Quantum key distribution uses the measurement properties of single photons or weak optical fields to let two parties grow a shared secret key whose secrecy rests on physics rather than computational hardness; the hardware now runs unattended in metropolitan fibre, but the gap between idealised security proofs and real devices remains the binding constraint.
Summary
Quantum key distribution (QKD) is a method for two parties to agree a symmetric encryption key over an untrusted channel. Instead of relying on a mathematical problem being hard, it encodes bits in quantum states of light. Any attempt to measure or copy those states disturbs them, so the legitimate parties can bound how much an eavesdropper could have learned by measuring the error rate, then compress the raw data down to a shorter key that is secret with high probability. The compression step (privacy amplification, typically universal hashing) and the accounting of how much secrecy survives finite data sets are where most of the theoretical work sits.
The field splits along several axes. Discrete-variable (DV) schemes such as decoy-state BB84 send weak laser pulses and count single-photon detections; the decoy-state trick uses deliberately varied pulse intensities to detect photon-number-splitting attacks. Continuous-variable (CV) schemes encode in the amplitude and phase quadratures and use homodyne detection with standard telecom components, trading distance for hardware familiarity and rate. Entanglement-based variants (E91, BBM92) distribute photon pairs and derive randomness from the correlations themselves rather than from a trusted transmitter. Measurement-device-independent and twin-field style protocols, including mode-pairing QKD, aim to remove trust in the detectors and to beat the repeaterless rate-versus-transmittance bound. High-dimensional encodings, for example in orbital angular momentum, raise bits per photon and noise tolerance at the cost of harder state generation and sorting.
The parameters that decide the technology are: channel loss in dB (which sets the key rate and, past a point, kills it entirely), detector noise and background light, clock rate, and the size of the data block available for finite-key statistics. Everything else is engineering around those four. A 1.25 GHz integrated-photonics prototype in 19-inch rack units has held continuous key exchange over more than 4 km of metropolitan fibre across more than 12 day-night cycles without intervention, and needs no chromatic dispersion compensation below 100 km. A free-space CV system with a local local oscillator and self-referenced passive state preparation reports an asymptotic secret key rate of 10.34 Mbps at up to 23.5 dB loss.
The unresolved problem is not whether keys can be produced but whether the security proof applied to a real box is actually valid. Recent work shows that high repetition rates create intensity correlations between successive pulses that leak encoding settings and violate a founding assumption of decoy-state QKD, with higher-order correlations mattering more than nearest-neighbour ones in two industrial prototypes, and that a gated avalanche detector’s click timing shifts by more than 2 ns as pulse energy varies over 50 dB, which an attacker can use to push clicks into adjacent bit slots and render standard proofs inapplicable.
Evidence base
- 29 May 2026: a 1.25 GHz integrated-photonics QKD prototype in standard 19-inch rack units maintained continuous, uninterrupted key exchange over more than 4 km of metropolitan fibre across more than 12 day-night cycles without manual intervention, and required no chromatic dispersion compensation below 100 km.
- 8 May 2026: a self-referenced, local-local-oscillator passive-state-preparation CV-QKD system reported a record asymptotic secret key rate of 10.34 Mbps over a free-space channel with up to 23.5 dB loss under turbulent conditions.
- 4 May 2026: intensity correlations were characterised in two industrial decoy-state BB84 prototypes; they leak encoding settings, invalidate a basic premise of the decoy-state method and significantly reduce the asymptotic key rate, with higher-order correlations mattering more than nearest-neighbour ones.
- 14 May 2026: an avalanche detector gated at 312.5 MHz showed click timing shifting by more than 2 ns as pulse energy varied over a 50 dB range, enabling proposed attacks that toggle clicks between adjacent key bit slots and render existing security proofs inapplicable.
- 14 May 2026: a numerical finite-key security framework valid against general coherent attacks was introduced, covering transmitter and receiver imperfections including non-IID signals from limited modulator bandwidth, and requiring only partial device characterisation.
- 14 May 2026: GHz sequential time-bin entangled states were distributed over an approximately 30 km, 9.5 dB link in the Vienna fibre network using off-the-shelf components.
- 29 May 2026: a decoy-state BB84 geostationary downlink was assessed with variable-length finite-key security across rural, urban and coastal environments and historical European cloud data, identifying positive-key regimes and bottlenecks rather than demonstrating operation.
Open questions
- Can the numerical finite-key frameworks now being proposed be applied end to end to a shipped commercial transmitter and receiver, including higher-order intensity correlations and detector timing side channels, without collapsing the usable key rate?
- What is the cost, rate and assurance comparison against classical and post-quantum key exchange for the same link? No supplied source addresses it, and it decides adoption.
- Does the integrated-photonics metro prototype hold its stability and key rate when scaled from the demonstrated 4 km to the 50-100 km links that real operator topologies require?
- Do geostationary downlinks yield enough annual secret key across realistic European cloud cover and daylight background to justify the space segment?
Assessment drafted 2026-08-31 from up to 18 KB sources using the technology-scorecard framework; scores are a draft read pending review.