A joint team composed of researchers from South Africa’s University of the Witwatersrand (Wits), in Johannesburg, and China’s Huazhang University of Science and Technology (HUST), in Wuhan, has successfully demonstrated the transmission of “multiple quantum patterns of twisted light” down a 250-m-long conventional optical fibre link.

Normally, such a conventional link could only carry a single pattern of light. The research team was led by the Wits School of Physics’ professor Andrew Forbes and HUST’s professor Jian Wang. The results of their work are contained in a paper, ‘Multi-dimensional entanglement transport through a single mode fibre’, recently published online in Science Advances.

Light is, of course, used to transmit data along optical fibres. This is very fast, but not very secure. The new discovery has very important implications for making that data transmission extremely secure.

The breakthrough makes use of a phenomenon known as quantum entanglement, famously described by Albert Einstein as “spooky action at a distance”. A simple definition of quantum entanglement, given by Charles Choi in a 2009 article in Scientific American, is that it occurs when subatomic “objects can become linked and instantaneously influence one another regardless of distance”. Other phenomena used are the polarisation of light, for which there are only two values (or aspects) and patterns of light (each pattern can be unique). In principle, there can be an infinite set of light patterns.

Combining these phenomena, the research team engineered quantum entanglement between two photons of light in what the team described as two degrees of freedom of light, that is, the polarisation and the pattern of the light. The polarised photon was sent down the fibre while the other photon accessed the many patterns of light. (Because the photons are entangled, it is not necessary for the both of them to go through the fibre.) The result is described as multidimensional entangled states.

“In essence, the research introduces the concept of communicating across legacy fibre networks with multidimensional entangled states, bringing together the benefits of existing quantum communication with polarised photons and the benefits of high-dimension communication using patterns of light,” said Forbes. “The trick was to twist the one photon in polarisation and twist the other in pattern, forming ‘spirally light’ that is entangled in two degrees of freedom. Since the polarisation entangled photon has only one pattern it could be sent down the long-distance single- mode fibre, while the twisted light photon could be measured without the fibre, accessing multidimensional twisted patterns in the free space. These twists carry orbital angular momentum, a promising candidate for encoding information.”

The basic unit for quantum computing and quantum communication is the quantum bit, better known as the qubit. This is the counterpart of the bit in classical computing. Dutch research centre QuTech defines a qubit as “a two-level quantum system” with two “basis states”, just as a bit can be either 0 or 1. The huge difference is that a bit must be either 0 or 1 but a qubit can be 0 or 1, or both at the same time.

“The novelty in the [newly] published work is the demonstration of multidimensional entanglement transport in conventional single-mode fibre,” he highlighted.