Quantum security for your credit card

Among the many unusual features that distinguish quantum mechanics from classical mechanics is the “no-cloning” theorem: It is impossible to perfectly replicate an unknown quantum state (see the Quick Study by Bill Wootters and Wojciech Zurek, Physics Today, February 2009, page 76 ). Quantum cryptography aims to exploit that property in order to keep communications secure from would-be eavesdroppers (see the article by Daniel Gottesman and Hoi-Kwong Lo, Physics Today, November 2000, page 22 ). Now Pepijn Pinkse and colleagues at the University of Twente and the Eindhoven University of Technology have shown how to provide fraud-proof physical identification using quantum-secure authentication (QSA). The keys for their experiment were thin layers of white paint sprayed onto the “cards” to be authenticated. When a light pulse comprising only a few photons is focused onto a key, the pattern of reflected light depends sensitively on the spatial shape of the incoming photon pulse, which can be programmed, and on the random, irreproducible positions of more than a million zinc oxide nanoparticles in the paint. After measuring and logging that dependence in an initial “enrollment” of the key, the researchers could interrogate the key with an arbitrary pulse shape and compare the expected response to what was actually observed. The correct key was clearly distinguishable from an incorrect key and even from an optimized forgery attempt based on stolen enrollment information. That the QSA technique does not depend on stored, secret data and can be implemented with current technology should add to its appeal. (S. A. Goorden et al., Optica 1, 421, 2014, doi: 10.1364/OPTICA.1.000421 .)