In his PhD thesis, theoretical physicist Joris Kattemölle (QuSoft, CWI & UvA) proposes a physical problem that could be the first one for a quantum computer to solve. The problem cannot be solved by a classical computer, but a quantum computer with about one hundred quantum bits can. On Wednesday 30 June Kattemölle received a doctorate from the University of Amsterdam for his PhD thesis entitled ‘Many-body physics meets quantum computation’.

A quantum computer can solve problems that a classical computer would never be able to calculate. Current quantum computers only exist in a few large research labs around the world and count at most a few dozen quantum bits, the elementary calculation units of the revolutionary new computer. In 2019, Google demonstrated a quantum computer consisting of 53 quantum bits that solved a problem that a classical computer cannot solve. This became world news, despite the fact that it was a toy problem with no applications.

In his doctoral thesis, theoretical physicist Joris Kattemölle describes a problem that is interesting for physicists to solve, one that cannot be solved by a classical computer but can be solved with ‘only’ around one hundred quantum bits. And a quantum computer consisting of one hundred quantum bits is already in sight.

The problem that Kattemölle proposes is called the kagome lattice (kagome is a Japanese word for a certain weaving pattern that looks exactly like the lattice). Reproducing this lattice on a computer can provide new insights into the behaviour 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 possible behaviours of atoms and molecules in solids.

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

The thread running through Kattemölle’s thesis is the interaction between many-particle physics (which, for example, explains why electrical conduction only occurs with many electrons and not with a single electron) and the quantum computer. A many-particle problem that some physicists believe is a practical obstacle to the construction of 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 lattice, also demonstrated that this super-noise does not pose any practical problem for the construction of a future quantum computer.

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The research was carried out at the Institute for Theoretical Physics of University of Amsterdam (UvA), and at the Algorithms and Complexity group at CWI the Centrum Wiskunde & Informatica (CWI), Amsterdam. It was made possible by a grant from the University of Amsterdam, in support of QuSoft and the research priority area ‘Quantum Matter & Quantum Information’. The PhD ceremony took place on Wednesday 30 June 2021 at the UvA. Supervisor: Prof. C.J.M. Schoutens (UvA), Co-supervisor: Dr. J. Van Wezel (UvA).

PhD thesis of Joris Kattemölle: Many-body physics meets quantum computation

**Website Joris Kattemölle:** http://www.kattemolle.com