  Digital payments have replaced physical banknotes in many aspects of our
daily lives. Similarly to banknotes, they should be easy to use, unique,
tamper-resistant and untraceable, but additionally withstand digital attackers
and data breaches. Current technology substitutes customers' sensitive data by
randomized tokens, and secures the payment's uniqueness with a cryptographic
function, called a cryptogram. However, computationally powerful attacks
violate the security of these functions. Quantum technology comes with the
potential to protect even against infinite computational power. Here, we show
how quantum light can secure daily digital payments by generating inherently
unforgeable quantum cryptograms. We implement the scheme over an urban optical
fiber link, and show its robustness to noise and loss-dependent attacks. Unlike
previously proposed protocols, our solution does not depend on long-term
quantum storage or trusted agents and authenticated channels. It is practical
with near-term technology and may herald an era of quantum-enabled security.
