From Macroscopic Quantum Phenomena to Artificial Atoms

Forty years ago, a fundamental inquiry into the possible quantum-ness of macroscopic variables—currents and voltages—launched the field of quantum superconducting circuits. This colloquium traces the journey from the foundational experiments at Berkeley that first uncovered macroscopic quantum tunneling and energy quantization to the current era of "artificial atoms." Unlike natural atoms, these engineered systems allow for precise control over energy levels and matrix elements by tailoring dimensional parameters. We will discuss how the ground and first excited states of these circuits are now utilized as qubits in processors exceeding 100 qubits, capable of executing quantum algorithms that challenge classical computation. Beyond computation, we highlight the transformative impact of superconducting circuits on quantum sensing, specifically their ability to amplify microwave signals at the fundamental limits of quantum physics.