Real-Life Schrödinger’s Cats Probe the Boundary of the Quantum World

Schrödinger’s kittens have never been very cute, and the latest litter is no exception. Images of nebulous clouds of ultracold atoms or microscopic strips of silicon are unlikely to go viral on the internet. All the same, these exotic objects are worth heeding, because they show with unprecedented clarity that quantum mechanics is not just the physics of the extremely small.

“Schrödinger’s kittens,” loosely speaking, are objects pitched midway in size between the atomic scale, which quantum mechanics was originally developed to describe, and the cat that Erwin Schrödinger famously invoked to highlight the apparent absurdity of what that theory appeared to imply. These systems are “mesoscopic” — perhaps around the size of viruses or bacteria, composed of many thousands or even billions of atoms, and thus much larger than the typical scales at which counterintuitive quantum-mechanical properties usually appear. They are designed to probe the question: How big can you get while still preserving those quantum properties?

To judge by the latest results, the answer is: pretty darn big. Two distinct types of experiments — both of them carried out by several groups independently — have shown that vast numbers of atoms can be placed in collective quantum states, where we can’t definitely say that the system has one set of properties or another. In one set of experiments, this meant “entangling” two regions of a cloud of cold atoms to make their properties interdependent and correlated in a way that seems heedless of their spatial separation. In the other, microscopic vibrating objects were maneuvered into so-called superpositions of vibrational states. Both results are loosely analogous to the way Schrödinger’s infamous cat, while hidden away in its box, was said to be in a superposition of live and dead states.

The question of how the rules of quantum mechanics turn into the apparently quite different rules of classical mechanics — where objects have well-defined properties, positions and paths — has puzzled scientists ever since quantum theory was first developed in the early 20th century. Is there some fundamental difference between large classical objects and small quantum ones? This conundrum of the so-called quantum-classical transition was highlighted in iconic fashion by Schrödinger’s thought experiment.

The poor cat is a much-misunderstood beast. Schrödinger’s point was not, as often implied, the apparent absurdity of quantum mechanics if extrapolated up to the everyday scale. The cat was the product of correspondence between Schrödinger and Albert Einstein, after Einstein had criticized the interpretation of quantum mechanics championed by the Danish physicist Niels Bohr and his colleagues.

Bohr argued that quantum mechanics seems to force us to conclude that the properties of quantum objects like electrons do not have well-defined values until we measure them. To Einstein, it seemed crazy that some element of reality depends on our conscious intervention to bring it into being. With two younger colleagues, Boris Podolsky and Nathan Rosen, he presented a thought experiment in 1935 that appeared to make that interpretation impossible. The three of them (whose work now goes by the collective label EPR) noted that particles can be created in states that must be correlated with each other, in the sense that if one of them has a particular value for some property, the other must have some other particular value. In the case of two electrons, which have a property called spin, one spin might point “up” while the other electron’s spin points “down.”

In that case, according to Einstein and his colleagues, if Bohr is right and the actual directions of the spins are undetermined until you measure them, then the correlation of the two spins means that measuring one of them instantly fixes the orientation of the other — no matter how far away the particle is. Einstein called this apparent connection “spooky action at a distance.” But such a phenomenon should be impossible, because Einstein’s theory of special relativity shows that no influence can propagate faster than light.

Schrödinger called this correlation between the particles “entanglement.” Experiments since the 1970s have shown that it is a real quantum phenomenon. But this doesn’t mean that quantum particles can somehow influence one another instantly across space through Einstein’s spooky action. It’s better to say that a single particle’s quantum properties are not necessarily determinate at one fixed place in space, but may be “nonlocal”: fully specified only in relation to another particle elsewhere, in a manner that seems to undermine our intuitive notion of space and distance.

Schrödinger’s cat arose out of his musings on the peculiarities of EPR entanglement. Schrödinger wanted to show how Bohr’s notion that nothing is fixed until it is measured could lead to logical absurdity if we imagined blowing entanglement up to everyday size. His thought experiment places the hapless cat in a closed box with a vial of lethal poison, which can be broken open by some mechanism that links it to — in fact, entangles it with — a quantum particle or event. The trigger could come from an electron, breaking the vial if it has upward spin but not if it has downward spin. You can then prepare the electron in a so-called superposition of states, in which both upward spin and downward spin are possible outcomes of a measurement. But if the spin is undetermined before the measurement, then so must be the status of the cat — there’s no way you can meaningfully say if it is alive or dead. And that’s surely nonsensical.

Schrödinger’s point was not simply that quantum rules lead to apparent nonsense when applied at the everyday scale — you don’t need a cat for that. Rather, he wanted to find an extreme demonstration of how deferring any assignment of a definite state (alive or dead) until measurement has been made (by opening the box to look) could lead to implications that seem not only odd but logically forbidden.

To Bohr this would have seemed an invalid scenario — measurement, such as opening the box and looking at the cat, was for him always a macroscopic and therefore a classical process, so quantum rules would no longer apply. But then how does measurement ensure that magical transformation from quantum to classical?

