# IFJ PAN: "quantum limit" are multiparticle phenomena

**Using simple theoretical models, systems can be built that faithfully reproduce even the most paradoxical predictions of quantum mechanics for single particles; the quantum "magic" begins only for many particles, argues a physicist from the IFJ PAN.**

World of quantumoin is full of paradoxoin incomprehensible to human intuition and inexplicable to classical physics – a thesis thatoThe rsis can be heard almost every time quantum mechanics is mentioned. Here are some examplesoin phenomena commonly considered to be typically quantum: a single electron generating interference bands behind two slits, as if passing through both simultaneously; particles residing at the same time in multiple ros of different states, so that at the time of observation "magically" appear in a single selected one; measurements without interaction; erasing the past with a quantum eraser or, finally, non-locality, making it appear as if entangled particles instantly interacted over an arbitrarily large distance. Just whether all these phenomena necessarily have to be purely quantum?

According to a press release from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Krakow, one of its employeesow, dr hab. Paweł Błasiak, has just published in a journal "Physical Review A" article, in whichorym showed how with "bricks" classical physics to construct broad optical interferometric systems, faithfully reproducing the strangest predictions of quantum mechanics for single particles.

The presented model helps to better understand why quantum mechanics is needed and what really new mowi about the reality around us. After all, if a quantum effect has a simple classical explanation, it should not be regarded as a special mystery.

The publication clearly indicates the limit, beyond whichoThe theory of quantumow is already essential: true quantum "magic" only begins for many particles.

– The controversy wokoł quantum mechanics is really a lot of. They are so strong that even today, when the theory is almost a century old, most physicistsow prefers to simply use it, avoiding uncomfortable questions of interpretation – mowi cited in the press material by Dr. Błasiak. – Our problems here stem from the fact that. we were too determined. Previously, we first observed certain phenomena and in order to explain them, we built a mathematical apparatus on the basis of well-established physical intuitions. In the case of quantum mechanics, the opposite has happened: from just a few experimental clues, we have guessed a highly abstract mathematical formalism, brilliantly describing the results of measurementoin the laboratory, but we don’t have the slightest idea what the physical reality behind it is,” states the.

American physicist Richard Feynman was firmly convinced that a phenomenon absolutely impossible to explain by classical physics is quantum interference, responsible, among other things, for the striations visible behind two slits, through which theore passes a single quantum object. Erwin Schrödinger, wspołtworca of quantum mechanics, had another favorite: quantum entanglement, which can remotely tie features of dwoch and more quantum particles.

A large number of physicistsow to this day, one wonders to what extent these non-intuitive phenomena of quantum mechanics are merely the result of our cognitive limitations – that is, the wayoin how we study the world. It is not nature, but our lack of full knowledge of the system that would cause the phenomena observed in it to take on the characteristics of inexplicable exoticism. This type of approach is proba to look at quantum mechanics as a theory with a well-defined ontology, leading to an answer to the question of what really odroThe new paper demonstrates that the theory of quantumow from classical theories.

The new article demonstrates the principles of building models of arbitrarily complex systemsoin optics, constructed from elementsoin operation according to the principles of classical physics, additionally taking into account the existence of some local hidden variables, for the existence of which theoof which the experimenter has only indirect access. Dr. Blasiak showed that for single particles, the presented model faithfully reproduces all phenomena commonly considered to be an obvious sign of quantumness, including wave function collapse, quantum interference or contextuality. Moreover, the classical analogies of these phenomena turn out to be quite simple. However, this model cannot reproduce the characteristic features of quantum entanglement, whichothe existence of which requires at least twooch quantum particles. This seems to indicate that entanglement and the associated nonlocality may be a more fundamental property of the quantum worldow than quantum interference.

– This type of approach avoids the dastardly practice of evasive answers and hand waving in discussions about the fundamentals and interpretation of quantum mechanics. We have the tools to formulate such questions and resolve them precisely. The model constructed is intended to show that information-limited ontological models have at least the potential to explain most of the exotic quantum phenomena within the broad framework of classical physics. The real quantum mystery would remain only quantum entanglement,” explains Dr. Blasiak.

As emphasized by representatives of the IFJ PAN, quantum entanglement thus goes to the very heart of quantum mechanics, indicating that "something", which forces a departure from the classically understood reality and pushes the boundary of mystery in the direction of multiparticle phenomena. This is because it turns out that quantum effects for single particles can be successfully reproduced within the framework of classical (i.e. Local) ontological models with limited access to information. So if multiparticle phenomena were ignored, we could basically do without quantum mechanics and its "ghostly" nonlocality. The described local model, reproducing quantum phenomena for a single particle, very clearly defines the limit, beyond ktorą statements regarding nonlocality lose their validity.

It seems, therefore, that it is Schrödinger went to the heart of quantum mechanics. But a quiet winner could be. Albert Einstein, whoory has never been satisfied with the generally accepted interpretation of quantum mechanics. Without his persistent questions, we would have neither Bell’s theorem nor quantum information today.

– This is why research in the foundations of quantum mechanics is so fascinating,” stresses Dr. Błasiak. – They range from the ever-recurring question of the nature of our reality, to the essence of true quantum, whichorej we owe the advantage of quantum technologies over their classical counterparts,” concludes the physicist.