Evrim Yazgin
Cosmos science journalist
Engineers have come up with a way that the famous thought experiment, “Schrödinger’s cat”, could be used to detect errors in quantum computers.
The thought experiment was derived by Austrian-Irish physicist Erwin Schrödinger. In 1925, he came up with the Schrödinger equation which describes how particles behave in quantum mechanics.
Just 10 years later, Schrödinger came up with his metaphoric cat conundrum to describe quantum mechanical superpositions. A superposition refers to the fact that a quantum particle exists in a multitude of physical states at the same time, only “collapsing” into one of those states when a measurement is taken.
The metaphoric and unfortunate cat in the thought experiment is in a box with a vial of toxic gas. Above the vial is a hammer. The hammer will fall, releasing the gas and killing the cat, if its trigger mechanism is activated. This mechanism is connected to a Geiger counter which measures radioactive decay coming from a small radioactive sample, also in the box.
If the Geiger counter registers a particle emitted due to radioactive decay – a random process – the cat will perish.
For an observer looking at the box, it is impossible to know whether the cat is alive or dead unless the box is opened (i.e. a measurement is taken). Before the box is opened, the cat is in a superposition of states – it is both dead and alive – according to the observer. It seems weird given our everyday experience, but this is the nature of the quantum world.
Now a team at the University of New South Wales (UNSW) in Sydney, has applied this fundamental principle of quantum mechanics to the development of quantum computers.
“No one has ever seen an actual cat in a state of being both dead and alive at the same time, but people use the Schrödinger’s cat metaphor to describe a superposition of quantum states that differ by a large amount,” says team leader UNSW Professor Andrea Morello and an author of the research paper published in Nature Physics.
“In our work, the ‘cat’ is an atom of antimony,” says Xi Yu, lead author of the paper.
“Antimony is a heavy atom, which possesses a large nuclear spin, meaning a large magnetic dipole,” Yu adds. “The spin of antimony can take 8 different directions, instead of just 2. This may not seem much, but in fact it completely changes the behaviour 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 2 branches of the superposition.”
This can be used by those working on quantum computers.
“Normally, people use a quantum bit, or ‘qubit’ – an object described by only 2 quantum states – as the basic unit of quantum information,” says co-author Benjamin Wilhelm.
“If the qubit is a spin, we can call ‘spin down’ the ‘0’ state, and ‘spin up’ the ‘1’ state. But if the direction of the spin suddenly changes, we have immediately a logical error: 0 turns to 1 or vice versa, in just one go. This is why quantum information is so fragile.”
Because antimony has 8 spin directions, a single error doesn’t completely scramble the quantum information.
“As the proverb goes, a cat has nine lives. One little scratch is not enough to kill it. Our metaphorical ‘cat’ has seven lives: it would take seven consecutive errors to turn the ‘0’ into a ‘1’!” explains Yu.
“If an error occurs, we detect it straight away, and we can correct it before further errors accumulate,” Morello says.
The antimony cat is embedded inside a silicon quantum chip.
“By hosting the atomic ‘Schrödinger cat’ inside a silicon chip, we gain an exquisite control over its quantum state,” says Dr Holmes. “Moreover, 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.”
