Have you ever seen “Schrödinger”? This living and dead cat is the hero of one of the most famous thought experiments in physics, demonstrating the concept of “quantum superposition”, according to which a microscopic system can be in several states at the same time. But in the microscopic world, this idea seems ridiculous. If it were possible, our daily life would be very strange: a cup of coffee would be hot and cold, the elevator would be on the ground floor, but at the same time on the third and fourth floors, etc. Superposition of quantum states is observed daily in the laboratory on microscopic systems, such as individual particles, but is completely absent in the macroscopic world. But nothing prevents her in principle! This is why physicists are trying to shed light on bigger and bigger things. Yiwen Chu’s team, from the Polytechnic School of Zurich, broke the record for the largest “Schrödinger’s cat”. Obviously not a real cat, but a mechanical oscillator, and this cat weighs barely 16 micrograms, or as much as a grain of sand… a being on the threshold of the macroscopic world!
If the superposition of states does not easily extend to macroscopic objects, it is because it is fragile. If the particle is placed in such a configuration, it suffices to make a measurement on this so that the superposition vanishes. At the moment the measurement is made, the particle adopts, randomly, only one of its superposition states. However, a macroscopic body is composed of an enormous number of particles (on the order of 1024) and interactions with its environment are sufficient to destroy any superposition of states. We talk about “decoherence”.
Erwin Schrödinger, one of the fathers of quantum mechanics, was not entirely satisfied with the idea of superposition of states. In 1935, he imagined a thought experiment to illustrate all the absurdities of it. “Cat” is placed in an opaque box containing a device containing a radioactive atom and a system that will release a poison lethal to the cat if the atom disintegrates. When the box is closed, it is impossible to know whether the atom has disintegrated after a certain time. We must assume that the atom is in a superposition of the ‘intact’ and ‘disjointed’ states. But the same applies to the bottle of poison, which is “open” and “closed”, and, accordingly, to the “live” and “dead” cat. An absurd situation in everyday life. Of course, when you open the box, you do a “measurement” and the state overlay “collapses” into one well-defined state: the cat is either alive or dead.
We’ve never observed a cat dead and alive at the same time, but how big of a system can fit in a superposition of states? In 2019, Marcus Arndt’s team at the University of Vienna, Austria, achieved this feat with a molecule containing nearly 2,000 atoms.
In 2010, Andrew Cleland of the University of Chicago and his colleagues took another approach. They made a kind of “quantum cylinder,” an aluminum nitride micromechanical oscillator containing about 1013 Atoms, whose vibrations produce an electric current with the piezoelectric effect. This oscillator was connected to a qubit which was used to measure the state of vibration. The group was then cooled below 0.1 K. Thanks to the external magnetic flux, physicists have succeeded in putting this oscillator into a superposition of two modes of vibration: vibrating and not vibrating at the same time! This device was inspired by Yuen Zhu and his team and expanded it to a much larger system: their resonator, with 1017 Atoms, is the largest object ever observed in superposition states.
By pushing the limit on the size of objects that exhibit quantum behavior, physicists hope to better understand how superpositions of states are destroyed. Interaction with the environment is not the only explanation offered. Other hypotheses exist, apart from standard quantum mechanics, and are based, for example, on spontaneous collapse or decoherence processes associated with gravitational interaction. But for now, Schrödinger’s cat is keeping all her secrets.
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