Science

In today's payment ecosystem, customers' sensitive data is substituted by sequences of random numbers, and the uniqueness of each transaction is secured by a classical cryptographic method or code. However, adversaries and merchants with powerful computational resources can crack these codes and recover the customers' private data, and for example, make payments in their name.

For enabling absolute secure digital payments, the scientists replaced classical cryptographic techniques with a quantum protocol exploiting single photons (particles of light). During the course of a classical digital payment transaction the client shares a classical code—called cryptogram—with their payment provider (e.g. a bank or credit card company).

This cryptogram is then passed on between customer, merchant and payment provider. In the demonstrated quantum protocol this cryptogram is generated by having the payment provider sending particularly prepared single photons to the client.

For the payment procedure, the client measures these photons whereby the measurement settings depend on the transaction parameters. Since quantum states of light cannot be copied, the transaction can only be executed once. This, together with the fact that any deviation of the intended payment alters the measurement outcomes, which are verified by the payment provider, makes this digital payment unconditionally secure.

The researchers successfully implemented quantum-digital payments over an urban optical fiber link of 641m, connecting two university buildings in down-town Vienna. Digital payments currently operate within a few seconds.