Practical quantum tokens: challenges and perspectives

TL;DR

Quantum tokens aim to provide information-theoretic authentication and secure transactions by encoding token data in quantum states stored in long-lived memories. The paper surveys optical and microwave realizations across platforms—rare-earth ion ensembles, diamond color centers, hybrid photonics, and room-temperature alkali-noble gas memories—addressing encoding, storage, transduction, and verification. It highlights memory lifetimes, interface efficiencies, multiplexing capabilities, and security frameworks including quantum-read physical unclonable functions, while outlining end-to-end token protocols and adversary models. The work emphasizes practical deployment pathways, particularly via nuclear-spin memory architectures and integrated photonics, and discusses how quantum tokens can complement post-quantum cryptography in critical infrastructure and high-assurance applications.

Abstract

The concept of quantum tokens dates back alongside quantum cryptography to Stephen Wiesner's seminal work in 1983[1]. Already this initial work proposes society-relevant applications such as secure quantum banknotes, which can be exchanged between a bank and a customer. This quantum currency is based on various physical states that can be easily verified but is protected from being copied by the fundamental quantum laws. Four decades later, these ideas have flourished in the field of quantum information, and the concept of quantum banknotes has not only adopted many varying names, such as quantum money, quantum coins, quantum-digital payments, and quantum tokens, but also reached its first experimental demonstrations. In this perspective article, we discuss the current state-of-the-art of quantum tokens in the field of quantum information, as well as their future perspectives. We present a number of physical realizations of quantum tokens with integrated quantum memories and their applicability scenarios in detail. Finally, we discuss how quantum tokens fit into the information security ecosystem and consider their relationship to post-quantum cryptography.

Figures (16)

• Figure 1: The loose structure of quantum information fields and sub-fields in relation to selected milestone ideas.
• Figure 2: (a) Simplified implementation of quantum token. Here, a flying quantum token is an authentication or transaction key that does not require any intermediate storage. The on-submission quantum token, on the other hand, requires a storage element. The on-submission quantum token can be verified by the holder at the latest by the time of expiry of the quantum storage element. (b) Structural representation of a quantum token, which in a general case requires an encoding protocol, a transmission channel, and a storage element. The encoding protocol is defined by the choice of type of variables (continuous, discrete or mixed) and multiplexing method: TDM (time-division multiplexing), FDM/WDM (frequency- or wavelength-division multiplexing) and mixed. The transmission channel defines the frequency of wavelength at which the respective quantum token is being generated and transmitted; it is bound to the available infrastructure. (c) Key elements of physical realization of quantum token, which include issuing and verifying parties, transmission channel, and storage element.
• Figure 3: Microwave quantum tokens show strong potential for the advancement of secure short-range quantum communication. Beyond traditional cable-based connections, wireless links in the 2GHz to 12GHz frequency range are feasible due to the exceptionally low absorption losses in open air. These links also exhibit strong resilience to weather-related disturbances, such as fog and rain. Recent studies Fesquet2024 report high fidelities for the wireless transmission of microwave quantum states. Taken together, these findings indicate that secure information transmission via quantum tokens can be seamlessly integrated into existing classical microwave communication infrastructures.
• Figure 4: Conceptual experimental setup for the coupling of propagating squeezed microwaves and quantum tokens to a spin ensemble. A Josephson parametric amplifier (JPA), controlled by a microwave pump signal, generates squeezed microwave states, which are subsequently displaced to form a quantum token. This signal is then transferred to a spin-resonator system engineered to transduce microwave excitations into spin excitations. The quantum properties of the emitted signal from the spin-resonator are assessed using Wigner tomography.
• Figure 5: (a) A photonic qubit is stored in a single REI qubit by cavity-enhanced absorption or cavity reflection. (b) Schematic level scheme for the spin-photon interface. (c) Inhomogeneously broadened optical transition (black) with narrow homogeneous linewidths (blue), enabling single- and multi-qubit addressing within a single cavity.