“It’s always a team effort”

Researchers in the MolQ project are venturing into uncharted territory in quantum physics, and their novel approach requires close monitoring through theoretical modelling. Jelena Klinovaja leads this part of the project with her colleague Daniel Loss. In the following interview, she discusses the role of theory and the most challenging parts of the undertaking.

Jelena Klinovaja, you and Daniel Loss are responsible for theoretical work in the MolQ project. What exactly does that entail?

In MolQ, molecules are constructed and then used for measurements. We theoreticians support both of these practical areas in a variety of ways. For instance, we help analyse the data generated by the measurements. Then, theories can also be used as a basis to recommend what should be measured in the first place—so we can actually locate the effect we want to identify.

 

What about the different molecules fabricated by the chemistry team?

That’s another one of our tasks. These molecules can be arranged in various ways. For some, we can theoretically predict that they’re interesting. But it’s always a team effort. The chemists might say the arrangement we theoreticians propose can’t be achieved in an experiment. This interaction is the best part of the project.

 

Which calculations and concepts are particularly challenging?

Topologically protected phases are hard to identify—researchers across the globe are seeking ways to do this. Right now, we’re working with individual electrons and their magnetic moments. But our ultimate goal is to create topological phases that arise from electron-electron interactions. These types of collective excitation are a tough nut to crack.

 

There are many ways to produce qubits. From a theoretical point of view, how do you rate the MolQ approach?

Well, it’s a very risky approach, because we don’t yet know a whole lot about this molecular topological system. At the same time, however, there are several theoretical indications as to why these kinds of qubits could be very interesting. That’s why it’s important to explore this molecular approach.

 

Do you also see any potential sticking points that might cause the concept to fail?

In physics, you always have to reckon with something going wrong. But often, the failure isn’t due to something not working. More often than not, we realise there’s a better way. Five years from now, we might see that our original configuration was suboptimal and that we should switch to a different superconducting platform. We have to remain flexible and be ready to adapt our strategy to our findings.

 

Topology is a key aspect of MolQ (see page 38). Your assumption is that topological protection will lengthen the coherence times of qubits from nano-seconds to microseconds. Even so, that’s still incredibly short.

It’s less about absolute times. What matters more is the number of computational operations that can be performed within this time. And the estimated numbers are actually at the lower limit of what we might expect from a theoretical point of view. But, of course, there’s one compromise that we’ll have to make.

 

What would that be?

A highly protected state isn’t readily affected by its surroundings. But that also means we researchers can’t easily manipulate the qubits, which is the objective. All qubit platforms face the same conundrum.

 

The quantum world stretches our perception of reality: there are particles that can be in two different states at once, and others that can be coupled even when they’re far apart. How does theory manage to penetrate these phenomena?

Quantum mechanics can be understood fairly well with the help of calculations. The underlying mathematics isn’t sorcery. It may take a little while to grasp, but once you begin studying it, you soon begin to understand it better. However, there are many ways to interpret the equations.And that’s when we enter the realm of philosophy. Another point is that quantum mechanics can’t be observed intuitively in our everyday lives.

 

How much progress has been made in quantum computing?

Huge advances have been made. Twenty years ago, many qubit platforms were either still unknown or in their infancy. Today, we have a certain degree of control over them. However, significant challenges remain. The focus is no longer on producing a single qubit. Now we need to figure out how to couple and scale them.

 

What will it take to achieve a major breakthrough?

Sometimes a very specific ingredient is needed to make the decisive leap forward. Take artificial intelligence, where the lack of sufficient computing power and high-performance chips long posed the biggest difficulties. In quantum computing, I believe a certain quality of material is imperative. If qubits aren’t functioning properly, it’s usually due to an imperfection or disorder in the material.

 

Is that where the MolQ approach has a major advantage?

That’s right. We’re working with molecules. And like atoms, molecules are reproducible. Compared to crystal lattices, they have far fewer defects. The molecular bonds always enable the same, predictable structures to form. This is an essential prerequisite for generating the sensitive quantum effects we need.