Frequency Comb Produces an Entangled Web

Entanglement underlies all of quantum information science. It is most familiar as a relation between two parts of a single system, in which the properties of each part—taken separately—are uncertain but at the same time correlated with the properties of the other part. This so-called “bipartite entanglement” is, however, just one way in which a system can be entangled. Much more complicated is the entanglement between multiple parts of a system. Such “multipartite entanglement” is of interest because it is known to improve certain quantum information protocols and enable others, like secret key sharing between multiple parties. However, until recently, no well-established criteria existed for this kind of entanglement, and it is still an open question how a quantum system should be split into parts to provide better resources for quantum computation. Stefan Gerke and his colleagues from the University of Rostock in Germany and their colleagues at the Kastler–Brossel Laboratory in France address this question [1]. They split the spectrum of a down-converted frequency comb into ten parts, or bands, and determined the entanglement between various combinations of these parts. The team utilizes a recently developed measure of entanglement called optimal witnesses. This work provides a recipe for creating multipartite entanglement and opens a perspective for using it in quantum information technologies.