Quantum key distribution (QKD) is back in the headlines and the rush of demonstrations has intensified the policy debate about whether militaries should retool their secure-comm arsenals around quantum physics. The headline claim is simple: physics, not math, can detect eavesdropping and thus enable key material that is in principle tamper-evident. The practical reality is far more complicated. Deployment choices for military communications will be driven less by theory and more by three factors: the physics-imposed operational envelope, engineering and supply-chain vulnerabilities, and lifecycle integration with existing crypto and command systems.

What QKD actually gives you and what it does not QKD produces symmetric key material by exchanging quantum states over a physical link. That key material still needs a classical cipher to protect traffic in transit and a signature scheme to authenticate end points. In short, QKD is a special-purpose key distribution technology, not a drop-in replacement for public-key infrastructure. It is best viewed as an additional layer in a larger cryptographic architecture rather than a panacea.

The operational constraints are real and measurable Two physical constraints frequently cited in the literature matter for military planning: fiber attenuation and satellite-pass dynamics. Terrestrial single-photon QKD over optical fiber is effectively limited to on the order of 100 kilometers without quantum repeaters; beyond that, attenuation drives key rates to near zero unless you accept trusted nodes. Satellite-based QKD extends reach across continents, but satellite passes are transient and key generation is bursty - early space experiments produced only kilobits to megabits of usable key material per pass, and entanglement experiments produced only a few coincident pairs per second during visibility windows. Those numbers influence whether you can support continuous voice, bulk data transfer, or only periodic rekeying of high-value links.

Security in theory versus security in practice The best-known theoretical security proofs assume idealized hardware. Real-world devices have consistently shown exploitable side channels: detector efficiency mismatches, timing and calibration attacks, bright-illumination attacks, and other implementation-level weaknesses. The literature has documented experimental attacks against commercial QKD systems and proposed countermeasures, which means that the security you get depends on rigorous hardware validation and continuous red-teaming. This is a meaningful operational risk for national security systems that demand very high assurance.

Trusted nodes, insider risk, and denial-of-service Long-distance QKD networks in current architectures rely on trusted relays or nodes - points where key material is processed in the classical domain and therefore becomes susceptible to compromise or insider attack. In addition, the fundamental sensitivity of quantum links makes them inherently vulnerable to denial-of-service attacks: a jammer or an adverse weather event can suppress photon counts and eliminate key generation. For a military operator, both the insider risk and availability fragility need explicit mitigation plans before QKD would be acceptable for mission-critical traffic. The U.S. NSA and other security authorities have explicitly warned that QKD is not recommended for National Security Systems until these limitations are addressed.

Where the technology is currently being proven: state actors and testbeds China has led operational demonstrations at scale, combining the Micius satellite experiments with regional fiber backbones to show intercontinental and mixed space-fiber demonstrations. Europe has pursued a testbed strategy to validate interoperability and use cases through projects such as OPENQKD and the EuroQCI initiative, which focus on controlled deployments across fiber and satellite interfaces and on building common interfaces for multiple vendors. Those efforts are deliberate: they accept that today QKD is an experimental infrastructure that must be integrated, standardized, and audited before being trusted for defence use.

How QKD compares with post-quantum cryptography for military adoption From a pragmatic procurement perspective, post-quantum cryptography (PQC) - algorithmic replacements for public-key functions designed to resist quantum-computer attacks - has three operational advantages: it runs on existing hardware and networks, it is upgradeable in software, and it addresses authentication needs that QKD leaves untouched. For most foreseeable military use cases where scalability and agility matter, PQC is the lower-friction path toward quantum resistance. This is why national agencies have concentrated on PQC standardization while treating QKD as a complementary capability for niche, high-assurance links.

Where QKD could make operational sense for defense QKD is not pointless for defense. Scenarios where physical control of the link is enforceable and where the marginal cost of dedicated optical infrastructure is justified are the most plausible near-term use cases. Examples include: hardened fiber between national command centers, ship-to-shore optical links in littoral basing where line-of-sight can be assured, and strategic satellite downlinks used for occasional, high-value key injection rather than continuous bulk traffic. In these roles QKD can offer a narrowly defined advantage - provided the system architecture addresses trusted-node exposure, hardware attestation, and key-lifecycle management.

Practical recommendations for military planners 1) Adopt an all-of-the-above crypto posture. Prioritize PQC for broad rollout while funding QKD testbeds to mature hardware assurance and space-fiber integration. 2) Invest in end-to-end hardware validation. Fund independent red-team exercises against vendor systems to discover side channels and harden designs. Document and certify mitigations rather than relying on theory alone. 3) Use QKD experimentally for narrow, well-instrumented links. Design requirements should include measured key-rate budgets, trusted-node hardening, and fallback PQC modes for availability. 4) Treat space-QKD as strategic R&D. Satellite constellations can change the operational calculus for long-distance key distribution, but they are an expensive and complex path that requires sustained investment in optical ground stations, scheduling, and orbital capacity. Do not rush wide operational adoption until hardware assurance and operational availability metrics are met. 5) Focus on standards and supply-chain diversity. Interoperability testbeds like OPENQKD reveal how important standard interfaces and multi-vendor ecosystems are to avoid single-source vulnerabilities. Military procurement should incentivize modular, auditable hardware and open interfaces.

Conclusion: niche capability, not a wholesale replacement As of this writing, QKD is a maturing set of techniques with impressive demonstrations but important, quantifiable limits. It is mission-relevant for narrowly defined, high-assurance links where physical control and defence-in-depth make sense. For broad fleet, tactical, and enterprise communications, post-quantum cryptography and classical engineering controls remain the more practical route to quantum resistance. The sensible path for defense organizations is parallel: accelerate PQC migration across systems while funding targeted QKD pilots that answer the hardware-assurance, availability, and lifecycle questions that still stand between laboratory demonstrations and operational trust.