Quantum Key Distribution Over Optical Communication Networks

11 Dec Quantum Key Distribution Over Optical Communication Networks

Quantum Key Distribution over Optical Communication Networks
Modern cybersecurity relies heavily on keeping digital information safe as it moves across the internet. Classical cryptography achieves this by using mathematical keys to encrypt and decrypt messages. One of the most widely used systems today is RSA encryption, the method that protects online banking, secure email, e-commerce, and most HTTPS traffic.
RSA works by generating two very large prime numbers, P and Q, and multiplying them to create a number N = P × Q. This number becomes part of the encryption public key. Anyone can use N to encrypt a message. However, only the recipient—who knows the original primes P and Q—can compute the private key needed to decrypt it. The strength of RSA comes from the difficulty of working backwards: although multiplying P and Q is easy, figuring out which two primes were used (prime factorization) is extraordinarily hard. For RSA keys that are thousands of digits long, even the fastest classical supercomputers would need millions of years to factor N.

But this security may not last forever. A sufficiently powerful quantum computer could factor large numbers almost instantly, potentially breaking RSA and exposing encrypted data. This looming threat has led researchers to develop Quantum Key Distribution (QKD)—a method of generating and sharing cryptographic keys that remains secure even against future quantum computers.
QKD takes advantage of the quantum properties of photons, the particles of light that travel through optical fibers. Photons have a property called polarization, which describes the direction of their electric field. In simple terms, a photon may be polarized horizontally or vertically, or in a combination of both states at the same time—a phenomenon called superposition.
In a polarization-based QKD system, the sender (Alice) uses photon polarization to encode bits of a cryptographic key. For example:
• Horizontal polarization = 0
• Vertical polarization = 1
Alice sends these polarized photons to the receiver (Bob). Bob measures each photon’s polarization using randomly chosen filters. When Bob uses the same basis that Alice used to send the photon, he gets the correct bit. When he uses the wrong basis, he gets a random result. After transmission, Alice and Bob publicly compare which bases they used (not the bit values), and they keep only the measurements that matched. These matching bits become their shared secret key.

The security comes from a fundamental principle of quantum mechanics: measuring a quantum state disturbs it. If an eavesdropper (Eve) intercepts the photons, she inevitably changes their polarization, introducing errors that Alice and Bob can detect. This guarantees that the final key is secure, independent of computing power.
Delivering QKD over real optical networks requires sending single photons—or extremely weak light pulses—through fiber. This introduces several engineering challenges. The biggest challenge is distance. In standard fiber, photons are gradually absorbed or scattered. Because QKD uses single photons, the usable range is typically 80–120 km, and with highly optimized systems, up to 150–200 km. Beyond this, too few photons reach the receiver to form a reliable key.

Another challenge is that quantum signals cannot pass through optical amplifiers such as EDFAs, which are standard in long-haul optical networks. Amplifiers add spontaneous emission noise and overwhelm the single-photon signals. Moreover, due to the quantum no-cloning theorem, the photon’s state cannot be copied or regenerated like classical signals.
QKD must also coexist with powerful classical data channels. Classical signals contain billions of photons that can generate noise through Raman scattering. To address this, QKD systems place the quantum channel on a separate wavelength, far from classical traffic, and use dense filtering to keep the quantum signal clean.
To overcome distance limits, operators use trusted nodes, where keys are received, regenerated, and forwarded. This allows secure key distribution across hundreds of kilometers, provided the nodes are physically secured. In the future, quantum repeaters—which create long-distance entanglement without measuring the quantum signal—will enable continent-wide and global QKD networks.
As optical networks continue to evolve, QKD stands out as a future-proof cybersecurity solution, providing security based not on mathematical hardness but on the unbreakable laws of physics. By harnessing the unique behavior of polarized photons, QKD transforms existing optical fibers into ultra-secured channels for the quantum era.

To learn more about optical networking, consider joining OTT optical network training programs, CONA (Certified Optical Network Associate) and CONE (Certified Optical Network Engineer).