From Qubits to Qubucks: Quantum Digital Currency

Quantum computing is often discussed in terms of its potential to revolutionize scientific discovery and to challenge cryptographic paradigms [1], but it could also change our relationship with money. By using quantum states, quantum currency could solve the double-spending problem and address throughput issues associated with distributed ledgers (blockchain), paving the way for the digital cash of the future.

Digital Banking vs. Digital Currency

To understand quantum currency, we should first distinguish “digital currency” from electronic banking. Credit/debit cards, electronic checks, and digital payments (e.g. Cash App, Venmo) are familiar examples of “digital money.” While such services entail electronic infrastructure, they involve credits/debits made in relation to digital money held by private banks and corporations. This system is contrasted with cash, which circulates peer-to-peer with oversight by an issuing authority to address counterfeiting and effect controls aligned with national policy. “Digital currency” has been defined as electronic cash [2, 3], a secure representation of value controlled by users in peer-to-peer transactions over electronic infrastructure. As electronic cash, digital currencies are faced with a unique challenge: How to make digital information that is easily transferred but not easily copied?

The “Double-Spending” Problem

Obviously, it shouldn’t be possible to spend money more than once. This “double-spending” problem is addressed in current banking systems by centralized ledgers and intra-institutional verification. In contrast, digital currencies that enable peer-to-peer transactions with no third parties require a different mechanism.

Distributed Ledger Technology

Blockchain cryptocurrencies (e.g. Bitcoin [4]) and many of their central bank digital currency (CBDC) counterparts [2] prevent double-spending by distributing transaction histories in nodes of a public network. Through a consensus mechanism, subsequent transactions are approved once a majority of the network has verified their uniqueness. While distributed ledger technology is effective and has gained efficiency since its inception, throughput remains an issue, particularly for CBDCs that are intended to encompass large portions of the economy. This scaling issue has caused CBDC researchers to lean towards incorporating centralized ledgers in hybrid systems [2], compromising privacy and security for the sake of efficiency. The current debate involving digital currencies therefore centers on trading security and privacy for throughput and scalability.

Quantum Digital Currency

Quantum currency [3] is created through a minting process that produces a banknote with a public key and an underlying quantum state. The public key facilitates destruction certificate generation and allows the public authority to maintain a classical record of the number of banknotes in circulation. In a transaction, the public key and quantum state combined are used to verify the legitimacy of the banknote during a transaction. This configuration has the following near-term advantages:

1. Security: The quantum principle of no-cloning [5, 6] means that the double-spending problem is solved, not mitigated.
2. Transaction independence: In contrast with distributed ledger technology, transactions are validated on an individual basis, addressing the performance bottleneck imposed by consensus mechanisms.
3. Stability and control: In contrast with cryptocurrencies, the issuing authority’s awareness of total currency in circulation allows quantum currency to serve as a national policy instrument in line with CBDC, affording it the stability and macroeconomic properties of national currency.

Additionally, in a future where quantum computing and infrastructure are ubiquitous, transaction privacy could also be realized. Instead of bit-wise copying, quantum banknotes could be physically transferred between the payer and receiver, facilitating offline transactions. In this way, a level of privacy and anonymity associated with cash could be extended to the digital realm alongside the stability of a national currency.

Near-Term Limitations

As with quantum computing in general, current technology forces compromised architectures for near-term implementations. Namely, the physical transfer of quantum currency between wallets for offline transactions requires quantum infrastructure that has yet to be developed. The middle-of-the-road solution proposed in [3] is to confine quantum functions to intermediary “Money Services Businesses” (MSBs) while maintaining classical communication with individuals and the issuing authority. MSBs mediate between users and quantum currency like credit/debit institutions of the present by using classical certificates of creation/deletion with the issuing authority to facilitate transactions (as opposed to transfer or copying). This near-term MSB approach bypasses the limitations of distributed ledgers while solving the double-spending problem with quantum states, but it reproduces the general setup of the existing banking system.

The Long View

While near-term quantum currency solves the double-spending problem and makes centralized digital currency more scalable than distributed ledgers with MSB intermediaries, “ideal” quantum currency on quantum infrastructure addresses some concerns of cryptocurrency proponents by making offline digital transactions possible, potentially re-creating the privacy and anonymity of a cash-based society. Quantum computing could therefore be as much a tool for social transformation as for scientific development.

Works Cited

[1] Scholten, T. L., Williams, C. J., Moody, D., Mosca, M., Hurley, W., Zeng, W. J., Troyer, M., & Gambetta, J. M. (2024). Assessing the benefits and risks of quantum computers. arXiv preprint arXiv:2401.16317. https://arxiv.org/abs/2401.16317 [2] Tang, Q., & Si, Y. W. (2025). Central Bank Digital Currencies: A Survey. arXiv preprint arXiv:2507.08880. https://doi.org/10.48550/arXiv.2507.08880 [3] Broadbent, A., Kazmi, R. A., & Minwalla, C. (2024). A Quantum Vault Scheme for Digital Currency. In 2024 IEEE International Conference on Quantum Computing and Engineering (QCE) (pp. 239-249). IEEE Computer Society. https://www.computer.org/csdl/proceedings-article/qce/2024/413701a239/23opYYtokUg [4] Nakamoto, S. (2008). Bitcoin: A Peer-to-Peer Electronic Cash System. https://bitcoin.org/bitcoin.pdf [5] Park, J. L. (1970). The concept of transition in quantum mechanics. Foundations of Physics, 1, 23-33. https://doi.org/10.1007/BF00708652 [6] Wootters, W. K., & Zurek, W. H. (1982). A single quantum cannot be cloned. Nature, 299, 802-803. https://doi.org/10.1038/299802a0