Instead of arguing about it, why not just do the experiment? The trouble is, while it was all very well for Schrödinger to imagine making a cat “quantum” by coupling it to some atomic-scale event, it’s not at all clear how — or indeed whether — we can do that scaling up in practice, or indeed what a superposition of alive and dead could mean in terms of quantum states.

But with modern techniques, we can imagine creating well-defined quantum superpositions of relatively big objects — not as big as cats, but much bigger than lone atoms — and probing their properties. This is what efforts to create Schrödinger’s kittens are all about.

“A lot of physicists don’t really expect any surprises at large scales,” said Simon Gröblacher of Delft University of Technology in the Netherlands. “But it is simply not known what will happen if you start making quantum states with around 10²³ atoms,” which is the typical scale of everyday objects.

The new experiments show that, despite what Schrödinger thought, relatively large objects can indeed exhibit counterintuitive quantum behavior.

Gröblacher and his colleagues created microbeams of silicon, each 10 micrometers long and 1 by 0.25 micrometers in cross-section. Each one featured holes along the beams that would absorb and trap infrared laser light. The researchers then excited those beams with light sent in a superposition of paths, one to each beam. By doing so, they were able to entangle two beams into a single quantum vibrational state. You could think of it as the very small equivalent of two entangled cats.

Another kind of entanglement between mechanical oscillators was reported, in back-to-back papers with Gröblacher’s team in Nature, by Mika Sillanpää of Aalto University in Finland and colleagues. They coupled two microscopic drumhead-like metal sheets via a superconducting wire. The wire can contain an electrical current oscillating at microwave frequencies (about 5 billion vibrations per second); its electromagnetic field exerts a pressure on the vibrating plates. “The electromagnetic fields act as a kind of medium that forces the two drumheads into the entangled quantum state,” Sillanpää said.

Researchers have long sought to achieve quantum effects such as superposition and entanglement in “large” micromechanical oscillators like these, which have billions of atoms in them. “Entangled states of mechanical oscillators have been discussed theoretically since the late 1970s, but only within the last few years has it been technically possible to create such states,” Sillanpää said.

What makes these experiments such a tour de force is that they avoid the process that generally transforms large objects from ones governed by quantum rules into ones that obey classical physics. This process seems to provide the missing part (at least, most of it) of the puzzle of measurement, which Bohr left so maddeningly vague.

It is called decoherence — and, rather neatly, it is all about entanglement. According to quantum mechanics, entanglement is an inevitable result of any interaction between two quantum objects. So if an object — a cat, say — starts off in a superposition of states, that superposition — that quantumness, you might say — spreads as the object interacts with its environment and becomes increasingly entangled with it. But if you want to actually observe the superposition, you’ll need to deduce the quantum behavior of all the entangled particles. This rapidly becomes impossible, in much the same way as it becomes impossible to trace all the atoms in a blob of ink as it disperses in a swimming pool. Because of interaction with the environment, the quantum nature of the original particle leaks away and is dispersed. That’s decoherence.

Quantum theorists have shown that decoherence gives rise to the kind of behavior seen in classical physics. And experimentalists have proved it in experiments that can control the rate of decoherence, where the characteristic quantum effects such as wavelike interference of particles gradually vanish as decoherence proceeds.

Decoherence, then, is central to the current understanding of the quantum-classical transition. The ability of an object to show quantum behavior, such as interference, superposition and entanglement-induced correlations, has nothing to do with how big it is. Instead it depends on how entangled it is with its environment.

Nevertheless, size does generally play a role, because the bigger an object is, the more easily it can become entangled with its environment and decohere. A large, warm, restless object like a cat doesn’t have a hope of remaining in a quantum-mechanical superposition of any sort and will decohere more or less instantly.

If you simply stick a cat in a box and link its fate to the outcome of some quantum event, you’re not likely to put it in a superposition of alive and dead, because decoherence will almost instantly force it into one state or the other. If you could suppress decoherence by removing all interaction with the environment (without killing the cat in an ultracold vacuum!) — well, then it’s another story and the arguments persist. It’s nigh impossible to imagine how to achieve that for a cat. But that’s in essence what the teams of Gröblacher and Sillanpää have achieved with their tiny oscillators.

Instead of working toward the quantum-classical boundary from the top down, seeing if we can conjure quantumness into a vibrating object when it’s small enough, we can come at it from the bottom up. Since we know that quantum effects like superposition and interference are readily seen in individual atoms and even small molecules, we might wonder how far those effects can be sustained as we keep adding more atoms. Three teams have now explored this question, achieving quantum states for clouds of up to tens of thousands of ultracold atoms by entangling them in a state called a Bose-Einstein condensate (BEC).

Einstein and the Indian physicist Satyendra Nath Bose pointed out that such a state may exist among bosons (named for Bose), one of the two general classes of fundamental particles. In a BEC, all the particles are in the same single quantum state, which means in effect that they act rather like one big quantum object. Because it is a quantum effect, Bose-Einstein condensation happens only at very low temperatures, and a BEC was only seen in its purest form — a cloud of bosonic particles — in 1995, in atoms of rubidium cooled to just a few billionths of a degree above absolute zero.

