Q Seminar: Archishna Bhattacharyya

Speaker: Archishna Bhattacharyya, University of Ottawa 

Title: The uncloneable bit exists 

Abstract: We establish quantum uncloneable encryption with unconditional security, preventing two non‑communicating adversaries from simultaneously decrypting a single ciphertext — even when both are given the key. Our construction achieves security that approaches the ideal limit at a rate that is exponentially small in the security parameter, without employing any assumptions. Our proof invokes quantum information principles in the fully quantum realm, in a novel setting of cryptography. A decoupling step certifies the statistical independence needed for randomness extraction, and monogamy of entanglement, formalised via strong subadditivity, rules out the sender being highly correlated with two non‑communicating adversaries at once. Consequently, no coordinated strategy beats random guessing of the encrypted bit, establishing unconditional uncloneability. This reveals the existence of an uncloneable bit in Nature and delineates a fundamental, physically enforced cryptographic primitive unavailable in classical settings.

This is joint work with Anne Broadbent and Eric Culf based on arXiv 2603.08916.