Quantum computing: How to calculate with individual atoms

Control of quantum mechanical phenomena underlies many modern technologies, including lasers and modern semiconductor materials. Both these examples depend upon controlling a large number of elementary particles – photons in the first case and electrons in the second. Furthermore, the targeted control of atoms opens up a whole new window on to the world of quantum mechanics and enables practical new applications.

As part of the events surrounding Quantum Year 2025, Prof. Johannes Zeiher will be giving a talk at Deutsches Museum on 16 October 2025 at 7:15 pm. The subject will be: “Quanta under the microscope: How we can calculate with individual atoms” (registration necessary).

Professor Zeiher, how did you come upon the idea of using atoms for calculating?

The corresponding theoretical ideas had already been formulated in the 1990s and early 2000s, but for a long time we lacked the technology to realize them in the laboratory. In recent years, however, we’ve learned how to handle individual atoms with ever greater control. This is a crucial prerequisite for experimental implementation of the ideas.

Important groundwork was done by LMU physicist and Nobel laureate Ted Hänsch, who formulated the idea of laser cooling in the 1970s. Building on this, scientists could investigate increasingly complex systems of atoms.

And then to be really able to calculate with atoms, there was another decisive event: Immanuel Bloch and his team – at the same time as similar work undertaken in Harvard by Markus Greiner – managed to make individual atoms visible in so-called quantum gas microscopes. Since then, we’ve improved the control of atoms to the extent that we can make calculations with them.

A new way of calculating

Is this a kind of calculating we’re familiar with from school?

[Zeiher laughs] When we control atoms, it’s a very different kind of calculating from what we learned at school. It’s not like calculating with numbers – addition, subtraction, multiplication, division. Even our classical computers work in a more abstract fashion, using the binary system to calculate – in other words, processing zeroes and ones.

And quantum computing goes beyond this again.

Exactly. Quantum computers do not just calculate with zeroes and ones, but also with superpositions of zeroes and ones. That’s what makes it so special. Under certain conditions, it can thus access a much greater possibility space – giving it an enormous computing advantage.

How do you actually go about calculating with atoms?

Like classical computing with bits of zeroes and ones, we can define two states in atoms – for example, internal electronic states – that are stable. To be able to perform quantum calculations, we also have to produce the necessary superposition states. From the discoveries of quantum optics, we know we can achieve this by shining laser beams on atoms.

Why do the atoms have to be cooled for this to work?

Atoms generally move extremely fast when they are not cooled. The air molecules here in this room – the nitrogen, say – fly around at around 450 meters per second. That would certainly make it hard to catch one. If we want to calculate using the internal states of an atom, first we have to precisely locate it. To do this, we cool the atoms. And there is a second trick besides the cooling: so-called optical tweezers. If an atom is cold enough, we can hold it in one place in a single laser beam – as if with invisible tweezers. Then we reach the point where we have perfect control.

In your basic research, you’re also working with systems of cold atoms and developing models for solid-state systems out of them. What’s the concept here?

I’ll explain with an example: A metal is a system which at the simplest level can be described by electrons zipping around in a crystal lattice. Calculating this system exactly in quantum mechanical terms means having to consider an extremely large number of particles in different states. According to the laws of quantum mechanics, moreover, the states of these particles are superimposed and influence each other. A classical computer simply cannot model this. Naturally, we can measure various macroscopic properties, such as conductivity or heat capacity. But microscopic information is generally hidden from us.

In our quantum optical laboratories, we can build models of such metals or other solid-state materials atom by atom, as it were, and then investigate what happens inside them at the microscopic level. With these analog quantum simulators, we can fundamentally investigate the underlying quantum mechanical phenomena inside materials.

The technologies employed in the laboratory can also be used for digital quantum simulations. This improves the control even further and makes it possible to solve more general problems.

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Staying with the analog system for the moment, what can we research with it?

