Practical quantum tokens: challenges and perspectives

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.

💡 Research Summary

The paper “Practical quantum tokens: challenges and perspectives” provides a comprehensive overview of the emerging field of quantum tokens, which are cryptographic primitives that combine the no‑cloning property of quantum mechanics with quantum memories to create unforgeable authentication and payment credentials. Starting from Stephen Wiesner’s 1983 proposal of quantum banknotes, the authors trace the evolution of the concept through the development of BB84 quantum key distribution and the modern terminology (quantum money, quantum coins, quantum‑digital payments, quantum tokens).

A quantum token is defined as a device that stores a codebook of quantum states in a quantum memory and allows on‑demand retrieval and verification. Tokens can be public or private, and their security relies on information‑theoretic guarantees rather than computational assumptions. The paper emphasizes that a practical token must include three components: (i) a long‑lived quantum memory, (ii) a transmission channel, and (iii) an encoding/verification protocol. The authors illustrate this architecture with schematic figures and discuss how the choice of memory dictates the suitable transmission frequency (optical, microwave, or hybrid) and encoding scheme (continuous, discrete, or mixed variables, with TDM/FDM/WDM multiplexing).

The core of the article surveys state‑of‑the‑art physical platforms capable of storing quantum states for seconds to hours. Four families are highlighted:

1. Rare‑earth‑doped optical memories (e.g., Eu³⁺, Pr³⁺) that achieve nuclear‑spin coherence times up to 18 h and are compatible with telecom wavelengths, offering a pathway to long‑distance fiber‑based token distribution.
2. Diamond color‑center ensembles ((Si,V,Sn,N)‑V centers) that couple electronic spins to nearby ¹³C nuclear spins, providing fast optical readout and coherence on the order of seconds.
3. Phosphorus donors in silicon where electron spins are controlled by microwave pulses while nuclear spins serve as long‑term storage, enabling integration with superconducting circuits.
4. Cesium‑xenon gas mixtures that exploit atom‑nuclear spin exchange to reach tens of seconds to minutes of coherence, suitable for free‑space or vapor‑cell implementations.

The authors also discuss alternative approaches that avoid long‑lived memories. Quantum‑read physically unclonable functions (QR‑PUFs) rely on the secrecy of a classical physical structure and use quantum probes for verification, while the S‑money protocol uses short‑range quantum communication and spacetime constraints to secure classical keys without any storage element. Both schemes reduce hardware complexity but have distinct security assumptions.

From a security standpoint, the paper contrasts quantum tokens with post‑quantum cryptography (PQC). PQC provides computational security that may be vulnerable to “harvest‑now‑decrypt‑later” attacks, whereas quantum tokens offer unconditional security grounded in the laws of physics. The authors argue that the two paradigms are complementary: quantum tokens can protect the authentication and payment phases, while PQC can safeguard the surrounding classical infrastructure (e.g., digital signatures, key exchange).

Key challenges identified for practical deployment include:

• Memory access latency and recyclability – fast write/read cycles and the ability to reuse a token without degrading security.
• Scalable issuance and management – standardized protocols for token generation, revocation, and lifecycle tracking at a banking‑scale.
• Robust error correction – mitigation of transmission loss, environmental noise, and decoherence through quantum error‑correcting codes and adaptive decoding.
• Standardized verification procedures – development of interoperable, possibly ISO/IEC‑approved, verification algorithms that can operate across different hardware platforms.
• Manufacturing cost and integration – mass‑production techniques for rare‑earth crystals, diamond color centers, and silicon donor chips, as well as on‑chip integration of photonic and microwave components.

To address these hurdles, the authors propose a hybrid quantum‑classical security framework: integrate on‑chip quantum memories with photonic interconnects, employ time‑division multiplexing for high‑throughput token distribution, and develop modular verification units that can be embedded in point‑of‑sale terminals or access‑control readers.

Potential application domains are outlined: (i) unconditional identity authentication for high‑security facilities, (ii) quantum‑enabled digital payments ranging from micro‑transactions to large‑scale settlements, (iii) distributed trust infrastructures where quantum tokens act as anchors for blockchain‑based smart contracts, and (iv) quantum network management where tokens certify the authenticity of quantum routers and repeaters.

In conclusion, the paper positions quantum tokens at the intersection of quantum cryptography, quantum memories, and information‑theoretic security. While experimental progress—especially in achieving hour‑scale coherence and efficient readout—has brought the concept from theory to proof‑of‑principle, significant engineering and standardization work remains. The authors forecast that within the next 5–10 years, coordinated multidisciplinary efforts could yield commercially viable quantum token systems that complement, rather than replace, existing post‑quantum cryptographic solutions.