One of the quietest places in the universe is an unremarkable room on the southern coast of the UK. Here, in one of the University of Southamptonâs physics labs, overseen by , a preposterous amount of effort has gone into eliminating every conceivable disturbance: a 1-tonne slab of granite absorbs all vibrations aside from the faintest tremors, while a pendulum repurposed from a gravitational wave observatory catches the last leftover wobbles and a fridge lowers temperatures to within a whisker of those in the deepest reaches of outer space. All of this is done in the slim hope we might answer a question that has plagued scientists since the advent of quantum mechanics a century ago.
In the microscopic quantum realm, reality seems to work differently than in the solid, predictable world we are used to. Hard boundaries melt into one another and objects can become deeply intertwined, or entangled, without physical contact. Quantum objects in what is known as a superposition seemingly inhabit more than one place at a time, at least if we arenât looking directly at them. But with the smallest of disturbances, entanglement vanishes and superpositions collapse â and the larger an object is, the more likely it is to succumb to certainty.
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This article is part of a special series celebrating the 100th anniversary of the birth of quantum theory. Read more here.
However, over the past few years, scientists have gone from putting tiny things like simple particles into a superposition to getting surprisingly large things into this state, including a sapphire crystal. As these quantum effects get bigger and bigger, the mystery intensifies: does the weirdness that takes over at the minuscule scales of the quantum world have an upper size limit?
The hunt for this boundary is happening in experiments across the world. The results could fill in the blanks left by the creators of quantum mechanics and perhaps help us redefine the meaning of this revolutionary theory 100 years after it was first proposed.
âThe original Schrödinger equation has no mass limit,â says Ulbricht, referring to a cornerstone of quantum mechanics put forward by Erwin Schrödinger in 1926. âThat equation says you can do whatever size, you can do all masses. [Anything] is possible. You only have the technological challenge that you have to isolate your system from the environment⊠thereâs nothing which prevents you [scaling up] fundamentally.â
How big is too big?
Start with a single particle, say a photon, and you will certainly see quantum effects. Such particles of light can be put into a state of entanglement with others, so anything that happens to one is immediately measurable in the other. Photons can exhibit the behaviour of both a particle and a wave simultaneously, as seen in the famous double-slit experiment. This consists of a beam of light shining at a barrier with two slits. If light were only particles, we would see two bands of light through the slits, but instead a pattern distinct to waves emerges. So we know that single particles can be governed by quantum rules. And in some cases, quantum effects like superpositions hold for five or 10 particles. But how about 500? Or 500 million?
âThe most successful mathematical framework for the description of nature â quantum mechanics â doesnât seem to be formally limited to the microscopic scale at all,â says at Queenâs University Belfast, UK.
Testing whether there is, in fact, some threshold and finding out where it may lie has proven extraordinarily difficult. Though the quantum world may be fuzzy, it readily collapses into something more concrete, or classical, through an effect called decoherence. Nudge a quantum object in some way with an errant particle, for example, and the strangeness vanishes.
To figure out why, physicists first need to understand whether quantum collapse is only caused by environmental disturbances or whether there is something deeper at work. âWe have to do these extreme experiments under very specific conditions to remove those known [environmental] effects and see if thereâs a fundamental effect there,â says Ulbricht.
If there is something more exotic disrupting the quantum qualities we see at small scales, it may be related to how gravity interacts with quantum mechanics or how dark matter shapes the quantum world, or it could arise from the quantum vacuum itself, with extremely short-lived particles popping into and out of existence just long enough to shake things up.
It is reasonable to question whether these extreme experiments have a finish line in sight or whether they are part of an infinite quest that will end only when the laws of physics say we can go no further. In one view of things, we live in a quantum world, so quantum effects should be everywhere.
But the experiments do have a north star. Starting in the 1980s, physicists began to propose ideas that could explain why, and when, a collapse of quantum states (also known as a wave function collapse) might happen that isnât driven by environmental factors. These are collectively known as spontaneous collapse models. Though they arenât based on any observations of spontaneous collapse â as we have none â they also make specific predictions for when one might happen, says at the University of Vienna, Austria.
The GhirardiâRiminiâWeber (GRW) model, for instance, one of the first and more widely studied of the models, says that collapse should happen for a particle around 1 micrometre in diameter. Until recently, experiments trying to catch a spontaneous collapse were working with quantum effects at scales that were far below the GRW modelâs predictions. But technological advances mean that a growing number of groups, including Ulbrichtâs and Arndtâs, are pushing into uncharted territory.
âInstead of just shooting in the blue sky with experiments, collapse models guide you to search for the best experiment,â says at the University of Trieste, Italy. However, quantumness shows up in many different ways, so hunters of the quantum realmâs boundaries are tackling the problem from many different angles â which could help us pin down exactly how the laws of quantum physics apply to the world we inhabit.
