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.

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## From the Interpretation of Quantum Mechanics to Quantum Technologies

### Olival Freire Junior

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## Philosophy of Quantum Mechanics: Dynamical Collapse Theories

### Angelo Bassi

Quantum Mechanics is one of the most successful theories of nature. It accounts for all known properties of matter and light, and it does so with an unprecedented level of accuracy. On top of this, it generated many new technologies that now are part of daily life. In many ways, it can be said that we live in a quantum world. Yet, quantum theory is subject to an intense debate about its meaning as a theory of nature, which started from the very beginning and has never ended. The essence was captured by Schrödinger with the cat paradox: why do cats behave classically instead of being quantum like the one imagined by Schrödinger? Answering this question digs deep into the foundation of quantum mechanics.
A possible answer is Dynamical Collapse Theories. The fundamental assumption is that the Schrödinger equation, which is supposed to govern all quantum phenomena (at the non-relativistic level) is only approximately correct. It is an approximation of a nonlinear and stochastic dynamics, according to which the wave functions of microscopic objects can be in a superposition of different states because the nonlinear effects are negligible, while those of macroscopic objects are always very well localized in space because the nonlinear effects dominate for increasingly massive systems. Then, microscopic systems behave quantum mechanically, while macroscopic ones such as Schrödinger’s cat behave classically simply because the (newly postulated) laws of nature say so.
By changing the dynamics, collapse theories make predictions that are different from quantum-mechanical predictions. Then it becomes interesting to test the various collapse models that have been proposed. Experimental effort is increasing worldwide, so far limiting values of the theory’s parameters quantifying the collapse, since no collapse signal was detected, but possibly in the future finding such a signal and opening up a window beyond quantum theory.

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## The Philosophical Significance of Decoherence

### Elise Crull

Quantum decoherence is a physical process resulting from the entanglement of a system with environmental degrees of freedom. The entanglement allows the environment to behave like a measuring device on the initial system, resulting in the dynamical suppression of interference terms in mutually commuting bases. Because decoherence processes are extremely fast and often practically irreversible, measurements performed on the system after system–environment interactions typically yield outcomes empirically indistinguishable from physical collapse of the wave function. That is: environmental decoherence of a system’s phase relations produces effective eigenstates of a system in certain bases (depending on the details of the interaction) through prodigious damping—but not destruction—of the system’s off-diagonal terms in those bases.
Although decoherence by itself is neither an interpretation of quantum physics nor indeed even new physics, there is much debate concerning the implications of this process in both the philosophical and the scientific literature. This is especially true regarding fundamental questions arising from quantum theory about the roles of measurement, observation, the nature of entanglement, and the emergence of classicality. In particular, acknowledging the part decoherence plays in interpretations of quantum mechanics recasts that debate in a new light.