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Even if you’re not a quantum physicist, you’ve likely heard of Schrödinger’s famous cat. Erwin Schrödinger came up with cats that could be both alive and dead at the same time in a thought experiment in 1935. The apparent contradiction — after all, in everyday life we only see cats either alive or dead — has prompted scientists to try to make sense of the situations Similar in vitro. So far, they have been able to do this by using, for example, atoms or molecules in quantum mechanical superposition states of being in two places at the same time.
At ETH, a team of researchers led by Yiwen Chu, a professor in the Solid State Physics Laboratory, created a dramatically heavier Schrödinger’s cat by placing a small crystal in a superposition of two states of oscillation. The results were published this week in the journal Sciencescould lead to more powerful qubits and shed light on the mystery of why quantum superpositions are not observed in the macroscopic world.
cat in a box
In Schrödinger’s original thought experiment, a cat is locked inside a metal box with radioactive material, a Geiger counter and a vial of poison. In a given timeframe—an hour, for example—an atom in matter may or may not decay through a quantum mechanical process with a certain probability, and the decay products may cause a Geiger counter to explode and trigger a mechanism that shatters the vial containing the poison that will eventually kill the cat.
Since an outside observer cannot know whether the atom has actually decayed, he also does not know whether the cat is alive or dead – according to quantum mechanics, which governs the decay of an atom, it must be in a living/dead superposition state. (Schrödinger’s idea is commemorated by a life-size cat figure outside his former home at Huttenstrasse 9 in Zurich.)
“Of course, in the laboratory we cannot achieve such an experiment with an actual cat that weighs several kilos,” says Zhu. Instead, she and her colleagues managed to create a so-called cat state using an oscillating crystal, representing the cat, with a superconducting circuit representing the original atom. This circuit is essentially a qubit or qubit that can take the logical states “0” or “1” or a superposition of both states, “0 + 1”.
The link between the qubit and the crystal “cat” is not a Geiger counter and poison, but rather a layer of piezoelectric material that creates an electric field when the crystal changes shape as it oscillates. This electric field can be coupled to the electric field of the qubit, and thus the superposition state of the qubit can be transferred to the crystal.
Simultaneous vibrations in opposite directions
As a result, the crystal can now swing in two directions at the same time – up/down and down/up, for example. These two directions represent the “alive” or “dead” state of the cat. “By superimposing the two states of oscillation in the crystal, we have effectively created a 16-microgram Schrödinger’s cat,” Zhou explains. That’s roughly the mass of a fine grain of sand and nowhere near as massive as a cat, but it’s still several billion times heavier than an atom or molecule, making it the fattest quantum cat yet.
In order for wobbles to be true cat states, it is important that they can be distinguished with the naked eye. This means that the separation between the “up” and “down” states must be greater than any thermal or quantitative fluctuations of the positions of the atoms within the crystal. Zhou and colleagues examined this by measuring the spatial separation of the two states using a superconducting qubit. Although the measured separation was only a billionth of a billionth of a meter—smaller than an atom, in fact—it was large enough to clearly distinguish the states.
Measure small disturbances with cases of cats
In the future, Chu would like to push the block limits of its crystal cats even further. “This is interesting because it will allow us to better understand why quantum effects disappear in the macroscopic world of real cats,” she says.
In addition to this somewhat academic interest, there are also potential applications in quantum technologies. For example, quantum information stored in qubits can be made more robust by using cat states made up of a large number of atoms in a crystal rather than relying on single atoms or ions, as is currently practiced. Also, the extreme sensitivity of massive objects in superposition states to external noise can be exploited to make precise measurements of small perturbations such as gravitational waves or to detect dark matter.
Marius Bild et al., Schrödinger’s cat refers to a 16 μg mechanical oscillator, Sciences (2023). DOI: 10.1126/science.adf7553
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