In 2019, Arndt and his colleagues broke the record for so-called macroscopicity, a way to measure and compare tests of the size of different quantum effects. This accounts for factors such as the number of atoms in a quantum state, their mass and the length of time they are in a superposition. Their experiment managed to demonstrate quantum interference â where the fuzzy natures of two quantum objects interact with each other â between clumps of thousands of organic compounds called oligoporphyrins, around the size of a small protein.
Since then, researchers have expanded the boundaries of the quantum world in myriad directions. In 2023, researchers at ETH ZĂŒrich in Switzerland put a sapphire crystal, 16 micrograms in mass, into a superposition state. The following year, a different group at ETH ZĂŒrich showed that a 100-nanometre glass bead, containing billions of atoms, had a wave-like nature. Researchers have experimented with more complex quantum states, too, with a group at the University of Basel in Switzerland demonstrating entanglement between two objects that each contained 700 rubidium atoms. Simply comparing the size of objects across these experiments can be misleading, as they have measured different facets of quantumness. But all these different tests are necessary to understand just what makes something quantum.
âIt is important to not do just one experiment, but to have a number of experiments in different labs probing the same physics, so that you can have some confidence that if thereâs an effect, that that is indeed something which is not an artefact,â says Ulbricht.
This proliferation of experiments was recognised in December at the inaugural Schrödinger Cats conference in Okinawa, Japan. There, the worldâs macroscopic quantum physicists came together to see just how large they might make a Schrödingerâs cat state, so named for the famous thought experiment in which it is impossible to tell whether a cat in a box is alive or dead, because it has been placed in a superposition of both states. âThis community is growing so much,â says Arndt. Ten years ago, there âwere not very many around doing this, and now itâs really vitalâ, he says.
The next generation of experiments, some of which are already under way, will put us within touching distance of the predictions of spontaneous collapse models. Ulbricht and his team, for instance, are currently using their ultra-silent room at the University of Southampton for an experiment that aims to put a 20-nanometre silicon sphere thousands of times smaller than a human hair, in a superposition. This will require extremely fine control of the bead, which will be levitated by a powerful laser, while minimising environmental effects to a whisper.
This is all made possible by advances in technology, such as more powerful and intricate laser systems, that give us precise control of complex systems in the lab, says Arndt. He and his colleagues have work coming out later this year that will also push the boundary further.
These experiments are typically still a couple of orders of magnitude below the size at which the GRW model predicts we might see some spontaneous collapse, admits Ulbricht, but it is closer than we have ever been before. And if such a collapse were to happen before the predicted boundary, it could rewrite our understanding of the limits of quantum mechanics.
Boundary hunters
The quantum world is notoriously secretive, its mysterious and bizarre traits disappearing as soon as they are observed. Perhaps the boundary hunters wonât be able to directly capture the threshold where the quantum world dissipates. So Bassi and his colleagues are taking another approach to catching quantum collapse in the act: a kind of eavesdropping.
Beneath the Gran Sasso mountains in Italy, where a kilometre of rock provides some of the shielding that Ulbricht and Arndt have spent so much effort creating, Bassi and his colleagues have been searching for high-energy X-ray emissions. According to some ideas, these could be a telltale sign of a spontaneously collapsing wave function. The from these experiments narrowed down the range of X-ray energies we would see if these models are correct, but Bassi is still hopeful that we might see something unexpected.
If we do start spotting indirect signs of unexplainable quantum collapses, then quantum mechanics would need to be adjusted, says Ulbricht. One hundred years after its advent, we would have finally found a crack in the armour. âThat would mean that quantum mechanics is not the final theory, there has to be some modification,â he says.
Signals indicating that collapse is happening would also confirm that Ulbricht and Paternostro are on the right track with their experiments. Ulbricht says it is possible that experiments able to get to the predicted ranges of the GRW models and find evidence of spontaneous collapse â if it exists â could be done within 10 to 15 years. However, Paternostro argues that making sure any findings hold across many tests will be far harder. âWe will have to devise such careful and incredibly accurate experimental settings in the right parameter range, with increasing levels of mass, with [an] increasing number of participating particles, which makes the control of such quantum experiments incredibly more difficult,â he says.
For the physicists hunting for the boundary between the quantum and classical worlds, technology seems to be the limiting factor. And as it improves, their efforts may reveal that the line isnât a feature of reality, but the border of our ability to probe it. In a quantum world, like the one we may live in, there should be signs of entanglement, superposition and other quantum effects on every scale imaginable.
The accepted assumption for most physicists, says Bassi, is that, as long as environmental effects are adequately controlled, there is no limit to how large we can make a quantum system. Proving it is the difficult part.
