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Quantum internet one step closer to reality with innovative wavelength switch

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Engineers from Purdue University have developed a device to address a complication which has stood in the path of developing quantum networks large enough to reliably support more than a handful of users.

The engineers' approach, described in Optica, could form part of the groundwork for establishing a ‘quantum internet’: a large number of interconnected quantum computers, quantum sensors and other quantum technologies exchanging data.

They developed a programmable switch which can be used to adjust how much data goes to each user in the network by selecting and redirecting wavelengths of light carrying the different data channels, making it possible to increase the number of users without adding to photon loss as the network grows. When photons are lost - which becomes more likely the further they have to travel through fibre-optic networks - their associated quantum information is lost.

“We show a way to do wavelength routing with just one piece of equipment – wavelength-selective switch – to, in principle, build a network of 12 to 20 users, maybe even more,” said Professor Andree Weiner, an electrical and computer engineer. “Previous approaches have required physically interchanging dozens of fixed optical filters tuned to individual wavelengths, which made the ability to adjust connections between users not practically viable and photon loss more likely.”

Rather than adding these fixed filters every time a new user joins the network – which makes scaling an awkward process – engineers can simply program the wavelength-selective switch to direct data-carrying wavelengths over to each new user. This would reduce operational and maintenance costs, in addition to making the quantum internet more efficient.

The switch could also be programmed to adjust bandwidth in response to a user’s needs; this is not possible with fixed optical filters. This is based on similar technology to that used for adjusting bandwidth for classical communication, a widespread practice today. Like classical light-based communications, the switch is also capable of using a flex grid to partition bandwidth to users at a variety of wavelengths and locations, rather than being restricted to a series of fixed wavelengths, each with a fixed bandwith.

Forming connections between users of a quantum internet and adjusting bandwidth means distributing entanglement: a quantum-mechanical phenomenon in which at least two particles are created with “entangled” states. This means that they have a fixed relationship to each other no matter the distance between them; change the state of one and the state of the others change instantaneously. Entanglement is one of the quantum phenomena at the core of quantum information and quantum computing.

“When people talk about a quantum internet, it’s this idea of generating entanglement remotely between two different stations, such as between quantum computers,” said PhD candidate Navin Lingaraju. “Our method changes the rate at which entangled photons are shared between different users. These entangled photons might be used as a resource to entangle quantum computers or quantum sensors at the two different stations.”

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