More is Less:Optimal Security for Haar Quantum Money and More

Abstract

Quantum cryptography leverages unclonability to enable a wide range of cryptographic applications that are impossible classically, such as digital currency protected against counterfeiting by quantum mechanics. Security requires that no efficient user can produce even one additional valid banknote beyond those already in their possession. However, existing security bounds weaken as the number of banknotes available to a user increases. In this paper, we study a construction of quantum money whose asymptotic query security does not deteriorate as long as the number of banknotes available to a user remains below the scale required for state tomography. In particular, we show that the construction based on an $n$-qubit Haar-random state and a reflection oracle achieves optimal query security: unless a user holds $Ω(2^n)$ banknotes, their existing banknotes cannot asymptotically speed up counterfeiting --- their best possible attack is the same as if they do not have any banknotes. To establish this result, we develop a framework based on the compressed-oracle technique for defining and analyzing progress measures for quantum tasks such as Haar state cloning. Furthermore, we prove a tight lower bound for generating $r$ additional copies, give a matching attack, and apply our results to quantum copy-protection.