High-speed quantum encryption may secure the future internet

The problem with many of these systems, said Nurul Taimur Islam, a graduate student in physics at Duke, is that they can only transmit keys at relatively low rates—between tens to hundreds of kilobits per second—which are too slow for most practical uses on the internet.

At these rates, quantum-secure encryption systems cannot support some basic daily tasks, such as hosting an encrypted telephone call or video streaming,” Islam said.

Like many QKD systems, Islam’s key transmitter uses a weakened laser to encode information on individual photons of light. But they found a way to pack more information onto each photon, making their technique faster.

By adjusting the time at which the photon is released, and a property of the photon called the phase, their system can encode two bits of information per photon instead of one. This trick, paired with high-speed detectors developed by Clinton Cahall, graduate student in electrical and computer engineering, and Jungsang Kim, professor of electrical and computer engineering at Duke, powers their system to transmit keys five to ten times faster than other methods.

It was changing these additional properties of the photon that allowed us to almost double the secure key rate that we were able to obtain if we hadn’t done that,” said Gauthier, who began the work as a professor of physics at Duke before moving to OSU.

In a perfect world, QKD would be perfectly secure. Any attempt to hack a key exchange would leave errors on the transmission that could be easily spotted by the receiver. But real-world implementations of QKD require imperfect equipment, and these imperfections open up leaks that hackers can exploit.

The researchers carefully characterized the limitations of each piece of equipment they used. They then worked with Charles Lim, currently a professor of electrical and computer engineering at the National University of Singapore, to incorporate these experimental flaws into the theory.

We wanted to identify every experimental flaw in the system, and include these flaws in the theory so that we could ensure our system is secure and there is no potential side-channel attack,” Islam said.

Though their transmitter requires some specialty parts, all of the components are currently available commercially. Encryption keys encoded in photons of light can be sent over existing optical fiber lines that burrow under cities, making it relatively straightforward to integrate their transmitter and receiver into the current internet infrastructure.

All of this equipment, apart from the single-photon detectors, exist in the telecommunications industry, and with some engineering we could probably fit the entire transmitter and receiver in a box as big as a computer CPU,” Islam said.

— Read more Nurul T. Islam et al., “Provably secure and high-rate quantum key distribution with time-bin qudits,” Science Advances 3, no. 11 (24 November 2017): e1701491 (DOI: 10.1126/sciadv.1701491)