Quantum Cheshire Cat: Even Weirder Than Schrödinger’s

[n2doc]

By George Musser | September 21, 2011 | 5



Just when you thought you’d heard every quantum mystery that was possible, out pops another one. Jeff Tollaksen mentioned it in passing during his talk at the recent Foundation Questions Institute conference. Probably Tollaksen assumed we’d all heard it before. After all, his graduate advisor, Yakir Aharonov—who has made an illustrious career of poking the Schrödinger equation to see what wild beasts come scurrying out—first discovered it in the 1990s and discussed it in chapter 17 of his 2005 book, Quantum Paradoxes. But it was new to me.

The situation is an elaboration of Schrödinger’s thought experiment. You have a cat. It is either purring or meowing. It is curled up in one of two boxes. As in Schrödinger’s scenario, you couple the cat to some quantum system, like a radioactive atom, to make its condition ambiguous—a superposition of all possibilities—until you examine one of the boxes. If you reach into box 2, you feel the cat. If you listen to the boxes, you hear purring. But when you listen more closely, you notice that the purring is coming from box 1. The cat is in one box, the purring in the other. Like a Cheshire Cat, the animal has become separated from the properties that constitute a cat. What a cat does and what a cat is no longer coincide.

In practice, you’d pull this stunt on an electron rather than a cat. You’d find the electron in one box, its spin in the other. Even by the standards of quantum mechanics, this is surprising. It requires what quantum physicists call “weak measurement,” whereby you interact with a system so gently that you avoid collapsing it from a quantum state to a classical one. On the face of it, such an interaction scarcely qualifies as a measurement; any results get lost in the noise of Heisenberg’s Uncertainty Principle. What Aharonov realized is that, if you sift through the results, you can find patterns buried within them.

In practice, this means repeating the experiment on a large number of electrons (or cats) and then applying a filter or “postselection.” Only a few particles will pass through this filter, and among them, the result of the softly softly measurement will stand out.

http://blogs.scientificamerican.com/observations/2011/09/21/quantum-cheshire-cat-even-weirder-than-schrodingers/

[aquart]

1. Scientists never do this to dogs.

[Speck Tater]

2. I love the closing quip...

Re: trying to understand the concept: "In the meantime, try sitting in a different room from where your confusion is."

[tama]

3. Related article

http://www.newscientist.com/article/mg21028104.700-quantum-probes-that-wont-kill-schrodingers-cat.html

"IT MAY soon be possible to extract information from a quantum object - and even manipulate it - without simultaneously destroying its delicate quantum state. The result would be a boon for quantum computing, which requires control over such states. It would also defy a thought experiment dreamed up by physicist Erwin Schrödinger: in principle it is now possible to peek inside his box without endangering the life of the precarious pussycat inside.

States that are mutually exclusive in classical physics can exist simultaneously in the weird world of quantum mechanics - a situation called a superposition. To illustrate this effect, Schrödinger imagined putting a cat in a box along with a device that would release poison to kill it, depending on the random decay of a radioactive atom. Because the atom's quantum state only takes a definite value when someone looks at it, the cat is both dead and alive until the box is opened.

Superpositions are fragile, however. Outside disturbances, including observations, tend to destroy the "coherence" of these states, forcing the system to collapse into just one of the possibilities. The larger the system, the harder it is to isolate it from outside influences.

In 2010, physicists put the largest system yet into a superposition: a 40-micrometre-long strip of piezoelectric material, which expands and contracts in response to voltage changes. They put it into a superposition of both minimal and more vigorous oscillation, but the method they used to observe the system caused it to lose this dual state."

More at link. A vibrating string... bit like guitar, or the line of real numbers vibrated by the imaginary unit!

[Thor_MN]

4. Isn't Schrödinger’s Cat being tortured? Does it know it's half dead

until it's examined, when it either is fine or all the way dead?