Published 4 July 2024 by Andrei Mihai

# Let’s Put the Weirdness to Work: the Long and Still Unfolding Journey of Quantum Mechanics

Quantum mechanics is one of the most promising and hotly debated fields of physics. The quantum principles governing the subatomic world are often counter-intuitive, yet researchers have managed to not only uncover some of the quantum secrets but even use the technology for practical applications. At #LINO24, several sessions addressed some of the progress (and challenges) that have marked the field.

### Quantum Leaps

In a lecture on Monday, Nobel Laureate Anton Zeilinger discussed his journey through the quantum world. Zeilinger is one of the key pioneers in modern quantum mechanics, widely hailed for the first realization of quantum teleportation of an independent qubit. Later on, Zeilinger expanded quantum teleportation to over 144 kilometers between two Canary Islands.

A significant portion of Zeilinger’s lecture was devoted to the role of information in quantum mechanics. He argued that the quantum state represents information about a system rather than a definitive physical reality. This perspective aligns with the modern interpretation that information is fundamental to the universe. Zeilinger illustrated this with the concept of quantum teleportation, where information about a particle’s state is transmitted instantaneously across distances using entanglement, without transmitting the particle itself.

With quantum teleportation and quantum information being such essential concepts in the quantum world, it was surprising to learn that the laureate was never formally taught quantum mechanics – he started by learning it independently.

“I want to say one thing for the young people: I have never attended a single course in quantum mechanics in my whole life. No seminar, no lecture, nothing. I learned quantum mechanics through various books.” However, it was this approach that sent him on a path to study the fundamental parts of quantum science.

“I realized immediately that this field is mathematically extremely beautiful and unbelievably precise; number three, the textbooks always escape what it really means. So I decided to work on the foundation for quantum mechanics.”

Anton Zeilinger’s work has been pivotal in demonstrating the bizarre properties of quantum entanglement and in the process, showing that entanglement is not just a theoretical curiosity but a cornerstone of quantum mechanics with practical applications.

This seemingly bizarre phenomenon is far from the only counter-intuitive one in the quantum world.

### Ghostly Action at a Distance

Nobel Laureate Alain Aspect also conducted experiments on quantum entanglement.

Aspect began his Lindau lecture by discussing the Einstein-Podolsky-Rosen (EPR) paradox, a thought experiment that questioned the completeness of quantum mechanics. Einstein and his colleagues argued that quantum mechanics could not be a complete theory if it relied on probabilistic outcomes and non-local correlations. They proposed that there must be hidden variables determining the outcomes of quantum measurements.

Decades later, John Bell’s theorem provided a way to test the EPR paradox experimentally. Bell showed that if hidden variables existed, the correlations predicted by quantum mechanics would be limited by certain inequalities. Aspect’s experiments in the 1980s tested these inequalities rigorously.

Using entangled photons, Aspect demonstrated violations of Bell’s inequalities, providing strong evidence against hidden variable theories and confirming the non-local nature of quantum mechanics. He proved that the ‘ghostly action at a distance‘ was very much real and entangled particles remain correlated even when larger distances separate them.

However, there are also “ghostly” things happening at small smaller scales, even inside the atom – albeit a different type of phenomenon.

David J. Gross, who was awarded the Nobel Prize for asymptotic freedom, is renowned for his groundbreaking work in quantum chromodynamics (QCD), the theory describing the strong force that binds quarks together in protons and neutrons. This theory within quantum field theory is now celebrating its 50th anniversary.

QCD was one of the puzzle pieces needed to complete the Standard Model, the classification of all known subatomic particles that also classifies all the four known fundamental forces. The strong force was especially intractable and gave many physicists headaches – so much so that Freeman Dyson once famously says that “the correct theory will not be found in the next one hundred years.”

Gross jokes about this, saying that “you should never trust theorists, because that just means they haven’t been able to explain things”.

“It only took 13 years. A revolution was needed. But revolutions happen,” the Laureate continued.

The revolution was not easy to carry out, however. Experiments showed that quarks, these fundamental particles that combine to form protons, neutrons, and other hadrons, almost behaved like free particles in the nucleus – but quarks were supposed to interact strongly, so how could this be?

It was Gross’ work that helped solve this conundrum, with his contributions in asymptotic freedom.

In asymptotic freedom, the nuclear force weakens at shorter distances, explaining why some experiments seem to only make sense if nuclear particles are made of non-interacting quarks. The closer quarks are to each other, the less the strong interaction between them. When quarks are in extreme proximity, they almost behave as free particles.

As surprising as these interactions were, they were confirmed by experiments. The judge of all physics is experiment, says Gross, echoing famous words by another Nobel Laureate, Richard Feynman. It doesn’t matter how beautiful a theory is, if it disagrees with experiment, it can only be wrong.

However, QCD is both beautiful and correct, Gross concludes. “It’s a perfect theory, with no infinities or adjustable parameters”

### Another Revolution That Affects All of Us

The quantum sessions culminated in a panel discussion on the potential and hype of quantum technologies.

Quantum mechanics already exists in technologies all around us, mentions William (Bill) Phillips. Phillips knows first hand how research can sometimes have unexpected applications. His work on trapping and cooling atoms with lasers is now applied in technologies such as GPS satellites.

Something similar happened with quantum mechanics, and while people may not realize it, quantum applications are already upon us.

“You can’t walk from here to the city hall without using quantum mechanics,” Phillips quips. However, he mentions that he assumes the current quantum revolution won’t be as impactful as the previous one.

“But at the same time I remember when the heads of huge corporations said there’s no real need for computers in our homes. So in one sense, my assumption is probably wrong. I don’t see how it’s gonna happen, but there’s no way we have a good enough imagnation to know what’s going to happen 50 years from now.”

Laureate Serge Haroche, also on the panel, agreed that it’s impossible to foresee where quantum research will take us in a few decades. He advocated the idea of fundamental research working without strict deliverables.

“We should work without the pressure of having to achieve some goal. Even to enter in this discussion is dangerous. Basic science doesn’t work that way. We have to direct it from the top down, not from the bottom up. This is a dangerous game and it’s bound to fail at some point.”

This triggered an exciting discussion among the panelists, who had diverse perspectives. Lene Oddershede, known for her work at the intersection of physics and biology, currently serves as Senior Vice President at the Novo Nordisk Foundation with responsibility for the foundation’s grant awarding activities. She mentions that it’s important for funders to see the big picture and fund long-term projects as well as more clear-cut ideas that can make a direct impact on technology. She mentioned that technologies like quantum sensors are already used in pilot projects to monitor things like heart flow or malnutrition, and that quantum computers are expected to make an even bigger impact in life sciences.

Heike Riel, from IBM, echoed this idea.

“No one would have ever imagined in the 70s what we can do with the technology today. I’m sure that it will happen in the same way with the quantum technology that we developed right now.”

Ultimately, the panel’s sometimes contrasting views on whether quantum research should be purely “blue sky” or also have an applied component seem to suggest that it’s important to strike a balance. Fundamental research and practical applications are not in competition, they are two sides of the same coin. In a healthy society, we need both.

There’s still plenty left to explore from the quantum world, who knows where the road will lead us? As Phillips concludes, we need to “put the weirdness to work.”