# Can quantum money exist?

Can a quantum money system exist in which bank (who possesses a secret key $k$) can create quantum states $S$ that anyone (in possession of public key $P$) can verify was issued by the bank without interacting with the bank, but nobody can spend twice?

• @PaulUszak No – Ella Rose May 1 '18 at 2:18
• @EllaRose Well I never. That's so cunning you could put a tail on it and call it a weasel. But what about the double spend problem? That paper is about simple verification against forgery, and only concerns national central banks. Spending genuine pounds in a sweet shop is entirely unrelated. And how can this money be transferred to the shop as only a central bank can actually create cash via monetary policy decisions. If ever there was too a broad question... Perhaps the question needs truncation? – Paul Uszak May 1 '18 at 3:17
• @PaulUszak Double spending is impossible because duplicating the money is impractical, so if I give you some money, then you know I no longer have access to it (because it was physically transferred to you). Same as with physical cash. – Demi May 1 '18 at 3:20
• @Demi Ah. The paper deals with authentication though doesn't it? It doesn't deal with ownership /transfer /conversion to-from paper. And what would you do with all the notes under my bed? A cashless authoritarian society? And do you realise that quantum thingies don't go down copper wires? And spending on board a plane /ship /submarine /car will be a right PITA. Recall how we all hate QKDNs? – Paul Uszak May 1 '18 at 3:39
• @PaulUszak I know. This is strictly of theoretical interest until we can store qbits indefinitely at negligible cost. And the intention is to replace physical cash, not electronic transfers. – Demi May 1 '18 at 20:49