Quantum mechanics emerged laden with issues and doubts about its foundations and interpretation. However, nobody in the 1920s and 1930s dared to conjecture that research on such issues would open the doors to developments so huge as to require the term second quantum revolution to describe them. On the one hand, the new theory saw its scope of applications widen in various domains including atoms, molecules, light, the interaction between light and matter, relativistic effects, field quantization, nuclear physics, and solid state and particle physics. On the other hand, there were debates on alternative interpretations, the status of statistical predictions, the completeness of the theory, the underlying logic, mathematical structures, the understanding of measurements, and the transition from the quantum to the classical description. Until the early 1960s, there seemed to be a coexistence between these two orders of issues, without any interaction between them.
From the late 1960s on, however, this landscape underwent dramatic changes. The main factor of change was Bell’s theorem, which implied a conflict between quantum mechanics predictions for certain systems that are spatially separated and the assumption of local realism. Experimental tests of this theorem led to the corroboration of quantum predictions and the understanding of quantum entanglement as a physical feature, a result that justified the 2022 Nobel Prize. Another theoretical breakthrough was the understanding and calculation of the interaction of a quantum system with its environment, leading to the transition from pure to mixed states, a feature now known as decoherence. Entanglement and decoherence both resulted from the dialogue between research on the foundations and quantum predictions. In addition, research on quantum optics and quantum gravity benefitted debates on the foundations. From the early 1980s on, another major change occurred, now in terms of experimental techniques, allowing physicists to manipulate single quantum systems and taking the thought experiments of the founders of quantum mechanics into the labs. Lastly, the insight that quantum systems may be used in computing opened the doors to the first quantum algorithms. Altogether, these developments have produced a new field of research, quantum information, which has quantum computers as its holy grail. The term second quantum revolution distinguishes these new achievements from the first spin-offs of quantum mechanics, for example, transistors, electronic microscopes, magnetic resonance imaging, and lasers. Nowadays the applications of this second revolution have gone beyond computing to include sensors and metrology, for instance, and thus are better labeled as quantum technologies.
