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.
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| 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: astronimate.com |
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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