Experimental demonstration of subcarrier multiplexed quantum key distribution system

Abstract

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.

Simultaneous transmission of 20x2 WDM/SCM-QKD and 4 bidirectional classical channels over a PON

“…1 combines 4 bidirectional classical channels which transport baseband data at 10 Gb/s and 20 WDM quantum channels based on SCM-QKD technology [6]. Note that each WDM quantum channel contains two SCM quantum channels.…”

“…Upstream channels are demultiplexed in the CN. Figure 2 represents a scheme which shows in detail the configuration of the SCM-QKD system [6]. Each quantum channel consists on a transmitter located at the CN and a receiver for each end-user which are named Alice and Bob, respectively.…”

“…While this solution is technically feasible there are other QKD multiplexing schemes which require at least one classical reference traveling together with the quantum signal through the same fiber for management and control tasks such as clock recovery, polarization, delay and phase tracking etc. This is the case for instance of Time-bin QKD [5] or SCM-QKD [6] systems.…”

“…The achievement of these goals is critical in order to spread the use of QKD to current deployed networks, especially in the access and metro areas. Among the reported contributions in the area of multiplexed QKD systems one can find Time Division Multiplexing (TDM)-QKD [4], Wavelength Division Multiplexing (WDM)-QKD [5] and Subcarrier Multiplexing (SCM)-QKD [6]. In principle the WDM-QKD approach is very attractive since it is compatible with the dominant techniques employed in the transmission of classical information channels along current optical networking infrastructures.…”

Quantum computer networks with the orbital angular momentum of light

“…The channels are given separate properties in the time [4], frequency and wavelength [5][6][7], coherent-state amplitude [8], or orbital angular momentum [9] domains so that they can be later divided again at the receiver.…”