This metaphorical cat is both dead and alive – and it will help quantum engineers find computing errors

A team led by quantum engineers at the University of New South Wales – that includes two UCalgary researchers – has demonstrated a well-known quantum thought experiment in the real world.

It has created a “Schrödinger’s cat” inside a silicon chip.

Their findings, published in the journal Nature Physics, deliver a new and more robust way to perform quantum computations – and they have important implications for error correction, one of the biggest obstacles standing between them and a working quantum computer.

Quantum mechanics has puzzled scientists and philosophers for more than a century. One of the most famous quantum thought experiments is that of the “Schrödinger’s cat” – a cat whose life or death depends on the decay of a radioactive atom. 

According to quantum mechanics, unless the atom is directly observed, it must be considered to be in a superposition – that is, being in multiple states at the same time – of decayed and not decayed. This leads to the troubling conclusion that the cat is in a superposition of dead and alive.

“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 Dr. Andrea Morello, professor at the UNSW.

The research used an atom of antimony, which is much more complex than standard ‘qubits,’ or quantum building blocks.

“My dream of a spin cat state on a single isolated particle, proposed 35 years ago, has come true by exquisitely controlling a single atomic nucleus that has been isolated from the rest of the universe,” says Dr. Barry Sanders, PhD, professor in the Faculty of Science at the University of Calgary.

The latest research has profound consequences for scientists working on building a quantum computer using the nuclear spin of an atom as the basic building block.

It was the result of a large international collaboration. 

Several authors from UNSW Sydney, plus colleagues at the University of Melbourne, fabricated and operated the quantum devices. Theory collaborators in the U.S., at Sandia National Laboratories and NASA Ames, and in Canada, at the University of Calgary, provided precious ideas on how to create the cat, and how to assess its complicated quantum state.

Illustration at top of article: The ‘dead’ state corresponds to the antimony nuclear spin pointing completely downwards; the ‘alive’ state is the spin completely upwards. A superposition of the two results in a striking quantum state that displays seven quantum interference fringes. The number of fringes corresponds to the number of ‘spin flips’ necessary to go from one extreme to the other. In quantum computing, this corresponds to the number of consecutive errors required to turn a ‘0’ into a ‘1’ or vice versa.