GHz-rate polarization-based QKD system for fiber and satellite applications

Quantum key distribution (QKD) leverages the principles of quantum mechanics to exchange a secret key between two parties. Despite its promising features, QKD also faces several practical challenges such as transmission loss, noise in quantum channels and finite key size effects. Addressing these issues is crucial for the large-scale deployment of QKD in fiber and satellite networks. In this paper, we present a 1550 nm QKD system realizing the efficient-BB84 protocol and based on the iPOGNAC scheme. The system achieved repetition rates up to 1.5GHz and showed an intrinsic QBER of $\sim 0.4%$. The system was first tested on a laboratory fiber link and then on an intermodal link in the field, consisting of both deployed fiber and a 620 m free-space channel. The experiment was performed in daylight conditions, exploiting the Qubit4Sync synchronization protocol. With this trial, we achieved a new benchmark for free-space BB84 QKD systems by generating a sustained secret key rate (SKR) above 1Mb/s for 1 hour. Finally, exploiting a recently discovered finite-size bound, we achieved a secure key rate of about 10 Mb/s at low losses (5 dB), and around 6.5~kb/s in the high-loss (38.5 dB), low block length ($N=10^4$) regime. The latter results demonstrate the system’s suitability for highly lossy and time-constrained scenarios such as QKD from low Earth orbit satellites.

💡 Research Summary

Quantum key distribution (QKD) promises unconditional security by exploiting quantum mechanics, yet practical deployment faces challenges such as channel loss, noise, and finite‑key effects. This paper introduces a 1550 nm, polarization‑encoded QKD system that operates at repetition rates up to 1.5 GHz and implements the efficient three‑state one‑decoy BB84 protocol using the iPOGNAC scheme. The source consists of a gain‑switched DFB laser followed by two symmetric iPOGNAC modules. The first iPOGNAC attenuates pulses in a cosine‑squared manner to generate decoy states, while the second encodes the three required polarization states (|D⟩, |L⟩, |R⟩). A 6 GSa/s DAC driven by an UltraScale+ SoC provides the high‑speed electrical control, and the balanced modulation architecture ensures a zero‑average voltage, improving resilience with AC‑coupled RF amplifiers.

In the laboratory fiber link (5 dB loss), the system achieved an intrinsic quantum bit error rate (QBER) of 0.38 % in the key basis and 0.27 % in the check basis, remaining stable over two hours at 1 GHz without active stabilization. When the repetition rate was increased to 1.5 GHz—close to the theoretical limit of 1.53 GHz—the optimized protocol parameters (μ = 0.28, ν/μ ≈ 0.44, p_Z = 0.9, p_μ = 0.5) yielded an estimated secret key rate (SKR) exceeding 5 Mb/s at 5 dB loss.

The system was then deployed on an intermodal link comprising two deployed fibers and a 620 m urban free‑space channel between two university buildings in Padova. The transmitter terminal integrated dual‑wavelength alignment (980 nm) and channel‑monitoring (1545 nm) lasers, a fast‑steering mirror (FSM), and a tip‑tilt correction loop to compensate atmospheric turbulence. The receiver terminal employed an f/8 Ritchey‑Chrétien telescope, a second FSM, and dichroic optics to separate the quantum channel from the alignment beam. Total channel loss amounted to ≈10.9 dB (fiber + free‑space + terminal). Even under daylight conditions, the QBER stayed below 0.4 %, and a sustained SKR above 1 Mb/s was maintained for one hour—a new benchmark for metropolitan free‑space BB84 QKD.

Crucially, the authors applied a recently derived finite‑size bound to evaluate performance under high‑loss, short‑block scenarios relevant to low‑Earth‑orbit (LEO) satellite QKD. With 38.5 dB loss and a block size of N = 10⁴, the system produced a secure key rate of about 6.5 kb/s, extracted roughly every 0.3 s. In the low‑loss regime (5 dB), the finite‑size analysis predicts a secure key rate of ~10 Mb/s. Synchronization was achieved using the Qubit4Sync protocol, which operates at the full 1.5 GHz rate, eliminating timing bottlenecks.

Overall, this work demonstrates the first integrated GHz‑rate, polarization‑encoded QKD platform that simultaneously achieves ultra‑low QBER, high SKR, flexible operation over both fiber and free‑space links, and robust finite‑key performance. The architecture is directly applicable to space‑qualified transmitters, paving the way for scalable global quantum‑secure networks that combine terrestrial fiber backbones with satellite links. Future directions include extending the range to true satellite‑to‑ground distances, multiplexing multiple users, and integrating quantum repeaters to further increase reach and key throughput.