The 2025 Nobel Prize in Physics was awarded to John Clarke, Michel H. Devoret, and John M. Martinis in recognition of their decisive contributions to realizing and controlling macroscopic quantum coherent states in superconducting circuits. Their work demonstrates that coherent superposition—a core feature of quantum mechanics—is not exclusive to microscopic particles but can also exist and be precisely manipulated in engineered artificial systems composed of billions of electrons. This article begins with the fundamental concept of quantum superposition, explains how decoherence blurs the line between the quantum and classical worlds, and describes how the laureates used superconducting circuits to achieve macroscopic energy-level quantization, coherent oscillations, and quantum tunneling—ultimately laying the physical foundations for modern quantum computing and quantum engineering.