Quantum key distribution over a 2 km free-space channel with a high secure key rate

arXiv:2607.24118 · quant-ph · Submitted 2026-07-27 · Read on arXiv

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Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: We're looking at the paper: "Quantum key distribution over a 2 km free-space channel with a high secure key rate".

Mira: The gist:

Kai: First, who's behind it and why it matters.

Title and authors: Kai: Now let's move into the specifics of what this paper is actually proposing, looking at the title "Quantum key distribution over a two km free-space channel with a high secure key rate <ref:2607.24118#pg1,Quantum key distribution over a 2 km free-space channel with a>."

Mira: The authors are focusing on proving that you can achieve a high secure key rate using decoy-state BB84 QKD specifically in an outdoor free-space environment. It's not just about the protocol, it’s about the practical deployment constraints.

Lev: What they are doing is demonstrating a system operating at one hundred MHz repetition rate with a two point five nanosecond pulse width over a two kilometer channel, which sets the baseline for high-speed performance in this domain.

Kai: They are using eight independent semiconductor lasers, four signal and four decoy photons, to manage the quantum states. The goal here is to maintain spectral indistinguishability across all those sources simultaneously while dealing with timing variations.

Mira: So they are addressing the challenge of matching the central wavelength and ensuring that even though you have eight different sources, they are all indistinguishable enough for the protocol to work as expected.

Lev: The core technical achievement seems to be managing that sub-nanosecond synchronization across all channels, which is necessary because pulse timing can vary due to internal electronic delays and path length differences.

Kai: And they’re getting tight sub-nanosecond precision on that synchronization, which page two shows is what you need for the temporal pulse profiles of each channel to look right.

Mira: So the paper essentially lays out a specific experimental realization of a free-space QKD system that tackles those fundamental hardware challenges head-on.

Lev: It’s showing that this level of synchronization is achievable, which is crucial because if your timing isn't tight, you can't properly implement the BB84 protocol without introducing errors.

Kai: And they are verifying the polarization states for both signal and decoy pulses, showing they can successfully generate and measure the required H V D and A states.

Mira: So, this summary is about confirming that all the necessary physical components are working together in a coherent system to perform quantum key distribution over this short distance.

Lev: It confirms that you can build the transmitter side correctly, which is a prerequisite before you even worry about dealing with atmospheric effects or receiver optics.

Kai: So they've built the source and the preparation stage successfully, which is always a good step when you’re trying to deploy something mobile.

Mira: And that means we can now move on to how they are handling those external environmental challenges, like turbulence, which is where things usually fall apart.

Lev: Because atmospheric turbulence causes beam fluctuations and amplitude scintillation, and the paper clearly states these are major challenges in practical free-space implementations.

Kai: And the authors immediately address this by incorporating an active beam-wander correction based on fast-steering mirrors and position sensitive detectors in the receiver setup.

The paper's summary: Kai: So to summarize what they found, this paper experimentally demonstrates a free-space decoy-state BB84 QKD system operating at one hundred MHz repetition rate with a two point five nanosecond pulse width over a two kilometer outdoor channel.

Mira: They achieved a secure key rate of one hundred sixty-four point eight kilobits per second under atmospheric turbulence, with the quantum bit error rate or QBER being approximately three point three percent <ref:2607.24118#pg1,a secure key rate of 164>.

Lev: So the main result is that when they activated the stabilization system, their average secure key rate increased substantially from one hundred three kilobits per second to one hundred sixty-four point eight kilobits per second.

Kai: That increase is directly tied to the active beam stabilization, and it also reduced their standard deviation by approximately fifty percent.

Mira: So the implication here is that active beam stabilization is absolutely indispensable for high-speed, reliable free-space quantum communications because without it, you don't get those good numbers.

Lev: And they project that this system can achieve a secure key rate of approximately one hundred bps at a transmission distance of thirty kilometers under clear atmospheric conditions based on numerical simulations.

Kai: So the summary is: they built a functional system, found that stabilizing it improved the key rate and reliability significantly, and they have projections for longer distances.

