The collaboration of TU Wien with research groups in China has resulted in a crucial building block for a new kind of quantum computer: The realization of a novel type of quantum logic gate makes it possible to carry out quantum computations on pairs of photons that are each in four different quantum states, or combinations thereof. The advancement is an important milestone for optical quantum computers. The study has now been published in Nature Photonics.

The basic idea of quantum computers is simple: While a classical computer only works with the values “0” and “1,” quantum physics allows for arbitrary combinations of these states. In a certain sense, a quantum bit (“qubit”) can be in the states 0 and 1 simultaneously. This makes it possible to develop algorithms that can solve some problems much faster than a comparable classical computer.

However, such superpositions can in principle involve more than two states. Depending on what degree of freedom one considers, a quantum system such as a photon may not just have two different settings—two different outcomes of a potential measurement—but many. In this case, one refers to the system as a “qudit” rather than a “qubit.”