Several weeks ago, I was huddled over a laptop with two researchers from IBM to learn about a Schr枚dinger鈥檚 cat they had made in one of their quantum computers. It was a big one 鈥� big enough, I learned, to be considered proof that their computer was worth paying attention to.
Now, this wasn鈥檛 a real cat with paws and whiskers, of course, but a metaphorical one made from tiny superconducting circuits called qubits. Quantum physicists use this language all the time, referencing the animal from the famous thought experiment that Erwin Schr枚dinger wrote about in 1935.
Schr枚dinger didn鈥檛 set out to write a paper about a cat, but rather one on the 鈥減resent situation in quantum mechanics鈥�. That situation included handwringing over how to interpret uniquely quantum combinations of states called superpositions. When a quantum object like an electron is in a superposition of two states, it is impossible to tell which of the states actually reflects the truth without directly measuring it.
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This is where the cat comes in. In Schrodinger鈥檚 thought experiment, a cat is confined within a steel chamber where there is a 50 per cent chance that it will survive and a 50 per cent chance that a 鈥渄iabolical device鈥� will kill it. Writing somewhat with tongue in cheek, Schr枚dinger concluded that, based on the rules of quantum mechanics, the cat is bound to end up in a superposition of being both dead and alive. To work out whether the cat is living or not, a physicist must crack open the chamber. 鈥淧ardon the expression,鈥� Schr枚dinger wrote before describing the animal as 鈥渕ixed or smeared out in equal parts鈥�.
Schr枚dinger called his cat a 鈥渜uite ridiculous case鈥�. The IBM researchers called it a benchmark. The quantum zeitgeist has clearly changed.
This quantum cat underscores how the relationship between mathematical theory and reality is nowhere near straightforward for quantum objects. And because we are large and warm, unlike a particle, we simply don鈥檛 experience this quantum strangeness in our everyday lives.
Physicists broadly agree that putting an actual cat into a quantum superposition state would be impossible. In 2021, a team of researchers claimed to have made a tardigrade astonishingly cold, then put it into a quantum state called entanglement, shared with a tiny superconducting circuit. But this rather extreme case wasn鈥檛 received well by everyone: critics contended that the researchers hadn鈥檛 been able to prove that a distinctly quantum property of the tardigrade was connected to the circuit in a way that isn鈥檛 possible in classical physics.
Letting the cat out of the bag
The appeal of Schr枚dinger鈥檚 cat has only intensified over the years. For about four decades, mentions of it really only appeared in essays on philosophy. But then, in 1974, the iconic science fiction author Ursula K. Le Guin published a short story titled Schr枚dinger鈥檚 Cat, where a sorrowful narrator encounters a yellow cat that may belong to Schr枚dinger himself.
In the story, the cat does end up in a chamber equipped with a diabolical device, but when the narrator opens it, the animal is nowhere to be found 鈥� and the roof of the narrator鈥檚 house blows off, inviting the reader to speculate on whether the narrator was subject to a diabolical device of their own all along. References abound in the science fiction written in the years following Le Guin鈥檚 story. Today, Schr枚dinger鈥檚 cat is a pop culture mainstay, gracing everything from T-shirts to coffee mugs to making appearances in animated series like Futurama, far more than as the symbol of existential uncertainty that Le Guin seems to have been after.
Physicists never got out of the Schr枚dinger cat business either. For one, they became extremely adept at making quantum superposition states, whether from extremely cold atoms, particles of light, qubits or even microscopic membranes not dissimilar to minuscule drums. In 2023, researchers placed a crystal comprising 10 quadrillion atoms 鈥� about a hundredth as many as make up a crystal of table salt 鈥� into a superposition state, setting a new record for macroscopic objects in cat-like states.
Pushing the number of atoms in such experiments to larger and larger values shows us just how far the reach of quantum effects can extend. But quantum states inspired by Schr枚dinger鈥檚 cat are also useful 鈥� this is what got me talking to those IBM researchers. Their cat鈥檚 official name was the Greenberger-Horne-Zeilinger (GHZ) state, which they created from 120 qubits.
These qubits were both put in a superposition and connected through the inextricable link of quantum entanglement. The effect was familiar: all of them were in a combination of two otherwise mutually exclusives states, representing computational values of 0 and 1, a scenario that simply can鈥檛 exist in non-quantum objects.
Entanglement is expected to be the key ingredient for getting quantum computers to leapfrog the world鈥檚 best supercomputers on several tasks in the future, and GHZ states are used in many of their computations. If you have a quantum computer, then you want it to be good enough to sustain large GHZ states, so these are often used as a benchmark. In 2024, I spoke to a researcher who had made a 60-qubit GHZ state, which he designated as the 鈥渇attest鈥� cat at the time. IBM鈥檚 cat, at twice the size, is therefore a veritable heavyweight champion.
The IBM team also made its GHZ state 鈥渓ogical鈥�, which means it could detect errors in the qubits during computation. This is important because one of the biggest obstacles that today鈥檚 quantum computers face on the path to becoming truly useful is that they simply make too many errors.
I once visited one of IBM鈥檚 quantum computing facilities. Instead of a steel chamber, I found myself standing inside what was essentially a very large fridge, which would usually contain a quantum computer because these devices don鈥檛 work unless they鈥檙e super cold. It鈥檚 a less ridiculous image than Schr枚dinger imagined, though I was still in a chamber of sorts.
Schr枚dinger died in 1961, two decades before the first Conference on the Physics of Computation where Paul Benioff and Richard Feynman separately argued that not only could quantum computers be built, but they may be necessary if we are ever to fully understand the quantum realm. All these years later, Schr枚dinger鈥檚 idea still crowds the thoughts of today鈥檚 quantum physicists and engineers. The quite ridiculous cat found a way to stay alive after all.