BECs made from such ultracold atoms have given physicists a new medium for investigating quantum phenomena. In the past, researchers have shown that such a cloud — perhaps several thousand atoms — can be placed in a state in which all the atoms are quantum-entangled together.

These aren’t strictly Schrödinger’s kittens, said Carsten Klempt of Leibniz University Hannover in Germany. Those are generally defined as superpositions of states that are as different as could be: for example, all with upward spin and all with downward spin (analogous to “alive” and “dead”). That’s not the case in these entangled clouds of atoms. Nonetheless, they still show quantum behavior at a relatively huge scale.

There’s a more important proviso, though, to the idea that they are “kitten-scale” embodiments of EPR-style entanglement. The atoms are all jumbled together in space and are identical and indistinguishable. This means that, even if they are entangled, you can’t see it in terms of a correlation between the property of one object here and another one there. “Bose-Einstein condensates of ultracold atoms consist of large ensembles of indistinguishable atoms, literally equal in any physical observable,” Klempt said. “Therefore, the original definition of entanglement [as portrayed in the EPR thought experiment] cannot be realized in them.” In fact, the whole concept of entanglement between indistinguishable particles has been theoretically disputed. “That is because the notion of entanglement requires the possibility to define the [distinct] subsystems that are entangled with each other,” said Philipp Kunkel of Heidelberg University in Germany.

A much clearer kind of entanglement, directly analogous to the entanglement of the spatially separated particles in the EPR thought experiment, has now been demonstrated in three separate experiments by Klempt’s team in Hannover, Kunkel’s group (led by Markus Oberthaler) in Heidelberg, and a team led by Philipp Treutlein at the University of Basel in Switzerland. “The conflict with classical physics is particularly striking when the entanglement is observed between such spatially separated systems,” Treutlein said. “This is the situation the 1935 EPR paper considers.”

All three groups used clouds of hundreds to thousands of rubidium atoms held in electromagnetic trapping fields (either produced by microscopic devices on an “atom chip” or generated by crossed laser beams). The researchers used infrared lasers to excite quantum transitions in the atoms’ spins and looked for the correlations between spin values that are the telltale sign of entanglement. While the Heidelberg and Basel groups addressed two different regions in a single large cloud, Klempt’s group actually split the cloud by inserting a region of empty space in the middle.

The Basel and Heidelberg groups demonstrated entanglement via an effect called quantum steering, in which the apparent interdependence of the two entangled regions is exploited so that measurements made on one of them allow researchers to predict the measurements of the other. “The term ‘steering’ was introduced by Schrödinger,” Treutlein explained. “It refers to the fact that, depending on the measurement result in region A, the quantum state we use to describe system B changes.” But this doesn’t imply that there is any instantaneous information transfer or communication between A and B. “One can’t steer the state of the distant system deterministically, since the outcome of the measurement is still probabilistic,” Kunkel said. “There’s no causative influence.”

These results are “very exciting,” said Jens Eisert of the Free University of Berlin, who was not involved in the work. “Entanglement in atomic vapors has been generated long before,” he said, “but what’s different here are the levels of addressability and control in these systems.”

Aside from the clearer demonstration of entanglement when it exists between spatially separated regions, there’s also a practical advantage to doing things this way: You can address the separate regions individually for quantum information processing. “It is not possible even in principle to address individual atoms in the BEC without affecting all other atoms, if they’re all in the same location,” Treutlein said. “However, if we can individually address the two spatially separated regions, the entanglement becomes available for quantum information tasks like quantum teleportation or entanglement swapping.” That, however, will require the physical separation of the clouds to be increased beyond what was done in the current experiments, he added. Ideally, Klempt said, you would divide the cloud further into individually addressable atoms.

“Large” quantum objects like these might also enable us to probe new physics: to find out, for example, what happens when gravity starts to become a significant influence on quantum behavior. “With this new way of controlling and manipulating large entangled states, there could be room for sophisticated tests of quantum effects in gravitational theories,” Eisert said. It has been proposed, for example, that gravitational effects might induce a physical collapse of quantum states into classical ones, an idea that is in principle amenable to experiment on superpositions or entangled states of large masses. Treutlein said that one way to test physical-collapse models involves interference between distinct atomic “matter waves” — and, he added, his group’s split, entangled BEC can act as such an atom interferometer. “Most physicists will probably not expect a sudden breakdown of quantum physics” as the system size increases, Klempt said. But Kunkel added that “it is still an open question, experimentally and theoretically, if there is a fundamental limit to the size of the objects that can be entangled with each other.”

“The most interesting question is if there is some fundamental size where one cannot in some sense make entanglement,” Sillanpää said. “That would mean that something else in addition to normal quantum mechanics enters the picture, and this could be, for example, collapse due to gravity.” If gravity does play a role, that might offer some hints for how to develop a theory of quantum gravity that unites the currently incompatible theories of quantum mechanics and general relativity.

That would be quite a coup for Schrödinger’s kittens. For now, they reinforce the general belief that there is nothing special about quantum behavior, beyond the fact that it spins itself into an ever more tangled cat’s cradle from which our classical web emerges. And no cat need be killed in the process.