Physicists from Poland, the Czech Republic, Japan and the United States have demonstrated a prototype of the optical quantum money they have created (such objects have not been created in full and are not in circulation at present). The authors published a preprint dedicated to the research on the arXiv.org website.
Photon polarization states
Image: Karol Bartkiewicz, Antonín Сernoch, Grzegorz Chimczak, Karel Lemr, Adam Miranowicz, Franco Nori
Physicists were able to experimentally demonstrate the capabilities of the quantum money prototype they created. The model bill was encoded with the states of polarization of single photons, so that its unauthorized reproduction (copying or, what is the same, cloning) by an unauthorized person is impossible without the consent of the owner.
Classical banknote and its different quantum counterparts
Image: Karol Bartkiewicz, Antonín Сernoch, Grzegorz Chimczak, Karel Lemr, Adam Miranowicz, Franco Nori
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In their study, scientists have demonstrated the fulfillment of security conditions for quantum money. The banknote in the scientists' approach was modeled by an object containing a sequence of qubits (quantum bits) that encoded the serial number of the banknote (in accordance with an idea proposed in 1970 by the American-Israeli physicist Steven Wisner). Unlike classical bits, in qubits a variable can take not only two values zero and one, but also any other value determined by the superposition of basic states (zero and one).
Quantum money offers a high degree of protection against fraud because quantum information cannot be completely copied by unknown persons. This is applied in quantum cryptography, in particular, the prospect of creating quantum money.