A team of researchers from Ludwig-Maximilian University of Munich, Germany, has been successful in implementing technology for transmitting quantum information and the phenomenon of quantum entanglement from one stationary quantum storage device to another through a conventional optical telecommunication channel. During the experiment, information contained in the state of an atomic quantum bit was converted to the state of a photon of light, which, having traveled a distance of 20 kilometers through a fiber optic cable, successfully transmitted this information to another atomic quantum bit.

This achievement will make it possible in the future to increase the distance separating the individual parts of quantum concentrators and quantum computers, physically separated, but remaining, at the same time, online. In addition, the technologies used can underlie the construction of a quantum repeater, a device that can push the boundaries of the use of quantum technologies to regional, national, and even international scales.

Note that right now, German researchers have implemented only half the functionality of a quantum communication system. To get a full-fledged communication channel, they will need to do another exactly the same transformation of information, only in the opposite direction.

The experiment on the transfer of quantum information was started with a rubidium atom enclosed in an optical laser trap and cooled to a temperature of several millionths of a degree above absolute zero. This atom was selected from a random cloud of rubidium atoms and moved into a trap using optical tweezers, focused in a special way by a beam of laser light that allows you to move tiny objects.

Then this atom was transferred to an excited energy state in which it became a quantum particle, and the quantum information contained in it was encoded in the spin, in the direction of rotation of the upper excited electron. When an atom spontaneously returned to a lower energy state, it emitted a photon of light whose polarization corresponded to the electron’s spin, and which was entangled with the atom at the quantum level.

The next steps were capturing this photon of light, converting it into an S-band photon and directing it into an optical fiber 20 kilometers long. Note that S-band light is able to pass through the optical fiber and a slightly larger distance before it is attenuated or distorted, which will lead to the loss of the quantum information contained in it. The boundary length of the fiber-optic line has led to the fact that only 78 percent of light photons retained entanglement with the rubidium atom.

Quantum entanglement

Now German scientists are working on a receiving unit, in which the photons of light transmit quantum entanglement and the quantum information they contain to another rubidium atom placed in the same conditions as the atom at the other end of the line. This site will be installed in the laboratory of the Max Planck Institute for Optics in Munich, which is located 20 kilometers from the University of Munich.

And, in the end, in order to make a really working quantum communication channel out of all this, German scientists have to develop and implement a suitable error detection and correction technology for this. This task is very difficult, but its solution is a necessary step for creating practical systems capable of transmitting the state of quantum entanglement from one stationary qubit to another over distances amounting to many tens and hundreds of kilometers.