Light from artificial atoms

Many objects that we normally deal with in quantum physics are only visible with special microscopes – individual molecules or atoms, for example. However, the quantum objects that Elena Redchenko works with at the Institute for Atomic and Subatomic Physics at TU Wien can even be seen with the naked eye (with a little effort): They are hundreds of micrometers in size. Still tiny by human standards but gigantic in terms of quantum physics.

Those huge quantum objects are superconducting circuits – structures in which electric current flows at low temperatures without any resistance. In contrast to atoms, which have fixed properties, determined by nature, these artificial structures are extremely customizable and allow scientists to study different physical phenomena in a controlled manner. They can be seen as ”artificial atoms”, whose physical properties can be adjusted at will.  

By coupling them, a system was created that can be used to store and retrieve light – an important prerequisite for further quantum experiments. This experiment was carried out in the group of Johannes Fink at ISTA, with theoretical collaboration from Stefan Rotter at the Institute for Theoretical Physics at TU Wien. The results have now been published in the journal ‘Physical Review Letters’.

Customized “atoms”

A key property of quantum physics is that certain objects can only assume very specific energy values. “An electron moving around an atomic nucleus can assume a lower energy state or a higher energy state, but never a state in between,” says Elena Redchenko, the lead author of the current publication. “All values in between are simply not physically possible. With our artificial atoms, however, we can choose which energy values should be allowed. For each artificial atom, we can set exactly how large the distance between the physically permitted energy values should be.”

Microwaves are sent through a special metal wire (a resonator) that runs directly past the superconducting artificial atoms. These microwaves now influence the superconducting artificial atoms: some of the microwave radiation can pass from the wire into the artificial atoms – and back again. The strength of this interaction can also be specifically adjusted.

“We can show that photons are exchanged between the microwave in the wire and the artificial atoms in a precisely predictable way,” says Elena Rechenko. “This is only possible because our artificial atoms give us a huge amount of engineering freedom to customize our system to our exact requirements. This means we can now achieve things that would be unthinkable with atoms or other natural quantum objects.”

Quantum light pulses and quantum memory

If the artificial atoms are adapted correctly, it is possible to create very special rhythms of light pulses. “We send a short classical microwave pulse into the wire, but the interaction with the artificial atoms can create a series of quantum pulses of light, separated by time intervals that we can control. It is like an on-chip quantum timer,” explains Elena Rechenko.

“In our work, we have shown how flexible this system is and how precisely it can be used for very different quantum experiments,” says Elena Rechenko. “For example, you can use it to generate individual, clearly separated photons – this is important for many experiments. But you can also use it to temporarily store photons for a certain period of time until they are released again – this is another technique that promises exciting new applications.”