Scientists from Heidelberg, Hamburg and Jena demonstrated a new method to produce very delicate quantum states which might be very important for the implementation of new quantum technologies in the future. In contrast to everyday experience, quantum mechanical objects can exist in multiple states simultaneously. However, these superposition states are extremely fragile and can already be destroyed by interaction with the vacuum, which – in terms of quantum mechanics – is not empty. The scientists now managed to manipulate the interaction with the vacuum in such a way that the vacuum produces and even stabilizes these superpositions instead of destroying them. This opens up a variety of future perspectives for quantum optics with novel X-ray light sources.

Detailed view of the experiment. The sample with the iron atoms embedded between mirror layers is irradiated with X-ray light from a flat angle, and the reflecting light is measured.

One of the most surprising predictions of quantum mechanics is the possibility that a quantum object can exist in multiple states simultaneously. Such a superposition of states is in contrast to our everyday experience, where each object always has clearly defined properties. This becomes especially evident in the famous thought experiment developed by Schrödinger, in which a cat – according to the rules of quantum mechanics – may be in the state of ‘dead’ and ‘alive’ simultaneously. Only a measurement decides upon the fate of the cat. In spite of the apparently absurd consequences, this superposition can be produced with quantum objects. They are essential for many quantum mechanics applications, e.g. future quantum computers.

Unfortunately, these superpositions are very fragile; therefore they can only survive in a completely isolated system. But even with the best experimental realisation there still is a quantum mechanical disturbing effect: whereas the vacuum is empty from the classical point of view, particles are permanently produced in the quantum mechanical vacuum and disappear after a very short time. Already the interaction of this inevitable vacuum fluctuation with a superposition state is often sufficient to destroy it. A promising solution in theory is already known for more than 40 years. At that time it was predicted that the interaction with the vacuum may be manipulated in such a way that it produces the desired states of superposition instead. Unfortunately, this is linked to rigorous conditions which so far hindered the experimental realisation.

Theoretical considerations of Kilian Heeg and Jörg Evers from the MPI for Nuclear Physics now showed how these rigorous conditions can be evaded. For this they devised two tricks. First, the superposition is realised in atomic nuclei that are surrounded by two mirrors. With this, it is possible to specifically influence the interaction of the particles with the vacuum. On the other hand, the theoreticians considered a large number of atomic nuclei between the mirrors, thus intensifying the occurring mechanisms via collective effects. Both tricks at once allowed producing a firm superposition between the different excitation states of the atomic nuclei.

Hans-Christian Wille and Ralf Röhlsberger from DESY headed an experiment which, in good agreement with the predictions, demonstrated the generation of such superposition states exploiting the vacuum. For this purpose, they embedded a large number of iron nuclei in a layer of 2.5 millionth of a millimetre between similarly thin layers of palladium, acting as mirrors. Subsequently, the prepared nuclei were investigated with X-ray radiation from the synchrotron radiation source PETRA III at DESY in Hamburg. With a so-called X-ray polarimeter developed by a group headed by Ingo Uschmann and Gerhard Paulus (University of Jena/Helmholtz Institute Jena), it was possible to detect the signal with so far unprecedented efficiency. The experimenters were able to successfully control the interaction between the vacuum and the atomic nuclei by applying an additional weak magnet field.

This method opens up a variety of possibilities for future experiments: the superpositions generated by the vacuum can be investigated systematically and utilised for applications, because the system applied now is not restricted to the X-ray range but also works with visible light. This offers the chance to realise the theoretically proposed applications reaching from novel laser mechanisms up to the efficiency increase of solar cells. It might also be possible to dynamically change the properties of atomic nuclei. At the same time, the successfully performed experiment shows how it is possible to realise undisturbed and highly configurable quantum optical model systems for applications with hard X-ray radiation.This offers an exciting future perspective for novel X-ray light sources as the European XFEL which is now under construction in Hamburg.

Original publication

Vacuum-assisted generation and control of atomic coherences at x-ray energies, K.P. Heeg, H.-C. Wille, K. Schlage, T. Guryeva, D. Schumacher, I. Uschmann, K.S. Schulze, B. Marx, T. Kämpfer, G.G. Paulus, R. Röhlsberger, J. Evers, Phys. Rev. Lett. 111, 073601 (2013), DOI: 10.1103/PhysRevLett.111.073601

Experimentally measured data (black) in comparison to the theoretical predictions (red). The image shows the light intensity reflected from the atomic nuclei as a function of the light energy relative to the resonance energy. The X-rays excite the atomic nuclei to different states which cause a maximum in the measured light intensity in the experiment. Energetically, the single states lie closely together; therefore one might expect that contributions of the different states would overlap. However, in the experiment it was observed that the intensity between the individual maxima disappears completely (blue areas). The theoretical analysis shows that such a behavior indicates the superpositions generated by the vacuum. The different possibilities to excite the superposition interfere and this makes the light intensity disappear.