In his doctoral thesis, theoretical physicist Joris Kattemölle (CWI / QuSoft) proposes a physical problem that could be the first to be solved by a quantum computer. The problem cannot be solved by a classical computer, but a quantum computer with a hundred quantum bits can. On Wednesday June 30, Kattemölle received a doctorate from the University of Amsterdam for his thesis entitled “Many-body physics meets quantum computing”.

A quantum computer can solve problems that a classical computer would never be able to calculate. Today’s quantum computers exist only in a few large research laboratories around the world and have at most a few dozen quantum bits, the elementary computing units of the new revolutionary computer. In 2019, Google introduced a quantum computer made up of 53 quantum bits that solved a problem that a classical computer cannot. This became global news, despite the fact that it was a toy problem with no apps.

In his doctoral thesis, theoretical physicist Joris Kattemölle describes an interesting problem for physicists to solve, which cannot be solved by a classical computer but can be solved with “only” a hundred quantum bits. And a quantum computer made up of one hundred quantum bits is already in sight.

The problem that Kattemölle proposes is called the kagome network (*kagome* is a Japanese word for a certain weaving pattern that looks exactly like the lattice). Reproducing this network on a computer can provide new information about the behavior of solids found in nature. For example, the kagome lattice describes the magnetic properties of the mineral Herbertsmithite, which was discovered by Herbert Smith in Chile in 1972. The mineral has no specific applications but is an interesting object for physicists to study in order to understand all of them. the possible behaviors of atoms. and molecules in solids.

The most exciting aspect of the kagome network is that it is a promising candidate to prove that there is a new kind of magnetism: a so-called *quantum spin liquid* (a new type of disordered magnetic state in which there is no order in the direction of the elementary magnets, as there is, for example, in a ferromagnetic, where all the elementary magnets point in the same direction ). Physicists believe that a *quantum spin liquid* exists, but have never proven or found it experimentally. In his thesis, Kattemölle showed that the kagome problem has exactly the right properties that make it very suitable to be solved with a quantum computer.

The common thread of Kattemölle’s thesis is the interaction between multi-particle physics (which, for example, explains why electrical conduction only occurs with several electrons and not with just one) and the quantum computer. A multi-particle problem that some physicists see as a practical obstacle to building a quantum computer is super-noise. Super-noise is the phenomenon that the noise of all quantum bits combined is greater than the sum of the noise of all individual quantum bits. In his thesis, Kattemölle, in addition to his work on the kagome network, also demonstrated that this super-noise does not pose any practical problem for the construction of a future quantum computer.

**More information**

- The research was carried out at the Institute for Theoretical Physics at the University of Amsterdam (UvA) and at the Algorithms and Complexity group of the CWI, Centrum Wiskunde & Informatica (CWI), Amsterdam. It was made possible by a grant from the University of Amsterdam, in support of QuSoft and the priority research area “Quantum Matter and Quantum Information”.
- The thesis ceremony took place on Wednesday, June 30, 2021 at UvA. Supervisor: Prof. CJM Schoutens (UvA), Co-supervisor: Dr J. Van Wezel (UvA).
- Joris Kattemölle’s doctoral thesis: N-body physics meets quantum computing
- Joris Kattemölle website: http://www.kattemolle.com

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