Superconducting circuits are being used to create new types of quantum systems that are much easier to control and much more tunable than natural quantum systems like atoms. (Microscopic picture of the structure.) Credit: TU Wien
Imagine being able to see quantum objects with your own eyes — no microscopes needed. That’s exactly what researchers at TU Wien and ISTA have achieved with superconducting circuits, artificial atoms that are massive by quantum standards.
Unlike natural atoms, these structures can be engineered to have customizable properties, allowing scientists to control energy levels and interactions in ways never before possible. By coupling them, they’ve developed a method to store and retrieve light, laying the groundwork for revolutionary quantum technologies. These engineered systems also enable precise quantum pulses and act as a kind of quantum memory, offering an unprecedented level of control over light at the quantum level.
Gigantic Quantum Objects – Visible to the Naked Eye
Many quantum objects, such as individual molecules or atoms, are so small that they can only be observed with specialized microscopes. However, the quantum structures that Elena Redchenko studies at the Institute for Atomic and Subatomic Physics at TU Wien are different — they are large enough to be seen with the naked eye, though only with some effort. Measuring hundreds of micrometers across, these objects remain tiny by everyday standards but are immense in the realm of quantum physics.
These large quantum objects are superconducting circuits — structures that allow electric current to flow without resistance when cooled to low temperatures. Unlike natural atoms, which have fixed properties dictated by nature, these artificial structures can be precisely customized. This flexibility enables scientists to manipulate and study various quantum phenomena in a controlled environment. Often referred to as “artificial atoms,” their physical properties can be engineered to suit specific experiments.
By coupling these artificial atoms, researchers developed a system capable of storing and retrieving light — an essential step for future quantum experiments. This breakthrough was achieved by the research group of Johannes Fink at ISTA, with theoretical contributions from Stefan Rotter at the Institute for Theoretical Physics at TU Wien. The findings were recently published in Physical Review Letters.
Customized “Atoms” – Engineering Quantum Properties
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 Controlling Time
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.”
Reference: “Observation of Collapse and Revival in a Superconducting Atomic Frequency Comb” by E. S. Redchenko, M. Zens, M. Žemlička, M. Peruzzo, F. Hassani, R. Sett, P. Zieliński, H. S. Dhar, D. O. Krimer, S. Rotter and J. M. Fink, 11 February 2025, Physical Review Letters.
