a trillion times shorter than a second. We see that this is compatible with high contrast quantum interference.”

The bars of the nanogratings look resemble the strings of a mini harp. One may thus wonder if the molecules induce vibrations in these strings. If this were so the grating bars could reveal the molecular path through the grating and quantum interference should be destroyed. Although the molecules give the grating a little kick in the diffraction process this recoil remains always smaller than the quantum mechanical momentum uncertainty of the grating itself. It therefore remains undetectable. This applies even to membranes 1 atom thick. http://medienportal.univie.ac.at/presse/aktuelle-pressemeldungen/detailansicht/artikel/quantum-diffraction-at-a-breath-of-nothing/

Modern fabrication methods allow to make atomically thin nanomasks which prove to be sufficiently robust for experiments in molecular quantum optics. Credit: Copyright Quantennanophysik, Fakultät für Physik, Universität Wien; Bild-Design: Christian Knobloch