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Acta Sinica Quantum Optica

Polarization Control for Optical Fiber Quantum Cryptography

Huan Zhao, Ma Hai-Qiang, Ling-An Wu

Published Jan 1, 2005• 1 citation
OpenAlex

Abstract

A method is proposed for measuring and controlling optical polarization by using circular fiber polarization controllers. A detailed matrix analysis of the modulation of the optical polarization state is given for the two-disc type controller. It is experimentally verified that this method is effective for controlling polarization in the quantum channel of a quantum cryptography system over a transmission distance of 25 km.

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A method is proposed for measuring and controlling optical polarization by using circular fiber polarization controllers. A detailed matrix analysis of the modulation of the optical polarization state is given for the two-disc type controller. It is experimentally verified that this method is effective for controlling polarization in the quantum channel of a quantum cryptography system over a transmission distance of 25 km.
Quantum cryptography is a new technique for transmitting quantum information. The information is securely transmitted due to the laws of physics. In such systems, the vehicle that transfers quantum information is a single photon. The problem with using photons is that the transmission distance is limited by the absorption of the photons by the optical fiber along which they pass. The maximum demonstrated range so far is approximately 100 km. Using free-space quantum cryptography between a ground station and a satellite is a possible way of sending quantum information farther than is possible with optical fibers. This is because there is no birefringence effect in the atmosphere. However, there is a complication in that the directions of the polarization basis between the transmitter and the receiver must coincide with each other. This polarization changes because the mobile terminals for free-space transmission continuously change their attitudes. If the transmission protocol is based on polarization, it is necessary to compensate for the change in attitude between the mobile terminals. We are developing a scheme to track the polarization basis between the transceivers. The preliminary result is presented.
We demonstrate distribution of polarization entangled photons over 100 km of optical fiber. Measurements of the two-photon visibility, without dark count subtraction, have shown that the quantum correlations (raw visibility 89%) allow secure quantum cryptography after 100 km of non-zero dispersion shifted fiber using commercially available single photon detectors. In addition, quantum tomography has revealed little degradation of entangled state indicating minimal loss in fidelity during the transmission.
The rapid increase on the information sharing around the world, leads to an utmost requirement for capacity and bandwidth. However, the need for security in the transmission and storage of information is also of major importance. The use of quantum technologies provides a practical solution for secure communications systems. Quantum key distribution (QKD) was the first practical application of quantum mechanics, and nowadays it is the most developed one. In order to share secret keys between two parties can be used several methods of encoding. Due to its simplicity, the encoding into polarization is one of the most used. However, when we use optical fibers as transmission channels, the polarization suffers random rotations that may change the state of polarization (SOP) of the light initially sent to the fiber to a new one at the output. Thus, in order to enable real-time communication using this encoding method it is required the use of a dynamic control system. We describe a scheme of transmission of quantum information, which is based in the polarization encoding, and that allows to share secret keys through optical fibers without interruption. The dynamic polarization control system used in such scheme is described, both theoretically and experimentally. Their advantages and limitations for the use in quantum communications are presented and discussed.
We perform a transmission of polarization encoded quantum bits through 16 km of single mode optical fiber in the 1550 nm window.The polarization drift caused by the changing birefringence of the fiber is kept stable through the use of a polarization control system we developed.This system employs 2 classical reference signals with 0.8 nm spacing from the quantum channel.We also send through the same fiber an extra reference classical signal used to trigger the single-photon detector.To the best of our knowledge this is the first time a quantum channel is multiplexed with 3 classical signals in the same optical fiber.