Headline-grabbing experiments by China’s satellite Micius have shown that quantum signals can reach Earth from satellites with their spooky and useful properties intact, pointing the way to building a global quantum internet. An international team led by researchers at the Centre for Quantum Technologies (CQT), National University of Singapore have now shown that nanosatellites might do the job, saving cost compared to using larger satellites.
The team described their nanosatellite SpooQy-1 in a paper that was published on 25 June 2020 in Optica. Data from the 2.6kg satellite, in orbit 400 km above the Earth, confirm that it creates entangled quantum signals in a compact instrument onboard.
Entanglement, once dismissed as ‘spooky action at a distance’ by Einstein, is a correlation in the properties of particles that can bring gains in privacy and computing power.
Both SpooQy-1 and Micius carry quantum sources that create pairs of entangled light particles, called photons. Today’s internet already uses photons to carry data through optical fibres. The challenge in building a quantum internet is that optical losses in fibre limit quantum signals to distances of a few tens of kilometres. Satellites could reach further.
“In the future, our system could be part of a global quantum network transmitting quantum signals to receivers on Earth or on other spacecraft,” said first author Aitor Villar, who worked on the quantum source for SpooQy-1 during his PhD at CQT. “These signals could be used to implement many types of quantum communication application, from quantum key distribution for extremely secure data transmission to quantum teleportation, where information is transferred by replicating the state of a quantum system from a distance.”
Results from Micius published in Nature on 15 June 2020 showed that the satellite sent entangled signals simultaneously to two ground stations 1,120km apart, creating a secure encryption key at a rate of 0.12 bits per second. Micius weighs 630kg in total, including a quantum light source weighing 23.8kg.
Back in the ‘90s, CQT Director Artur Ekert was the first to propose that entangled signals could be exploited for secure communication. He contributed to the new experiments with Micius, working on some theoretical aspects of the paper.
A country the size of Singapore could have a fibre-based quantum network – and in separate projects the Infocomm Media Development Authority, Singtel, and ST Engineering have been looking at the technology – but innovation is needed to go global.
“We are seeing a surge of interest in building quantum networks around the world. Satellites are a solution to making long range networks, creating connections across country borders and between continents,” said Alexander Ling, Principal Investigator at CQT and an Associate Professor in the NUS Department of Physics. He leads CQT’s satellite programme.
The project is supported by the National Research Foundation’s (NRF) Competitive Research Programme (CRP) funding scheme, which fosters use-inspired research projects. NRF Executive Director (Academic Research) Dr Lim Khiang Wee, said, “We are pleased that the NRF’s CRP award enabled the team to develop the science needed to deploy quantum technologies in space through nanosatellites. The research findings can potentially be used for data encryption, offering the superior level of security that the technology is known for. As Singapore is a highly digital society, this development could help build a resilient and reliable quantum network that ensures digital privacy is more secure than ever.”
Most of the paper’s authors were based at CQT, where the satellite was built. Other collaborators came from FHNW University of Applied Science and Arts Northwestern, Switzerland, The University of New South Wales Canberra, Australia and University of Strathclyde, UK.
A smaller satellite
Nanosatellites like SpooQy-1 are cost-effective to build and launch because they follow industry standards for CubeSats and have a small size. SpooQy-1 launched first to the International Space Station from the United States in April 2019, and then into orbit with help from the station’s astronauts on 17 June 2019. CQT made these arrangements for space travel with the Singapore Space and Technology Association and the Japan Aerospace Exploration Agency.
Before the satellite left Earth, much of the team’s work went into miniaturising the quantum source to fit into SpooQy-1’s shoe-box sized frame, optimising it to work with minimal power and making it rugged enough to survive a rocket launch and conditions in space. The source consists of a blue laser diode that shines on nonlinear crystals to create pairs of photons.
“At each stage of development, we were actively conscious of the budgets for mass, size and power,” said Aitor. “By iterating the design through rapid prototyping and testing, we arrived at a robust, small-form factor package for all the off-shelf components needed for an entangled photon-pair source.” Trials of earlier prototype instruments were carried out by flying equipment on a weather balloon to the edge of the atmosphere and in another NUS-built nanosatellite called Galassia.
The team reported that the instrument in SpooQy-1 successfully generated entangled photon-pairs over temperatures from 16 °C to 21.5 °C. The satellite has now been operational for a full year, completing about 5,800 orbits around the Earth, and continues to work.
Space to ground
What Micius has achieved highlights the challenges ahead. The Chinese satellite first reported entanglement distribution to ground stations in 2017, but so few photons were detected on Earth that it was not practical for communication. To achieve the newly reported quantum key distribution rate of 0.12 bits per second, the team upgraded the ground stations to improve detection efficiency and improved the signal processing.
SpooQy-1 is controlled from ground stations in Singapore and Switzerland, but it hasn’t attempted to send its quantum signals to Earth. That’s the goal of a next phase, announced in 2018 as a joint project between Singapore and the UK.
For that mission, researchers at CQT are collaborating with RAL Space in the UK to design and build a quantum nanosatellite similar to SpooQy-1 with the capabilities needed to beam entangled photons from space to a ground receiver. Some of this work is happening with SpeQtral, a venture-funded startup that spun-off from CQT to commercialise satellite-based quantum communication systems.
Robert Bedington, a co-author on the work with SpooQy-1, is Chief Technology Officer at SpeQtral and one of the project managers for the satellite’s successor. He said, “In our next mission, we are working towards CubeSat-to-ground quantum communications for sharing secret encryption keys across the globe. This capability is attractive to organisations who need to keep their networks secure from the most sophisticated hackers.” This next quantum nanosatellite is slated for launch in 2022.
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