New Devices for Global Quantum Networks

Quantum device in combination with an artist's illustration of nanosatellites establishing a space-based quantum network (Source: NUS)

Quantum device in combination with an artist’s illustration of nanosatellites establishing a space-based quantum network (Source: NUS)

You can’t sign up for the quantum internet just yet, but researchers have reported a major experimental milestone towards building a global quantum network in space. With a network that carries information in the quantum properties of single particles, you can create secure keys for secret messaging and poten­tially connect powerful quantum computers in the future. But scientists think you will need equipment in space to get global reach. Researchers from the National Univer­sity of Singapore (NUS) and the University of Strath­clyde, UK, have become the first to test in orbit technology for satel­lite-based quantum network nodes.

They have put a compact device carrying components used in quantum commu­nication and computing into orbit. The team’s device dubbed SPEQS creates and measures pairs of photons. Results from space show that SPEQS is making pairs of entangled photons with correlated properties. Team-leader Alexander Ling at the Centre for Quantum Techno­logies at NUS, said: “This is the first time anyone has tested this kind of quantum techno­logy in space.” The team had to be inventive to redesign a delicate, table-top quantum setup to be small and robust enough to fly inside a nano­satellite only the size of a shoebox. The whole satellite weighs just 1.65-kilogramme.

Making correlated photons is a precursor to creating entangled photons. Artur Ekert, Director of CQT, invented the idea of using entangled particles for crypto­graphy. He said: “Alex and his team are taking entanglement, literally, to a new level. Their experiments will pave the road to secure quantum commu­nication and distr­ibuted quantum computation on a global scale. I am happy to see that Singapore is one of the world leaders in this area.”

Local quantum networks already exist. The problem Ling’s team aims to solve is a distance limit. Losses limit quantum signals sent through air at ground level or optical fibre to a few hundred kilometres, but we might ultimately use entangled photons beamed from satellites to connect points on opposite sides of the planet. Although photons from satel­lites still have to travel through the atmosphere, going top-to-bottom is roughly equivalent to going only 10 kilometres at ground level.

The group’s first device is a techno­logy path­finder. It takes photons from a BluRay laser and splits them into two, then measures the pair’s proper­ties, all on board the satellite. To do this it contains a laser diode, crystals, mirrors and photon detectors carefully aligned inside an aluminum block. This sits on top of a 10 centimetres by 10 centimetres printed circuit board packed with control electronics. Through a series of pre-launch tests the team became more confident that their design could survive a rocket launch and space conditions. The team had a device in the October 2014 Orbital-3 rocket which exploded on the launch pad. The satel­lite containing that first device was later found on a beach intact and still in working order.

Even with the success of the more recent mission, a global network is still a few mile­stones away. The team’s roadmap calls for a series of launches, with the next space-bound SPEQS slated to produce entangled photons. SPEQS stands for Small Photon-Entangling Quantum System. With later satellites, the researchers will try sending entangled photons to Earth and to other satellites. The team are working with standard “CubeSat” nano­satellites, which can get relatively cheap rides into space as rocket ballast. Ulti­mately, completing a global network would mean having a fleet of satellites in orbit and an array of ground stations.

In the meantime, quantum satellites could also carry out funda­mental experiments for example, testing ent­anglement over distances bigger than Earth-bound scientists can manage. “We are reaching the limits of how precisely we can test quantum theory on Earth,” said Daniel Oi at the University of Strathclyde. (Source: NUS)

Reference: Z. Tang et al.: Generation and Analysis of Correlated Pairs of Photons aboard a Nano­satellite, Phys. Rev. Applied 5, 054022 (2016); DOI: 10.1103/PhysRevApplied.5.054022

Link: Centre for Quantum Technologies, National University of Singapore, Singapore • SUPA Department of Physics, University of Strathclyde, Glasgow, UK

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