Experimental quantum forgery of quantum optical money

Unknown quantum information cannot be perfectly copied (cloned). This statement is the bedrock of quantum technologies and quantum cryptography, including the seminal scheme of Wiesner’s quantum money, which was the first quantum-cryptographic proposal. Surprisingly, to our knowledge, quantum money has not been tested experimentally yet. Here, we experimentally revisit the Wiesner idea, assuming a banknote to be an image encoded in the polarization states of single photons. We demonstrate that it is possible to use quantum states to prepare a banknote that cannot be ideally copied without making the owner aware of only unauthorized actions. We provide the security conditions for quantum money by investigating the physically-achievable limits on the fidelity of 1-to-2 copying of arbitrary sequences of qubits. These results can be applied as a security measure in quantum digital right management.

An illustrative example of a a classical banknote. In panels b and c, the simplified banknote from panel a with the decreased number of colors and resolution is encoded experimentally in two ways to form two examples of quantum banknotes.

¹Faculty of Physics, Adam Mickiewicz University, Poznań, PL-61-614, Poland
²RCPTM, Joint Laboratory of Optics of Palacký University and Institute of Physics of Academy of Sciences of the Czech Republic, 17. listopadu 12, Olomouc, 772 07, Czech Republic
³CEMS, RIKEN, Wakoshi, 351-0198, Japan
⁴Institute of Physics of Czech Academy of Sciences, Joint Laboratory of Optics of PU and IP AS CR, 17. listopadu 50A, Olomouc, 772 07, Czech Republic
⁵Department of Physics, The University of Michigan, Ann Arbor, MI, 48109-1040, USA