Digital twin technology has gained increasing attention across various sectors due to its ability to create virtual replicas of physical systems, enabling real-time monitoring, optimization, and simulation. This paper explores the integration of digital twins within defence applications, focusing on key use cases ranging from system design and development, operational planning and training, to mission execution and debriefing. By examining the application of digital twin technologies across defense platforms, we highlight their key advantages such as enhanced operational performance, predictive capabilities, and increased system uptime. Additionally, we introduce a novel characterization framework for digital twins that aims to standardize and unify their application across different defence domains to facilitate interoperability. Thereafter, we discuss the main challenges, gaps and limitations in implementing and adopting digital twins within defence organizations by analyzing a combination of scientific literature, current industry practices, governmental strategies, and the findings from a comprehensive survey of industrial stakeholders and ministries of defense. Finally, we outline future research directions and development opportunities, emphasizing the need for robust frameworks and interdisciplinary collaborations to fully realize the potential of digital twins in the defence sector.

We propose a model describing the formation of both dual (quantum) and classical Shapiro steps in small Josephson junctions. According to this model, the dual Shapiro steps are formed at relatively low frequency of the microwave signal and low microwave power, while the classical steps are formed in the opposite limit of high frequency and power. The crossover between the two regimes is controlled by a single parameter - the effective relaxation time of the environment. The model accounts for the effect of a large inductor in the bias circuit, which has been used in recent experiments to protect the junction from the high frequency noise of the environment. We predict the possibility of observing both types of steps in the same sample. Our model describes the I-V curves observed in the experiments with reasonable accuracy, thus opening up an opportunity for quantitative fitting of the data.