Listening to Senior University Lecturer in Theoretical Physics **Esko Keski-Vakkuri**, you become less certain about how you view the universe.

“I conduct basic research, basically looking for a better understanding of natural laws. There are plenty of open-ended questions. Even though quantum mechanics and the general theory of relativity, which explains gravity, were discovered in the last century, no one has succeeded in combining the two,” says Keski-Vakkuri.

Keski-Vakkuri studies quantum gravity, one of the unanswered questions in physics. It has no relevance to everyday life right now, unless you wish to know what will happen when you enter a black hole. There are alternative theories on this. When you step over the edge of a black hole, you either don’t notice anything or end up in a sea of fire. What everyone agrees on is that you cannot get out.

Besides black holes, the quantum theory of gravity is also needed when considering how the universe began. Going further back from the Big Bang, you end up with the conclusion that, in the beginning, distances were zero, which makes gravity extremely strong and lets quantum phenomena take over.

In physics, answers to open-ended questions are sought through what are known as toy models. One such model is a mathematical model studied by Professor of Mathematics **Antti Kupiainen** at the University of Helsinki and his partners, where the theory of gravity and the unified theory of quantum mechanics were successfully solved with mathematical precision. However, the assumption was that spacetime was two-dimensional instead of four-dimensional, as in reality. This was a significant step forward, for which the team was awarded the esteemed George Pòlya Prize in Mathematics in 2022 (Mitä hyötyä on puhtaasta matematiikasta? article in Finnish only).

## What if the universe is held together by quantum entanglement?

The Nobel Prize in Physics in 2022 was awarded to scientists specialising in quantum entanglement. **Alain Aspect**, **John Clauser** and **Anton Zeilinger** were able to experimentally test a theory that Einstein called ”spooky action at a distance”. Quantum entanglement is easier to understand if you start from a simple example.

“We have two entangled particles, and we conduct a measurement that can result in 0 or 1. We don’t know the result for either particle, but the entanglement of the particles leads to a situation where, if we measure one of the particles and get 0 as the result, we know for certain what the result for the other particle will be, even if the other particle is on the Moon,” Keski-Vakkuri says.

There are billions of particles in actual bodies, and we do not know in how many different ways they can be entangled. In space, there are even more points, making it possible for entanglement to be infinite, but this is not a satisfactory answer.

According to Keski-Vakkuri, a hypothesis presented 15 years ago on quantum entanglement (**Mark van Raamsdok** is one of the architects of this idea) posits that the theory of gravity is only a consequence of entanglement. In fact, quantum entanglement is the glue that holds spacetime together, while gravity is a description of something even more fundamental.

This leads to complex calculations. The leap in computing power needed for this is expected to come from quantum computers.

## A master's student develops a new quantum measurement algorithm

Quantum computers are based on quantum mechanics, a theory of physics. Many theoretical physicists have ended up focusing on quantum computers, after originally investigating the information paradox associated with black holes. The development of quantum computers requires coders and engineers, as well as mathematicians and physicists.

Quantum computers generate the correct answer with a certain probability. The answer is read by measuring the final state. The computing process and measurements are repeated in several cycles until the correct answer can be identified with sufficient certainty. An important element of the process is error correction, to ensure that computing can be carried out reliably for a sufficiently long time.

“Quantum error correction is where theoretical physicists have done a great deal of valuable work,” says Keski-Vakkuri, who teaches quantum physics at the University of Helsinki.

A course organised by the Master’s Programme in Theoretical and Computational Methods reviews, among other topics, the functioning of quantum computers and the challenges related to their development.

**Otto Veltheim** took the course, going on to complete his master’s thesis on measuring algorithms for quantum states. The aim was to explore the algorithms currently available, but Veltheim ended up developing a new one and continues to test it in connection with his doctoral thesis at the University of Helsinki’s Department of Physics. (*Otto Veltheim: Quantum State Tomography with Observable Commutation Graphs*** **)

Esko Keski-Vakkuri also works at InstituteQ, a network established in 2021 in cooperation with the VTT Technical Research Centre of Finland and Aalto University that promotes research and education relating to quantum technology as well as its commercial application in Finland.