Physicists Set New Quantum Record With Heaviest 'Schrödinger Cat' Yet

Via u/H0agh

The famous thought experiment of Schrödinger’s Cat neatly sums up a complex quantum phenomenon, highlighting how bizarre that unseen world is by putting it in terms we can visualize. Now, scientists have created the heaviest Schrödinger’s Cat to date, probing the boundaries between quantum and classical physics.

Particles on the quantum scale can behave in ways that don’t sound possible according to our everyday experience. For instance, it’s perfectly normal for particles to exist in a superposition of two states at once, or even be in multiple places simultaneously, neither of which are possible up here on the macro scale. But why can’t we have our cake and eat it too? Where exactly is the line that separates the realms of quantum and classical physics?

Enter Schrödinger’s Cat. In the theoretical scenario, a cat is sealed in a box with a Geiger counter, a hammer, a flask of poison and a radioactive source. If an atom in the radioactive source decays, the Geiger counter detects it and drops the hammer, which shatters the flask, releases the poison and kills the cat. However, the radioactive atom can exist in a superposition of two states, according to quantum physics. But by extension that superposition should also apply through the whole system, so the cat is also both alive and dead at the same time. It should only be when an observer opens the box and peeks inside that the superposition collapses into one state or the other.

The famous feline was first conjured up in 1935 by theoretical physicist Erwin Schrödinger, originally to highlight what he saw as the absurdities of quantum mechanics, but which eventually became a cornerstone question: at what point does quantum superposition end and reality “choose” one possibility or another?

To help find an answer, scientists at ETH Zurich have now created the heaviest “Schrödinger’s Cat” so far – a crystal weighing 16 micrograms, or about that of a fine grain of sand. That’s obviously still far smaller than a cat, but it’s a few billion times heavier than an atom or molecule, which have previously been used in these kinds of experiments. Even one that involved 2,000 atoms was still far lighter.

Of course, the question here isn’t whether the crystal is alive or dead but whether it’s oscillating “up” or “down.” Like the cat, the crystal’s state is linked to a quantum trigger – in this case, a superconducting circuit that generates an electric field, which interacts with another electric field created by the oscillations of the crystal on a material between them.

In the ETH Zurich experiment, the cat is represented by oscillations in a crystal (top and blow-​up on the left), whereas the decaying atom is emulated by a superconducting circuit (bottom) coupled to the crystal. (Image: Yiwen Chu / ETH Zurich)

And sure enough, the team was able to measure the oscillations of the crystal, and found that they settled into a superposition of both states. That brings the realm of quantum physics closer to the macro scale than ever before, which could help scientists better figure out where the line lies.

“This is interesting because it will allow us to better understand the reason behind the disappearance of quantum effects in the macroscopic world of real cats,” said Yiwen Chu, lead researcher on the study.

It’s not purely theoretical either. The team says that the crystals could make for more robust quantum computers, or potentially future detectors for dark matter and gravitational waves.


Marius Bild, Matteo Fadel, Yu Yang, Uwe von Lüpke, Phillip Martin, Alessandro Bruno, and Yiwen Chu: Schrödinger cat states of a 16-​microgram mechanical oscillator. Science 2023, DOI: 10.1126/science.adf7553

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