All-Fiber Device Could Enable Free-Space-Based Quantum Key Distribution

Researchers from the University of Padova have developed an all-fiber device to generate the quantum states necessary for quantum key distribution (QKD), a method of protecting data that uses properties of light, such as polarization, to encode data and send a random key needed to decrypt the encoded data. The fiber device can switch the polarization of light more than 1 billion times per second. It is insensitive to temperature and other environmental changes.

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. Courtesy of Marco Avesani, University of Padova.

Polarization is well suited for encrypting free-space communication links because it is not perturbed by the atmosphere, and the decoding at the receiver end can be performed without funneling the data into single-mode fiber. The Padova team’s new polarization encoder, called 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.

Inside the fiber loop, the researchers used a commercially available electro-optics modulator to change the polarization to create the quantum states necessary for QKD. Because the clockwise and counterclockwise components arrive at 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 professor Giuseppe 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% to 2% QBER typical of QKD systems. “Our results show that data can be encoded using the polarization of light in a simple and efficient way,” Vallone said. “We were able to accomplish this using only commercially available components.”

The current components of POGNAC could fit into a package measuring 15 × 5 × 5 cm, with further miniaturization possible, the team said. The researchers plan to perform further tests to see how the POGNAC performs when encoding quantum keys for encryption.

The POGNAC could help simplify QKD for free-space communication, such as from satellites to Earth or between moving terminals. “Our goal is to develop a quantum encryption scheme to use between a satellite and the ground, where the keys are generated in orbit,” Vallone said.

The research was published in Optics Letters, a publication of OSA, The Optical Society (https://doi.org/10.1364/OL.44.002398).