Making mobile transactions more secure with a quantum key system
light from their matching LEDs and convert the photons into the key.
It is important not to let a potential adversary know which channel has which polarization, because that would reveal which bits were being sent, but there will always be some slight variation in the wavelength emitted by each LED, which could serve to identify them and give a hacker a way to break the code. The researchers solved this problem by equipping 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.
Steering the beam
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 such a high data transmission rate in turn requires that most of the photons get to where they’re supposed to go.
As a result, Choi said, the prototype’s most important innovation is the 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 jittered 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 this 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 from brands such as Nokia, which participated in the research. Improving the protocol while keeping the same hardware could also increase the transmission rate, and other changes could be made to let the device work over longer distances to, for instance, connect with a Wi-Fi hub.
— Read more in H. Chun et al., “Handheld free space quantum key distribution with dynamic motion compensation,” Optical Express 25, no. 6 (2017): 6784-95 (DOI: 10.1364/OE.25.006784)
