First Observation of a Quantum Cheshire Cat

Physicists have measured the location of neutrons in one place but found their magnetic properties in another location, just like Alice’s Cheshire cat

In Alice’s Adventures in Wonderland by Lewis Caroll, the eponymous heroine has a bizarre conversation with a Cheshire cat who gradually disappears until nothing is left apart from its grin.

‘Well! I’ve often seen a cat without a grin’, thought Alice, ‘but a grin without a cat! It’s the most curious thing I ever saw in my life!’

Of course, nothing as weird as that can happen in the real world. Or so it’s easy to imagine.

Last year, however, physicists predicted that the quantum equivalent of a Cheshire cat ought to be possible too. By this, they meant that in the quantum world, it ought to be possible to separate the location of an object from its properties.

Now, Yuji Hasegawa at Vienna University of Technology in Austria and a few pals say they’ve observed a quantum Cheshire cat for the first time. These guys have performed a paradoxical experiment in which they measure the location of neutrons in one part of the set up while detecting their spin in another part.

“The results exhibit the characteristics of a quantum Cheshire Cat,” they conclude.

First some background. One of the well known consequences of quantum mechanics is that a measurement always changes the state of the thing it is measuring. Measure the position of a photon, for example, and you can be sure that the measurement must have influenced the photon, changing its position in the process.

But less well known is another form of measurement that does not change the state of the thing it is measuring. That’s possible if the measurement provides only a small amount of information about the quantum object, which it can extract without changing the system’s state.

For example , it is possible to measure whether a photon has travelled down one arm of an interferometer by looking for the deflection it causes to a mirror it bounces off. That doesn’t significantly change the state of the photon because it produces only a small amount of information: whether the photon is present or not. This is known as a weak measurement.

Another of the strange properties of quantum mechanics is that objects can exist in a superposition of states. So for example, a photon can travel down both arms of an interferometer at the same time.

That gave a group of theoretical physicists an idea. They suggested that an experiment could be devised to make two types of weak measurements in different places.

And that would lead to the paradoxical state of affairs in which a quantum object could be located in one place while its properties are measured somewhere else.

Today, Hasegawa and co say they’ve done just that by passing neutrons through a matter wave interferometer. In this experiment, the neutrons pass through a magnetic field to ensure that the spins are aligned in the same direction. They then enter the interferometer where the beam is split so that the neutrons pass down both arms of the device before recombining to produce an interference pattern picked up by a pair of detectors.

The paradox arises when the team carried out two weak measurements. The first found the presence of neutrons in one arm while the second noted their magnetic properties in the other arm. “The neutrons behave as if particle and magnetic property are spatially separated while travelling through the interferometer,” they say. In other words, they observed a quantum Cheshire cat.

That’s an interesting result that at first sight appears to confirm the predictions made by theorists last year. But this experiment is likely to be subject to significant scrutiny.

At issue is whether the result is really paradoxical or simply an ordinary consequence of the way the experiment is set up. For example, perhaps the experiment measures the properties of different neutrons in each of these places.

That’s for the reviewers to work out. In the meantime, it’s worth pointing out that the last year’s theoretical paper generated its own share of controversy, taking over a year to pass through the peer review process.

So it would be no surprise if the current work was subject to the same level of scrutiny and debate.

Ref: arxiv.org/abs/1312.3775 : Observation Of A Quantum Cheshire Cat In A Matter Wave Interferometer Experiment