This paper describes progress in gigahertz-clocked quantum key distribution systems. It details current advances in both
point-to-point and network applications. We will discuss possibilities for practical quantum key distribution using single-photon
sources, and discuss the experimental system performance of GHz-clocked quantum key distribution systems
focusing on issues of quantum bit error rate, net bit rate and transmission distance with different detector structures,
concentrating on single-photon avalanche diode detectors, but also examining superconducting nanowire-based
structures. The quantum key distribution system is designed to be environmentally robust and an examination of long-term
system operation will be presented.
In this paper we demonstrate the application of multi-user quantum key distribution (QKD) to typical broadband fibrebased
passive optical access links to metropolitan area networks. We propose a technique to utilize the currently unused
850nm waveband in standard telecommunications fiber for QKD in two network architectures. Net bit rates of up to
100's of kilobits-1 were achieved for each receiver, depending on the network topology. The transmission distances
between sender and receivers were compatible with the typical span of optical access links ( ≤ 10km).
We present a fiber-based Quantum Cryptography (QC) system in which data is acquired by utilizing a new Time-Correlated Single Photon Counting (TCSPC) instrument. This device captures single photon events on two synchronized channels with picosecond resolution over virtually unlimited time spans and with extremely short dead-times (<95ns). The QC system operates at a wavelength of 1550nm and employs an interferometric approach in which quantum-level information is encoded in the relative phase shift between pairs of faint optical pulses generated by a strongly attenuated semiconductor laser. The QC channel and three additional conventional data channels are carried over a single transmission fiber using a coarse wavelength division-multiplexing (CWDM) scheme with a 20nm channel separation. We assess the impact of the various sources of errors in the system, such as imperfect interference visibility, detector dark counts and Raman scattering in the transmission fiber. Secure key distributions with mean photon numbers of 0.1 and 0.2 per pulse pair were demonstrated for transmission distances up to 25km and 38km respectively.
In recent years quantum information research has lead to the discovery of a number of remarkable new paradigms for information processing and communication. These developments include quantum cryptography schemes that offer unconditionally secure information transport guaranteed by quantum-mechanical laws. Such potentially disruptive security technologies could be of high strategic and economic value in the future. Two major issues confronting researchers in this field are the transmission range (typically <100km) and the key exchange rate, which can be as low as a few bits per second at long optical fiber distances. This paper describes further research of an approach to significantly enhance the key exchange rate in an optical fiber system at distances in the range of 1-20km. We will present results on a number of application scenarios, including point-to-point links and multi-user networks.
Quantum key distribution systems have been developed, which use standard telecommunications optical fiber, and which are capable of operating at clock rates of up to 2GHz. They implement a polarization-encoded version of the B92 protocol and employ vertical-cavity surface-emitting lasers with emission wavelengths of 850 nm as weak coherent light sources, as well as silicon single-photon avalanche diodes as the single photon detectors. The point-to-point quantum key distribution system exhibited a quantum bit error rate of 1.4%, and an estimated net bit rate greater than 100,000 bits-1 for a 4.2 km transmission range.
The potential for sharing infrastructure costs between a large
number of customers and the high data rates allowed by optical
fibres make passive optical networks (PONs) an attractive solution
to the problem of upgrading current copper-based access networks.
Optically-amplified, long reach, time division multiple access
(TDMA) PONs or 'SuperPONs' offer the potential to further reduce bandwidth transport costs by enabling the direct connection of access networks and inner core networks, thereby eliminating the costs of the outer core/metro backhaul network. The use of dense wavelength division multiplexing (DWDM) could also allow sharing the same feeder fibre and PON head end equipment between a number of such TDMA SuperPONs, each working at different ITU-grid wavelengths. However, a cost effective access solution should employ a customer optical network unit (ONU), which is independent of the PON wavelength, or colorless, in order to reduce the high inventory and deployment costs of using expensive, wavelength-specified sources at the customer. In this paper we demonstrate for the first time the use of a monolithically-integrated, electroabsorption modulator-semiconductor optical amplifiers (EAM-SOAs) as a colorless ONU in a high performance DWDM SuperPON system. These compact devices offer the potential for low-cost optoelectronic integration with other ONU components together with the ability to modulate at rates up to 10Gbps and beyond. We have used this approach to investigate the feasibility of supporting up to 17 SuperPONs from a single feeder fibre and PON head end, each of 100km-reach accommodating 512 users at 2.5Gb/s or 128 at 10Gb/s.
Quantum cryptography exploits the fact that an unknown quantum state cannot be accurately copied or measured without disturbance. By using such elementary quantum states to represent binary information it is possible, therefore, to construct communication systems with verifiable levels of security that are 'guaranteed' by fundamental quantum mechanical laws. This paper describes recent progress at BT Laboratories in the development of practical optical fiber- based quantum cryptography system. These developments include interferometric systems operating in the 1.3 micrometers - wavelength fiber transparency window over point-to-point links up to approximately 50km in length and on multi-user passive optical networks. We describe how this technology performs on fiber links installed in BT's public network and discuss issues such as cross-talk with conventional data channels propagating at different wavelengths in the same fiber.