Move Over Schrödinger, the Quantum Cheshire Cat Is Here to Carve Up Reality

Here's a thought experiment: think about yourself, your body, your proper you. Now, consider the properties of yourself that independent of that proper you—the you that would result if you got rid of all the physical, squishy stuff that makes you, you.

But it's all physical stuff, you might say. Except it's not, or not quite. And when it comes to quantum mechanics, it appears separating physical objects from the properties of those objects—having the Cheshire Cat leave his smile behind, while also bringing said smile with him—is actually possible.

As defined here, "properties" are attributes that are carted around by a physical something, but don't exist; they're actually abstractions applied to said physical thing.

For example, you have a heartbeat, which is a physical thing of electricity and muscle contraction. Yet it's also possible to look at a heart's rhythm as an abstract thing, a property independent of muscle and electricity, that could continue on as a property of some other physical system, like a metronome or Ableton Live drum pattern.

The proper term, and one familiar from computer science, is instantiation. There is a heart rhythm existing conceptually just out there in the ether of ideas, but if it were to be produced by an actual heart, it becomes an heart rhythm instance: physical stuff.

It's a weird concept, and I'll give the Stanford Encyclopedia of Philosophy a second go at it: "we would typically say that [properties] characterize objects or, conversely, that objects instantiate or exemplify them. To illustrate, if apple is recognized as a property, it is a property that characterizes all apples."

Likewise, you could take "smiling" and apply it to a bunch of people with perfect accuracy, but how it's exemplified person to person might be quite different. A smile, in this philosophical sense, is a property; it's both an attribute of some specific individual, but something more. As the Cheshire Cat fades away, taking along its proper physical self, an intangible property is impossibly allowed to persist as a tangible abstraction: the grin.

If I haven't yet induced a headache in the reader, this notion has a peculiar extension into quantum mechanics, in the suggestion, made in a paper released this week in Nature Communications, that for systems of particles it may be possible for a quite real, quantum version of the Cheshire Cat to occur, as has been previously posited.

It turns out that in the quantum world it's possible to separate an object from its properties. Imagine if we were standing together and I suddenly left the room. As you breathe a sigh of relief for having been freed of my babble of nonsense, you might notice something highly strange: my heartbeat was still in the room, beating away, with no muscle, no electricity, and certainly no me. Particles can do this in real life.

The fundamental quantum principle needed to understand how this can be is "superposition." Simply, this is the bizarre ability of particles and systems of particles to be many things at the same time: here and there, up and down, left and right. What's more, a quantum object can be everything in between too: some parts up, some parts down, etcetera. Again: all at the same time.

The experiment described in the Nature paper takes a beam of neutrons split at some point by a silicon crystal into two paths. If I was driving a car along such a route, I'd have to make a choice at that split, left or right, but a single particle can say whatever and just take both paths. Similarly, as shown in the experiment, the particle may take one path, while a property of the particle may take another.

The work comes courtesy of a team at the Vienna University of Technology, which is where we find our grin/cat separation taking place. Instead of a grin, the property cleaved away from our neutrons is spin. Spin, a property itself tied to a particle's magnetic moment, is intrinsic in the quantum world, as heartbeats are to humans.

So, let's imagine a stream of neutrons, the electrically-neutral particles that share nuclear real estate with protons, heading along the aforementioned pathway toward some split. A twist is added, however, and neutrons that take the upper split have their spins aligned in one direction (parallel to the beam), while the neutrons that pass through the lower channel have their spins aligned in the other (at a right angle to the beam).

On the other side of the split, the researchers decide to only count the particles that had their spins aligned parallel, ignoring any others. These particles must have taken the upper path, right? That's correct, but what the researchers found was that the same particles registered magnetically as if they were products of the lower path.

"Along one of the paths, the particles themselves couple to our measurement device, but only the other path is sensitive to magnetic spin coupling," said study co-author Tobias Denkmayr in a Vienna Institute statement. "The system behaves as if the particles were spatially separated from their properties."

The particle travels through the upper channel, while the particle's magnetic property travelled through the lower. That the particles being registered do in fact come from the upper stream can be verified by adding a filter to the lower stream and observing if the total number of particles changes further down the stream. It doesn't.

Implications? Well, it's really weird, for one. More tangibly, the authors point to the possibility of better measurements in beam-splitting/recombining experiments (interferometery) in general.

"For example, one could imagine a situation in which the magnetic moment of a particle overshadows another of the particle's properties which one wants to measure very precisely," the paper says. "The Cheshire Cat effect might lead to a technology which allows one to separate the unwanted magnetic moment to a region where it causes no disturbance to the high-precision measurement of the other property."

But we might be more satisfied returning to philosophy and letting the concept settle there. We're not quantum objects, of course, but imagining a world in which we might be carved up like neutrons—heartbeats and hearts beating, smiles and faces—is worth the detour.