Title: Long Distance High Data-Rate Quantum Key Distribution
Abstract: Photonic crystal fibers (PCFs) are attractive for the implementation of long-distance high data-rate quantum key distribution. In this project we experimentally explore the cabability of PCF technology for the next generation quantum communication and cryptography applications.
Keywords: Photonic Crystal Fibers, Quantum Cryptography
Collaborators: David A. Meyer (UC San Diego), Paul G. Kwiat (University of Illinois Urbana-Champaign), Onur Hosten (University of Illinois Urbana-Champaign), Sahin Kaya Ozdemir (Osaka University, Japan), Rasim Dermez (Kocatepe University, Turkey)
Single photon interference with high fringe visibility for quantum key distribution (QKD) after 150km transmission over telecom fiber has been demonstrated by Kimura, et al [1] using integrated optical interferometers based on planar light-wave circuits. Despite the experimental progress in distance and bit-rate, however, security of QKD due to real-life imperfections is still a crucial issue for practical applications. Especially photon-number splitting (PNS) attack renders most experiments insecure. In the PNS attack, an eavesdropper, say Eve, can in principle count the photons in the signal and suppress the single photon pulses while reserving a copy for herself for later use by splitting multiphoton pulses. Given this attack, the secure key generation rate in most experiments is severely reduced.
A weak pulse quantum key distribution system based on a time/polarization division Mach-Zender interferometer [2] using the BB84 protocol has been demonstrated by C. Gobby, et al. [3] to be secure against all individual attacks, including photon number splitting (PNS) atack. They generate weak pulse photons by a 1.55 um distributed feedback diode laser. By carefully controlling the beam intensity maximum secure bit rate as a function of fiber length has been demonstrated. In other words, for each fiber length there is an optimal 'mu' value (i.e., photon flux per clock cycle used by sender, say Alice) for maximum secure bit rate against the PNS attack. Unconditionally secure keys for fiber lengths of over 50 km (attenuation constant of 0.21dB/km) is obtained. The shorter is the fiber, reasonably stronger photon flux can be used. Over 30 km, QBER increases. Because erroneous detector count is no more negligible at weaker photon flux due to higher initial attenuation and fiber attenuation. The secure bit rate decreases from 300 bits/s at shorter lengths to 2.1 bit/s for 44 km. The longest fiber length which allows unconditionally secure bit rate is 50.6 km. In this experiment syncronization of Bob's detector is achieved by a 1.3 um clock laser using wavelength division multiplexing (WDM).
Figure 1 Schematic of the experimental set-up for the decoy pulse QKD system proposed by Y. Zhao, et al. in 2005 [4]. Bob and Jr. Alice consist of the commercial id Quantique QKD sytem, which includes laser diode (LD), avalanche photodiode (APD), phase modulators, polarization beam splitters (PBS), classical photodetector (PD), delay line, and Faraday mirror. To implement the decoy pulse method, Alice additionally includes two acousto-optical modulators (DA and CA) driven by two function generators (DG and CG). 15 km SMF28 single mode optical fiber was used as transmission medium.
To circumvent the PNS attack at relatively higher secure data-rate Hwang introduced the first decoy pulses method in 2003, for Bennett-Brassard 1984 (BB84) QKD protocol [5]. In the decoy state idea, the sender, say Alice, prepares a set of decoy states in addition to standard BB84 states. These states are used to detect a possible intrusion in the link and the BB84 states are used for key generation. Based on this idea, a decoy state QKD has been recently proposed by Lo, et al [6], which has dramatically enhanced the secure bit-rate with essentially using the same hardware in the literature.
In the same year, the first experimental implementation of the decoy state QKD was reported by essentially the same group [4] (Figure 1). They have shown that secure key generation rate of 165bit/sec can be obtained over a 15km of telecom fiber. Under the same experimental parameters not even a single bit of secure key can be generated without decoy states. Despite promising improvement achieved by the decoy state QKD, however, the secure key generation rate and the transmission distance are still far from those of the state of art classical systems. Improved security due to QKD technology at the expense of much lower data-rate than it is possible classically would be certainly unpleasant. Therefore, all possible improvements in protocols and hardware need to be researched for practical quantum communication. The hardware improvements will not only enhance the performance of the QKD systems at a level of unprecedented security, but will also significantly boost the repeaterless transmission distance in conventional optical communications.
Attenuation of CTF is about 0.2dB/km and to the best of our knowledge the minimum attenuation obtained experimentally is about 0.15dB/km [7]. There is little prospect for much improvement of CTFs. Regarding photonic crystal fibers (PCF), on the other hand, holey fibers appear promising, at least in the long run, because the electromagnetic field mode is confined inside the air core, which prevents the absorption that exists in solid core optical fibers. Attenuation currently obtained in these PCFs is still worse than that of CTFs. The record to date is 0.28dB/km. This is mainly due to fabrication imperfections and and surface modes residing between air-cladding interface. P. Roberts, et al. have predicted a minimum PCF attenuation of 0.1dB/km by eliminating the surface states and improving material processing [8]. Using this value transmission distance of a given system may be extended by a factor of 2.
In this project we follow two strategies toward long distance high data rate secure quantum communications:
(i)Photonic crystal fibers (PCF) are attractive for the implementation of next generation quantum communication and cryptography applications. We explore the PCF technology to replace conventional telecom fibers (CTF) in weak laser pulse QKD systems similar to GYS or decoy pulses.
(ii)We evaluate up to date QKD systems and protocols.
References
[1] T. Kimura, Y. Nambu, T. Hatanaka, A. Tomita, H. Kosaka, and K. Nakamura, Single-photon Interference over 150 km Transmission Using Silica-based Integrated-optic Interferometers for Quantum Cryptography, Japanese J. Appl. Phys. 43, L1217 (2004).
[2] C. Gobby, Z. L. Yuan, and A. J. Shields, Quantum Key Distribution over 122 km of Standard Telecom Fiber, Appl. Phys. Lett. 84, 3762 (2004).
[3] C. Gobby, Z. L. Yuan, and A. J. Shields, Unconditionally secure quantum key distibution over 50 km of standard telecom fiber, Electronics Lett. 40, 1603 (2004).
[4] Y. Zhao, B. Qi, X. Ma, H.-K. Lo, and L. Qian, Experimental Quantum Key Distribution with Decoy States, arXiv: quant-ph/0503192.
[5] W.-Y. Hwang, Quantum Key Distribution with High Loss: Toward Global Secure Communication, Phys. Rev. Lett. 91, 057901 (2003).
[6] H.-K. Lo, X. Ma, and K. Chen, Decoy State Quantum Key Distribution, Phys. Rev. Lett. 94, 230504 (2005).
[7] Nagayama, K. Kakui, M. Matsui, M. Saitoh, I. Chigusa, Y., Ultra-low-loss (0.1484 dB/km) pure silica core fibre and extension of transmission distance, Electronics Lett. 38 1168 (2002).
[8] P. Roberts, F. Couny, H. Sabert, B. Mangan, D. Williams, L. Farr, M. Mason, A. Tomlinson, T. Birks, J. Knight, and P. St. J. Russel, Ultimate low loss hollow-core photonic crystal fibers, Opt. Express 13, 236 (2005).