Imagine a network where data transmission is completely secure from hacking. This will be possible if physicists get their way. It is possible with the quantum mechanical phenomenon called entanglement. The rule for entangled particles is that if you measure one particle’s state, you can automatically determine the state of the other. It doesn’t matter how far apart the entangled particles may be from one another. This is a great way to transmit information long distances without eavesdropping.
Two atomic quantum memories have been linked by Prof. Harald Weinfurter, LMU, and Prof. Christoph Becher, Saarland University. This is a team of physicists. This is the longest distance that anyone has managed to entanglement using a telecom fiber.
Photons are emitted from the two quantum memories and mediate quantum mechanical entanglement. The researchers made a decisive step by shifting the wavelength of the emitted particles to a value suitable for conventional telecommunications. Weinfurter says, “By doing so, we were in a position to reduce the loss of photons significantly and create entangled quantum memory even over long distances using fiber optic cable.”
In general, quantum networks are composed of nodes that contain individual quantum memories, such as atoms or ions. These nodes can receive, store and transmit quantum states. Light particles can be used to mediate between nodes. These light particles can be exchanged over the air or through a specific fiber optic connection. The researchers used a system consisting of two optically trapped rubidium-atoms from two LMU campuses for their experiment. These two locations are linked by a 700-meter long fiber optic cable that runs beneath Geschwister Scholl Square, in front of the university’s main building. You can make connections up to 33 km long by adding fibers to the coils.
After being excited by a laser pulse, the atoms spontaneously return to their ground state and emit a photon. Because of conservation of angular momentum, spin of an atom is bound to the polarization of its emitting photon. These light particles can be used to create a quantum mechanic coupling between the two atoms. The scientists used the fiber optic cable to send them through to a receiver station. A joint measurement of photons indicated an entanglement in the quantum memories.
Quantum memories emit light in the visible and near-infrared ranges. Christoph Becher explains, “In fiber optics these photons make a few kilometers before being lost.” Saarbrucken’s physicist and his team optimized wavelengths of photons to ensure that they travel in the cable. They used two quantum frequency convertors to increase the wavelength from 780 nanometers down to 1,517 nanometers.
Becher says, “This is very close to the so-called telecom wavelength which is approximately 1,550 nanometers.” The telecom band refers to the frequency range where fiber optics transmit light with the lowest losses. The conversion was achieved by Becher’s team with an astonishing 57% efficiency. They were able to preserve the high quality information in the photons, which is a condition for quantum coupling.
Tim van Leent, the lead author of Nature’s paper, said that “the significance of our experiment was that we actually entangle 2 stationary particles” (that is, atoms that act as quantum memories). This is a more complex task than entangling photons but opens up many other application possibilities.
Researchers believe that the system they have developed could be used for large-scale quantum networks as well as secure quantum communication protocols. Harald Weinfurter says that the experiment is an important step in the development of a quantum internet using existing fiber optic infrastructure.
