Learning about quantum phenomena with quantum machines

Phew! This is a lot of abstract words – so let’s take them step by step. Quantum phenomena are observations made in nature which can only be explained by effects of quantum mechanics. Generally speaking, quantum mechanics gives our nowadays best description of the fundamental processes in nature on very small length scales. This might sound very abstract, but actually a lot of modern technologies rely on quantum mechanical effects. The better and better understanding of quantum mechanics has led to innovations, like for example the satellite positioning (GPS), and it still holds big promises for the future. One prominent technological goal you may have seen in the media is to build a new type of computer which makes use of quantum mechanical effects – the so-called quantum computer. This is currently a very hot topic and already some other of our writings touched upon this. The focus of this text is more directed towards the fundamental aspects because there still exist many interesting open questions as well.

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Thermometer with logarithmic scale. This means the ticks are spaced by a factor of 1000. The temperatures of the systems we create in the laboratory are as low as the energies created at LHC are high.

We want to learn something about the fundamental processes in quantum mechanical systems with machines using quantum effects. Let me explain this in more detail: Our understanding of quantum mechanical properties is very good when we are dealing with systems consisting of only a few particles. These particles can be for example electrons, nuclei or atoms. One speaks of many-body systems if many of those particles come together and interact. Transferring our understanding to these situations is hard; it is actually exponentially hard. This means that the complexity for twice as many particles does not double but grows super-fast — it quickly becomes impossible to simulate these problems even on the largest computers. 

Nature isn’t classical, dammit, and if you want to make a simulation of nature, you’d better make it quantum mechanical, and by golly it’s a wonderful problem, because it doesn’t look so easy.

Richard Feynman

In nature we typically find systems that are made up of many particles. To understand the consequences of quantum mechanical effects in complicated materials like superconducting devices or in our observable universe, one has to use approaches different from brute-force computations using the understanding of small systems. Since a decade, experimental physicists aim to simulate quantum mechanics involving many particles with newly developed machines — so-called quantum simulators. The ideas for that go back to the famous physicist Richard Feynman. Already in the 80’s he pointed out that quantum simulators (he did not use that name at this time) would incorporate quantum mechanics naturally as the building blocks themselves are quantum. My work is concerned with improving these quantum simulators. For example, I want to make them more efficient and develop new concepts for learning as much as possible from the large amount of generated experimental data. I will now give you a brief idea of what is done with these machines to gain insights in otherwise unsolvable problems.

In many laboratories around the world scientists use cold atoms to build analog quantum simulators: We create dilute clouds of thousands of atoms which are trapped inside a vacuum chamber, i.e., a glass cell which is very properly emptied. The vacuum chamber ensures isolation from the environment and thus allows us to study the influence of quantum mechanical effects. The atoms are cooled to temperatures close to absolute zero — actually they are hundred billion times colder than room temperature. This is hard to imagine so we made an illustration for that. At these low temperatures the atoms’ properties are solely influenced by quantum mechanics. Using lasers, we build flexible “cages” where we can trap the atoms in different spatial configurations – you find some examples in the second illustration. With that we can study what happens if the space of the atoms is not three- but two- or one-dimensional. Here, new phenomena can emerge. In the real world, outside the lab, such phenomena are important for example in solid state devices where electrons are confined in two-dimensional planes. In our experiments we can not only tailor the space, but also manipulate the internal structure of the atoms. This changes for example the way they interact. Over the last thirty years an enormous amount of control has been achieved. If you are interested in that in more detail stay tuned for my next article.

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Using laser light and optical components we engineer potentials to confine the atoms in different locations. With grids, so-called lattices, the atoms are pinned to discrete locations. This mimics the situation in condensed matter systems.

 

For a real-world material, like an insulating piece of metal, there are many different processes that affect its properties. There are not only electrons interacting with each other, but also the atom cores binding the electron, and many other things.  To perform an analog quantum simulation, we, however, build toy-model systems. This means that we do not rebuild exactly the systems of interest but first we break down the complex interplay into single building blocks. We then study their quantum mechanical properties by implementing the corresponding physics “analog” to the complex material in our simulation device. Hence, our systems do not have an exact counterpart in nature. 

Having accomplished the hard task of implementing these building block in the lab, we can do two things. First, we study their properties employing different conditions. This leads to new insights on quantum mechanical phenomena and helps creating new intuition about the phenomena in many-body quantum systems. After that, we can put different building blocks together. This is like a quantum-lego, where the combination of rather simple bricks may lead to really complex ‘objects’.

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We can also create situations where space is continuous. This allows us to study for example physics that appears in heavy-ion collisions at the LHC.

Now let us come to the experimental, hands-on, side of the work. I used the word machine – with that, you might associate a single purpose tool that is user-friendly and ready to do the same job over and over again. However, our quantum machine is not at all like this! It consists of many different components which have been assembled by generations of researchers in a dynamical process of discovering new phenomena. Imagine it like being in the laboratory and looking at what the experiment tells you about quantum mechanics. Then you realize: Heh, for more insights I have to find out more about this new property X. Now you have to think how you use your knowledge about optics, electronics, mechanics, and plumbing to build something probing property X. By assembling many of such projects we get into the position to precisely study quantum effects and their role in for example the emergence of large-scale structures in the early universe.

Physicists always aim at describing things observed in nature. In our case, these observations might not be immediate for everyone, but the invention of quantum mechanics historically also was motivated by observations which could not be explained otherwise. And even nowadays there is still so much to learn about nature and its underlying quantum processes. Thus, in my opinion fundamental questions are a great motivation for doing science! That’s why there is a large community of researchers using quantum simulators to investigate many different properties of quantum mechanics. Even though there might be ‘only’ insights for a special community of researchers in the first place, the past teaches us that eventually many inventions tailored for fundamental questions lead to technological big steps. Besides cold atoms there exist many other platforms, like for example ions or superconducting devices. The variety of platforms and the creativity of the involved people make me confident that the occurrence of the next ground-breaking technology is just a matter of time.

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