Quantum communication over long distances is essential for information security, and it has been shown in free space and fiber using two-dimensional states, most notably across distances of more than 1200 km between satellites. However, utilizing only two states lowers the photons’ information capacity, making the communication safe but sluggish. To make it safe and quick, a higher-dimensional alphabet is required, such as utilizing light patterns, of which there are an unlimited number. The orbital angular momentum (OAM) of light is one such pattern set.
By employing OAM as the information carrier, bit rates may be increased. However, such photon states degrade when transmitted over long distances, for example, due to mode coupling in fiber or turbulence in free space, necessitating an amplification method. Unfortunately, such “amplification” is not permitted in the quantum realm, but an analogue, known as a quantum repeater, is feasible, similar to optical fiber repeaters in classical optical networks.
The core element of the quantum repeater is a cube of glass. Two independent photons were put in, and as long as two photons coming out the other sides are detected, we know that entanglement swapping can be performed.
The capacity to entangle two photons that have never interacted before is an essential component of a quantum repeater, a process known as “entanglement switching.” This is achieved by interfering two photons from separate entangled pairs, causing the remaining two photons to become entangled. This enables the formation of entanglement between two distant sites without requiring a single photon to travel the full distance, minimizing the effects of decay and loss. It also means that there is no need for a direct line of sight between the two locations.
The information of one photon can be transmitted to the other, a process known as teleportation. Information is “teleported” from one location to another, similar to how individuals are “beamed” from one location to another in the science fiction series Star Trek. When two photons are entangled, if you alter the value of one of them, the other one changes as well. This occurs despite the fact that the two photons are never linked and are in reality at two distinct locations.
As a result, the researchers carried out the first experimental demonstration of entanglement switching and teleportation for orbital angular momentum (OAM) states of light. They demonstrated that quantum correlations could be formed between previously independent photons and that this could be utilized to transfer data via a virtual connection. The method is also scalable to larger dimensions, paving the path for long-distance quantum communication with great information capacity.
Long distance communications through quantum teleportation of light patterns, Prof. Jonathan Leach, Heriot-Watt University, Scotland and Prof. Andrew Forbes, University of the Witwatersrand
Published: October 2017