OPTICAL COMMUNICATIONS FEATURE
“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”
Quantum memories containing many atoms have been entangled via commercial fibre links
‘It is already a challenge to phase- stabilise single spatial modes, which is required for specific quantum network types,’ he said. ‘This 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.’
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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. They used light with telecom-appropriate wavelengths around 1,550nm. ‘The two works used the same basic principles to generate spatial entanglement and numerically reconstruct quantum states from the set of measurement data,’ continued Park. These 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-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. These often interact with microwave-range photons that have centimetre wavelengths, putting 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. ‘This is one reason why also considerable investments are done in the development of optical quantum computers, where this transduction would be much simpler and efficient.’ He therefore hopes that researchers will
be able to improve systems to achieve photon coupling good enough in order to demonstrate nontrivial quantum networks in a few years. ‘Many exciting technologies
June 2020 Electro Optics 19 g
Quantop
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