The quantum artists: Terbium duet and other quantum art

In the fairytale, Cinderella enlists birds to help her sort lentils: “The good ones go into the pot, the bad ones into your crop,” she tells them. When it comes to quantum computers, too, the good must be separated from the bad – at the level of atoms and their electrically charged siblings, the ions. For researchers, the “good” are the qubits: atoms or ions, for example, capable of performing calculations in a quantum computer. Qubits are usually very sensitive; even the smallest mechanical or magnetic disturbance from outside can cause the so-called coherence to collapse in a fraction of a second. Then the qubits are thrown out of sync, and the quantum information is lost. They become “bad” qubits, a hindrance to a quantum computer’s calculations. 

In current quantum computers, qubits are relatively far apart, so they don’t interfere with each other. While this works well with 50 or 100 qubits, it becomes problematic when you envision future quantum computers with many millions of qubits that need to be packed together very densely, like the bits on today’s computer chips. They would be likely to interfere with each other, turning “good ones” into “bad ones.” 

Simon Gerber and Gabriel Aeppli at the PSI Center for Photon Science know a way to keep even densely packed qubits in the “good” pot for significantly longer. Their terbium ion qubits are embedded in the atomic lattice of yttrium lithium fluoride crystals. The astonishing result of this is that the qubits produced by the terbium ions are more stable than expected, in other words their coherence is much greater. 

“The trick is that the qubit states are now stored in the interaction of two ions rather than individual ions, as is customary,” explains Gerber. “These ion pairs form naturally when you add a lot of terbium to the crystal.” The advantage is that ion pair qubits communicate at a very specific frequency that cannot be disrupted by individual terbium ions or other atoms in the crystal. It’s like using a new radio frequency far from those used by existing transmitters. The old frequencies don’t cause interference. In terms of the qubits, this means that you can communicate with them unimpeded, and their coherence is maintained many times longer. This makes the PSI approach extremely interesting for the construction of future quantum computers.

A protective shield made of microwaves

But things can get even better. Another threat to terbium ion qubits is external magnetic disturbances. Irradiating them with microwaves, however, provides a protective shield. The paired qubits then have a lifetime up to a hundred times longer than qubits made of individual, non-irradiated ions. “With the right material, the coherence could last even longer,” says Gabriel Aeppli, head of the PSI Center for Photon Science. The team now wants to use their knowledge of this phenomenon to further optimise the setup.

The experiments, published last year in the journal Nature Physics, open up an intriguing new avenue for building quantum computers. However, simply scattering terbium ions into a crystal lattice won’t be enough. “The trend is towards a quasi-surgical placement of atoms or ions,” says Aeppli.

Wenchao Xu is familiar with the arrangement of individual atoms. She too is a scientist at the PSI Center for Photon Science. However, her atoms are not located in a solid, but float in the vacuum inside a compact chamber. Xu aims at positioning up to 5,000 atoms with high precision using an array of focused laser beams. “We refer to this technique as optical tweezers. Each laser beam can capture and position a single atom,” Xu explains. Her team uses the individual atoms as qubits and, from many atoms positioned in this way, builds quantum processors.

Hybrids of light and matter

Xu is also investigating possible ways to set up connections between quantum systems. For this purpose, she uses so-called polaritons. These are quasiparticles that simultaneously possess the properties of light and matter. They could potentially serve as interfaces between atomic qubits and light, carrying quantum information over longer distances. Using polaritons to connect multiple quantum processors to each other, larger quantum computers could become a reality

Polaritons are also of interest to Dominik Sidler, a theoretical physicist at the PSI Center for Scientific Computing, Theory and Data. He studies the quasiparticles not in the laboratory, but in computer simulations. He is investigating the strong coupling of light with matter. When molecules are locked in a tiny hall of mirrors and light is allowed to bounce back and forth, the light can alter the molecular structure and thus its chemical properties as well. The remarkable thing is that no external light is needed. The quantum effect occurs even in absolute darkness. However, why does this alter the chemical properties? This is still largely unknown, because current physical models don’t actually allow for it. Here, too, polaritons might be involved.

Sidler is trying to uncover the secrets of polaritonic chemistry. If successful, his work could for example lead to more energy-efficient ways of producing the active ingredients for drugs, because the light traps would allow the chemical structures to be selectively modified.

Cat qubits

While researchers like Simon Gerber and Wenchao Xu use use what nature provides as the smallest building blocks for their qubits – atoms and ions – Alexander Grimm takes a different approach: he creates qubits artificially. To do so, he uses so-called microwave resonators, in which an electrical signal can oscillate back and forth like a pendulum. Grimm’s research group is able to precisely control the oscillation state in these resonators. What’s more, at minus 273 degrees Celsius, Grimm can create a quantum mechanical superposition – that’s when two opposing oscillations occur simultaneously. It’s as if a classical pendulum were swinging in opposite directions at the same time. “In this way, we create in the laboratory something like Schrödinger’s cat,” says Grimm, referring to Erwin Schrödinger’s famous thought experiment. The advantage of these cat qubits is that the two opposing states are resistant to disturbances, so Grimm’s qubits naturally stay longer in Cinderella’s “good” pot.

The ideal place for quantum research

The many different approaches pursued in quantum research are particularly successful when researchers from different fields work together. “Every discipline has its own language, its formalisms. So it’s important that we transcend our own disciplinary boundaries and explore new possibilities,” says Dominik Sidler. “PSI is the ideal place for this – also thanks to our close cooperation with the universities in the ETH Domain and our many contacts with leading international institutions.”

“We mutually enrich each other’s research,” agrees Simon Gerber. The experiment with pairs of terbium ions, for example, could also lead to the development of new types of sensors. Essentially, understanding what is happening at the levels of atoms and electrons, large “microscopes” are needed. “The large research facilities at PSI, which are unique in the world, can help us understand the properties of materials,” says Gerber, “thus opening up a multitude of new possible applications.”