Quantum Device Theory explores how quantum principles like superposition and entanglement can be used to design advanced technologies. Simulating theoretical designs helps predict and refine their performance before experiments, making development more efficient, more intuitive and less costly. The proof of concept stage in experiment will respond to our creativity in the domain of theory, ensuring what we predict will be next improvement or revolutionary concepts will work in practice and paving the way for real-world applications in computing, sensing, and communication.