A record-breaking experiment by a Chinese satellite has taken the weird world of quantum physics to new heights and is likely to spur other nations, including Canada, in their efforts to develop an unhackable form of long-distance communication.
“We have done something that was absolutely impossible using conventional approaches,” said Jian-Wei Pan, project leader and physicist at the University of Science and Technology of China in Hefei. Dr. Pan suggested his team’s achievement had “kick-started a new quantum space race.”
The tour de force experiment featured a device on the satellite beaming pairs of photons – individual particles of light – linked by a mysterious effect known as quantum entanglement to two separate receiving stations on Earth. The stations were located 1,203 kilometres apart, a tenfold increase in distance over ground-based tests. Because such quantum signals are effectively impervious to eavesdropping, the results have implications for efforts to create more secure communication networks.
“I believe this will lead to an increase of interest in other countries to follow up,” said Thomas Jennewein, a researcher at the University of Waterloo, who called the Chinese result the “first proof of concept” for satellite-based quantum communications.
Dr. Jennewein has long been working toward a Canadian ground-to-space version of the experiment, and last fall demonstrated that a system he built at the university’s Institute for Quantum Computing was able to establish a link with a passing aircraft. In March, the federal government allocated an additional $80.9-million to the Canadian Space Agency’s budget over the next five years, part of which is earmarked for the development of a quantum encryption satellite demonstration mission. Research groups in Germany, Singapore, the United States and Britain, among others, are also pursuing their own variants of quantum communications in space.
The results of the quantum experiment were reported Thursday in the journal Science, the same day that China successfully launched an X-ray telescope into orbit. Both are examples of the growing use of China’s space capabilities for scientific research, an emphasis that Dr. Pan has championed.
The quantum satellite, dubbed Micius, appears to have worked without a hitch. In an e-mail to The Globe and Mail, Dr. Pan revealed that he and his team were able to get the experiment working within a couple of months of the satellite’s launch last August. The results bolster the view that entanglement – an effect that is so strange it was originally considered a reason why quantum physics must be wrong – may one day form the foundations for a secure quantum Internet.
The potential usefulness of quantum physics to secure communication is based on the principle that, when photon pairs are entangled, it is possible to measure one photon received at one station to determine the characteristics of the other, even though there is no apparent physical connection between them. The phenomenon, which Albert Einstein once called “spooky action at a distance,” is analogous to tossing a coin and using the result to correctly predict the outcome of another coin-toss taking place in a different room.
Despite its apparent violation of the rules of cause-and-effect, there are real-world consequences because entangled photons can be used to communicate the unique key to decode an encrypted message sent over a conventional network. Anyone trying to eavesdrop on the signal would disturb its delicate quantum properties, thereby revealing the intrusion.
In recent years, quantum entanglement has has been demonstrated in laboratory experiments using photons sent through optical fibres up to 100 kilometres long. This distance has remained a practical limit because the photons are eventually absorbed by the medium they are moving through. They cannot be amplified or copied without sacrificing their entangled state.
To send quantum signals farther, scientists have looked to the vacuum of space, which allows entangled photons to travel for hundreds or even thousands of kilometres before re-entering the atmosphere.
Yet, the technical challenges are formidable. Micius is armed with telescopes that can independently track the two ground stations with high precision while spitting out carefully timed photons at a rate of millions per second.
“What they did is a major undertaking,” Dr. Jennewein said. “They’re clearly leading this whole field.”
Paul Kwiat, a professor of physics at the University of Illinois at Urbana-Champaign, cautioned that there were still many hurdles to be surmounted to create a workable quantum encryption system because a fast-moving satellite has a limited amount of time to beam its message, while the number of photons that get through to the receiver is relatively low. That means it might take several passes of a satellite to transmit enough information to successfully decode a message.
He said his team is planning to test a system that can create photons with more than one form of entanglement at the same time, thereby increasing the amount of information that can be sent with each photon. That system is set to be tested on board the International Space Station in the next few years.
Meanwhile, Dr. Pan said he is already thinking about follow-on missions to push China’s version of the technology further.
“The current Micius satellite only has five minutes coverage time in each ground station,” he said. “To increase the time and area coverage, we plan to launch satellites at higher orbit, which require us to develop new techniques to further increase the link efficiency.”