Mira: That means we can start thinking about how to integrate these concepts into mobile platforms because this framework provides a practical path forward for deploying high-rate quantum communication in realistic turbulence environments.

Lev: The caveat they mention is that the results are under atmospheric turbulence, so it's an important qualifier when you consider deployment conditions.

The paper's improvements: Kai: Now let’s talk about what improvements the paper suggests they made to this system, because it wasn't just about building it; there were specific fixes they implemented.

Mira: One major improvement is the active beam-wander correction based on fast-steering mirrors and position sensitive detectors, which they use to correct for atmospheric turbulence.

Lev: This correction directly addresses the issue of amplitude scintillation and beam wander, which severely degrades the coupling efficiency of single photons into receiver detection systems.

Kai: They also improved that coupling efficiency by more than ten percent when active stabilization was enabled, which is a key metric here.

Mira: So that ten percent improvement in coupling efficiency is directly attributed to the stabilized MMF coupling efficiency, showing a measurable physical effect from the correction mechanism.

Lev: From an engineering perspective, this means that if you can reliably implement that tracking loop with a bandwidth of more than five hundred hertz, you see tangible gains in system performance.

Kai: So it’s not just about having the hardware; it’s about how effectively you manage the dynamic environment around your quantum components.

Mira: They also confirmed that their average QBER was three point three percent when stabilization was active, which is better than what you might expect in a turbulent setting.

Lev: That QBER of three point three percent when stabilized shows that the link remains reliable even with the turbulence present, which is a solid indicator for real hardware testing.

Conclusion: Kai: So wrapping up this discussion on "Quantum key distribution over a two km free-space channel with a high secure key rate," this paper confirms they built a high-rate decoy-state BB84 system that works in the field <ref:2607.24118#pg1,Quantum key distribution over a 2 km free-space channel with a>.

Mira: The main implication is that for deploying these systems, you need active beam stabilization to get those results, and it’s indispensable for achieving reliable performance over short outdoor links.

Lev: And while they project thirty kilometers under clear atmospheric conditions, we have to keep in mind that that’s a projection based on numerical simulations of their experimental parameters.

Kai: So the system is capable of scaling to support secure key distribution over distances up to thirty kilometers under clear atmospheric conditions, which gives us a target for future network design.

Mira: It really shows how this work provides a practical framework for integrating these concepts into mobile platforms and ground stations in next-generation global quantum networks.

Lev: And from an engineering view, the final thing is that you need to build that kind of fast tracking loop with high precision if you want those one hundred sixty-four point eight kilobits per second results on actual hardware <ref:2607.24118#pg1>.

Kai: So the paper on "Quantum key distribution over a two km free-space channel with a high secure key rate" gives us concrete data on how to handle the real world challenges of turbulence in QKD deployment <ref:2607.24118#pg1,Quantum key distribution over a 2 km free-space channel with a>.

Kyungdeuk Park, Dongkyu Kim, Dong-Gil Im, Yonggi Jo, Jisu Kim, Jonguk Choi, Yong Sup Ihn

Agency for Defense Development

quant-ph

Submitted: 2026-07-27

Updated: 2026-10-16

Comments: 8 pages, 4 figures

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 83/100

The gist: The gist: This work experimentally demonstrates a free-space decoy-state BB84 QKD system operating at a 100 MHz repetition rate with a 2.5 ns pulse width over a 2 km outdoor channel, achieving a

Key concepts

Decoy State BB84 Protocol: This is a specific method used in QKD where the sender (Alice) sends multiple pulses, some with known photon numbers and others with unknown numbers. This allows the receiver (Bob) to estimate the actual number of photons sent, even when noise or eavesdropping occurs. It helps ensure that only secure key material is extracted from the quantum signals.
Atmospheric Turbulence: This refers to rapid, unpredictable fluctuations in the atmosphere, such as temperature variations and air density changes. These fluctuations cause the laser beam traveling through free space to bend and wander erratically, which severely degrades how well photons couple into the receiver's detector.
Active Beam-Wander Correction: This is a real-time feedback system using fast-steering mirrors (FSMs) and position-sensitive detectors (PSDs). When turbulence causes the beam to drift, these components detect the misalignment instantly and automatically adjust the mirror angles to keep the beam centered on the receiver's detector, maintaining a stable link.
Secure Key Rate: This is a measure of how many secure bits of information can be generated per second. In this experiment, it shows that with active correction against turbulence, the system achieved a high rate of 164.8 kbps over 2 km, confirming its viability for practical quantum networks.

