All-Fiber Device Simplifies Quantum Key Distribution

Researchers have developed a simple and stable device to generate the quantum states necessary for quantum key distri­bution. The device could make it more practical to develop a global data network that uses this very secure method of encryption to protect everything from credit card transactions to texts. New encryption techniques are needed because computers powerful enough to crack today’s algorithm-based encryp­tion codes will likely be available in the next decade or two. Rather than relying on math, quantum key distri­bution uses quantum properties of light such as polarization to encode and send a random key needed to decrypt encoded data. The method is excep­tionally secure because any third-party intrusion is detectable.

Researchers developed a fiber optic device that can switch the polarization of light more than 1 billion times per second. The device could be useful for quantum encryption data transmission in free-space links. (Source: M. Avesani, U. Padova)

Researchers from the University of Padova in Italy report that their all-fiber device can switch the polari­zation of light more than 1 billion times per second. The device is also self-compen­sating, making it insensitive to temperature and other environ­mental changes. “Quantum key distribution is expected to have a deep impact in the privacy and security of citizens,” said Giuseppe Vallone, who led this research within the Quantum­Future research group. “Our scheme simplifies quantum key distribution for free-space communi­cation – such as from satellites to Earth or between moving terminals – which is required to achieve a global quantum network.”

Because quantum encryption doesn’t work well over long-distance fiber networks there is now a push to develop a satellite-based quantum communi­cation network to link various ground-based quantum encryption networks around the world. Although various properties of light can be used to create quantum states for quantum encryption, polari­zation is parti­cularly well suited for free-space links because it is not perturbed by the atmo­sphere and the decoding at the receiver can be performed without the challenging task of funneling the data into single mode fiber.

“Our goal is to develop a quantum encryp­tion scheme to use between a satellite and the ground, where the keys are generated in orbit,” said Vallone. “However, today’s polari­zation encoders aren’t ideal for use in space because they are unstable, expensive and complex. They can even exhibit side-channels that undermine the security of the protocol.” The new polarization encoder – which the researchers call POGNAC for POlari­zation SaGNAC – can rapidly rotate the polari­zation of incoming laser light thanks to a fiber-loop Sagnac inter­ferometer. This setup splits the light into two beams whose polarizations are at right angles relative to each other. The beams then travel through the fiber-loop in clockwise and counter­clockwise directions. The current components could fit into a package measuring 15 x 5 x 5 centimeters, with further miniaturi­zation possible if smaller components were incorporated.

Inside the fiber loop, the researchers used a commer­cially available electro-optics modulator to change the polari­zation to create the quantum states necessary for quantum key distri­bution. Because the clockwise and anti­clockwise components arrive to the modulator at different times, they can each be modulated independently. Modulators use an applied voltage to change the optical phase. However, the absolute value of the phase shift depends on many parameters that change with time. “In the POGNAC, only the relative shift between the two polarization components is relevant – this relative phase shift corresponds to a change in output polarization – while shifts that arise from tempera­ture changes and other factors are self-corrected,” said Vallone. “This makes the POGNAC very stable and eliminates polari­zation drifts that have affected other devices.”

The researchers tested their new device by measuring the polari­zation of quantum states generated by the POGNAC and comparing them with the expected values. They measured an intrinsic quantum bit error rate (QBER) as low as 0.2 %, well below the 1-2 percent QBER of typical quantum key distri­bution systems. “Our results show that data can be encoded using the polari­zation of light in a simple and efficient way,” said Vallone. “We were able to accomplish this using only commer­cially available components.” The researchers are continuing to improve on their approach and plan to perform further tests to see how the POGNAC performs when encoding quantum keys for encryption. (Source: OSA)

Reference: C. Agnesi et al.: All-fiber self-compensating polarization encoder for quantum key distribution, Opt. Lett. 44, 2398 (2019); DOI:

Link: Dept. of Information Engineering, Università degli Studi di Padova, Padova, Italy

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