Digital signatures attempt to guarantee the validity of internet transactions. Their security rests on the mathematical difficulty of factorising large numbers into primes. This difficulty cannot be proved, so there is a possible flaw in all current schemes. Quantum digital signatures have security guaranteed by the laws of physics. We have developed test systems that run digital signature protocols in laboratory situations [1,2] and are exploring moving these towards trials in fibre networks. The diagram shows a digital signature system based on unambiguous quantum state elimination.
 P.J. Clarke et al., Experimental demonstration of quantum digital signatures using phase-encoded coherent states of light, Nature Comm. 3, 1174 (2012).
 R.J. Collins et al., Realisation of Quantum Digital Signatures without the Requirement of Quantum Memory, Phys. Rev. Lett. 113, 040502 (2014).