Quantum optimization with an atom-light simulator

When we are faced with a choice between many options, it is sometimes not so easy to decide. There are especially many options in situations where the order or arrangement of objects is at stake, so-called combinatorial problems. A well-known example occurs when packing a suitcase for a flight: suppose we would like to take our favourite clothes with us, but unfortunately their total weight exceeds the luggage weight allowed by the airline. So we try to fit as many clothes as possible in the suitcase without exceeding the allowed weight. Finding the best solution among all the many combinations is not easy: even with a dozen clothing items, the number of possibilities can run into the millions. Combinatorial optimisation problems do not only occur when packing suitcases, but also, for instance, in logistics, medicine or chemistry. At the same time, they are among the most challenging problems for our computers, because all the countless possibilities have to be searched through and evaluated individually in order to find the solution. Fortunately, quantum physics provides a remedy: if the problem could be programmed into a quantum mechanical system, it would be possible to solve optimisation problems of unprecedented complexity. The idea is that the quantum system can be brought into a superposition of several possibilities; this makes it possible to search through all possibilities simultaneously and thus find the optimal solution considerably faster. In recent years, extraordinary progress has been made in the realisation of such quantum systems. However, one central building block continues to cause difficulties: in order to solve optimisation problems, a special coupling within the quantum system is required that can also link distant particles. This is in contrast to most interactions in physics, because they normally only work between neighbouring particles. It remains a challenge how to realise such a long- range interaction. This is where this START project comes in. The goal is to couple all particles in the quantum system simultaneously to a single light field, which then conducts information through the entire system. This can be thought of as a bus connection that can move data just like passengers between all the stops. Such a quantum processor would produce exactly the long- range interaction needed to solve optimisation problems. In addition to its broad practical applications, such a quantum system also raises fundamental questions in physics. The influence of long-range interactions on quantum states remains one of the most exciting chapters in solid-state physics today. This project will contribute to our understanding of such systems, with the potential to produce materials with new, exotic properties in the future.