Ensemble-Based Quantum-Token Protocol Benchmarked on IBM Quantum Processors

Abstract: Quantum tokens envision to store unclonable keys in quantum states that can be used for personal authentication in a physical device issued by a bank, for instance. Still, its experimental realization faces many technical challenges. In this work, we propose an ensemble-based quantum-token protocol, having the potential to make these applications technologically less-demanding. Therefore, in order to demonstrate the safety of the protocol, a simple and minimal model based on an observable operator and measurement uncertainty is developed to describe the quantum token hardware, while the protocol is fully benchmarked and compared on five different IBM quantum processors. First, the uncertainties of the hardware are characterized, from which the main quality parameters that describe the token can be extracted. Following that, the fraction of qubits which the bank prepares and measures successfully is benchmarked. These fractions are then compared with the values obtained from an attacker who attempts to read the bank token and prepare a forged key. From which we experimentally demonstrate an acceptance probability of 0.059 for a single forged token, in contrast to 0.999 for the bank's own tokens. These values can be further optimized by increasing the number of tokens in the device, where even in the worst IBMQ, with 49 tokens the acceptance probability of forged tokens is below $10^{-22}$. Finally, we show that minor improvements in the hardware quality lead to significant increases in the protocol security, denoting a great potential of the protocol to scale with the ongoing quantum hardware evolution. This work demonstrates the overall security of the protocol within a hardware-agnostic framework, further confirming the interoperability of the protocol in arbitrary quantum systems and thus paving the way for future applications with different qubits.