The exoskeleton of a tiny organism has been used as a diffraction grating by researchers in Vienna, who have carried out a molecular interferometry experiment using it. The team showed that a coherent molecular beam could be diffracted from the silicon-based cell walls of a marine alga. Algae are cheap and easily available, so replacing costly nanodevices with them in interferometry experiments would be beneficial, according to the researchers.

Contrary to classical mechanics, quantum physics states that a particle can act like a wave and vice versa – an idea that was first proposed by Nobel-prize-winning physicist Louis de Broglie back in 1923. While the idea that tiny particles such as electrons could behave like a wave came as a shock, scientists now know that even objects a million times more massive than electrons, such as complex molecules, also show quantum interference. Massive molecules have very small wavelengths and therefore a grating with extremely thin and closely spaced slits is needed to observe their diffraction. Currently, such sophisticated devices are specially fabricated using nanotechnology techniques.

Natural grating

Now physicists Michele Sclafani, Markus Arndt and colleagues from the University of Vienna have shown that a simple, inexpensive and natural grating is close at hand in the form of diatoms. These are a group of mostly unicellular marine algae that are encased within a cell wall made of silica that is known as a frustule. In the experiment, Sclafani used Amphipleura pellucida – an alga that can be found on the beach. It has a wall thickness of 90 nm and a surprisingly regular pore distance of about 200 nm. The researchers found that they could use the diatom to measure molecular De Broglie wavelengths as small as a few billionths of a millimetre by firing beams of molecules at it.

Lighting up molecules

To do this, the Amphipleura pellucida frustule was suspended in a vacuum with its pores oriented towards the coherent molecular beam of phthalocyanine molecules – a commonly used blue-green dye. Incidentally, a phthalocyanine molecule is about a million times more massive than an electron. Phthalocyanine was chosen because it gives off fluorescent light when illuminated with a laser and so can be easily detected. The beam was created by shining a 1.5 μm focused laser onto a dye-coated glass window. The tiny spot size of the laser ensured that the beam was coherent. The beam was then sent through the alga and each individual molecule was counted on a screen behind the sample. This was done by observing the location of the fluorescent light with a microscope. After many molecules were passed through the alga, the ensemble of their locations made up the diffraction pattern.

"We actually managed to delocalize and coherently propagate a particle through the pores of a biologically grown nanostructure," exudes Sclafani, explaining that here, coherently means without loss or scrambling of quantum phases. Instead, the wavefunction of each molecule is delocalized over several of the alga's pores. He also points out that "most of the apparatus was very easily sourced – if you are making a matter–wave interferometer in school, this might be the way to go". Sclafani went on to tell physicsworld.com that the diatoms used were bought online and cost a mere £12. You can see more about how the researchers carried out their experiment in the video abstract (above) of their paper published in New Journal of Physics.

Compare and contrast

To dissect and understand their experimental results, the researchers took a photograph of the alga and used a set of formulae (that normally model the propagation of light in similar circumstances) to calculate the molecular intensity distribution that should appear on the screen behind the alga. Upon comparing the two, the researchers noted something odd – while their theory predicted 2D interference, experimentally they had observed a 1D pattern.

Sclafani says that the simple explanation for this dimensional anomaly is gravity: as the molecules travel from the source to the detector they "fall" to some extent, thanks to the Earth's gravitational field. This means that molecules with different velocities end up contributing to the same spatial region along the y-axis and the pattern is smeared out. "If we repeat the interference experiment and twist by 90°, you would see a 2D pattern," says Sclafani.

Missing orders

The researchers also noticed that in their theoretical simulation, the higher-order interference effects are almost missing, but they see many more higher-order interference effects in their experiment. They believe this can be explained by an interaction potential between the single molecules and the real material surface of the pores of the alga. Sclafani says that this interaction is described by the Van der Waals potential that comes into play as the molecule goes past the alga's walls, inducing a small dipole moment as it does so. So, the Van der Waals potential generates an additional momentum kick.

Mathematically, this interaction can be mimicked by reducing the size of the alga's pores, as this results in a larger population of high interference orders. As an alternative, the team also extended its theoretical model by adding a phase term to the integral that estimates the interaction constant between the phthalocyanine and the inner surface of the alga. This results in good qualitative agreement between theory and experiment.

Sclafani is keen to point out the many advantages of using diatoms. In addition to that fact that there are regular structures in most algae that could be used in matter–wave interference, one could use atoms or different molecules to probe the internal properties of the algae via their dephasing influence on the quantum-interference pattern. Diatoms are less exotic than it may appear at first glance: they have many uses including drug delivery and as natural filters, thanks to their "very nice and regular structures", according to Sclafani.

Scalfani says that, in the long run, the focus of the group is to probe the matter–wave duality of an object that has mass and complexity. "Matter–wave duality still puzzles people – is there any limit to quantum mechanics, mass-wise?" he asks. Understanding this is crucial.

The research is published in the New Journal of Physics.