FEATURE: NEXT GENERATION OPTICAL NETWORKS


1,550nm. ‘Te two works used the same basic principles to generate spatial entanglement and numerically reconstruct quantum states from the set of measurement data,’ continued Park. Tese studies show the feasibility, benefits


and also the shortcomings of fibre transport of high-dimensional quantum states. ‘Such high-dimensional states, carrying more than one bit-per-photon, can increase the capacity limit of quantum communications imposed by the operation speeds of single-photon sources and detectors,’ said Park. ‘Our work also identifies the practical limitations of transmission distance due to non-idealities of the fibres and presents the methods to characterise those limiting factors.’ However, Park stressed how important high-


Andrew Forbes Two photons are entangled, one in polarization and the other in orbital angular momentum - twisted light. By passing the polarization photon through the fibre and keeping the twisted light in air, multi-dimensional entanglement transport is possible even over single mode fibre.


Innsbruck, Austria transported polarisation- entangled photons 50km through single-mode fibre at the most commonly used telecom wavelength, 1550nm. ‘Tat’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. Tis 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’ paterns 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 paterns down fibre because paterns 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!’


Spatial delivery Te team exploited transverse mode components of light’s electromagnetic field that arise in order to ensure light remains within optical fibre. Tey were the first to transport spatially entangled photons through a hollow core photonic crystal optical fibre. Tey preserved entanglement over 30cm using light wavelengths around 826nm. Loeffler stressed


10 FiBRE SYSTEMS n Issue 27 n Spring 2020


that transporting high-dimensional spatial modes over longer distances is difficult. ‘It is already a challenge to phase-stabilise


single spatial modes, which is required for specific quantum network types,’ he said. ‘Tis is even harder for many spatial modes, so I think that for quantum network applications, single- mode will be the transport of choice. Not even classical information is transported via long- distance multimode fibres due to intermodal dispersion. But, who knows? If the advantages of high-dimensional quantum states weigh stronger than technical issues, the case may be different. High-dimensional quantum entanglement can be made also amongst spectral modes, not only spatial modes, so it can be sent through single- mode fibres.’ Park’s team also exploited spatial


entanglement, having worked on linearly polarised (LP) modes in research published in 2012. ‘Photons guided by optical fibres are in discrete modes having particular spatial field distribution in analogy with electrons residing in discrete energy levels in a potential well,’ he explained. ‘Each LP mode becomes a logical quantum state of photons, and photons can be present generally in superposition states of those modes.’


Split the difference Using this approach, the KRISS team split two entangled photons with wavelengths near 810nm and sent them down separate 40cm- long two-mode fibres. ‘To the best of my knowledge, we are the only group that has demonstrated propagation of two spatially entangled photons respectively through two different fibres,’ Park said. In 2019, the KRISS researchers extended this


approach into two 50cm-long multicore fibres to entangle four spatial modes. Tey used light with telecom-appropriate wavelengths around


dimensional entanglement could be in quantum networks. ‘By transferring the quantum state of entangled photons to the qubits of quantum computers, two physically separated quantum processors can be, in principle, connected by entanglement to build a distributed quantum computer,’ he explained. ‘High-dimensional encoding helps us to increase the amount of quantum links that can be generated by single photons. Of course there are a huge lot of technical challenges regarding connection between different types of qubits and long- distance transmission of photons.’


On the right wavelength Existing quantum computers are based on superconducting or ion-trap qubits. Tese oſten interact with microwave-range photons that have centimetre wavelengths, puting them far outside the telecom window. Park noted that there are theoretical proposals to inter-convert such photons to optical wavelengths. ‘I can only say that they are not impossible,’ he observed. ‘Quantum transduction to fibre-compatible


frequencies is very challenging and an active research field,’ agreed Loeffler. ‘Tis is one reason why also considerable investments are done in the development of optical quantum


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


www.fibre-systems.com @fibresystemsmag


Wits University


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32