FEATURE OPTICAL COMMUNICATIONS


Making a quantum leap into commercial fibre networks


Progress is being made on entanglement methods that can work in mainstream telecom wavelengths, in conventional networks finds Andy Extance


A


fter more than a decade’s work, the prospect of full quantum networks exploiting entangled photons is drawing closer. That’s


thanks to the work of researchers like Hee Su Park from the Korea Research Institute of Standards and Science (KRISS). From 2005 to 2010 he built up a quantum optics lab to develop measurement techniques for quantum information technologies. He wanted to find out to what extent optical fibres guiding multiple spatial modes can be used for quantum communications. Quantum networks encourage ‘collaboration by scientists in diverse fields, and surely inspire a lot of novel and interesting ideas,’ Park explained.


And, findings from scientists like Park


are already suggesting ways in which quantum networks could be integrated with fibre networks. Experimental approaches previously used different wavelengths of light and different types of fibre to the predominantly single-mode, single core fibre infrastructure that today underlies the internet. But, new methods increase the chances that quantum networks might integrate with existing systems. It may be at least 20 years until that becomes a reality – but findings are already being published


18 Electro Optics June 2020


Optical polarisation of defect spin in silicon carbide could enable qubits that link with quantum networks at conventional telecom wavelengths.


Spatial delivery The team exploited transverse mode components of light’s electromagnetic field that arise in order to ensure light remains within optical fibre. They were the first to transport spatially entangled photons through a hollow core photonic crystal optical fibre. They preserved entanglement over 30cm using light wavelengths around 826nm. Loeffler stressed that transporting high-dimensional spatial modes over longer distances is difficult.


@electrooptics | www.electrooptics.com


that put the process in motion. For quantum networks, the key physical phenomenon to harness is entanglement, where properties of at least two different particles are correlated. In quantum computers, entanglement enables calculations that are extremely difficult for classical computers, in applications such as code-breaking and simulating chemical systems.


Spin city Andrew Forbes from the University of the Witswatersrand in Johannesburg, South Africa, groups the ways quantum communication experiments entangle photons into two extremes. The most common approach exploits correlation between the polarisation, or spin angular momentum, of two photons. In 2019 researchers at the Institute for Quantum Optics and Quantum Information (IQOQI) in Innsbruck, Austria transported polarisation-entangled photons 50km through single-mode fibre at the most commonly used telecom wavelength, 1550nm. ‘That’s relatively easy to set up,’


he said. However, it entangles only two states, specifically opposite spin directions, which limits the amount of quantum information that photons can carry. At the other extreme, observed Forbes,


is spatial mode entanglement transport. This involves spatial light modulation, which integrates two light beams. It uses one beam to modulate the other’s spatial information, including its phase, polarisation state, intensity and propagation direction. Forbes noted that this can create ‘high-dimensional’ patterns that potentially include an infinite number of entangled states. ‘You have now lots of information for every photon, but it’s extremely hard to get those patterns down fibre because patterns tend to couple into one another,’ he said. Wolfgang Loeffler’s team, at Leiden


University in the Netherlands, decided to study such spatial light modulation techniques of photon entanglement as far back as 2011, when they were relatively new. ‘Most work was done in theory and not much about fibre transport of complex modes was known,’ Loeffler explained. ‘Nobody could answer the question “can we transport spatially quantum-entangled photons through an optical fibre?” So, we had to try this!’


Courtesy of Tom Bosma, the University of Groningen.


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