Part #01: When physics reinvented itself

The evolution of modern electronics – from transistors and computers to quantum computers – would never have been possible without modern quantum physics. Our understanding of the universe, which extends almost back to the Big Bang, is also based on quantum physics and the theory of relativity. Quantum physics describes the laws of physics on a microscopic scale – in the realm of molecules, atoms, atomic nuclei and elementary particles.

The emergence of quantum physics took place in stages. It began with the formulation of Max Planck’s radiation formula in 1900, in which he attributed the transmission of light energy to a “quantum of action.” In this view, the transmission of light can only take place in standardized packets, or in “quantized” form. Just five years later, Albert Einstein deciphered another previously unexplained effect, namely the photoelectric effect, by attributing a particle-like quality to light – which until then had been assumed to be a wave. Einstein instead interpreted light as consisting of “localized energy quanta,” or photon particles.

Shortly afterwards, Ernest Rutherford and others developed the first more precise ideas concerning the structure of atoms, with positively charged protons in the nucleus and corresponding negatively charged electrons in the shell. In order to understand the long-known optical line spectra of atoms, which were now interpreted as electrons’ abrupt (discrete) changes of state in the atomic range, Niels Bohr assigned corresponding discrete properties to the electrons. Bohr postulated the “quantization” of their kinematic properties and characterized the potential states of electrons in the atom using quantum numbers – a concept you won’t find in classical mechanics, but one that has since been proven experimentally. Nevertheless, the limitations of Bohr’s atomic model, according to which the electrons move like planets in circles around the atomic nucleus, soon became apparent. As such, it seemed that the conceptual system of physics would need to be rebuilt from scratch.

Einstein’s explanation of the photoelectric effect led the young Louis de Broglie to transfer the astonishing wave-particle duality of light to free electrons in his doctoral thesis (1924) and to attribute both wave-like and particle-like properties to them. Here, the electrons in atoms do not follow circular paths, but instead “wobble” around Bohr’s circular paths in the form of matter waves.

In 1925, the Gordian knot that had previously blocked physically consistent access to the quantum world was finally cut: Werner Heisenberg, Max Born and Pascual Jordan developed a formalism they called “quantum mechanics” to describe the states of electrons in the atom. In quantum mechanics, the physical measurands (observables) of classical physics correspond to so-called operators, whose special mathematical properties allow quantum physical phenomena to be described and calculated. Although the laws of conservation of momentum or energy also apply in quantum mechanics, they have to do without exact descriptions of the movements of atomic particles in the sense of classical mechanics, because only probabilities can be given for measurement results. Further, these probabilities are not due to a lack of knowledge, but represent a fundamental property of the quantum world. However, Heisenberg’s quantum mechanics, which was based on matrices, i.e., rectangular arrangements of numbers or variables, was mathematically demanding and not very clear. Shortly afterwards, Erwin Schrödinger developed a different approach, transferring the well-known relationships from wave optics and geometrical optics to de Broglie’s aforementioned matter waves.

At first, it was unclear whether Heisenberg’s matrix mechanics and Schrödinger’s wave mechanics were simply different expressions of the same quantum mechanical theory. Max Born and Paul Dirac quickly recognized the similarities between the two approaches and confirmed that they were based on common, general quantum mechanical principles that could be formulated mathematically in different ways. In a very short space of time, quantum mechanics was applied to a host of previously unsolved problems in atomic physics – successfully! Though it sparked a breakthrough in quantum physics, this sensational development was also accompanied by a fundamental revision of the epistemological principles of classical physics, according to which everything that happens in nature is essentially determined from the outset and can therefore be predicted. But the quantum mechanics proposed by Heisenberg and Schrödinger had to abandon this strict determinism, as it showed that many physical effects were inherently indeterminate at the quantum mechanical level. Heisenberg formulated this natural indeterminacy two years later in his famous uncertainty principle. However, if one considers the probability statements of quantum mechanics to be actual statements, these are in turn strictly determined.

Over the course of this year, we will be closely following the development of quantum physics and its vital role in modern research. Stay tuned!

Quantum Technologies