Quantum Theory

Quantum Superposition Bridges the Classic World


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Cracks have begun to show in one of quantum’s biggest controversies. The well-known Schrödinger’s cat thought experiment, which sought to illustrate the absurdity of applying quantum theory to the macro-physical world ruled by classical physics has been challenged by a recent advancement in quantum physics. An international team, led by Markus Arndt of the University of Vienna, successfully placed a large molecule of 2,000 atoms—the biggest object yet—into a state of quantum superposition. The advancement shows that quantum effects can be translated into the classical world, establishing the foundations for scientists to continue to demonstrate how the gap between these seemingly disparate worlds might be reconciled.

Quantum theory tells us that particles in superposition can shift between a wave-like state and a particle-state, meaning they can be in two places at once. Of course, from what is observable in the classical world, this cannot be true. If it were, our understanding of what we understand to be “real” would be challenged, opening the door for a whole host of quantum weirdness that classical theory keeps at bay. Essentially, as Schrödinger tried to prove with his thought experiment, if quantum mechanics is reflected on a macro-physical scale, it signifies that human beings could also exist in two places at once. It does not take long for this information to snowball into theories of time travel and multiple worlds, both of which find basis in quantum theory.

On a fundamental level, the new work published in Nature illustrates that the multi-state paradox of quantum mechanics, known as superposition, functions on a larger scale than previously demonstrated. In theory, we already knew this to be true, but the experiment proves it at the largest scale yet, having only been demonstrated previously using the smallest possible particles; atoms, photons and electrons. The experiment used by Arndt and his team, essentially a souped-up the double slit experiment, has been used regularly since 1801 in quantum mechanical experiments to observe the effects of superposition.

The simple experiment involves particles of light (photons) beamed toward a barrier with two slits in it. On a screen behind the barrier, the effects of quantum superposition are displayed in the form of what is known as an interference pattern. It looks something like this:

This striped pattern that results is interesting, as one might assume that a single beam of photons would produce a representative pattern of a solitary line, indicating their fall along a single path. However, the striped pattern that is produced shows that all of the photon’s possible paths are taken and eventually interfere with each other, suggesting the particle in fact also acts as a wave. This describes the probabilistic nature of quantum phenomena, challenging Einstein’s famous claim that “God does not play dice with the universe”.

In order to pull their super-sized version of this experiment off, the international team had to create not only the perfect environment but also synthesized the massive molecule itself in order to ensure it met the requirements for complex quantum activity to occur. The team built a custom interferometer—which, as the name suggests, is a tool that works by merging two or more sources of light in order to create an interference pattern—called the Long-Baseline Universal Matter-Wave Interferometer (LUMI). The team’s LUMI also beats a record: it is the longest interferometer ever built, with a baseline length of 2 metres. Use of this specialised machine permitted the researchers to fire the beam of heavy molecules (some more than 25,000 times the mass of a hydrogen atom) at the multiple-slit apparatus and observe the resulting interference pattern, confirming the molecule’s state of superposition.

With records being broken in the quantum space with what feels like near-weekly regularity, this advancement especially marks a unique turning point in the disagreement between quantum mechanics and general relativity. These two frameworks we use to understand the world around us have come as close to being bridged as ever before. While the success of this experiment does serve to wedge the door open for a number of seemingly bizarre theories like time travel and multiple worlds, it is doubtful that human beings or planets will be time traveling through multiple realities any time soon, if ever. However, this new, scalable research pushes the limit that scientists seek in quantum interactions of superposition further along, enabling and encouraging future research to continue to explore these limits.

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