Terminology

Steering — 1× in this paper · explained in 2 episodes

Summary

The gist: This work experimentally demonstrates a free-space decoy-state BB84 QKD system operating at a 100 MHz repetition rate with a 2.5 ns pulse width over a 2 km outdoor channel, achieving a secure key rate of 164.8 kbps under atmospheric turbulence with active beam-wander correction.

System Architecture and Components

The free-space QKD system is structurally divided into two main configurations: the transmitter (Alice) and the receiver (Bob). The transmitter light source utilizes eight independent pigtailed semiconductor lasers (Topica, iBeam smart PT805 nm), where four units generate the signal photons and the other four produce the decoy photons. To ensure identical pulse arrival times at the final output, individual optical delay lines are implemented in each channel to compensate for electronic timing jitter and path length variations. The system demands a pointing resolution of 7.5 µrad or finer to satisfy the spatial margin requirement for reliable tracking.

Transmitter Implementation Details

The transmitter employs a specific setup for generating and preparing the quantum signals. Each light source is a continuous-wave (CW) laser operating at a central wavelength of 805 ± 1 nm, directly modulated by pulsed input voltages to generate short optical pulses. The spectral overlap of the eight independent lasers allows for matching both the central wavelength and ensuring spectral indistinguishability. Furthermore, all emitted laser pulses must arrive at the transmitter output aperture simultaneously, which is achieved by tightly synchronizing pulse timing across all channels with sub-ns precision.

Receiver Optics and Stabilization

The receiver system incorporates a complex optical assembly designed for collection and measurement. It includes an off-axis parabolic mirror, dichroic mirrors (DMs), fast-steering mirrors (FSMs), position-sensitive detectors (PSDs), beam splitters (BSs), polarization beam splitters (PBSs), half-wave plates (HWPs), quarter-wave plates (QWPs), and band-pass filters. To correct for rapid beam wander induced by atmospheric turbulence, the system uses a high-speed tracking loop consisting of two sets of FSMs and PSDs. This closed-loop control bandwidth was set to approximately more than 500 Hz to suppress rapid beam-wander caused by atmospheric turbulence.

Performance Evaluation and Key Results

The experiment was conducted over a 2 km outdoor link using the decoy-state BB84 protocol with mean photon numbers configured as µ = 0.8 and ν = 0.1. The performance comparison between operating without and with active beam stabilization is summarized in Table I. Upon activating the stabilization system, the average secure key rate increased substantially from 103 kbps to 164.8 kbps, while its associated standard deviation was reduced by approximately 50 %. The results confirm that active beam stabilization is indispensable for high-speed, reliable free-space quantum communications. Numerical simulations based on experimental parameters project that the system can achieve a secure key rate of approximately 100 bps at a transmission distance of 30 km under clear atmospheric conditions.

Post-Processing and Security Analysis

The QKD post-processing program involves sifting, reconciliation (error correction), and privacy amplification. A hybrid error correction approach combining the Winnow [12] and LDPC (lowdensity parity-check) codes [13, 14] is employed depending on the QBER. The error correction efficiency f represents the performance of the reconciliation process, defined as f(QBER) = leakEC·H(QBER). Privacy amplification is performed using a Toeplitz hash function, which is commonly used in QKD [15]. The analysis of the finite-key effects utilizes the Chernoff bound with Kullback–Leibler (KL) divergence for finite-key effects [21] and the Azuma-Hoeffding (AH) inequality for the nonindependent and identically distributed (non-i.i.d.) assumption [22]. The system achieved an average QBER of 3.3% when stabilization was active, demonstrating enhanced link reliability.

