Physicists find a way to observe Schrödinger's cat without risk of killing him

To illustrate the principle of quantum superposition and the problem of measurement, in 1935 physicist Erwin Schrödinger invents a thought experiment that later became known as the Schrödinger Cat. In its box, the cat is both alive and dead, and only the observation (the action of looking inside the box) selects one of the two states. However, in a recent study, theoretical physicists have highlighted a way to be able to observe the cat without "risking to kill it".

"We generally think that the price we pay to observe our environment is nothing," says the lead author of the study, Holger F. Hofmann, an associate professor of physics at Hiroshima University in Japan. It's not correct. To look, you must have light, and the light changes the object. Indeed, even a single photon of light transfers energy to the object you are watching.

Hofmann and Kartik Patekar have developed a mathematical framework that separates the initial interaction (looking at the cat) from the result of this interaction (living or dead). "Our main motivation was to look very carefully at how a quantum measurement is done. And the key point is that we separate the measurement in two steps" explains Hofmann. The article was published in the journal New Journal of Physics.

In the experience of Schrödinger's cat, as long as no observer looks inside the box, the cat is simultaneously dead and alive, because the radioactive atom (detected or not by the Geiger counter that triggers the killing mechanism the cat) is simultaneously intact and disintegrated. Credits:

 Preservation of information on the condition of the cat

In doing so, Hoffman and Patekar may assume that all the photons involved in the initial interaction are captured without losing information about the condition of the cat. So before reading this information, all there is to know about the status of the cat (and how it has changed) is always available. It is only when we read the information that we lose some of it. "What's interesting is that the reading process selects one of the two types of information and completely erases the other."

Suppose the cat is still in the box, but rather than looking inward to determine if it's alive or dead, you're putting a camera out of the box, which can somehow take a picture at the inside. Once the photo is taken, the camera has two types of information: how the cat has changed as a result of taking the photo (what researchers call a quantum tag) and whether the cat is alive or dead after the interaction.

None of this information has been lost yet. And depending on how you choose to "develop" the image, you retrieve one or the other information.

Think of a coin. You can choose to find out if a coin has been returned or is currently stacked. But you can not know both. Moreover, if you know how a quantum system has been modified and if this change is reversible, then it is possible to restore its initial state.

A compromise between resolution and disruption

Crucially, the choice of reading comes with a compromise between the resolution of the measure and its perturbation, which are exactly equal. The resolution refers to the amount of information extracted from the quantum system and the disruption to the amount of irreversible changes made to the system. In other words, the more you know about the current status of the cat, the more you have irreversibly changed it.

"What surprised me was that the ability to cancel the disturbance is directly related to the amount of information you get on the observable," says Hofmann. Although previous work has shown a compromise between resolution and quantum perturbation, this article is the first to quantify the exact relationship, according to Michael Hall, theoretical physicist at the Australian National University.


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