New All-fiber Devices Promote Global Quantum Encryption Network Construction

- Jun 25, 2019-

New All-fiber Devices Promote Global Quantum Encryption Network Construction


Quantum key distribution does not rely on mathematics, but uses quantum properties of light such as polarization, decoding, and transmitting random keys that decrypt encoded data. This method is especially safe because any third-party intrusions are detected.


New All-fiber Devices Promote Global Quantum Encryption Network Construction

Researchers at the University of Padova in Italy reported in the Optical Society of America (OSA) journal Optics Letters that their all-fiber devices have more than one billion polarizations per second of switchable light. The device is also self-compensating and is not sensitive to temperature and other environmental changes.

In the QuantumFuture research group, Giuseppe Vallone, who led the research with the co-author Paolo Villoresi, said: "Quantum key distribution is expected to have a profound impact on citizens' privacy and security. Our The scheme simplifies the quantum key distribution used in free-space communication. For example, satellite-to-earth or communication between mobile terminals. Realizing global quantum networks requires free-space communication."


Because quantum keys do not work well across long-haul fiber-optic networks, it is extremely urgent to develop a satellite-based quantum communication network that connects different ground-based quantum-encrypted networks around the world.

Although the various characteristics of light can be used to create the quantum states required for quantum encryption, polarization is particularly well suited for free-space linking because it is not affected by the atmosphere and is decoded at the receiver without the need to collect data. Going to single mode fiber (this is a challenging task).

Vallone said: "Our goal is to develop a quantum encryption scheme that can be used between satellites and the ground. Keys are generated in orbit. However, today's polarization decoders are not suitable for use in space because they are unstable and expensive. And complex. They even show side channel vulnerabilities that can weaken the security of the protocol."

Researchers say the new polarization encoder is "POGNAC" and POGNAC is a combination of POlarization and SaGNAC. With the help of a fiber-optic ring-shaped Sagnac interferometer, this polarization encoder can rapidly rotate the polarization of the incident laser. The device divides the beam into two, and the polarization of the two beams is perpendicular to each other. The two beams then pass through the fiber loop in clockwise and counterclockwise directions, respectively. Current components can be placed in a 15 x 5 x 5 cm package, and if the included components are smaller, the package can be further miniaturized.

In the fiber optic loop, researchers used commercially available electro-optical modulators to change the polarization and create the quantum state required for quantum key distribution. Because the clockwise and counterclockwise rays arrive at the modulator at different times, they are modulated independently of each other.

The modulator uses an applied voltage to change the optical phase. However, the absolute value of the phase shift depends on a number of parameters that vary over time. Vallone said: "In POGNAC, only the relative displacement between two polarized lights is meaningful. This relative displacement corresponds to the change in the output polarization. At the same time, the displacement caused by temperature changes and other factors is self-correcting. This makes POGNAC very stable and eliminates polarization drift that affects other devices."



The researchers tested their new devices by measuring the polarization of the quantum states produced by POGNAC and comparing them to the expected values. They measured a quantum error rate (QBER) of guilt as low as 0.2%, much lower than the quantum error rate of 1% to 2% of a typical quantum key distribution system.

“Our results show that data can be encoded in a simple and efficient way using the polarization of light,” Vallone said. “We can do this with only commercially available components.”

Researchers are continually improving their methods and plan to conduct further testing to observe how POGNAC behaves when encoding quantum keys for encryption.