We are entering the second quantum revolution. This is how we got here.

Quantum mechanics has guided us to a revolutionary new understanding of nature at its smallest scale. Some of the greatest minds of 20th-century physics have contributed to hammering out the basics. Albert Einstein, of course; Erwin Schrödinger, of cat fame; Max Planck, Wolfgang Pauli and numerous others. Below the radar of public perception, however, a second quantum revolution has been shaping up since the mid-1990s: the advent of quantum technology. The underlying idea might sound deceptively simple. Quantum mechanics has shown that nature behaves in ways that simply cannot be explained in the framework of classical physics. So why not make these phenomena the heart of a new breed of unique technological devices?

This idea underlies a string of ground-breaking new concepts that have emerged over the past quarter of a century, proposing fundamentally new approaches to computation and cryptography, to measuring physical quantities, and other applications. Translating these concepts into practical devices has proved immensely challenging. Nevertheless, over the years, capabilities have been developed that now enable researchers to create prototype quantum devices that start challenging their classical counterparts. And this step from the drawing board to the lab is creating tremendous excitement.

At ETH Zurich, some 20 professors are working in quantum science, collaborating in centres such as NCCR QSIT and the just-founded Zurich Quantum Centre. And the school is currently systematically expanding in the field of quantum technologies, for example by currently training its first generation of Quantum Engineers in a dedicated Master programme, one of the first worldwide.

But how did we get here, from a theory that is often perceived as ‘mystical’ to the verge of real-world devices? The history is complex and multi-layered — and has intimate connection points to Switzerland.

It all started, one might argue, with a distinctly applied problem: developing a standard for measuring the intensity of lamps. That problem led in the late 19th century to a string of experimental and theoretical works on radiation from an idealised heat source, known as a ‘black body’. Outstanding results were produced, but there was a catch. A good description was found for short wavelengths of the radiation, and another one for long ones. But neither described the entire spectrum of thermal emission.

In 1900, the German physicist Max Planck found a way to interpolate between the two. His theory contained one assumption that would prove revolutionary: that electromagnetic energy could be emitted and absorbed only in discrete packets, ‘quanta’.

Planck was an unlikely father of quantum theory. When he decided, in the 1870s, to study theoretical physics, his professor in Munich, Philipp von Jolly, offered discouraging advice. “The edifice of theoretical physics is fairly complete”, von Jolly said. “There will be a mote to wipe out in a corner here or there, but something fundamentally new you won’t find.” Undeterred, Planck followed his path, and the changes he was to bring about were sweeping.

Far beyond thermal emission, over the decades following Planck’s work, the steadily expanding framework of quantum mechanics provided insight into the structure and behaviour of atoms, molecules and solid materials, as well into the constituents and interactions of atomic nuclei and their interactions.

But the path to this wealth of knowledge has been anything but easy. Wolfgang Pauli famously wrote in 1925, when struggling with the description of a property known as ‘spin’: “At the moment physics is again terribly confused. In any case, it is too difficult for me, and I wish I had been a movie comedian or something of the sort and had never heard of physics.”

Fortunately for physics, Pauli didn’t change his career path. In early 1928, he came to ETH Zurich, where he attracted a remarkable fellowship of young physicists and remained, interrupted by stays abroad, until his death in 1958. Shortly before Pauli’s arrival, another giant in the history of quantum mechanics left Zurich, after having worked across the road at the University of Zurich: Erwin Schrödinger. While there, he made a momentous contribution by giving quantum mechanics a powerful mathematical framework in the form of wave mechanics.

It was 1935 when Albert Einstein, working with Boris Podolsky and Nathan Rosen published a paper in which they questioned whether quantum mechanics could fully describe ‘physical reality’. The three physicists argued that quantum mechanics must be incomplete, as it ‘permits’ forms of correlations between particles that did not seem to make sense. Even when the particles are in different places, they seemed to share an amount of information that classical (that is, non-quantum) entities simply cannot.

The Einstein–Podolsky–Rosen (EPR) paradox was long surrounded by obscurity. For more than 40 years the paper describing it was barely referenced by other researchers. But then, citations started to balloon, making it by far Einstein’s most highly cited paper. What had happened?

In the mid-1970s, experiments began to be conducted in the lab that related to key aspects of the EPR paradox. These experiments showed not only that ‘non-local’ quantum correlations do indeed exist, it also pointed to ways for working with them.