Comparison of Free-Space and Fiber-Based Transmission Systems in Quantum Cryptography

Abstract

The distance for quantum key distribution using optical fibers is limited to around 100 km by the transmission loss, background level in the optical fibers, and birefringence in the fibers. Free space is an ideal medium for transmitting photon-based quantum information; this is considered to be the key advantage of quantum cryptography. In fiber-based systems, there is an optical loss in the fiber of about 0.2 dB/km for a telecom wavelength of 1.5 µm. That is, after a propagation of 150 km, the transmission loss in the fiber becomes 30 dB. On the other hand, free-space transmission obeys the diffraction theory of laser light; therefore, the free-space transmission loss is inversely proportional to the square of the distance, and this transmission has an advantage at longer distances over fiber-based transmission. The final key rates for free-space and fiber-based transmission systems are compared with respect to the parameters for the dark count rate, the imperfection of the optical system, and so on. In this paper, the trade-off of free-space and fiber-based systems is presented.