Quantum machines consist of single atoms or modes of light, and they obey the physical theory of quantum mechanics. They are able to simulate complex systems that cannot be represented differently. "This leads to a question that does not lack a certain amount of irony," says Eisert, the leader of a research group at Freie Universität that deals with quantum many-body theory and quantum information. "If it is not possible to find out what the machine is doing internally, how is it possible to check, whether it is functioning correctly?" The accuracy of the results cannot be calculated directly. Eisert says this paradox is similar to the famous answer 42 in the cult book, The Hitchhiker’s Guide to the Galaxy, where 42 was (jokingly) given as the answer to life, the universe, and everything. Here, the number 42 is the answer, but what is the question that the computer solved?

In a paper published in Nature Communications, the scientists demonstrated how they solved this paradox for a large class of quantum machines. They show how it is possible to certify the correct preparation of quantum states using an efficient and experimentally friendly method. The procedures they developed are particularly well-suited for optical systems in which single photons obey the quantum laws.

The research done in Eisert's group was sponsored by the European Union (RAQUEL, SIQS, AQuS, Marie-Curie), the European Research Council (TAQ), the German Federal Ministry of Education and Research, and the German Academic Scholarship Foundation.