FEATURE OPTICAL COMMUNICATIONS
measurement in the memory, prior to being sent from the memory down the next fibre link. In this way, the repeater progressively entangles systems that have not interacted previously. The Witwatersrand team, together
The IQOQI uses nonlinear crystals in a polarisation interferometer pumped by powerful lasers to convert the wavelength of entangled photons to the optimal wavelength for long- distance travel in fibre
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will be developed on the side – this already is justifying the efforts for me,’ Loeffler said. ‘From a pure progress of research point of view, worldwide entangled quantum states would enable a number of very exciting new experiments, from precision metrology to tests of fundamental physics.’ The IQOQI team experiment using polarisation entangled photons has already shown what’s possible for transduction to telecom wavelengths. It collects 854nm photons from a trapped atomic ion qubit. To get the 50km range, the researchers converted these photons to 1550nm by combining them with 1902nm pump-laser photons in a polarisation.
Keep it simple The work of Caspar van der Wal’s group at the University of Groningen is one of several teams developing semiconductor- based qubits that could simplify such transductions. Often the semiconductor is diamond and the qubits occupy nitrogen vacancy (NV) sites in their structure, explained Tom Bosma, a member of van der Wal’s team. NV centres occur when a nitrogen atom substitutes for a carbon atom, carrying an extra electron whose spin can be used as a qubit. ‘Their spin lifetime and coherence times are sizeable even at room temperature,’ Bosma said. ‘It’s more scalable because you can basically make every defect act as a qubit throughout a very large piece of material. Then still, you need to entangle them with each other, which is going to be a challenge. But, one other thing with semiconductor qubits is that you can integrate them more with existing silicon electronics.’ Bosma noted that NV centres also
produce photons with wavelengths around 600nm. While this is closer to telecom wavelengths than better-developed ion- trap and superconducting qubits, this wavelength has attenuation lengths in
20 Electro Optics June 2020
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.
fibre below a kilometre. ‘You would lose your photons that carry the entanglement,’ Bosma said.
Silicon carbide (SiC)-based materials are
similar, he added, but cheaper and easier to produce, and better suited to integration with existing telecom infrastructure. While it’s less well studied, researchers can form qubits where either silicon or carbon atoms are absent, or where one of both types of atom are missing immediately adjacent to each other. Most important for tuning the wavelength of a SiC-based qubit is the fact that it’s easier to dope than diamond, by intentionally adding metal impurities. ‘We’re looking into vanadium defects that actually emit at 1300nm telecom wavelengths,’ Bosma said. ‘And they have, it seems, promising spin properties.’
Link and repeat Researchers at Delft University in the Netherlands are already building quantum networks based on NV centres in diamonds, Bosma said. ‘Then, just like in normal fibre communications, at some distance you need to repeat the signal,’ he noted. ‘We think that silicon carbide might actually fit in as a quantum repeater, that you could in principle, emit at the right wavelength. You could do some entanglement swapping, and then repeat your entanglement through much larger distances.’ Such repeaters could enable quantum
systems interacting ‘like the internet of today,’ Forbes suggested. The erbium-doped fibre amplifiers that serve as repeaters roughly every hundred kilometres in a classical fibre network ‘copy’ the beam to boost its signal. ‘That’s not allowed in the quantum world,’ Forbes said. A leading potential approach that quantum repeaters might adopt would be to separate polarisation entangled photons, sending one to a quantum memory and the other to a Bell state measurement. This stores the entangled outcome of the
Measure for measure ‘We can make it look as if one photon is only entangled through polarisation and the other photon is only entangled through pattern,’ Forbes said. ‘We can take the photon that looks like it’s only polarisation entangled and send it down conventional fibre.’ Then, at the other end, a measurement extracts the spatial pattern information, he added. ‘In the quantum world, the photon doesn’t know about that until you make the measurement.’ Forbes stressed that his group’s aim
“We can take the photon that looks like it’s only polarisation entangled and send it down conventional fibre”
is to use such findings to build a toolkit that helps push information capacities higher. That would build on cutting edge polarisation entanglement studies, like the Nature paper from Jian-Wei Pan’s team at the University of Science and Technology of China, Hefei, China in February 2020. The team established a link between two quantum memories comprising around 100 million laser trapped and cooled atoms over 11km of commercial optical fibre. Similarly to the IQOQI team, they shift the 795nm photon output from the quantum memory using a 1,950 nm pump laser to move it to the 1,342nm telecom O band. Forbes called these achievements ‘crazily difficult experiments’, and used them as a measure for how soon quantum networks will be a reality. ‘How far is it from being commercially realisable? If you can get your work into Nature, it means that it’s 20 years away from being deployable.’ EO
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with Scottish collaborators, were first to publish work on quantum repeaters for more complex, information-rich orbital angular momentum entanglement, Forbes claimed. Their all-optical approach uses an interference phenomenon from quantum optics called the Hong-Ou-Mandel effect to entangle photons that had not previously interacted. He is now working with collaborators on quantum repeaters that mix patterned spatial entanglement modes and polarisation entanglement. And now, as an offshoot of this work, Forbes and his colleagues have found ways to pass information-rich spatially entangled photons down conventional fibre.
IQOQI Innsbruck/Harald Ritsch
Wits University
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