IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 53, NO. 5, OCTOBER 2017 9300408 Analysis of the Public Channel of Quantum Key Distribution Link Miralem Mehic, Oliver Maurhart, Stefan Rass, Dan Komosny, Filip Rezac, and Miroslav Voznak, Senior Member, IEEE Abstract—Quantum key distribution (QKD) relies on the laws of physics to establish a symmetric binary key between remote parties. A QKD link involves the realization of a quantum channel for the transmission of quantum key material encoded in certain photon properties, as well as a public channel for verification of the exchanged key material. This paper deals with the mutual dependence of these channels and analyzes the impact of performance of both channels on the overall key material establishment process. This paper presents measurement data obtained under laboratory conditions as well as the results obtained by establishing a virtual QKD link. Despite the common beliefs that increased quantum bit error rate implies a larger amount of traffic on the public channel, our measurements prove the opposite. The obtained data clearly show that the public channel has a major impact on the overall performance of the QKD link. Index Terms— Quantum key distribution, network traffic, throughput, delay. I. I NTRODUCTION Q UANTUM Key Distribution (QKD), based on the laws of physics rather than the computational complexity of mathematical problems, provides an information-theoretically secure (ITS) way of establishing symmetrical binary keys between two geographically distant users. The keys are secure from eavesdropping during transmission and QKD ensures that any third party’s knowledge of the key is reduced to a minimum [1], [2]. QKD employs two distinct channels between communicating parties: a quantum channel, which is used for transmission of quantum key material encoded in certain photon properties such as polarization or phase, and a public channel, which is used for verification of exchanged key material. The combination of these two channels forms a QKD communication link, over which QKD allows two remote users to exchange specific type of data, for example, secret keys (Fig. 1). Manuscript received April 29, 2017; revised July 12, 2017; accepted August 22, 2017. Date of publication August 22, 2017; date of current version September 5, 2017. This work was supported by the SGS, VSB- Technical University of Ostrava, Czech republic, under Grant SP2017/174. (Corresponding author: Miralem Mehic.) M. Mehic, F. Rezac, and M. Voznak are with the Department of Telecom- munications, VSB-Technical University of Ostrava, 708 00 Ostrava, Czech Republic (e-mail: miralem.mehic.st@vsb.cz). O. Maurhart is with the Digital Safety & Security Department, AIT Austrian Institute of Technology GmbH, 1220 Vienna, Austria. S. Rass is with the System Security Group, Institute of Applied Informatics, Alpen-Adria Universität Klagenfurt, A-9020 Klagenfurt, Austria. D. Komosny is with the Department of Telecommunications, Brno Univer- sity of Technology, 61600 Brno, Czech Republic. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JQE.2017.2740426 Fig. 1. Overview of a QKD link which consist of an optical quantum channel (continuous red line) and a public/classical channel (dashed blue line). Quantum networks have been studied extensively by research teams and laboratories in recent years focusing mainly on basic characteristics of the network such as the greatest possible distance of a quantum channel [3], [4], the maximum key generation rate [5], [6] or research in the field of optimal network topologies [7], [8]. However, a public channel was mainly neglected, and to best of our knowledge, no investigations on the traffic over a public channel were made or published beyond studying confidentiality, authentic- ity and the correct QKD protocol functionality. So, it is not known how a QKD link behaves in the presence of congestion on a public channels and what are the values of the usual performance of these channels such as throughput or delay. Therefore, this paper addresses the question of traffic analysis of communication over a public channel of a QKD link in QKD post-processing process. The remainder of the paper is organized as follows. Section II covers related work while section III outlines the fundamentals of a QKD link. Section IV discusses the results obtained in laboratory testing while section V discusses the results obtained from measurement of the Virtual QKD link. Section VI concludes this study and outlines our future work. II. STATE OF THE ART Rapid progress in theory and experiment [9]–[12] has led to the release of quantum technology from the lab into several test beds [6], [13]–[15], which mainly test the interoperability of different quantum techniques. On 1st April 2004, a bank transfer between the bank headquarter and the Vienna city hall has been realized to demonstrate a successful distribution of secret keys in an urban environment [16], [17]. In 2006, a one-time pad (OTP) encrypted surveillance video appli- cation was developed to demonstrate the speed, robustness and sustainability of existing QKD system [18]. In Octo- ber 2007, the Geneva Sate Chancellery installed a QKD system to secure a link transmitting ballot papers to the counting station during the federal elections in Switzerland [19], while 0018-9197 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.