The Vienna Doctoral Program on Complex Quantum Systems (CoQuS) –cooperation partner to this year's Ars Electronica festival "Origin - how it all begins"– was represented in Linz by students of the Robert Fickler, Marissa Giustina, Mateusz Kotyrba, Robert Polster, Christoph Schäff, and Sascha von Egan-Krieger which are all supervised by Prof Anton Zeilinger engaged visitors of the exhibition ‘Symmetries’ in interactive demonstrative experiments exhibiting fascinating phenomena of quantum physics.

The exhibition which was frequented by around 5.200 visitors in total offered an inspiring environment to convey on a personal level the fundamentals of modern quantum science and technology to the interested non-specialist audience. The demonstrative experiments served as an ideal starting point to provide an insight into the motivation and working methods of scientists, to communicate cutting-edge research results and, at the same time, to capture the enthusiasm of the young generation of researchers.

Light is a wave and a particle—both in one but never at the same time. The particle nature of light is manifested, for example, by the photoelectric effect. If light is shone upon a suitable metal plate, even an individual photon—the smallest quantum of light—can release an electron from the plate. Modern secondary electron multipliers are capable of producing, out of this single electron, a current of many millions of electrons, and commercially available electronic components can convert this into the crackling sound of a loud-speaker. Although this does not enable us to see individual light particles, we can thus hear them. Individual photons are the basis for modern experiments demonstrating the entanglement of light and for optical quantum computers and quantum simulations.

The wave nature of light becomes apparent when we investigate its propagation. Numerous analogies to classical physics are valid here. When a laser beam encounters an aperture, it is narrowed. If the aperture is made smaller, the beam becomes narrower. Below a certain aperture size the beam spreads out again, its momentum (here: its direction) becomes ‘uncertain’. In the wave picture, this is the phenomenon of diffraction.

If you position two narrow apertures right next to each other and shine the same laser beam upon them, then you can observe the phenomenon of interference. The partial waves emerging from each slit overlap with one another. At certain locations, they amplify each other; at others, they cancel each other. This results in the seemingly paradoxical situation—that there are locations on the (detection) screen that get darker when a second slit is opened in addition to the first one. The really amazing thing about the double slit experiment with light is that you still see the interference even when only individual light quanta reach the detector. You might well ask: How can an individual light particle ‘know’ whether both slits are open if it only goes through one of them? The quantum answer: Quantum systems equally realize all possibilities that are, in principle, indistinguishable. If there were a method of finding out which path the light took in passing through the aperture, the interference pattern would be destroyed. This can be seen most directly when one slit is closed—the interference disappears and, despite the fact that less light now reaches the screen, some of the points on the screen that were initially dark become bright again.

The observation of quantum phenomena is by no means limited to the world of the microcosm; instead, it depends much more on our ability to isolate a quantum object from its environment. With light in fiber optics, this isolation can approach perfection. A photon is ‘seen’ and destroyed only when it reaches the detector. But along its way, it can be subjected to numerous transformations—for example, it is possible to arrange a situation in which it reaches a fiber optic beam splitter, where it decides to take two paths at the same time and delocalizes over the great distances spanned by the optic fibers. The beam is split wide enough for you to stick your hand ‘between a quantum of light’. We are once again confronted with the question: What is the reality of the position of a photon?”