Smart contracts meet quantum cryptography
This work addresses the challenge of secure and efficient quantum payment systems for users in decentralized networks, representing a novel application rather than an incremental improvement.
The authors tackled the problem of integrating quantum cryptography with classical blockchain systems by proposing a hybrid classical-quantum payment system that uses quantum states as banknotes and a classical blockchain for dispute resolution and tracking. The result is a decentralized system with fast quantum communication-based payments and recovery mechanisms for lost or damaged quantum banknotes.
We put forward the idea that classical blockchains and smart contracts are potentially useful primitives not only for classical cryptography, but for quantum cryptography as well. Abstractly, a smart contract is a functionality that allows parties to deposit funds, and release them upon fulfillment of algorithmically checkable conditions, and can thus be employed as a formal tool to enforce monetary incentives. In this work, we give the first example of the use of smart contracts in a quantum setting. We describe a simple hybrid classical-quantum payment system whose main ingredients are a classical blockchain capable of handling stateful smart contracts, and quantum lightning, a strengthening of public-key quantum money introduced by Zhandry [Eurocrypt 2019]. Our hybrid payment system uses quantum states as banknotes and a classical blockchain to settle disputes and to keep track of the valid serial numbers. It has several desirable properties: it is decentralized, requiring no trust in any single entity; payments are as quick as quantum communication, regardless of the total number of users; when a quantum banknote is damaged or lost, the rightful owner can recover the lost value.