Video: Three quantum paradoxes illustrated with candy
Can you separate a bell from its ring? You can in the quantum world – the Cheshire cat experiment has shown neutrons splitting from their spins
AS WEIRD as the quantum world is, something happened last year in the shadow of the French Alps that caused even hardened quantum physicists to do a double take. At the in Grenoble, France, where a nuclear reactor spews out the world’s most intense beam of neutrons, physicists made these particles perform a trick that until now had only existed in the fevered imaginations of theorists. In doing so, they have upset our notion of reality.
The international team of physicists coaxed the neutrons to shed their quantum properties, getting the particles to go one way and their spins another way. It’s as if you took one path and your personality another. Theorists have predicted the possibility of such strange behaviour for more than a decade. They even named it the quantum Cheshire cat phenomenon, after the cat in Alice’s Adventures in Wonderland, which would disappear leaving only its grin behind. Now, for the first time, the , posing fundamental questions about the nature of the quantum world.
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Quantum theory emerged in the 1920s and it remains wildly successful. No experiment has ever disagreed with its predictions and we can be confident that it is an accurate description of the microscopic world of atoms and their constituents. It is a strange description, for sure: quantum particles can be in two places at once, spin clockwise and anticlockwise at the same time, or instantaneously influence each other from across the universe.
(Image: Matt Murphy)
Why the microscopic world should behave this way while everyday objects do not is still very much a mystery. “We don’t believe that there is a deep intuitive understanding of quantum mechanics, and that’s the reason why we keep making these shocking discoveries,” says of Chapman University in Orange, California.
“We keep making these shocking discoveries about the quantum world”
That leaves theorists and experimentalists free to push the boundaries of quantum theory, looking for clues as to what makes it tick. One such push began 50 years ago, when , who is also now at Chapman, asked a very basic question: does time in quantum mechanics have to flow from the past to the present, as our intuition suggests? The answer, at least mathematically, is no.
Two-way time
Aharonov and his colleagues Peter Bergmann and Joel Lebowitz fashioned a theory called the time-symmetric formulation of quantum mechanics, which defies common-sense notions of time. In their 1964 paper, they showed how the state of a quantum system could be affected both by events in the past and events in the future. Time flowed both ways. The theory was mathematically equivalent to standard quantum mechanics, where time flows one way, yet suggested that nature was different. “It was the first paper that eliminated the assumption that at the deep microscopic level there was an asymmetry in time similar to the way we human beings experience it,” says Tollaksen.
So much for the theory. When it came to testing the idea, experimentalists faced a serious problem. In the classical world, an object can exist in one state or another: it can either spin clockwise or anticlockwise. But in the quantum world, particles can spin clockwise and anticlockwise at the same time. Experiments have confirmed that such “superpositions” really do exist, yet we don’t see them directly because the very act of measuring them forces the particle into one state or the other. Such “strong” measurements obliterate the quantum nature of the particle and, because of this, it is impossible to test whether an event in the future will affect a particle in the present.
This might have been the end of the story, had Aharonov and colleagues not discovered that reports of death in the quantum world are an exaggeration. In a seminal theory paper in 1988, they showed that it is possible to make a different kind of measurement, one that doesn’t destroy the quantum state. As long as the device that’s doing the measuring interacts extremely weakly with the particle, you can glean something more about it without forcing it into one state or the other. The trade-off for saving the particle’s quantum nature is that this “weak measurement” comes with a great deal of uncertainty.
This means that one weak measurement isn’t particularly useful. But who says you can’t do more? The trick, Aharonov’s team found, is to make weak measurements on an ensemble of identical particles. The uncertainty means that each generates a different value. But if you plot lots of them, they produce a bell-shaped curve whose peak denotes the state of the quantum particles. So weak measurements offer a way to test time symmetry.
Needless to say, the weak measurement idea did not go down well. “At first there was complete disbelief,” recalls Aharonov.
But time and technology have helped his case. Over the past decade, weak measurements have gone from theory to reality. Such experiments have allowed us to study aspects of the quantum world that were previously thought impossible, such as measuring the wave function that describes a particle. Jeff Lundeen of the University of Ottawa in Ontario, Canada, and his colleagues did exactly this in 2011 on a .
While experimentalists were figuring out how to do weak measurements in the 1990s, Tollaksen began working for his PhD with Aharonov on the theoretical foundations of quantum mechanics. Aharonov had shown that both the past and future can conspire to influence a particle in ways that are truly bizarre. In one example, his mathematics showed that a particle whose spin is exactly +½ or -½ when you make a strong measurement on it can have a spin of 100 when you make a weak measurement.
Even stranger, Aharonov and Tollaksen found that the past and future can lead to a particle and its properties going their separate ways. The quantum Cheshire cat was born.
Lost property
When Tollaksen published his PhD thesis in 2001, he laid out their thought experiment. It begins with a step called pre-selection that involves preparing a large number of neutrons with identical spins.
These particles are sent one by one into a device called an interferometer. The first stop in the interferometer is a beam splitter, which as the name suggests splits the beam into two. Each neutron is now in that crazy state of quantum superposition where it traverses both paths at once. Both paths contain equipment that lets you make a weak measurement. The paths are eventually brought together and the beams recombined in such a way that some neutrons exit through one output path, while the others exit via a different path (see illustration). On one of the output paths, the particles are subjected to a strong measurement, such as measuring what spin direction the neutrons now have. Only some will have the desired value. These particles are said to be post-selected. The experimenter discards all other neutrons that don’t satisfy the post-selection criterion.
