A quantum interface based on an ultrathin glass fiber that connects light particles and atoms has been realized by physicists at the Johannes Gutenberg University of Mainz. According to the team, this is an essential prerequisite for quantum communication, which will be used for secure data transmission via quantum cryptography. "Our quantum interface might also prove useful for the realization of a quantum computer," said Dr. Arno Rauschenbeutel, professor at the Institute of Physics at Mainz University. Today, telephone and Internet rely primarily on the optical transmission of data using glass fiber cables. In that sense, glass fiber networks can be considered the backbone of the modern communication society. The light that travels through them is not a continuous flow of energy. As was discovered by Albert Einstein, it rather consists of indivisible energy quanta, or photons. Each photon can then transmit one bit of information, corresponding to a zero or a one. The Mainz quantum interface. Laser light that travels through a tapered glass fiber is used to trap cesium atoms along its ultrathin waist. The central part of the fiber is thinner than the wavelength of the light itself. As a consequence, the latter protrudes into the space surrounding the fiber and couples to the trapped atoms. (Image: QUANTUM) In addition to being very efficient, this method opens the route to entirely new ways of communication because photons, being quantum objects, can exist simultaneously in both states, zero and one. As an example, this property is what makes quantum cryptography possible and thereby enables absolute protection against eavesdropping. To fully exploit the potential of quantum communication, however, one additionally needs the possibility of storing the quantum information that is encoded on each photon. Photons themselves are not well-suited for this purpose because one cannot hold them at a given position. Therefore, it would be much more advantageous to transfer the quantum information to atoms. For this purpose, one requires a quantum interface between photons and atoms that should ideally be easily integrated into glass fiber networks. As reported in the journal Physical Review Letters, the team’s glass fiber was heated and stretched until it measured only one-hundredth the diameter of a human hair. The nanofiber is thinner than the wavelength of the light it guides. As a consequence, the light is no longer restricted to the inside of the nanofiber but laterally protrudes into the space surrounding the fiber. Using this so-called evanescent field, the scientists trapped cesium atoms after they had been cooled to a few millionths of a degree above absolute zero by irradiation with suitably chosen laser light. When trapped, the atoms are arranged in a regular pattern and are levitated 200 nm above the surface of the nanofiber. This distance might seem very small, but it is big enough to protect the atoms from the spurious influences of the fiber surface. At the same time, the atoms reside in the evanescent field and thus interact with the photons propagating through the nanofiber. As was demonstrated by the Mainz researchers, this process is so efficient that only a couple of thousand atoms should suffice for a close to lossless transfer of quantum information between photons and atoms. Further possible applications for the Mainz quantum interface include the connection of different quantum systems. As an example, the trapped atoms could be brought into close proximity to a superconducting quantum circuit to combine the advantageous properties of both systems. This would be an important step toward the realization of a quantum computer. For more information, visit: www.uni-mainz.de/eng