Conclusion and Future Scaling

In conclusion, the work successfully demonstrated a high-rate, decoy-state BB84 free-space QKD system operating over a 2 km outdoor channel under realistic turbulence conditions. The high-speed tracking system corrected rapid beam-wander and improved single-photon coupling efficiency into MMFs by more than 10 % when active stabilization was enabled. These results provide a practical path forward for the integration of mobile platforms and ground stations into next-generation global quantum networks. The architecture is shown to be capable of scaling to support secure key distribution over distances up to 30 km under clear atmospheric conditions.

Acknowledgments

This work was supported by an Agency for Defense Development grant funded by the Defense Aquisition Program Administration. The references cited include foundational works on quantum cryptography [1–3], practical free-space demonstrations [4–7], and specific techniques for turbulence correction [8–10].

References

[1] C. H. Bennett and G. Brassard, “Quantum cryptography: Public key distribution and coin tossing,” Proc. IEEE Int. Conf. Comput. Syst. Signal Process., 1, 175 (1984).

[2] A. K. Ekert, “Quantum cryptography based on Bell’s theorem,” Phys. Rev. Lett., 67, 661 (1991).

[3] N. Gisin, G. Ribordy, W. Zbinden, and H. Zbinden, “Quantum cryptography,” Rev. Mod. Phys., 74, 145 (2002).

[4] C. J. Pugh, S. Kaiser, J.-P. Bourgoin, et al., “Airborne demonstration of a quantum key distribution receiver payload,” Quantum Sci. Technol., 2, 024009 (2017).

[5] X.-H. Tian, et al., “Experimental demonstration of drone-based quantum key distribution,” Phys. Rev. Lett., 133, 200801 (2024).

[6] A. Conrad, et al., “Drone- and vehicle-based quantum key distribution,” arXiv:2505.17587v1.

[7] Y. Li, et al., “Microsatellite-based real-time quantum key distribution,” Nature, 640, 47 (2025).

[8] A. Carrasco-Casado, N. Denisenko, and V. Fernandez, “Correction of beam wander for a free-space quantum key distribution system operating in urban environment,” Opt. Eng., 53(8), 084112 (2014).

[9] M. Chen, C. Liu, and H.

Improvements for AI systems

The improved AI system can achieve robust deployment of quantum communication networks by integrating active beam-wander correction based on fast-steering mirrors (FSMs) and position sensitive detectors (PSDs). This corrects for atmospheric turbulence, which causes amplitude scintillation and beam wander, severely degrading the coupling efficiency of single photons into receiver detection systems.
The system can operate at a high secure key rate of 164.8 kbps under realistic turbulence conditions by employing an active beam-wander correction based on fast-steering mirrors (FSMs) and position sensitive detectors (PSDs) configuration, leading to a significant performance enhancement is directly attributed to the stabilized MMF coupling efficiency.
The AI system can scale QKD networks over longer distances by utilizing numerical simulations based on experimental parameters, projecting that the system is projected to achieve a secure key rate of approximately 100 bps at a transmission distance of 30 km under clear atmospheric conditions.

Abstract

Free-space quantum key distribution (QKD) provides crucial advantages, including mobility and deployment exibility, for securing next-generation communication networks. However, atmospheric turbulence and beam wander severely degrade single-photon coupling effciency and link stability over outdoor channels. In this work, we experimentally demonstrate a free-space decoy-state BB84 QKD system operating at 803 nm with a 100 MHz repetition rate over a 2 km outdoor link. By employing an active beam-wander correction based on fast-steering mirrors (FSMs) and position sensitive detectors (PSDs) with a 500 Hz control loop, we enhance the ?ber coupling effciency and achieve an average ?nite-key secure key rate of 110 kbps with an average quantum bit error rate (QBER) of 3.28 %, representing a 2.5 times secure key rate enhancement under an active beam stabilization. This work demonstrates the indispensability of active beam stabilization and provides an operational reference for robust, long-range free-space quantum communication.

Sources

Drone- and Vehicle-Based Quantum Key Distribution