Features of the famous Schrödinger’s Cat thought experiment have been applied in the real world using a single antimony atom embedded in a silicon chip. Surprising as it may seem, the atom has a more complex quantum life than even the theoretical cat manages. Whether it can match the endless variation of real cats remains unmeasured.
Physicist Erwin Schrödinger proposed the thought experiment for which he is now best known in an effort to show why he considered quantum mechanics ridiculous, despite having greatly contributed to its invention. If quantum mechanics was right, he argued, it would be possible to create a situation where a cat was simultaneously alive and dead until we opened a box in which it was placed. Whether this could apply to actual cats is still debated, but at the subatomic scale, superposition – where an object can exist in two states at once – is an accepted fact.
The potential of quantum computers lies in harnessing this capacity, allowing a superposition of one and zero to replace a traditional computer bit that can only be a one or a zero. Vast resources have been poured into building quantum computers, but numerous obstacles have caused progress to lag far behind what has frequently been promised.
A major impediment is how frequently quantum computers produce errors because some random process flips the state of a component, known as a qubit. Particles’ spins make popular qubits to store information, with spin up representing one state and spin down the other. However, noise that causes a spin reversal can disrupt the data and whatever is done with it.
To address this, a team led by Professor Andrea Morello of the University of New South Wales turned to antimony atoms, which allow the creation of what are known as high-dimensional quantum systems.
“Antimony is a heavy atom, which possesses a large nuclear spin, meaning a large magnetic dipole. The spin of antimony can take eight different directions, instead of just two,” lead author Xi Yu said in a statement. “This may not seem much, but in fact it completely changes the behavior of the system. A superposition of the antimony spin pointing in opposite directions is not just a superposition of ‘up’ and ‘down’, because there are multiple quantum states separating the two branches of the superposition.”
A superposition of eight different states sounds like the sort of chaos only a kitten let loose on infinite balls of string could create, but the team think it will instead be more robust. Specifically, they hope it will allow the identification of errors that creep in and the prevention of new ones.
“A single, or even a few errors, do not immediately scramble the information,” Morello said. Stretching the cat metaphor to a remarkable degree, he continued; “It’s as if we saw our cat coming home with a big scratch on his face. He’s far from dead, but we know that he got into a fight; we can go and find who caused the fight, before it happens again and our cat gets further injuries.”
Benjamin Wilhelm, Xi Yu, Andrea Morello, and Danielle Holmes with four cats that are definitely alive, and will never be experimented on like that.
Image Credit: Lee Henderson
Error-detecting quantum computers have captured the imagination of physicists every bit as much as that bag of food on an unreachable shelf has done to a hungry housecat. Most approaches have been built around entangling several particles, but the authors think the higher dimensionality offered by antimony makes a better approach.
The quantum equivalent of finding the feline neighborhood bully and scaring it away is still just a hope, but the team are making it their next goal. They say up to three flip errors can be corrected in an antimony qubit unlike in lower dimensions.
Two papers announcing similar results with higher dimensional qubit states based on different materials were submitted for review while the team were working on their paper.
Nevertheless, the authors think their design has advantages in forming the basis of much larger systems other models lack.
“Hosting the ‘cat’ in silicon means that, in the long term, this technology can be scaled up using similar methods as those we already adopt to build the computer chips we have today,” said Dr Danielle Holmes.
On the other hand, claims quantum computers are just around the corner have been made for at least 25 years, and their presence is as elusive as the other feline frequently associated with quantum behavior, Lewis Carroll’s Cheshire Cat.
The study is published in Nature Physics.
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