Researchers from Caltech and Ben-Gurion University 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 their work, they give the first example of the use of smart contracts in a quantum setting. They have described 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.
Their hybrid payment system employs 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.