Quantum communication over long distances is seen as the future of information security and has been demonstrated in free space and fibre with two-dimensional states, recently over distances exceeding 1200 km between satellites. But using only two states reduces the information capacity of the photons, so the link is secure but slow. To make it secure and fast requires a higher-dimensional alphabet, for example, using patterns of light, of which there are an infinite number. One such pattern set is the orbital angular momentum (OAM) of light.

Increased bit rates can be achieved by using OAM as the carrier of information. However, such photon states decay when transmitted over long distances, for example due to mode coupling in fibre or turbulence in free space, thus requiring a way to amplify the signal. Unfortunately, such ‘amplification’ is not allowed in the quantum world, but it is possible to create an analogy, called a quantum repeater, akin to optical fibre repeaters in classical optical networks.

Figure 1. The core element of the quantum repeater is a cube of glass. The researchers put two independent photons in, and as long as they could detect two photons coming out the other side they knew that they could perform entanglement swapping.

An integral part of a quantum repeater is the ability to entangle two photons that have never interacted – a process referred to as entanglement swapping. This is accomplished by interfering two photons from independent entangled pairs, resulting in the remaining two photons becoming entangled. This allows the establishment of entanglement between two distant points without requiring one photon to travel the entire distance, thus reducing the effects of decay and loss. It also means that a line of sight is not necessary between the two places.

Figure 2. An alphabet of OAM modes. OAM modes are sometimes called twisted light as the light appears as a ring with a vortex in the middle. The light can be twisted once, twice, three times and so on to create a high-dimensional alphabet.

An outcome of this is that the information of one photon can be transferred to the other, a process called teleportation. Like in the science fiction series, Star Trek, where people are ‘beamed’ from one place to another, information is teleported from one place to another. If two photons are entangled and the value of one of them is changed, the other one automatically changes too. This happens even though the two photons are never connected and, in fact, are in two completely different places.

Figure 3. A schematic of the experiment. Four photons are created, one pair from each entanglement source (BBO). One from each pair (B and C) are brought together on a beam splitter. When all four photons are measured together one finds that photons A and D, which previously were independent, are now entangled.

In this latest work, the team performed the first experimental demonstration of entanglement swapping and teleportation for orbital angular momentum (OAM) states of light. They showed that quantum correlations could be established between previously independent photons, and that this could be used to send information across a virtual link. Importantly, the scheme is scalable to higher dimensions, paving the way for long-distance quantum communication with high information capacity.

Figure 4. An experiment being conducted in the Structured Light Laboratory at Wits University.