Abstract:We report the transmission of 40 quantum-key channels using WDM/SCM-QKD technology and 4 bidirectional classical channels over a PON. To our knowledge the highest number of quantum key channels simultaneously transmitted that has ever been reported. The quantum signal coexists with classical reference channel which is employed to process the qbits, but it has enough low power to avoid Raman crosstalk and achieving a high number of WDM-QKD channels. The experimental results allow us to determine the minimum rejection ratio required by the filtering devices employed to select each quantum channel and maximize the quantum key rate. These results open the path towards high-count QKD channel transmission over optical fiber infrastructures. Sasaki, and A. Tajima, "A high-speed wavelength-division multiplexing quantum key distribution system," Opt. Lett. 37(2), 223-225 (2012). 6. J. Mora, A. Ruiz-Alba, W. Amaya, A. Martínez, V. García-Muñoz, D. Calvo, and J. Capmany, "Experimental demonstration of subcarrier multiplexed quantum key distribution system," Opt. Lett. 37(11), 2031-2033 (2012). 7. P. Townsend, "Simultaneous quantum cryptographic key distribution and conventional data transmission over installed fibre using wavelength-division multiplexing," Electron. Lett. 33(3), 188-190 (1997) ©2012 Optical Society of America

We provide, to our knowledge, the first experimental demonstration of the feasibility of sending several parallel keys by exploiting the technique of subcarrier multiplexing (SCM) widely employed in microwave photonics. This approach brings several advantages such as high spectral efficiency compatible with the actual secure key rates, the sharing of the optical fainted pulse by all the quantum multiplexed channels reducing the system complexity, and the possibility of upgrading with wavelength division multiplexing in a two-tier scheme, to increase the number of parallel keys. Two independent quantum SCM channels featuring a sifted key rate of 10 Kb∕s∕channel over a link with quantum bit error rate <2% is reported. , we theoretically proposed the distribution of more than one key per wavelength by means of subcarrier multiplexed quantum key distribution (SCM-QKD) [17]. Here, we experimentally demonstrate, for the first time to our knowledge, the feasibility of the SCM-QKD approach to achieve the simultaneous distribution of two parallel keys by using frequency channels very closely packed in the optical spectrum.The operation principle of SCM-QKD can be explained referring to Fig. 1, which shows a scheme of the experimental setup assembled to demonstrate the feasibility of the SCM-QKD approach by multiplexing different independent keys. Four main blocks can be distinguished, which correspond to the quantum transmitter (Alice), the quantum receiver (Bob), both interconnected by an 11 Km access network link [10 Km single-mode fiber (SMF28e) followed by 1 Km dispersion compensating fiber (DCF)], the classical reference channel, and the overall electronic control system.In the experiment, Alice's transmitter produced weak coherent-state pulses by strong attenuation of a laser source previously pulsed using a time gating electronic signal to drive a 20 dB extinction ratio electro-optic Mach-Zehnder modulator. The output pulses had 1.3 ns FWHM and a repetition rate of 1 MHz. The nominal 3 dB laser linewidth was 10 MHz and the emission wavelength was 1557.30 nm. Quantum states to encode the binary secret keys were prepared at Alice's location by amplitude modulating the faint laser pulses using a 20 GHz bandwidth external electro-optic modulator (AM), biased at quadrature. We employed two RF subcarriers (f RF1 10 and f RF2 15 GHz), generated by independent voltage controlled oscillators (VCOs). The laser source radiation was externally modulated by the RF subcarriers.For parallel key distribution, each RF subcarrier transmits a different key, which is generated by randomly phase modulating the output of each VCO among four Fig. 1. (Color online) Experimental SCM-QKD system to distribute different keys in parallel: AM, amplitude modulator; PM, phase modulator; VCOs, voltage controlled oscillators; Att 1∕2A∕B , electrical attenuators; CWDM, coarse wavelength division mux/demux; EDFA, erbium doped fiber amplifier; RFA, RF amplifier; SPAD, single photon avalanche detector; and Φ 1∕2A∕B , RF phase shifters.

No abstract

The incorporation of multiplexing techniques used in microwave photonics to quantum key distribution (QKD) systems brings important advantages by enabling the simultaneous and parallel delivery of multiple keys between a central station and different end-users in the context of multipoint access and metropolitan networks, or by providing higher key distribution rates in point to point links by suitably linking the parallel distributed keys. It also allows the coexistence of classical information and QKD channels over a single optical fiber infrastructure. In this paper, we show, for the first time to our knowledge, the successful operation of a two-domain (subcarrier and wavelength division) multiplexed strong reference BB84 QKD system. A four-independent channel QKD system featuring a sifted key rate of 10 kb/s/channel over an 11-km link with quantum bit error rate (QBER) G 2% is reported. These results open the way for multi-QKD over optical fiber networks.

No abstract