npj Quant. Inf. http://dx.doi.org/10.1038/s41534-017-0010-x (2017)

Unknown quantum states cannot be perfectly copied. This is the no-cloning theorem, making quantum information an ideal medium for cryptography. By embedding a quantum token into a banknote, it may be possible to create a currency that is invulnerable to any cloning attacks. In reality, however, quantum data are always prone to external influence, and some level of noise has to be tolerated in order to harness the benefits of quantum technologies. This inevitably leads to the possibility of forgery, placing the safety of quantum money under question.

Karol Bartkiewicz and colleagues have reported an experimental realization of quantum money. Instead of focusing on a practical implementation, though, they analysed the possibility of attacking their setup by exploiting its imperfections. The authors applied optimal cloning strategies and tried to find the conditions under which quantum money is vulnerable. Their results — and the solutions they pose — may cause us to rethink the security of quantum communication and quantum technologies.