Physicists Can Finally Peek at Schrödinger's Cat Without Killing It Forever

From Live Science.com (Nov. 7):

There may be a way of sneaking a peak at Schrödinger's cat — the famous feline-based thought experiment that describes the mysterious behavior of subatomic particles — without permanently killing the (hypothetical) animal.

The unlucky, imaginary cat is simultaneously alive and dead inside a box, or exists in a superposition of "dead" and "alive" states, just as subatomic particles exist in a superposition of many states at once. But looking inside the box changes the state of the cat, which then becomes either alive or dead.

Now, however, a study published Oct. 1 in the New Journal of Physics describes a way to potentially peek at the cat without forcing it to live or die. In doing so, it advances scientists' understanding of one of the most fundamental paradoxes in physics.

…………..

Hofmann and Kartik Patekar, who was a visiting undergraduate student at Hiroshima University at the time and is now at the Indian Institute of Technology Bombay, wondered if there was a way to look without "paying the price." They landed on a mathematical framework that separates the initial interaction (looking at the cat) from the readout (knowing whether it's alive or dead).

…………..

Here's how they described their work in terms of Schrödinger's cat. Say the cat is still in the box, but rather than looking inside to determine whether the cat is alive or dead, you set up a camera outside the box that can somehow take a picture inside of it (for the sake of the thought experiment, ignore the fact that physical cameras don't actually work like that). Once the picture is taken, the camera has two kinds of information: how the cat changed as a result of the picture being taken (what the researchers call a quantum tag) and whether the cat is alive or dead after the interaction. None of that information has been lost yet. And depending on how you choose to "develop" the image, you retrieve one or the other piece of information.

Think of a coin flip, Hofmann told Live Science. You can choose to either know if a coin was flipped or if it's currently heads or tails. But you can't know both. What's more, if you know how a quantum system was changed, and if that change is reversible, then it's possible to restore its initial state. (In the case of the coin, you would flip it back.)

"You always have to disturb the system first, but sometimes you can undo it," Hofmann said. In terms of the cat, that would mean taking a picture, but instead of developing it to see the cat clearly, developing it in such a way as to restore the cat back to its dead-and-alive limbo state. [read more]