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Quantum Key System Uses Unbreakable Light-Based Encryption to Secure Data

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OXFORD, England, March 21, 2017 — A device that can send unbreakable secret keys from a handheld device to a terminal could keep users' personal financial information more secure and safer in the event of a cyber-attack.

Researchers from Oxford University are using ultrafast LEDs and moveable mirrors to send a secret key from a device at a rate of more than 30 kilobytes per second over a distance of 0.5 meters.

"The idea is that this gadget would be a mobile object that talks to something that is fixed," said Iris Choi of Oxford University. “If integrated into a cellphone, for example, the device could allow secure links to near-field communications mobile payment systems and indoor Wi-Fi networks.”

Quantum Key System Developed With Unbreakable Light-Based Encryption
This handheld device for transmitting and receiving quantum cryptographic keys was built from off-the-shelf components. The device could be miniaturized for use in a mobile device. Courtesy of Iris Choi, Oxford University.

Choi said the device could also improve the security of ATMs and help prevent ATM skimming attacks, which are estimated to cost the industry more than $2 billion annually.

The technology is a quantum key distribution system that relies on characteristics of a single photon to provide a bit — a 1 or a 0 — to build up a cryptographic key that can encrypt and decrypt information. Quantum keys are considered secure; if someone intercepts the quantum bits and then passes them on, the very act of measuring them alters them.

The system contains six resonant-cavity LEDs, which provide overlapping spectra of light. Each of the six is filtered into a different polarization, split into pairs to represent 1s and 0s. The circularly polarized LEDs provide the bits for the key, while the other pairs are used to measure the security of the channel and provide error correction. Every four nanoseconds, one of the channels produces a one-nanosecond pulse in a random pattern. On the other end, six polarized receivers pick up the light from their matching LEDs and convert the photons into the key.

The researchers equipped both the transmitter and the receiver with filters that select only a portion of the light, so they all shine with the exact same color, regardless of which polarization they produce. This feature in and of itself should deter hackers from breaking the code.

A quantum key must be long enough to ensure that an adversary cannot hack it simply by guessing randomly. This requires the system to transmit a large number of bits in less than a second. Achieving that high data transmission rate also requires that most of the photons get to where they're supposed to go. The Oxford prototype addresses this need through its innovative steering system.

Even someone trying to hold perfectly still has some motion in his hand. The research team measured this motion by looking at how the spot of a laser pointer moved as a person tried to hold it steady. They then optimized design elements of the beam-steering system, such as bandwidth and field of view, to compensate for hand movement.

To help the detector properly align with the transmitter and further correct for hand movement, both the receiver and the transmitter contain a bright LED with a different color than the quantum key distribution LED that acts as a beacon. A position-sensing detector on the other side measures the precise location of the beacon and moves a microelectromechanical systems (MEMS) mirror to align the incoming light with the fiber optics of the detector.

The team tested their idea with a handheld prototype made from off-the-shelf equipment. Choi said the design likely could be easily miniaturized in order to turn the system into a practical component for a mobile phone.

The Oxford team’s research has been published in the Optical Society (OSA) journal Optics Express (
Mar 2017
FiltersmirrorsOxford UniversityOxfordreserach & technologylaserseducationLEDslight sourcesIris ChoiEuropeopticsSensors & DetectorsTest & Measurementdefensemobile devicebeam-steering system

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