Now comes the weirdness. Pre- and post-selection amount to tinkering with the past and future respectively. If you look at only those neutrons that were post-selected, the maths says that all these neutrons took one path inside the interferometer whereas their spins went along the other path. It’s as if you had two boxes – one full of the particles without their properties and the other full of their properties but not the particles. “That definitely should cause you to pause,” says Tollaksen.
That’s all very well on paper, but would the quantum Cheshire cat appear in reality? To find out, researchers would have to make weak measurements on neutrons – something that no one has done to this kind of particle or any other kind of matter. So Tollaksen teamed up with experimenters from the Vienna University of Technology in Austria who specialise in .
In Grenoble, the Vienna team used a feeble magnetic field and a weakly interacting neutron absorber to make the weak measurements. They found that when they put the absorber in one path of the interferometer (say left), there was a discernible effect at the output. But when they put it in the right path, it had no such effect. The neutrons were travelling in one path only.
Next, the experimenters introduced a weak magnetic field near each arm of the interferometer, to interact with the spin of the neutrons. When they did this in the left path, there was no change in the interferometer’s output. If they introduced the magnetic field in the right path, though, there was a change: the magnetic field had interacted with the spin. In other words, they had confirmed that the spin had chosen the path not taken by the parent neutron. They had separated the cat from its grin.
The results are so new that no one really knows what this might lead to. However, some are willing to speculate. Say you want to measure something called the electric dipole moment of a neutron, which reflects how the charged quarks that make up the particle are distributed. Theories that try to explain why the universe has matter but no discernible antimatter left from the big bang also predict that the neutron has a tiny electric dipole moment.
All efforts to measure it have thus far found nothing. But the neutron has a strong magnetic moment – and this potentially messes up such experiments. “If I can separate the magnetic moment of the neutron from the neutron, I can do a very delicate experiment to check whether it has a very weak electric dipole moment,” says Aharonov.
Peter Geltenbort of the Institut Laue-Langevin studies the fundamental properties of neutrons and is intrigued by the promise of the quantum Cheshire cat. “This is a really nice piece of experimental physics,” he says. “In terms of ongoing attempts to measure the neutron’s electric dipole moment, it would be extremely exciting to apply such a technique and, if successful, it would undoubtedly increase the sensitivity of our measurements.”
And these aren’t the only properties that neutrons can lose, according to Aharonov. “The only thing that cannot be separated from a particle is its mass. What defines where the particle lives is its mass,” he says. “Everything else is like the smile of the cat. You can separate it from the cat.”
Because there is nothing to stop you from making quantum Cheshire cats from photons, electrons and even atoms, Tollaksen thinks there might be uses in quantum computing. One of the challenges in quantum computing is to isolate particles from external disturbances that destroy the all-important superposition of states. The quantum Cheshire cat phenomenon could separate the more susceptible properties of particles from the particles themselves, leading to more stable quantum computers.
“We can open a completely new kind of physics. I’m only beginning to understand the implications,” says Aharonov.
“We’re only just beginning to understand the quantum Cheshire cat’s implications”
There is of course the bigger question of what all this says, if anything, about the nature of reality. It depends on what you make of the values thrown up by weak measurements. “That’s the elephant in the room,” says of the University of Toronto, Canada, who studies weak measurement experiments. “What do these values really tell us about the physical world?”
If you ascribe physical reality to weak values, a particle really can have an absurd spin. A neutron can really be in one path and its spin in the other. “I believe that these are true physical properties of quantum systems,” says Aharonov. “When you pre and post-select quantum systems, you see a completely new reality.”
A new reality
Not everyone is ready to accept this strange new reality. “The interpretation of these measurements is non-trivial, even tricky,” says of the Max Planck Institute of Quantum Optics in Garching, Germany. “The Cheshire cat paradox arises only when you give a physical meaning to the observed weak values – which is challenged and debated in the community.”
Where does this leave the time-symmetric formulation of quantum mechanics in which the future influences the present? Steinberg remains cautious. “I think it’s easy to overinterpret that language. I’m not going to claim that when I dig up a dinosaur bone today, it causes that dinosaur to have gotten killed 65 million years ago. I wouldn’t say that the future is influencing the past. I’d say that information about the future gives us information about the present or the past.”
Tollaksen, however, points out that with the time-symmetric formulation, the calculations needed to predict the weak values in experiments like the quantum Cheshire cat are simple and elegant. Trying to do the same with traditional quantum mechanics, where time flows one way, is unwieldy, even inelegant. Often in physics the quest for elegance has led to a clearer understanding of reality. But it’s too early to tell if that reality involves time symmetry, Steinberg says.
What’s clear is that finally observing the quantum Cheshire cat is challenging our intuitions about matter. What does it mean for an atom to be separated from its properties? No one quite knows. The first answers might come when experimentalists succeed in measuring the electric dipole moment of a neutron by separating it from its magnetic moment. That would be momentous in more ways than one, giving us a glimpse of a reality that, as with almost everything in quantum mechanics, defies common sense. Curiouser and curiouser, indeed.
This article appeared in print under the headline “Lost and found”
