It is a milestone and a significant step toward the quantum internet that researchers from LMU and Saarland University were able to entangle two quantum memories across a 33-kilometer fiber optic link.
a network where data transfer is 100% safe from hacking? This will eventually become a reality thanks to the quantum mechanical phenomena known as entanglement, if scientists get their way. If you measure the state of one of the entangled particles, you instantly know the state of the other particle. No matter how far apart the entangled particles are from one another, nothing changes. This is the best situation for securely delivering information over long distances while preventing eavesdropping.
Professors Harald Weinfurter from LMU and Christoph Becher from Saarland University are leading a team of scientists that has successfully connected two atomic quantum memories via a 33-kilometer fiber optic connection. This is the furthest distance over which entanglement over a telecom fiber has ever been achieved to date. The two quantum memories' photons act as a medium for the quantum mechanical entanglement. The researchers' decision to change the wavelength of the light particles they were emitting to one appropriate for traditional telecommunications was a key move. Through this method, Weinfurter explains, "we were able to drastically lower the loss of photons and establish entangled quantum memories even over great distances of fiber optic cable."
In general, quantum networks are made up of nodes that are made up of discrete quantum memories, such as atoms, ions, or flaws in crystal lattices. These nodes have the capacity to acquire, hold, and send quantum states. Light particles transferred over the air or in a targeted manner via a fiber optic link can mediate communication between the nodes. The researchers conduct their experiment in two labs on the campus of LMU using a system made up of two rubidium atoms that have been optically trapped. A 700-meter fiber optic cable that connects the two locations runs below Geschwister Scholl Square in front of the university's main building. Connections of up to 33 kilometers in length can be made by adding new fibers to coils.
The atoms are excited by a laser pulse, and when they return to their ground state on their own, each one emits a photon. The spin of the atom is entangled with the polarization of its released photon as a result of the conservation of angular momentum. The two atoms can then be coupled quantum mechanically using these light particles. To do this, the researchers transmitted them via fiber optic cable to a receiver station, where a combined measurement of the photons reveals a quantum memory's entanglement.
However, the majority of quantum memory produce light with visible or near-infrared wavelengths. According to Christoph Becher, "In fiber optics, these photons travel just a few kilometers before they are lost." The physicist from Saarbrücken and his group tuned the photons' wavelength for travel through the cable because of this. They extended the initial wavelength from 780 nanometers to 1,517 nanometers using two quantum frequency converters. According to Becher, "this is quite near to the so-called telecom wavelength of about 1,550 nanometers." The frequency range in which there are the fewest losses in fiber optic light transmission is known as the telecom band. The conversion was completed by Becher's team with an exceptional efficiency of 57%. In addition, they were able to maintain the photons' high level of information quality, which is necessary for quantum coupling.
According to Tim van Leent, the paper's principal author, "the importance of our work is that we genuinely entangle two stationary particles — that is, atoms that serve as quantum memory." Although far more challenging than entangling photons, this offers a wide range of new application possibilities. The researchers believe that the method they created might be utilized to build expansive quantum networks and to put secure quantum communication protocols into practice. According to Harald Weinfurter, "the experiment represents a crucial step toward the quantum internet based on current fiber optic infrastructure."
