A modern take on Schrödinger's cat featuring an oscillating crystal

"Is the cat alive? Or dead? Or is it both?" – Schrödinger, 1935.
Amal Jos Chacko
Scientists at ETH Zurich have made progress in creating heavier Schrödinger cats, which can be alive (top) and dead (bottom) at the same time. 
Scientists at ETH Zurich have made progress in creating heavier Schrödinger cats, which can be alive (top) and dead (bottom) at the same time. 

Yiwen Chu / ETH Zurich 

Imagine a cat, a flask of poison, and a radioactive source kept in a closed box. If a Geiger counter detects the radioactive source to have decayed, the flask shatters, killing the cat. 

This was an experiment devised by Erwin Schrödinger in 1935 in a discussion about the Copenhagen interpretation of quantum mechanics with Albert Einstein. And yes, it’s just a thought experiment: no cat’s getting poisoned.

Since an outside observer could not know if the radioactive source had decayed, they couldn’t know if the cat was alive or dead. 

The Copenhagen interpretation of quantum mechanics states that until a system is measured, it exists in a superposition of all possible states. 

This prompted Schrödinger to question whether the cat was simultaneously dead and alive.

Despite these contradictions, the Copenhagen interpretation continues to be the most widely accepted interpretation of quantum mechanics. 

Also continuing are questions in line with Schrödinger’s. Are particles in quantum states in a quantum superposition until they are measured? 

A team of researchers led by Yiwen Chu, professor at the Laboratory for Solid State Physics at ETH Zürich, has now created a bigger, heavier cat— a crystal put into a superposition of two oscillation states. Their observations, published in Science, the scientific journal, shed light on why these quantum superpositions aren’t visible in the everyday world.

“In the lab, we can’t realize such an experiment with an actual cat weighing several kilograms,” says Chu. Instead, an oscillating crystal has been cast as the cat, with a superconducting circuit, fundamentally a quantum bit (qubit), standing in for the radioactive source. This circuit is capable of taking on logical states “0”, “1”, and superposition of these states “0+1”. 

A modern take on Schrödinger's cat featuring an oscillating crystal
A representation of the cat by oscillations in a crystal (top and blow-​up on the left), and the decaying atom emulated by a superconducting circuit (bottom) coupled to the crystal

Replacing the Geiger counter and poison is a layer of piezoelectric material that creates an electric field when the crystal changes shape amidst oscillating. This electric field can be coupled to the electric field of the qubit, thereby transferring the superposition state of the qubit to the crystal.

Chu and her team observed the crystal oscillate in two different directions simultaneously –up/down and down/up. “By putting the two oscillation states of the crystal in a superposition, we have effectively created a Schrödinger’s cat weighing 16 micrograms,” she explained. 

Although far from an actual cat, 16 micrograms make it several billion times larger than an atom or molecule, making it the “chunkiest” quantum cat ever.

Chu, however, seems intent on pushing the mass limits of her chunky crystal cats even further. “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,” she remarked. The topic is believed to have potential applications in quantum technologies beyond academic interest.

Study Abstract:

According to quantum mechanics, a physical system can be in any linear superposition of its possible states. Although the validity of this principle is routinely validated for microscopic systems, it is still unclear why we do not observe macroscopic objects to be in superpositions of states that can be distinguished by some classical property. Here we demonstrate the preparation of a mechanical resonator in Schrödinger cat states of motion, where the ∼1017 constituent atoms are in a superposition of two opposite-phase oscillations. We control the size and phase of the superpositions and investigate their decoherence dynamics. Our results offer the possibility of exploring the boundary between the quantum and classical worlds and may find applications in continuous-variable quantum information processing and metrology with mechanical resonators.

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