With the continuous advancement of multi-core fiber (MCF) preparation technology, multiple signals can now be transmitted simultaneously through different cores within a single fiber, enabling space division multiplexing (SDM) co-transmission of quantum and classical signals. This addresses the previous issue of quantum signals monopolizing individual fibers in quantum key distribution (QKD) systems. Despite advancements, previous SDM-QKD experiments using MCF have encountered limitations: limited fiber length, higher attenuation coefficients compared to standard single-core fibers, and lower inter-core crosstalk in laboratory-customized MCFs. We pioneer the practical industrial feasibility of SDM-QKD using commercial 4-core low-loss MCF and phase-coded QKD, demonstrating SDM of quantum and classical signals under realistic urban conditions. This verification provides crucial feasibility for future large-scale deployment of SDM-QKD in urban fiber optic networks.

The SDM-QKD experimental setup utilizes a commercial 4-core MCF and phase-coded QKD. Quantum and synchronous QKD signals occupy one core, while classical data signals occupy another core within the 4-core MCF. The MCF has a length of 21.39 km with a cladding diameter of 125 μm and core-to-core spacing of 43 μm. The cores are sequentially numbered clockwise as 1#, 2#, 3#, and 4#. Each core exhibits an attenuation coefficient of 0.182 dB/km@1550 nm with inter-core crosstalk coefficients ranging from 10^-7 km^-1. Spatial coupling and decoupling of signals across cores are achieved using 1×4 fan-in/fan-out devices with an insertion loss of 0.9 dB and an isolation degree of 50 dB. The experiment employs a commercial QKD device based on the phase-encoding decoy-state BB84 protocol with a Faraday-Michelson interferometer. The emission frequency of quantum signals is 50 MHz, with a distribution ratio of 14∶1∶1 among signal, decoy, and vacuum states. The average photon numbers for these states are 0.6, 0.2, and 0, respectively. Quantum and synchronous signals at wavelengths of 1549.32 nm and 1550.92 nm are co-propagated via dense wavelength division multiplexing before being connected to the 1# core of the 4-core MCF through fan-in/fan-out devices. For classical post-processing, QKD-T’s electrical signal is converted to 1490 nm optical signal by the optical line terminal, connected to other cores, while QKD-R’s electrical signal is converted to 1310 nm optical signal by the optical network unit, connected to the same core as the 1490 nm classical signal.

Quantum and synchronous signals occupy the 1# core, while the classical signals occupy cores 2/3/4# sequentially. The average secret key rate (SKR) of SDM-QKD at 21.39 km is 2.90 kbit/s with an average quantum bit error rate (QBER) of 0.88% over continuous operation exceeding 4 hours. Compared to non-SDM QKD, SKR is reduced by 0.68% and QBER is increased by 2.33 percentage points (Table 2). A loop test connecting cores 1# and 3# achieves an SDM-QKD experiment over 42.78 km with an average SKR of 0.75 kbit/s and QBER of 2.15%. Compared to non-SDM QKD, SKR is reduced by 8.54 percentage points and QBER is increased by 7.50% (Table 3). When quantum and synchronous signals occupy the 1# core and classical signals occupy the 2# core, the average SKR of SDM-QKD is 2.90 kbit/s with a standard deviation of 0.36 kbit/s. Average QBER is 0.89% with a standard deviation of 0.18% (Figs. 9 and 10). This experiment with commercial MCF reflects the influence of inter-core crosstalk noise on QKD performance in urban environments, addressing the deficiencies of previous SDM-QKD experiments and demonstrating the stable operation of SDM-QKD using commercial MCF and QKD devices.

We build an SDM-QKD model based on MCF and analyze background noise changes for SDM-QKD. It experimentally verifies the feasibility of SDM-QKD in urban environments under near-real conditions using commercial 4-core MCF and phase-coded QKD, alongside classical communication equipment. Compared to existing SDM-QKD experiments, results show that inter-core crosstalk noise in commercial MCF minimally influences SDM-QKD performance. Inter-core crosstalk noise remains a crucial factor affecting SDM-QKD performance; minimizing inter-core crosstalk coefficients is essential to improving the SDM-QKD signal-to-noise ratio. While the commercial MCF used in this paper effectively eliminates inter-core crosstalk noise influence on SDM-QKD performance when classical signal wavelengths are non-adjacent to quantum signal wavelengths, further reduction in inter-core crosstalk coefficient may be necessary for adjacent wavelength scenarios. Moreover, during the actual installation of fiber optic links, multiple fiber segments are typically fused to extend the link’s length. Both this paper and prior SDM-QKD experiments have focused on single MCF deployments. Future research should investigate how inter-core crosstalk changes at these fusion joints influence SDM-QKD performance. This will enhance the theoretical and experimental framework necessary for developing and implementing quantum secure communication systems based on MCF.