Have you ever been forced to enter confidential payment information on an unknown seller’s website? Would you like to put your credit card information or passwords into untrustworthy hands? Scientists from the University of Vienna have developed an absolutely safe system for shopping in such conditions, combining modern cryptographic methods with the fundamental properties of quantum light. A demonstration of such “quantum digital payments” in a realistic setting has been published. Nature Communication.

Digital payments have replaced physical banknotes in many areas of our daily lives. Like banknotes, they must be easy to use, unique, tamper-proof and traceable, but also resistant to digital attackers and data leakage.

In the modern payment ecosystem, customer confidential data is replaced by a series of random numbers, and the uniqueness of each transaction is ensured by a classical cryptographic method or code. However, attackers and traders with powerful computing resources can crack these codes and recover customers’ personal data and make payments on their behalf, for example.

A research team led by Professor Philip Walter of the University of Vienna has demonstrated how the quantum properties of light particles, or photons, can ensure the absolute security of digital payments.

During the experiment, the researchers showed that not every transaction could be copied or manipulated by attackers, and the user’s sensitive data remained private. “I am really impressed with how the quantum properties of light can be used to secure new applications related to our daily lives, such as digital payments,” says Tobias Guggemos.

To ensure the absolute security of digital payments, scientists replaced classical cryptographic methods with a quantum protocol that uses individual photons. In a classic digital payment transaction, the customer shares a classic code, called a cryptogram, with the payment provider (such as a bank or credit card company).

This cryptogram is then transmitted between the customer, the vendor and the payment provider. In the quantum protocol shown, this cryptogram is generated by the payment provider, which sends individually crafted photons to the customer.

For the payment procedure, the customer measures these photons with the measurement parameters depending on the transaction parameters. Because quantum states of light cannot be replicated, an operation can only be performed once. This makes digital payment absolutely safe, as any deviation from the intended payment changes the measurement results verified by the payment provider.

Researchers have successfully implemented quantum digital payments over a 641m long urban fiber optic line connecting two university buildings in central Vienna. Currently, digital payments are made in seconds.

“Currently, our protocol requires several minutes of quantum communication to complete a transaction. This guarantees security in the presence of noise and losses,” said Peter Szianski, first author of the paper. “However, these time limits are purely technological in nature,” explains Matthieu Bozio.