Fig. 3: Experimental quantum-digital payments.
a The Trusted Token Provider (TTP) creates entangled photon pairs using a Spontaneous Parametric Down Conversion (SPDC) source. One photon’s polarization is randomly measured by the TTP in either a linear or diagonal basis, creating the classical description \((b,\, {{{{{{{\mathcal{B}}}}}}}})\), which remotely prepares the quantum token \(\left|P\right\rangle\) on the second photon. The latter is sent to the Client through a 641 m long optical fiber link, who measures its polarization in a basis mi = MAC(C, Mi) specified by a Message Authentication Code (MAC) of the Merchant’s ID Mi and the Client’s private token C, and thereby obtains the cryptogram that is \({\kappa }_{i}\mathop{\leftarrow }\limits^{{m}_{i}} \, \left|P\right\rangle\). Classical communication between the TTP, Client and Merchant is used to verify the compatibility of κ, Mi and C with \((b,\, {{{{{{{\mathcal{B}}}}}}}})\). Red (blue) lines indicate quantum (classical) channels. The arrow numbering indicates the steps from Fig. 2. b Satellite image of the two buildings housing the TTP, Client, and Merchant. A 641 m optical fiber link connects the parties.
