NATIONAL CAT DAY
„Save the date” is a series of articles that have been written to celebrate various unusual holidays. The authors of the presented materials are students, doctoral students and employees of the Faculty of Science and Technology of the University of Silesia.
17 February is celebrated as National Cat Day.
We are constantly trying to understand and define the world. We are getting better and better at it too, but will we ever be able to understand… cats? Prof. Jerzy Dajka from the Institute of Physics tells us all about the mysterious four-legged creature from the scientific point of view.
Fot. archive of UŚ
Prof. Jerzy Dajka
Institute of Physics
Curiosity killed the cat. Or did it? On quantum felinology
Atoms and molecules, as small as hydrogen and as big as fullerenes, may appear in nature in the form of a superposition of distinguishable states (e.g. states of specific energies) and only when a measurement is carried out, an observer can determine, with a probability that can be calculated, which of the elements of superposition he is dealing with. Already in the 1930s, Erwin Schrödinger, one of the creators of quantum mechanics and the author of the equation later named after him, knew very well that every cat can be either alive or dead, and in which state he currently is (cat-living or cat-dead) can be distinguished (for instance by its smell).
Schrödinger, therefore, asked a seemingly innocent question – could a cat (known since then as Schrödinger’s cat) be in a superposition of states cat-alive and cat-dead, just like atoms or molecules of which it is built? Schrödinger’s question triggered an avalanche of other questions and to this day physicists and philosophers are able to give at least a few different explanations as to why in nature there are rather living cats or dead cats, while you can live to be a hundred and never encounter a zombie-cat or a nosferatu-cat (distinguishable by different eating habits). There is a very popular opinion saying that it is decoherence, which is caused by an unconscious, though vigilant observer, the environment, that is responsible for the forms of cats preferred by nature.
Schrödinger’s cats, meanwhile, have settled in physics for good. In quantum optics, Schrödinger cats describe superpositions of macroscopically distinguishable coherent states of light. States of light prepared in this way are excellent for qubits and serve as “quantum information letters” in quantum computing and quantum communication. Much, much later, Yakir Aharonov, Sandu Popescu, Daniel Rohrlich, and Paul Skrzypczyk observed (New Journal of Physics, 15 (2013) 113015) that under certain circumstances a photon of a certain polarisation, having only two choices, will be seen in one of the two paths and its polarisation in the other. If the photon is a cat and the polarisation is a smile, any reader of Alice in Wonderland will recognise that, lo and behold, the quantum Cheshire Cat has entered onto the scientific scene. On one of the two paths, the cat is seen without the smile (the photon without polarisation), while on the other the smile is seen without the cat (polarisation without the photon).
The effect of spatial separation of polarisation and photon position did not have to wait a long time to be experimentally confirmed. The quantum Cheshire Cat and the related notions of weak measurement and weak values of observables contributed to the flourishing of quantum counterfactual reasoning (“if A would happen, then B would happen”) somewhat in spite of orthodox physicists adhering to the Copenhagen interpretation of quantum mechanics given by Niels Bohr. When the exchange of information is not accompanied by the exchange of a physical (material) information carrier between the communicating parties, the quantum counterfactual communication [Nature Communications, (2021) 12:4770 https://doi.org/10.1038/s41467-021-24933-9] is one of the directions in which the physical chariot of progress pulled by the two cats is heading today.