As quantum mechanics turns 100, a new revolution is under way

One hundred years ago on a quiet, rocky island, German physicist Werner Heisenberg helped set in motion a series of scientific developments that would touch nearly all of physics. There, Heisenberg developed the framework of quantum mechanics. At the time, quantum theory was just a loose collection of ideas about the quirks of physics on the scale of atoms.

In June 1925, the 23-year-old Heisenberg cloistered himself on the island of Helgoland, in search of relief from a nasty attack of hay fever. With pollen scant in the sea breezes, the island, 60 kilometers off the coast of Germany, was a healing refuge. It also happened to be a distraction-free place to ponder the mysteries of atoms.

Early one morning, Heisenberg had a breakthrough. “I had the feeling that, through the surface of atomic phenomena, I was looking at a strangely beautiful interior, and felt almost giddy at the thought that I now had to probe this wealth of mathematical structures nature had so generously spread out before me,” he later recounted. “I was far too excited to sleep, and so, as a new day dawned, I made for the southern tip of the island, where I had been longing to climb a rock jutting out into the sea. I now did so without too much trouble, and waited for the sun to rise.”

Physicists are now gazing upon the dawn of a new quantum era. The work of Heisenberg and his contemporaries transformed how scientists understood matter and led to new technologies based on that understanding. Current research — what some call the second quantum revolution — involves a new level of precision control over quantum systems, including building them from scratch and wielding them as needed. Scientists are bending quantum systems to their will to push technology further and unlock secrets of the universe.

This revolution is a collective effort of physicists around the world, chipping away at different quantum frontiers. Likewise, the first quantum revolution was no one-man show. Heisenberg’s romantic and perhaps embellished narrative was only a sliver of the tale of the birth of quantum mechanics.

Upon his return from Helgoland, Heisenberg discussed his ideas with other physicists before publishing a famously inscrutable paper that July. Later, physicists Max Born and Pascual Jordan crystallized the mathematics in a paper submitted in September and in another, in collaboration with Heisenberg, in November. And physicist Erwin Schrödinger published his own influential quantum framework in 1926, which would prove mathematically equivalent to Heisenberg’s. These and many other hands turned a confusing muddle of quantum effects into a cohesive mathematical framework.

The impact quantum mechanics has had on physics is difficult to overstate.

“The theory has … been explored, developed and applied to a spectacular variety of phenomena and represents our basic current understanding of the nature of physical reality,” says physicist Carlo Rovelli of the Centre de Physique Théorique of Aix-Marseille Université in France. “It has explained phenomena ranging from the basis of chemistry to the color of objects, from the processes that give rise to the light of the sun to the formation of galaxies.”

Quantum mechanics also underlies innumerable technologies, including lasers, the transistors that are integral to smartphones and other miniaturized electronics, solar panels, LEDs, MRIs and the atomic clocks that make GPS navigation possible.

To kick-start that second quantum revolution, scientists must now harness some of the most beguiling aspects of the theory: superposition and entanglement.

In quantum mechanics, particles’ positions, velocities and other qualities are described by probabilities, not certainties. That means particles can be suspended in a weird purgatory known as a superposition. For example, a particle can have a chance of being found in one place or in another location entirely — a situation often colloquially described as being in two places at once. A hypothetical feline in a superposition of alive and dead, known as Schrödinger’s cat, highlights the utter peculiarity of this concept.

Entanglement is another mind-boggler, in which the fates of two particles are intertwined, their properties correlated in a manner impossible in classical physics. Measuring one particle in an entangled pair instantaneously reveals the state of another, even if they’re separated by a large distance.

By improving their ability to precisely manipulate superposition and entanglement, physicists are building the techniques needed to construct intricate devices such as quantum computers, which could allow for new types of calculations that are impossible with standard classical computers. Similarly, quantum sensors are beginning to enable new types of measurements, and quantum communication networks promise more secure ways to transmit information.

This revolution also has scientists closing in on some of the big mysteries of quantum physics, like whether there’s a fundamental limit to how much quantum effects can scale up, and if so, where that dividing line between quantum and classical lies. And they’re investigating how quantum mechanics can be melded with the general theory of relativity — Einstein’s theory of gravity.

Science News spoke with five physicists pushing the quantum envelope to get their takes on the state of quantum science. These interviews have been edited and condensed for clarity.

Supersizing superposition

The larger an object is, the more difficult it is for it to retain quantum properties. Interactions with the surroundings can wrench away its fragile quantumness and thrust it back into the everyday realm. Improved techniques for isolating larger objects have allowed researchers to scale up — even to borderline macroscopic objects. Some physicists believe there is a hard limit to how far that enlargement can go; others believe it can continue indefinitely.

Physicist Yiwen Chu of ETH Zurich is going big. In 2023, Chu and colleagues placed a vibrating sapphire crystal, with the mass of about half an eyelash, in a “cat state,” a superposition akin to Schrödinger’s cat. It’s the most massive cat state ever made. Here, the superposition is in the motion of the crystals’ atoms; it’s as if they’re moving in two directions simultaneously.

SN: What are you excited about now?