At the fundamental level, our goal is to better understand the quantum mechanics of many particles, so-called quantum many-body systems. For example, we’re interested in properties that individual particles do not have yet, but that arise collectively in systems of many particles. In the laboratory, we have entirely new control possibilities and can expose the particles, say, to extremely strong artificial magnetic fields, which are not so easy to obtain in classical solid-state bodies.

Does this allow us to understand phenomena like magnetism and superconductivity as well?

For us, it’s a big motivation to understand the microscopic relationships that underlie macroscopic phenomena like superconductivity and magnetism. We know what a superconductor does at the macroscopic level. You cool it and from a certain temperature it loses its electrical resistance. Then it can transport charge with hardly any loss. But in some cases we don’t know exactly why this happens at the microscopic level, especially when this so-called transition temperature occurs at particularly high temperatures. Our quantum simulators can give fresh impetuses to our microscopic understanding here.

You mentioned digital quantum computing a little earlier. That is your second main area of interest. You founded the startup “planqc” with some colleagues, which has now received the German Entrepreneur Award. How did this idea come about?

Many years ago, we were already considering whether we might be able to market the technologies we were developing in a start-up. For a long period, however, the time was not yet ripe for digital quantum computing based on neutral atoms. In 2022, various things eventually converged and we founded planqc. At the scientific level, there were wonderful advances in the control of individual atoms, while at the same time the commercial conditions were propitious through Munich Quantum Valley. And most of all, we had the right team with my co-founders Alexander Glätzle and Sebastian Blatt.

Been thinking about starting a start-up for a long time

Do you come up with an idea like that over a few beers?

[Zeiher laughs] Alex had already been playing around with the thought of a start-up for a long time. We know each other from our PhD days and even then he had taken part in entrepreneurial competitions. This passion came across clearly in our talks prior to the foundation of planqc. At the same time, we were aware that new technological developments were needed in areas like laser and optical systems before we could build scalable quantum computers. This calls for engineers – to improve vacuum systems, for instance, or the robustness of optomechanical and laser systems. Such tasks are beyond the capabilities of academic projects – they require specialists.

Quantum computers at room temperature

You want to operate quantum computers at room temperature, which sounds pretty neat. How easy is it to accomplish?

Yes, we want to do without cryogenic cooling. Unlike other platforms, such as superconducting qubits, we’re not dependent on keeping our qubits in contact with a coolant. In our system, laser cooling performs this function. We cool with light. This has major advantages as regards scaling, because we don’t have to put the whole system into a cooling apparatus.

Yes, you’ve run with your very own technical concept. Many competitors, large competitors like Google and IBM, have gone with cooling technologies – superconducting qubits and so forth. What made you so sure, or makes you so sure now, that you’re on the right track?

Commercial providers like Google, IBM, and others have done brilliant work to date in the field of quantum computing. They’re now able to control their qubits very, very well. Google reached an important milestone in 2019 with its quantum supremacy experiment. In the meantime, there are even chips with up to 1,000 qubits and promising initial results for quantum error correction.

What makes you optimistic that you can keep pace?

Neutral atoms have a large scaling advantage. The huge challenge for many other platforms is that although they can control hundreds of qubits well – as Google and IBM, for example, have demonstrated – going much beyond this will be increasingly difficult. With each additional qubit, more calibration needs to be done. This is because each of these superconducting qubits is slightly different from the others, as they are complex structures.

Control over one million qubits

And this doesn’t apply in your case?

Atoms are, so to speak, the most natural qubits. Atoms of a certain element are all identical. The challenge for us is to achieve good control in the range of a few atoms. The subsequent scaling is considerably easier. We can increase the number of atoms without fundamentally changing anything about our system. This is a big advantage even now, but will become decisive when it comes to further scaling.

And we will have to scale up system sizes if we are to be able to employ quantum computers for commercially relevant applications. We’re talking here about perhaps a million qubits or more that would have to be controlled. I see major challenges for all platforms in this regard. Our work, however, and also the progress in the field, show that neutral atoms have big advantages.

How do you solve the problem of scalability?

