Quantum cryptography uses quantum mechanics to enable provably secure key distribution and encryption. This post reviews foundational protocols and implementations up to 2021, from BB84 through satellite QKD and advanced schemes like measurement‑device‑independent QKD.

Origins of Quantum Key Distribution

The concept began with Stephen Wiesner’s conjugate coding in the early 1970s, introducing non‑orthogonal encodings for secure communication [1]. Building on that, Bennett and Brassard proposed the first QKD protocol, BB84, in 1984, using photon polarization and two conjugate bases [2]. Independently, Artur Ekert introduced an entanglement‑based scheme (E91) in 1991, leveraging Bell’s theorem for security [3].

Early Experimental Demonstrations

In 1992, Charles Bennett demonstrated QKD experimentally using attenuated laser pulses over short fiber links [4]. That same era saw the first free‑space entanglement QKD by Rarity, Tapster, and Ekert over 1 km in 1991 [5], and Los Alamos/NIST achieved 148.7 km fiber QKD in 2007 [6]. In 2008, the University of Cambridge with Toshiba reached 20 km at 1 Mbit/s using decoy‑state BB84, and 100 km at 10 kbit/s [7].

Decoy‑State and Device‑Independent Protocols

To counter photon‑number‑splitting attacks, decoy‑state methods were introduced around 2003–2006, enhancing security by varying pulse intensities [8]. Measurement‑device‑independent QKD (MDI‑QKD), proposed circa 2012, removes trust assumptions on detectors, boosting practical security [9].

Satellite and Long‑Distance QKD

In 2016, China’s Micius satellite performed the first space‑to‑ground QKD over distances up to 1,200 km, demonstrating entanglement distribution and secure key rates up to ~1 kbit/s [10]. This milestone opened the path toward global quantum networks, with follow‑up experiments confirming Bell violations under strict locality conditions [11].

Commercial and Network Deployments

Several companies, including ID Quantique, MagiQ Technologies, and Toshiba, commercialized QKD systems by 2020, deploying fiber‑based networks in Europe, North America, and Asia [12]. Integrated continuous‑variable QKD platforms also emerged, using homodyne detection for high secret‑key rates over metropolitan distances.

Challenges and Future Directions

Key challenges include photon loss, finite‑key effects, and integration with existing infrastructure. Advances in trusted‑node architectures, quantum repeaters, and hybrid classical‑quantum networks aim to extend secure range. Measurement‑device‑independent and continuous‑variable protocols are promising for robust, high‑rate QKD on commercial hardware.

References

[1] Wiesner, S. (1983). Conjugate coding. ACM SIGACT News, 15(1), 78-88.

[2] Bennett, C. H., & Brassard, G. (1984). Quantum cryptography: Public key distribution and coin tossing. Proceedings of IEEE International Conference on Computers, Systems and Signal Processing, 175-179.

[3] Ekert, A. K. (1991). Quantum cryptography based on Bell’s theorem. Physical Review Letters, 67(6), 661-663.

[4] Bennett, C. H., Bessette, F., Brassard, G., Salvail, L., & Smolin, J. (1992). Experimental quantum cryptography. Journal of Cryptology, 5(1), 3-28.

[5] Rarity, J. G., Tapster, P. R., Gorman, P. M., & Knight, P. (2002). Ground to satellite secure key exchange using quantum cryptography. New Journal of Physics, 4(1), 82.

[6] Hughes, R. J., et al. (2007). Quantum key distribution over a 148.7 km fiber link. New Journal of Physics, 9(6), 193.

[7] Dixon, A. R., et al. (2008). High speed prototype quantum key distribution system and long term field trial. Optics Express, 16(23), 18790-18797.

[8] Scarani, V., Acín, A., Ribordy, G., & Gisin, N. (2004). Quantum cryptography protocols robust against photon number splitting attacks for weak laser pulse implementations. Physical Review Letters, 92(5), 057901.

[9] Lo, H.-K., Curty, M., & Qi, B. (2012). Measurement-device-independent quantum key distribution. Physical Review Letters, 108(13), 130503.

[10] Yin, J., et al. (2017). Satellite-based entanglement distribution over 1200 kilometers. Science, 356(6343), 1140-1144.

[11] Yin, J., et al. (2017). Satellite-to-ground quantum-limited communication using a 50-kg-class micro-satellite. Nature Photonics, 11(8), 502-508.

[12] Peev, M., et al. (2009). The SECOQC quantum key distribution network in Vienna. New Journal of Physics, 11(7), 075001.