Prize recipient 2024 | Klaus Mølmer

What is your research field?

I study the quantum-mechanical behaviour of microscopic physical systems, and how modern technologies can make use of their peculiar quantum behaviour. In the quantum world, we describe particles as if they are waves and appear to be in different places at the same time. But this is only until we measure them or disturb them, because then they ‘collapse’ and become like classical particles.

When I’m wearing my philosophical hat, I study the fundamental properties of these collapses; when I’m wearing my practical hat, I research strategies to avoid them or, even better, make use of them for various purposes.

I’m a theoretical physicist at an exciting time when we can trap and control atoms, both individually and in groups in rows and columns, and when we can build electrical and mechanical components that behave according to the laws of quantum mechanics. This has spawned ideas for new technologies, such as powerful quantum computers and ultra-sensitive quantum sensors. It’s an amazing research field that needs me to don both my research hats.  

What are the challenges and prospects for your research?

The quantum computer will not build itself and will remain science fiction unless we get to grips with several technological and theoretical challenges. There are challenges with quantum systems’ tendency to ‘behave classically’ on the slightest contact with their surroundings, and with quantum mechanics’ formal structure which prevents us from measuring systems without disturbing them, thus stopping us from making back-up copies of quantum states in the same way as with normal data. My research specifically concerns describing quantum systems that interact with their surroundings and can be exposed to measurements. It therefore goes hand-in-hand with engineers and experimental physicists creating better components in the laboratory.

Perhaps the greatest challenge is that the ‘measurement problem’ in quantum mechanics means that even a perfect quantum computer will only be able to solve very specific problems. Niels Bohr said of quantum mechanics: ‘If we should one day wake up and realise it was all just a dream, we would still have learned something.’ This applies equally to research into quantum technologies and starting from the physical mechanisms I hope to be able to expand the prospects for what we can and want to get from quantum computers and other quantum technologies.

How did you become interested in this research field?

I’d completed my Master’s degree at Aarhus University and been awarded a Ph.D. fellowship to study the dynamics of electrons in atomic collisions. One day, I stumbled over a research article about a quantum phenomenon when measuring light and found it so exciting that I immediately had to trawl through the literature on so-called quantum optics, which I hadn’t come across in my studies.

Quantum optics appealed to me by being driven by original and surprising questions that were not immediately answered by textbook methods and formulae – I could see a completely new kind of physics here as it was being created. I received generous support from my supervisor Knud Taulbjerg to change subject, and with help and advice from Ove Poulsen, a professor at Aarhus University, I was able to spend a long period abroad in Paris, where I took part in research into quantum optics and laser cooling, built a professional network and made friends for life. As luck would have it, quantum optics would set the agenda for both national and international research initiatives in the 1990s and came to be at the heart of research into quantum technologies through to today.

What are the most important realisations or discoveries you have made?

My most influential discovery, together with French colleagues, is a method for calculating the time evolution of a quantum system exposed to loss and decay. In contrast to the textbook equation for a so-called density matrix, we were able to simulate the time evolution with stochastic wave functions. These take up less space in the computer’s memory – n rather than n2, where n is often a large number –, and the method is now standard in many modern textbooks and numerical software libraries.

We derived the method using a thought experiment where we assumed that the amount of energy lost could be measured in the system’s surroundings, and this method lends fresh mathematical support to a modern quantum mechanics of Niels Bohr’s so-called quantum jumps. With different descriptions of the measurements of the surroundings, some of the paradoxes that Einstein presented to challenge Bohr’s interpretation of quantum mechanics are unfolded. The ‘Copenhagen interpretation’ has stood the test of time – and now with formulae to back it up!

Over the years, my colleagues and I have regularly discovered new aspects and applications of the theory. For example, our theory describes actual systems under constant observation, such as hyper-sensitive quantum sensors. We’ve also been able to define the concept of hindsight in the quantum world: ‘What do we know about a quantum system’s state at a given time based on later measurements of the system?’ We have demonstrated the answer to this question with experimenters in France and the US, and in recent years we have shown its applications to precision measurements together with colleagues in China. 

What does it mean to you to receive the Carlsberg Foundation Research Prize?

I feel honoured and moved to receive the Carlsberg Foundation Research Prize. My dear family are my anchor in life, but it’s been important to me throughout my professional career to view my closest colleagues as my friends. I see the fact that my colleagues nominated me for this prize as confirmation of the depth of our professional relationship and a reminder of the best aspects of my working life.

I’ve had the privilege of supervising numerous students and young researchers whose projects have produced exciting results and always given me new insights. I’ve always been keen for my younger colleagues to pursue their own ideas, and I have been grateful to be part of their research careers. I consider it important to stress that it’s for this joint effort, and to a great extent for their work, that I’m now being awarded the Carlsberg Foundation Research Prize.