Practical quantum tokens without quantum memories and experimental tests

Abstract

Unforgeable quantum money tokens were the first invention of quantum information science, but remain technologically challenging as they require quantum memories and/or long-distance quantum communication. More recently, virtual “S-money” tokens were introduced. These are generated by quantum cryptography, do not require quantum memories or long-distance quantum communication, and yet in principle guarantee many of the security advantages of quantum money. Here, we describe implementations of S-money schemes with off-the-shelf quantum key distribution technology, and analyse security in the presence of noise, losses, and experimental imperfection. Our schemes satisfy near-instant validation without cross-checking. We show that, given standard assumptions in mistrustful quantum cryptographic implementations, unforgeability and user privacy could be guaranteed with attainable refinements of our off-the-shelf setup. We discuss the possibilities for unconditionally secure (assumption-free) implementations.

Demonstration of quantum-digital payments

“…This is far beyond state-of-theart quantum storage times, which range from a few microseconds to a few minutes [28][29][30] . Recently, an interesting alternative was proposed, replacing quantum storage by a network of trusted agents and authenticated channels, positioned at precise spatial locations with respect to the spending points 31,32 . From a practical standpoint, this approach presents new drawbacks, as customers and online shoppers do not have the means to securely set up complex trust networks for everyday transactions.…”

“…As shown in Fig. 1, we generate and verify i.t.-secure quantum cryptograms, in such a way that the unforgeability and user privacy properties from previous experimental works holds 32 , but all intermediate channels, networks, and parties are untrusted, thus significantly loosening the security assumptions. Only one authenticated communication (between the client and their payment provider) has to take place at an arbitrary prior point in time.…”

Enhancing quantum cryptography with quantum dot single-photon sources

“…We wish to encourage future quantum key distribution experiments with optimal pumping schemes, taking into account the security assumptions provided by the quantum cryptography community. Finally, we hope to stimulate experiments that explore the full potential of quantum dot-based single-photon sources for other quantum network primitives like unforgeable tokens [32][33][34] , coin flipping 35,36 and bit commitment [38][39][40] . We believe our analysis can be extended in future works to multipartite entanglement-based quantum network primitives, such as secret sharing 73 and anonymous messaging 74 .…”

“…This primitive allows a central authority to issue tokens, comprised of quantum states, whose unforgeability is intrinsically guaranteed by the nocloning theorem, thus requiring no hardware assumptions. One famous application is quantum money, which, in its private-key form, can prevent banknote forgery 9 , double-spending with credit cards 32,66 and guarantee features such as user privacy 34 .…”

“…Our findings are designed to bridge the gap between the quantum dot and quantum cryptography communities: we optimize and benchmark QDS optical pumping schemes for four main quantum cryptographic primitives, exploiting the combined advantage of brightness, single-photon purity, and photon-number coherence. The studied primitives include quantum key distribution (standard BB84, decoy and twin-field) 5,16,30,31 , unforgeable quantum tokens [32][33][34] , quantum strong coin flipping [35][36][37] , and quantum bit commitment [38][39][40] under storage assumptions.…”