“We’re in the second quantum revolution”

Titus Neupert, the UN has declared 2025 the International Year of Quantum Science and Technology. What exactly is being celebrated?

Titus Neupert: From a historical standpoint, we celebrate that quantum mechanics (glossary) was formulated as a physical theory 100 years ago. Several physicists contributed significantly to this – most notably Werner Heisenberg, who formulated this theory with the help of matrices, and Erwin Schrödinger, who developed the equation named after him here at the University of Zurich. That’s why we at UZH bear a special responsibility with regard to this centennial and are planning a number of events to commemorate it (see box). The Schrödinger equation is one of the cornerstones of quantum mechanics.

Quantum mechanics

Quantum mechanics completely redescribes what matter is and how it interacts with the forces of nature. One key insight of quantum mechanics is that quantum objects such as electrons or photons, for instance, can be interpreted as both waves or particles depending on how they are measured. With his equation, Erwin Schrödinger succeeded in mathematically visualizing this wave-particle duality back in the 1920s.

Quantum leap

If a pendulum is held still at a certain height, it has potential energy. According to classical physics, that energy is continuous and can take any value. Quantum mechanics says that’s not true. According to quantum theory, there are discrete gradations, so not every energy value can be set. Those discrete energy levels are quanta. The concept of a quantum leap denotes a jump from one energy level to another. Energy differences within an atom are small, but the levels are categorically distinct. For this second reason, the term “quantum leap” or “quantum jump” stands for a distinct change.

Quantum computing / quantum bits

Quantum computing is a concept that promises exponentially greater computational power compared to that of classical computers. A quantum computer is composed of quantum bits, which have to be gated individually and coupled to each other. Quantum bits (qubits) can exist simultaneously in several states (superposition) and be entangled with each other. This enables extremely fast calculations for complex problems.
Learn mor in the: DSI-Spotlights about Quantum Computing

Quantum entanglement

Quantum entanglement enables photons or atoms, for instance, to become linked and to share a single quantum state even when separated by vast distances. Today, that’s possible across a distance of 1,000 kilometers – from a satellite to the Earth, for example – and it opens up entirely new technological prospects such as secure communication impervious to eavesdropping on a future quantum internet.

What fundamental change did that bring about?

Neupert: Physicists at that time were horrified that they were suddenly faced with philosophical problems. Up until then, theories of physics had been entirely deterministic. Now physicists realized that measurement results depend on chance just like a lottery does and that the people conducting the measurements cannot be viewed separately from the system. Conceptually, this presented a challenge. Moreover, the length, time and energy scales that we know from everyday life are almost completely irrelevant. This means that quantum mechanics completely escapes our intuition and sometimes seems almost magical.

In what way has quantum mechanics radically altered our conception of the physical world?

Neupert: It was a radical change in perspective that affects all of physics. In principle, almost everything that physicists do today is based on quantum mechanics. Quantum mechanics allows us to understand fundamental things like the stability of matter, for instance. The fact that the table we’re sitting at right now doesn’t collapse in on itself can only be explained with the aid of quantum mechanics. One must also bear in mind that the development of particle physics would have been impossible without quantum mechanics. And quantum mechanics has opened up a vast range of technical possibilities for the semiconductor industry, the pharmaceutical sector and the fields of chemistry and materials science. We’re currently in the midst of the second quantum revolution.

What does this second quantum revolution entail?

Neupert: During the first quantum revolution, one of the aims pursued by the likes of Erwin Schrödinger was to describe quantum mechanics as a natural phenomenon as accurately as possible. The second quantum revolution is now focused on putting quantum mechanical systems to use in technology. Scientists and engineers today are working on quantum cryptography and on how to build quantum computers (glossary) and develop quantum sensors.

What, for example, can quantum cryptography do that conventional cryptography can’t?

Neupert: Classical cryptography methods are based on logic. That’s why it’s possible in principle to crack a code. Quantum cryptography, in contrast, is based on principles of physics. The quantum entanglement (glossary) enables information to be transmitted absolutely securely and completely impervious to eavesdropping. Initial commercial quantum cryptography products already exist, but we certainly still are a long way from them becoming more widely adopted.

The hot topic in quantum technology is the development of quantum computers. How far along is their development, and what can quantum computers do better than classical computers?

Neupert: Quantum computers already exist today on the market. The problem, though, is that quantum computers have to be of a certain size in order to enable exponential growth in computing power and to outperform our conventional computers. This requires quantum computers to be two to three times larger than we are currently capable of building them.

What will it take to build such computers?

Neupert: Many factors are at play here. What substance makes for the best quantum bits (glossary) is still an open question – there are several competing concepts. Materials research can contribute valuable input here.

In what direction could that go?

Neupert: Silicon is unbeatable for classical computer chips. There is not yet anything comparable for quantum computers. Small superconducting components or ultracold atoms are currently being worked with, for example. Engineers today concern themselves with quantum physics in order to develop quantum computing devices, for example. They have to comprehend how quantum mechanics works if they want to build powerful computers.

Do they already possess that knowledge?

Neupert: Not sufficiently to date because it’s not yet part of their training. But that’s changing, for instance with the rollout of courses on quantum mechanics for aspiring engineers. The same goes for computer scientists, because somebody has to program quantum computers. Google and Microsoft have already developed quantum programming environments.

We’ve talked about quantum computers and quantum cryptography. You also mentioned the pharmaceutical industry. What prospects does quantum technology open up for medicine?

Neupert: Diseases like cancer, for example, manifest themselves also at the molecular chemical level in individual cells. Advanced quantum sensors can measure changes of that kind – such as the formation of radicals, for instance – very precisely, enabling not only better detection of diseases, but also a more accurate prognosis of disease progression and therapy outcome. Quantum computers ultimately also can be used to develop customized drugs that target specific disease phenotypes, which ties in with the wider subject of personalized medicine.

In what areas is research in quantum mechanics being conducted right now at UZH?

Neupert: One line of research is particle physics, and it’s about verifying and broadening fundamental laws of nature by making very precise measurements under extreme conditions. The second area is quantum materials, a class which includes superconductors that could someday be used in quantum computers. The work being done here is highly experimental and complex with an uncertain outcome. And it has the potential to bring something new and unexpected into existence.

Why are superconductors so important?

Neupert: Superconductivity is a phenomenon that exists only thanks to quantum physics, but it has dramatic perceptible consequences. In a state of superconductivity, electrical resistance vanishes completely from materials at a certain temperature. Described in quantum mechanical terms, the electrons in the superconductor enter a special wave state in which they are able to glide unimpeded through the system without friction. However, superconductivity can only be achieved at very low temperatures at present.

Where do you see potential for quantum technology in the future?

Neupert: In medicine, for example, where greatly improved sensor technology and computing could enable the production of personally customized drugs in the future. Custom manufacturing of very expensive personalized drugs is not yet feasible at present, though. For now, it’s more realistic to use existing drugs in a more targeted manner to improve efficacy and minimize side effects.

Medicine is one application area that could greatly benefit humanity. However, quantum technology can also be deployed in areas that are potentially very dangerous, such as for military aims. What’s your take on that?

Neupert: Quantum technology could completely revolutionize military technology. Consider, for example, sensors, which are important for drones. Quantum sensors measure much more precisely and could improve navigation.

You said that we’re currently in the midst of the second quantum revolution. Why did it take 100 years to get to this second stage?

Neupert: Other technologies like classical computing first had to become sophisticated enough to advance quantum technology. It simply took that long. But now we’re on the verge of taking another giant step. I’m optimistic that it will succeed.