The Most Famous Physics Cat Now Has Competition

The Most Famous Physics Cat Now Has Competition

By: Elena Timakova

Thursday, June 4^th, 2015

In the well-known thought experiment proposed by Schrödinger in 1935, he presented a cat that is both alive and dead (in physics terms the cat is in a state of quantum superposition). The experiment goes a little like this; a cat is placed in a sealed box with a flask of poison and a radioactive material. If an internal monitor detects radioactivity (so a single atom decaying), it would trigger the flask to break releasing the poison, which kills the cat.

The most commonly used interpretation of the experiment is the Copenhagen interpretation. It states that the object (the Schrödinger’s cat in this case) in superposition stops being in superposition once observation takes place. This means that the felis catus is both alive and dead when the box is sealed, but it is either alive or dead once the box is opened and observation of the event may take place.

The quantum Cheshire Cat now follows in the paw prints of the famed Schrödinger’s cat.

In quantum mechanics; the study of the extremely tiny such as subatomic particles and the concepts of energy quantization—the concept of quantum superposition is very much accepted. The fact that a particle can be in two different states at the same time or in two different positions at the same time is strange and mind boggling, but according to physics, this concept is quite possible.

As the Cheshire cat slowly disappears and its smile remains, Alice exclaims, “Well! I've often seen a cat without a grin, but a grin without a cat! It's the most curious thing I ever saw in my life!"

Here the Cheshire Cat effect is demonstrated. Initially you have a cat with its smile, then it enters the interferometer and the cat is separated from its smile in two beams, then the cat and the smile rejoin as they exit the interferometer. 

Physicists were able to create a particle modeled after the Cheshire cat in Lewis Carroll’s Alice in Wonderland. The properties associated with a particle may now be separated from that particle. This is similar to separating the smell of roses from the rose itself.

A team from Vienna University of Technology, led by Tobias Denkmayr; a PhD student, experimented with neutrons and were able to briefly separate the magnetic moment from the particle. A neutron has no net charge, yet because of its composition it contains a magnetic moment. One quark spins in an upward direction and two others spin in the opposite direction allowing the neutron to be susceptible to spin due to an outside magnetic field. This susceptibility is called the magnetic moment.

When a beam of neutrons was fired into an interferometer, it was split into two beams. The upper beam of neutrons had spins parallel to their trajectory, the lower beam of neutrons had spins in the opposite direction. After, the two beams recombined only the neutrons with parallel spins were chosen. This implies that these neutrons must have travelled through the top beam only.

If a magnetic field is applied, then the spins may be altered. So when a magnetic field was applied to the lower beam, it was ether amplified or canceled out, but nothing happened to the top beam. Meaning that the top beam couples to the measurement device and not the magnetic field itself, while the lower beam was sensitive to the magnetic field created by the measurement device.  

These results showed that neutrons went through the upper beam and their magnetic moments went through the lower beam, hence they became temporarily separated.

This phenomenon of separating the properties from the particles is called the Cheshire Cat effect. This can be useful if one wants to measure a property of a particle which is overshadowed by its magnetic moment.