We use optical lattices, which are effectively crystals of light in which we trap our atoms. Then we control the atoms using optical tweezers. To do this, the lasers must be of high quality with sufficient optical performance. In the Munich ecosystem we have two world leaders in laser technology, Toptica and Menlo, with whom we can tackle these problems.

You sound very confident. Are there no major obstacles?

Certainly, as systems become more complex, so the task becomes more formidable. But I’m optimistic we’ll find good solutions and see no fundamental roadblocks over the coming years. On the contrary, the progress of recent years has been heartening. We’ve advanced a lot faster than originally anticipated.

In the meantime, planqc employs 90 experts at its company headquarters in a former home improvement center in Garching. In your view, what is the biggest challenge for an aspiring start-up in generating a viable concept from basic research?

Naturally enough, we faced many challenges, from finding the right team and suitable premises to negotiating a viable concept for IP transfer. We received a lot of support in all these areas and got useful tips from experienced founders. The most important thing in my view is enthusiasm for the idea you want to realize.

Of course, the start-up world also requires you to change the way you think. Compared to work at university, start-ups place a lot more value on creating sustainable structures that can grow. We strive to develop technology faster and better than would be possible in a university setting. Our way of working is also somewhat different, as we have to be much more targeted in our efforts to create a product under time pressure. All the while, we’re in permanent competition with other companies.

That being said, we also seek out close cooperation with academic research, where we can operate with more freedom and even pursue unconventional approaches. I see this as a highly promising way of mastering truly difficult problems like the building of a quantum computer.

Perfect Munich ecosystem

You’ve announced the first freely programmable quantum computer for 2027. Does this schedule still hold?

Specifically, this is a system that was commissioned by the German Aerospace Center (DLR) and is being built at the Ulm Innovation Center. It will have 100 qubits. Together with DLR, we’re already exploring whether we can optimize, say, battery design using quantum computers. In addition, we’re pursuing quantum-inspired approaches based on so-called tensor networks, in order to improve things like hydrodynamic simulations. Another major focus is on quantum error correction.

Why is that so important?

Even the best qubits, such as atoms, do not work without errors. The basic idea of error correction is to build a functional new processing unit out of multiple so-called physical qubits – that is to say, our atoms. We call this a logical qubit. Several physical qubits are involved in each logical qubit. Interestingly, it’s possible to improve the performance of logical qubits more and more under certain conditions as the number of physical qubits grows. Out of multiple logical qubits, we can build error-corrected quantum computers that calculate much better than their non-corrected counterparts.

Quantum error correction is also the object of current basic research. Ultimately, logical qubits are quantum many-body systems that need to be controlled very precisely. Furthermore, we’re convinced that with error-corrected machines of around 100 qubits we’ll be able to investigate exciting scientific problems.

Could you give an example?

Here we come full circle to analog simulators: Even with digital quantum computers, material science applications remain interesting. We can simulate elementary magnetic systems or electron systems and explore their properties. So there is a close connection between analog and digital quantum simulators.

Is it fun to compete with giants like Google?

We’re in a strong position in that we’re pursuing a different technology to Google and other players. It’s not so easy for Google to switch horses. Naturally, we’re also in competition with other companies in the United States that use neutral atoms as qubits. We feel we’re in a good position here in terms of technology and scaling. We’re embedded in the excellent Munich ecosystem with Munich Quantum Valley and the MCQST cluster and are receiving lots of support from all sides. Munich is a hotspot for quantum technologies. There are very successful educational programs, such as the master’s degree in quantum science and technology jointly offered by LMU and TUM, which attracts students from all over the world. In Germany, and especially in Munich, we’ve got photonics and laser companies which offer products that even our competitors buy. All these factors give us a clear edge as regards our location.

So soon we’ll really be able to calculate with atoms?

I’m convinced of it. On a small scale, we’re already doing it.

Johannes Zeiher is Professor of Physics at LMU and a research group leader at the Max Planck Institute of Quantum Optics. He is also founder of the start-up “planqc,” which has just received the German Entrepreneur Award.