NEC Metropolitan Quantum Key Distribution Network in Tokyo

Quantum key distribution exploits a foundational property of quantum mechanics to distribute encryption keys with information-theoretic security: any attempt to intercept a quantum communication necessarily disturbs the quantum states being transmitted, leaving a detectable trace. Classical cryptographic key exchange relies on computational hardness assumptions (most of which would fall to quantum computers), whereas QKD’s security derives from the laws of physics themselves. NEC has been developing QKD hardware for over a decade, and in 2023 the company deployed a production metropolitan-area QKD network across Tokyo, integrating with existing fiber infrastructure already used by government agencies and major financial institutions.

The original QKD protocol, BB84, encodes key bits in the polarization states of individual photons transmitted over optical fiber. Each photon carries one bit of key information, and the key rate is fundamentally limited by the rate at which single photons can be generated and detected, which drops sharply with distance as fiber attenuation reduces photon flux. NEC’s Tokyo deployment instead uses continuous-variable QKD (CV-QKD), which encodes information in the amplitude and phase quadratures of coherent light pulses rather than in single photons. The receiver uses homodyne or heterodyne detection, the same coherent detection technology already embedded in modern 100G and 400G fiber optic transceivers, making CV-QKD systems more compatible with existing telecom infrastructure and capable of higher key rates at metropolitan distances (roughly 50-100 km) compared to single-photon discrete-variable (DV-QKD) systems. At longer distances, DV-QKD with superconducting nanowire single-photon detectors regains the advantage, but for a metro network spanning city districts, CV-QKD is the practical choice.

NEC integrated the QKD network into the classical network infrastructure using a key management layer that sits between the quantum channel and the application layer. The quantum channel continuously generates a stream of symmetric key material; the key management system distributes this material to endpoint devices, which use it to refresh AES-256 session keys at regular intervals. At 10 kbps of secure key throughput over a 50 km link, the system generates a fresh 256-bit AES key every 3 seconds, far exceeding what would be needed to protect communications traffic at rates typical of government and financial applications. The quantum channel runs over dedicated dark fiber where available and over wavelength-division-multiplexed channels on shared fiber where dedicated fiber is not economical, with the QKD signal occupying a wavelength outside the conventional C-band to minimize interference with classical traffic. Japan’s national quantum strategy, formalized in its Quantum Technology and Innovation Strategy, explicitly targets quantum communication infrastructure as a priority, and the Tokyo network serves as the flagship demonstration of that policy commitment.

Expanding from initial pilot nodes to 12 nodes across the Tokyo metro area required solving practical network engineering challenges that laboratory QKD demonstrations never confront: splices, patch panels, variable fiber quality, and the need to route keys through intermediate trusted relay nodes where fiber runs exceed the direct transmission limit. Each relay node necessarily holds key material in plaintext, making its physical and operational security as important as the quantum channel itself. NEC’s network architecture treats relay nodes as high-security facilities equivalent to telco central offices. The 12-node deployment provides a realistic testbed for the trusted-relay architecture that will characterize near-term quantum networks before quantum repeaters, which would allow truly end-to-end quantum security without trusted relays, reach maturity.