Chu: We’re looking into new physical platforms for making quantum sensors and quantum processors. I’m getting excited about using these systems to test some fundamental physics. Quantum mechanics works really well for a lot of things, but there’s still so much that we don’t understand.

SN: What are some of those questions?

Chu: Does quantum mechanics apply to macroscopic objects in our everyday world? This question has been around since the early days of quantum mechanics. We showed that these — you could call them macroscopic — crystals can, in fact, behave quantum mechanically. So the question is, just how far can we push that? I don’t know if we’ll ever get to the level of “cat” in my career. (And maybe it shouldn’t be a cat — that probably is not very ethical.) But something really complex and macroscopic, if we can see the quantum mechanical behavior of that, I think that would be superexciting and would answer this question that’s been around for such a long time.

SN: What else are you planning to do with these devices?

Chu: We’re moving toward using these systems as detectors in measurements of gravity or other forces. If you had a very weak gravitational wave that hits this object, it would excite vibrations. And then if we could detect that, then we could say, “Oh, something came by, maybe a gravitational wave.” These devices would be used to detect gravitational waves at much higher frequencies than, say, the Laser Interferometer Gravitational-Wave Observatory.

Testing quantum gravity

Scaled-up quantum devices like Chu’s also provide the opportunity to test how quantum mechanics interacts with general relativity. The two theories are incompatible with one another and resolving that clash tops many physicists’ list of pressing problems. Vlatko Vedral of the University of Oxford is one of the physicists behind a proposal to test gravity for quantum effects. The test requires creating a superposition with an object with enough mass that its gravity will tug on another object in a superposition. That could cause the two objects to become entangled, solely due to their gravitational interaction. Confirming or refuting this effect would reveal whether gravity is quantum.

SN: What’s so fascinating about testing quantum gravity?

Vedral: Testing the quantum nature of gravity is a completely open problem. My guess is, within the next five to 10 years at most, we are going to violate general relativity. Gravity will prove to be quantum mechanical — that’s my bet. But I know that there are some formidable opponents to that view. That already tells you that this is an extremely interesting experiment to do because there is a huge disagreement about what to expect.

SN: How would you perform this test?

Vedral: You take two massive objects and put each in a superposition of being in two different states, in two places at the same time. If gravity is quantum mechanical, each of these states will gravitationally couple to each of the other states. You are basically going to have four interactions happening simultaneously. That would be my prediction and that would be the prediction of quantum gravity. However, some people believe that gravity will force these superpositions to collapse and to go into one definitive state. And that’s what the experiment is meant to test.

To me, this is possibly the most exciting experiment in physics because we’ve had a hundred years of huge successes, both quantum mechanically and in general relativity. But now we are able to test whether there will be a deviation in the domain where both really matter.

SN: What’s needed to perform it?

Vedral: There is a race going on; I think there are three or four teams trying to implement this proposal. You need a massive enough object. Rough calculations suggest a nanogram. It’s a very challenging experiment.

Thermodynamics goes quantum

It’s not just gravity that’s being mashed together with quantum physics. So too is thermodynamics, the discipline that governs engines, heat and entropy, a measure of disorder. The study of quantum thermodynamics could suggest ways of making machines with increased efficiency by harnessing quantum principles. Physicist Marcus Huber of the Institute for Quantum Optics and Quantum Information in Vienna works in this field, as well as on quantum communication. That’s a technique that uses quantum rules to send information securely, and it’s already being demonstrated outside of laboratories.

SN: What’s the current state of quantum physics?

Huber: I am super-enthusiastic about the questions that we can more and more experimentally access. I am worried, though. People have recognized the massive commercial potential of quantum technologies. And with this recognition come the grifters and oversellers and the hype machine, which is doing a disservice to basic science and research. And with that recognition comes a geopolitical aspect, where suddenly quantum technologies and research are deemed in the interest of national security. Instead of scientists unimpededly exploring the universe together, it starts casting all of these basic science questions in terms of geopolitical advantage.

SN: What are some of the more legitimate applications on the horizon?

Huber: There are many legitimate applications being drowned out by the noise. For one, precision measurements will be useful: We’re on the verge of building more accurate clocks, more sensitive sensors. These things don’t receive as much hype. Then of course, in quantum communication, in terms of data privacy and security, the applications are far advanced. We have the technological capabilities to have encrypted and secure communication between any two points. Granted, a lot of that is already possible with classical means. This extra bit of security is against very targeted attacks or against future quantum computing devices.

SN: What quantum experiments are you excited to see in the future?

Huber: One of the big questions we had is about the fundamental limits of timekeeping. There’s this old idea of the thermodynamic arrow of time, basically telling you, the way I can make a clock tick is by increasing the entropy of the universe. From a classical perspective, there’s a very precise relation showing that the more precise or the more accurate you want to make a clock, the more entropy you have to dissipate. We did a bit of theory showing that quantum clocks could be exponentially more efficient. We investigated this as a fundamental question: What is the fundamental cost of letting a clock tick? But the answer also makes me excited for possible experiments, because that could be useful if we can create insanely energy-efficient clocks.

