By his own admission, Christopher Pugh is not one for thrill rides. Even a regular commercial flight can make him a bit queasy.
But on a moonlit night last September, Mr. Pugh found himself strapped into a Twin Otter with its side door removed so he could monitor a telescope that was pointed down toward the darkened rural landscape of Eastern Ontario in the hope it would capture particles of laser light called photons fired up at the plane.
Despite the noise and rushing wind, Mr. Pugh, a PhD student at the University of Waterloo’s Institute for Quantum Computing, did not think to feel nervous on his own behalf.
“I was too scared for the equipment to work,” he said.
As it happens, it worked beautifully. Colleagues operating the laser from an airstrip near Smiths Falls, south of Ottawa, soon heard Mr. Pugh’s excited cry of “We have photons!” over an air-to-ground radio.
The success, reported in a scientific paper posted online on Tuesday, was a pivotal test for a system designed to transmit numerical keys that will unlock coded messages no future computer can hack.
It also means the group is ready to take on an even bolder challenge: putting the same system on a Canadian satellite so it can transmit secure information across long distances.
“If we got the go-ahead today, the system could be ready for launch in something like two years,” said Thomas Jennewein, an associate professor of quantum information at Waterloo and the experiment’s leader.
The flight, conducted with the help of a National Research Council plane, is another sign that quantum communication technologies that once lived only in physicists’ imaginations or as table-top experiments are beginning to work at useful scales. And one of the commercial drivers of those technologies is the need to protect information being encrypted using today’s methods against the anticipated future power of quantum computers.
Today, secure information is encrypted with numerical keys using algorithms too arduous for any conventional computer to overcome by crunching through a long list of possible answers. But computers based on quantum principles would operate in a realm that defies everyday intuition, in which a single bit of data can be both one and zero rather than either one or zero. In a sense, a quantum computer could be thought of as tapping multiple realities simultaneously. This would allow it to divvy up an impossibly long series of calculations and quickly arrive at an answer that decodes a message encrypted with non-quantum technology.
While such computers do not yet exist in practical form, researchers say they may be only a decade or two away. At that point, any personal, corporate or government data that have been encrypted using current means will be at risk.
“You have to ask yourself, if you encrypt something today, how long is that information going to be secure?” said Wolfgang Tittel, who specializes in quantum information at the University of Calgary.
Dr. Jennewein’s approach involves sending photons between two locations one at a time. Each photon has its own set of quantum properties that is unknown until it is received. Some photons will not make it, but those that do are used to construct a unique “quantum key” to unlock a coded message that can then be sent through normal public channels to the authorized receiver. The virtue of the system is that any attempt to eavesdrop will disrupt the quantum state and alert the receiver someone is trying to steal the information.
The problem is that over very long distances, too many photons are absorbed by the medium through which they are passing, whether it is a fibre optic cable or the air. Conventional digital communications overcome this with nodes spaced out along the way that repeat the signal. But that will not work for quantum transmissions, thanks to a principle called the “no-cloning theorem,” which states that it is impossible to duplicate a quantum state.
Looking for a solution, Dr. Jennewein and his team turned skyward. When a laser is fired up toward a satellite, the light has to penetrate only a relatively thin layer of air – a few tens of kilometres at most – before it is moving unimpeded through a near vacuum. The satellite generates the quantum key with the transmitting location and a second quantum key with the receiving location. Someone who already has one of the keys – either the sender or receiver – can work out what the other key is and use it to decode a message that would otherwise be impervious to decryption.
The technical challenge in getting the system to work was daunting, including the laser and telescope system engineered by Mr. Pugh that could send the photons from ground to plane.
“We only had five flight hours available to us,” Dr. Jennewein said. “But leading up to that took years.”
The group is now hoping to get an opportunity with the Canadian Space Agency to build the system into a small satellite.
Dr. Tittel, who was not involved in the Waterloo experiment, called the team’s achievement “very impressive.”
As for Mr. Pugh, whose PhD thesis is based on the system succeeding, the flight time was worth every second.Report Typo/Error
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