Press embargo lifts on 04 March at 1800 London time / 1300 US Eastern time A quantum wedding of two crystals A team of researchers lead by Nicolas Gisin and Mikael Afzelius at the University of Geneva are developing a new approach to bring quantum networks to reality, and their latest finding has just been published in Nature Photonics. “What we have demonstrated is that two solid macroscopic objects, namely two 1cm long crystals, can be entangled together. This means that both crystals essentially form a single quantum entity, and that one cannot fully describe the state of each crystal alone,” says Mikael Afzelius. The result is reminiscent of the famous cat of Erwin Schrödinger, except that here, we have two cats such that if one is alive, then the other one is dead, or viceversa. Entanglement means that the two possibilities coexist simultaneously, until someone tries to “look”, which forces one possibility to emerge over the other. In the laboratory, the cats are embodied as rareearthions doped crystals, the same type of crystals that are nowadays widely used in solidstate lasers, and notably in the green laser pointers that everyone can buy. Entanglement in itself is not new; it is routinely produced with photons in many laboratories around the world. However, to go from entanglement between two elementary particles of light, to entanglement between to macroscopic objects, is a challenging task. “Each crystal contains about ten billion rareearth ions that can collectively absorb a single photon. If this single photon is first sent onto a halfsilvered mirror, it emerges as an entangled state of light that can be absorbed by the crystals, thereby transferring the entanglement from light to two solid objects,” explains co author Félix Bussières. The main difficulty is in fact to prove that the crystals have indeed been entangled, which the team achieved by transferring back the entanglement into light and then performing sophisticated measurements on the light particles. But entanglement is not just a striking feature of quantum mechanics: it is a much needed resource for quantum communication. What is envisioned by the researchers is a “quantum internet” where nodes can exchange and processing quantum bits of information. Quantum physics then allows nodes to be entangled, and thus gain the ability to perform important tasks, such as provably secure encryption of messages over distances of a thousand kilometres or more, which is not known to be feasible otherwise. The achievement of the research team is a crucial step towards the realization of practical quantum networks.