Putting quantum biology on the map

Physicist Clarice Aiello is on a mission to get scientists to take quantum biology seriously. The idea that quantum effects are important in living things has been proposed in a few specific areas: Quantum mechanics might play a role in photosynthesis, and birds might use a quantum compass to sense magnetic fields. But Aiello, of the Quantum Biology Institute in Los Angeles, wants to push beyond those examples. She’s seized on impacts of weak magnetic fields such as Earth’s. Because that field is so weak, its effect on living things can be difficult to explain via classical means. But there’s potential for those effects to be explained by a concept called an electron spin superposition. The quantum property of spin makes an electron act like a tiny magnet. If that magnet’s orientation is in a superposition of directions, it could result in certain chemical reactions that are sensitive to tiny magnetic fields.

Aiello is starting from the basics, aiming to show the importance of Earth’s magnetic field on life before determining the cause. One of her team’s recent experiments suggested that tadpoles shielded from Earth’s magnetic field developed more quickly.

SN: What could cause magnetic field effects in biology?

Aiello: The most likely explanation is a chemical reaction that depends on electron spin superpositions. If magnetic field effects in biology are explained by this type of phenomenon, the implication is that electron spin superpositions survive inside cells for long enough to be functional. The tinier the field that you want to sense, the longer those electron spin superpositions should survive with their quantumness. For example, to sense the magnetic field of the Earth, this is about 750 nanoseconds.

SN: What’s the killer experiment you hope to do?

Aiello: We want to take a cell at room temperature, learn how to “talk” to the spins of relevance inside the relevant proteins, and measure how long those spin superpositions last. If we take a tadpole cell and find that the electron spin superpositions are only alive inside the cells for 100 nanoseconds, then it’s probably not what’s mediating the tadpoles sensing the Earth’s magnetic field. On the other hand, if you find that the quantumness of the superposition is alive for two microseconds, then all of a sudden you give credence to the idea that it is possible that the electron spin superposition mediates the sensitivity of the tadpoles to the shift in the magnetic field of the Earth.

SN: What’s been the reception so far to your work?

Aiello: There is a communication problem. We try to tell people that it’s not only about frogs; there’s evidence that this is correct in flies, worms, bacteria. I don’t think the biology community gets that. This is why I advocate for quantum literacy, because if everyone with a high school degree had a little bit of quantum, we might have more people with training in biology who might be able to make the connection between biology and quantum, or materials science and quantum. We need to have people to realize how quantum interlaces with many other disciplines.

Making quantum computing work

Quantum computers garner perhaps the most hype of any quantum technology. They function based on quantum bits, or qubits. These sensitive units can be made of a variety of materials, from tiny bits of silicon to individual atoms. They perform computations like the standard bits do in classical computers, but they are designed to use the rules of quantum mechanics to calculate. Qubits are so sensitive that they’re prone to errors. The promise of quantum computes rests on scientists devising ways to fix those errors, says Barbara Terhal, a physicist at QuTech in Delft, Netherlands. A technique called quantum error correction combines multiple error — prone qubits to create a more reliable, “logical” qubit. Scientists have recently demonstrated a variety of milestones toward error-corrected quantum computers.

SN: Why do we need quantum error correction?

Terhal: Without error correction, we can’t build a quantum computer. I wouldn’t say that the experiments that are being done in labs right now are quantum computers. What I call a computer is a reliable machine that can add large numbers and so on. Error correction enables the building of reliable computers which may be of interest for applications in the long term.

But it’s more than that. It’s also just a fundamental addition to our understanding of physics. What quantum error correction tells us is, if we very carefully control these quantum systems, we can have macroscopic quantum behavior, because these are logical qubits and they work according to the laws of quantum mechanics.

SN: In what sense are logical qubits macroscopic?

Terhal: The traditional difficulty with creating a superposition of a cat being dead and alive has little to do with the precise size of the cat. Rather due to its size, the cat consists of many “degrees of freedom.” That’s a feature of many macroscopic systems comprised of many atoms. We are trying to build something which has many degrees of freedom, but each physical qubit is quite well controlled and monitored for errors. So in this sense, we get quantum behavior at a macroscopic scale. It’s not literally about size.

SN: Are there still skeptics who are not convinced that reliable quantum computers are possible?

Terhal: There will always be naysayers. It’s a funny thing in quantum computing because it’s a mix of complete overhype, of people who know absolutely nothing, and then there are skeptical people. It’s good to be skeptical. It’s not like there’s one team that did error correction, and now we’re there. Because at every scale-up, there may be new problems that pop up. But there’s no theorem that says it’s not going to be possible, and because that doesn’t exist you have to try. You’ll bump into the limits when they arrive, and those will be interesting challenges to overcome.

SN: What do you think is coming in the next 100 years of quantum physics?

Terhal: Maybe the quantum ideas will have spread further, in terms of being a common language. Or maybe we’ll have built a quantum computer or quantumlike computers. There will probably be new theories that don’t make other theories completely wrong but widen the applicability of what we have right now. If you had asked people before the invention of quantum mechanics, they thought physics was almost done. And now we feel like maybe we have to unify quantum mechanics and gravitational forces, but other than that it’s kind of done. That might very well not be correct. That’s a bit too naïve.