Independence as an Architectural Property: A Research Direction for Multi-Source Cryptographic Entropy

True random number generators that seed cryptographic systems are evaluated on two properties: the entropy density of the physical noise being sampled and the independence of the samples produced. NIST SP 800-90B specifies that a multi-source entropy generator may combine multiple noise sources only when the sources are independent. In current practice, independence is achieved by substrate engineering: chaotic laser arrays and microcomb-based parallel chaos sources are designed to be independent at the optical substrate. This paper argues that the next architectural direction for multi-source entropy generation in critical-infrastructure cryptography is to design for verifiable independence at the joint-observation level — to build entropy architectures in which the independence of sampled noise is a measurable architectural property rather than an assumed substrate property.