Tracing Quantum’s First Steps: Physicists Return to Helgoland
In June of 1925, a young postdoc named Werner Heisenberg absconded to a small island in the archipelago of Helgoland, off Germany’s northern coast. There, he made a conceptual breakthrough that led to modern quantum mechanics. One hundred years later, roughly 300 physicists traveled there for the 2025 Helgoland workshop, where they would thrash out what a century’s worth of experience with the theory had taught them and where it might lead next.
Of Helgoland’s two islands, the larger—which is smaller than New York’s Central Park—rises from the North Sea like a wedge, with a harbor on one side and a wall of jagged cliffs dotted with nesting seabirds on the other. The conference’s talks were confined to mornings and evenings, leaving afternoons free for attendees to explore the rugged landscape and seek their meals at small, tourist-filled restaurants on quiet, carless streets bordered by brightly colored buildings. Some of the physicists took in the sights with their families, while others camped out on the smaller island’s beach.
The meeting had an official theme—the intersection of quantum foundations and technology—but it could equally have been summed up by the words of physicist Gemma De les Coves of the University of Pompeu Fabra, Spain, who at the end of the week characterized the history of quantum physics as “remarkable progress despite massive disagreement.”
Much of the progress presented at the conference was related to quantum computing. Mikhail Lukin of Harvard University described breakthroughs achieved by making qubits from the Rydberg states of neutral atoms, which can be shuffled in space while performing calculations. Nathalie de Leon of Princeton University recounted how her group has improved superconducting qubit performance by crafting them from tantalum and by carefully preparing their surfaces.
Much of the buzz during coffee breaks came from physicists marveling at the precisions being reached by quantum measurements. David Moore of Yale University described how levitated optomechanical systems have become sensitive enough to feel forces imparted by single atomic particles (see Viewpoint: Nuclear Decay Detected in the Recoil of a Levitating Bead and Special Feature: Sensing a Nuclear Kick on a Speck of Dust). He predicted that his group will detect single neutrinos within a year, which could be helpful in searches for heavy neutrinos, a dark matter candidate. Jun Ye of the University of Colorado Boulder explained how his nuclear clocks can discern gravity’s tiny tweak to the flow of time when the devices are lowered or raised by the width of a human hair.
And yet, massive disagreement about quantum theory’s deeper implications—what it tells us about the nature of the world—was on full display at the workshop’s first panel discussion. “It is just embarrassing that we can’t say what reality is like,” proclaimed moderator Carlton Caves of the University of New Mexico while displaying a long list of questions that physicists are still at loggerheads about, including whether the wave function represents reality, whether quantum randomness is fundamental or apparent, and whether quantum theory implies nonlocal effects that instantaneously connect particles, even at great distances.
“The quantum state only describes our knowledge,” pronounced Nobel laureate Anton Zeilinger of the University of Vienna. Therefore, Zeilinger argued, the apparent nonlocality in quantum phenomena such as entanglement does not correspond to actual, superluminal effects of one system on another.
“There is a quantum world,” retorted Nobel laureate Alain Aspect of the Institute of Optics, France, making the opposite case that quantum theory describes reality and not just what’s known about it. “And in my quantum world there is nonlocality, and it is useful,” he concluded, alluding, likely, to applications such as quantum cryptography.
A procession of theorists took the stage over the ensuing days to offer proposals for settling such debates. Robert Spekkens of the Perimeter Institute, Canada, outlined his program to “salvage realism” by defining a new, quantum version of causation. Angelo Bassi of the University of Trieste, Italy, argued that quantum mechanics is only an approximation to a less paradoxical theory and pointed out that spontaneous collapse models—which explain why quantum superpositions cannot be observed in the macroscopic world—remain viable alternatives. “There is still a long way to go to exclude these models [experimentally],” he said. “We are very ignorant about superposition at macroscopic scales.”
A recurring theme of the workshop was the connection of these foundational questions with a major outstanding problem in physics: the unification of quantum theory and general relativity. “Quantum mechanics may already know something about gravity,” said Renato Renner of the Swiss Federal Institute of Technology (ETH) Zurich to introduce work he’s done with Ladina Hausmann, also of ETH Zurich, suggesting that the “firewall” paradox of black hole physics and the “Wigner’s friend” paradox of quantum physics are effectively one and the same. Inspired by general relativity’s unification of Newton’s gravitational theory and special relativity, Lucien Hardy of the Perimeter Institute encouraged a search for a new interpretation of quantum mechanics that incorporates a quantum version of Einstein’s equivalence principle.
Several attendees noted that all observations to date can be explained by assuming that both quantum theory and general relativity are correct and expressed hopes that the recent profusion of tabletop experiments for testing quantum gravity will yield a revealing surprise. Markus Aspelmeyer of the University of Vienna, who runs one of those programs, explained how his team is working to grow the size of spheres that can be put into a quantum state. At the same time, they are shrinking the size of objects whose gravitational attraction they can measure. Once they’ve closed the gap between the two sizes, they’ll try to entangle two objects via their mutual gravitational interaction. If successful, such demonstration would prove that gravity cannot be fully described by a classical theory such as general relativity.
In a final panel discussion on “The Next 100 Years,” Lorenzo Maccone of the University of Pavia, Italy, emphasized the positive side of the disagreements—for example, students are often drawn to fields where there is a lack of consensus and a recognition that current understanding may just be scratching the surface. On an overhead screen he projected a picture that he’d taken the day before of a jellyfish floating off Helgoland’s shore. “We have to imagine,” he said, “that we may not know that much more than the jellyfish.”
–Bob Henderson
Bob Henderson is a freelance science writer based in Red Hook, New York.