BOSTON — It’s long been the Holy Grail of communications: technology that not only maximizes privacy, but also reveals when a message had been intercepted or copied.
The quest began in antiquity, with encryption and with the humble envelope — which not only kept out prying eyes but also showed if a message had been opened by someone other than its intended recipient.
Despite centuries of innovation, today’s methods for secure communication are basically the same — and in some ways are even more vulnerable, given how easy it is to copy, store, and search electronic data.
Scientists say a solution for truly private, tamper-free digital communication is underway, and should be commercially viable within a decade.
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For theoretical physicists, the solution has already existed for several decades, but the technology needs refining before it’s available on a mass scale across the internet. Still, the pieces of this ultra-secure, high-speed communications web are beginning to take shape in labs around the world.
The system is based on quantum physics, and more specifically on the concept of “entanglement.”
Entanglement is a topic that even hardened scientists discuss with a degree of wonder. “It’s quite mysterious, in fact,” said Félix Bussières, a senior researcher in the Group of Applied Physics at the University of Geneva in Switzerland.
Physicists have long struggled to come up with metaphors and analogies to describe entanglement, which is so hard to actually wrap the mind around that even Albert Einstein gave up and settled for calling it “spooky.” It involves creating two photons (particles of light) that, while independent of each other and free to travel long distances apart, are still tightly interrelated, almost as if they are not two separate photons but one indivisible photon pair. As photons travel, they spin; each part of the entangled photon pair spins in the exact opposite direction from the other. If something happens that causes either of the pair to change its spin, the other instantaneously changes its spin to compensate.
Entangled photons act like a tripwire for any outside tampering — which is what makes a quantum internet so secure. In other terms, “quantum mechanics tell us that if you look at a quantum state you perturb it,” wrote Thomas Jennewein, an associate professor at the Institute for Quantum Computing and in the physics and astronomy department of Ontario's University of Waterloo, in the institute’s 2013 annual report. (If you want to read more on the science, start by looking up the Heisenberg Uncertainty Principle and the Schrödinger’s cat thought experiment.)
The good and bad of the quantum internet
So in the ideal case, wiretapping a quantum message system is impossible, Bussières said, because the wiretap will disturb the system, and the disturbance can be detected by the sender and recipient.
“The principle is perfectly secure,” Bussières said. “One can use in principle the quantum properties of light ... to ultimately cipher communication ... in a way that is ... provably unbreakable.”
This now works in the lab. It has even gone commercial: There is a small industry doing what is called quantum-key distribution — using quantum methods to generate encryption keys that are substantially more secure than more conventional ones. But the keys can only be shared across relatively small distances, no more than about 125 miles of optical fiber.
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The challenge is that the technology depends on photons (instead of electrons), and photons attenuate, or lose, their momentum over distance.
It also means that quantum connections are quite slow (about one megabit per second, Bussières said) compared to standard internet communications speeds. That’s why the technology is being used for keys instead of entire messages. And as such, while messages with quantum keys are more secure than others, they can still be monitored and copied for storage and later cracking by hackers or spies.
Quantum-key distribution could be poised for widespread commercialization right away, Bussières said, if technological advances threatened the security of conventional electronic encryption.
“If we want to go beyond these distances” with actual quantum connections, “other technologies are being intensely researched around the world,” he said. It would take several years to develop quantum-enabled devices that are small enough, cheap enough, and efficient enough to be mass-produced and widely used, “but considering the amount of research put in that direction, there is a great chance that it will become a reality,” Bussières said.
For nerds: solving the quantum quandary
To transform quantum communications from a lab project to a commercial application, three major approaches are in development: wavelength optimization, quantum repeaters, and satellite connections.
Scientists say the progress is encouraging, in part because much of the research involves adapting existing, conventional optical-communications gear to quantum uses, rather than inventing all-new equipment.
First, it’s not enough to simply connect photon-entanglement sources and detectors to opposite ends of optical-fiber cables. Because eventually photons attenuate — getting absorbed or scattering away from their detectors — even non-quantum-carrying fibers need help to keep the signal alive across long distances.
Steven Olmschenk, an associate professor of physics at Denison University in Ohio, is working to lengthen the distance entangled photons can travel in optical fiber. While previously he had also been working on quantum repeaters at the Joint Quantum Institute, he and others realized they were researching themselves into a bit of a corner.
Most of the photons used in quantum research so far, he said, are in ultraviolet wavelengths, which attenuate too quickly to be truly useful in fiber-optic transmissions. Internet and telecom companies already use infrared signals in fibers, because they attenuate more slowly.
Olmschenk’s research focuses on taking existing capabilities for UV quantum communications, and adapting them to infrared transmission and reception. It has only been a couple of years, though, and he told GlobalPost that while he is optimistic, he does not yet have any results to report.
If he is successful, he and others will also have to translate into UV the accomplishments of other researchers figuring out how to extend signals in other ways.
Bussières is working to improve quantum repeaters, which combine a photon detector, a quantum memory, and a photon source so that when a quantum signal needs to be transmitted, say 600 miles, that trip can be split by repeaters into shorter segments with less attenuation.
But for distances (or geographic features) too large to handle usefully with optical fiber, there is another option: sending quantum signals by satellite.
Jennewein, of the Institute for Quantum Computing, is on that task. He and his team have set their sights on sending entangled photons to satellites in low Earth orbit, likely somewhere around 300 to 360 miles above the ground. At present, open-air quantum transmissions have been achieved at around 100 miles, using transmitters and receivers that are very precisely aligned. (This gets harder when involving a satellite moving 15,000 miles per hour.)
Jennewein’s current work covers several aspects of the puzzle, including aiming photons accurately at distant receivers that are moving, determining how much attenuation will happen in the atmosphere as it thins at higher altitudes, and improving detection of the weak signals that will arrive.
One crucial challenge has not yet been undertaken: Because quantum sources need to be smaller and more energy-efficient before they are ready to fly in space, nobody has yet sent a quantum signal from a satellite back to Earth.
Other efforts, which would expand bandwidth over those extended distances, are also in the works. "Quantum dots," nanocrystals that conduct electricity, can simplify and even automate the process of emitting photons with particular entanglements on demand, which could help increase transmission rates, as would using light from LEDs instead of lasers. And repeaters capable of handling multiple quantum signals simultaneously would speed things up as well.
But the crucial part is building the connections that can span the world so that people can, it is hoped, finally communicate with complete privacy.