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Two building blocks for quantum computers are so-called Rydberg atoms and trapped ions. So far, these systems, each with its own advantages, had been studied separately, but physicists from the UvA Institute of Physics have now shown that the two can be combined in a useful way. The results were published in Physical Review Letters this week.
Several Rydberg atoms surrounding a trapped ion.

Ordinary computers use bits. Similarly, quantum computers do their computations using quantum bits or ‘qubits’. Atoms are excellent qubits, since each atom of a particular species is exactly the same as the others, eliminating manufacturing errors when building a future quantum computer. Efforts to create quantum devices ‘atom-by-atom’ have in the past focused on using either crystals of ions or so-called Rydberg atoms as qubits. Rydberg atoms are atoms in highly excited states – that is, with an electron very far from the nucleus. This allows such atoms to strongly interact with their neighbors, which is a crucial ingredient for building quantum computers.

Combining advantages

Both trapped ions and Rydberg atoms have been successfully used to create small scale quantum devices in a growing number of research groups. However, each approach has a few inherent drawbacks. For instance, scaling up the trapped ion system to include hundreds of qubits has turned out to be very hard, whereas the Rydberg system has not been able to come close to the required accuracy for building a meaningful quantum computer. This begs the question whether the two approaches could be advantageously combined in a single system in order to reap the benefits of each.

Physicist Norman Ewald and his co-workers in the group of Rene Gerritsma have now for the first time observed interactions between Rydberg atoms and trapped ions. They have immersed single trapped ions into an ultracold cloud of Rydberg atoms, only about 15 millionth of a degree above the absolute zero temperature. The researchers measured that the Rydberg atoms collide with the ion at a rate that is more than a thousand times larger than that of normal atoms, indicating a strong interaction between ions and atoms controlled by the Rydberg excitation of the latter. They also showed that the presence of the ion changes the Rydberg atom’s spectrum – that is, its potential excited states. In particular, the researchers have found that the presence of a trapped ion allows a transition between excited states that would otherwise be forbidden.

The results constitute the first steps towards controlling the interactions between Rydberg and ionic qubits and towards building a hybrid quantum system out of the two. The authors describe their work in a paper that was published in Physical Review Letters this week.


Observation of Interactions between Trapped Ions and Ultracold Rydberg Atoms,
N. V. Ewald, T. Feldker, H. Hirzler, H. Fürst and R. Gerritsma, Physical Review Letters 122, 253401 (2019).