Keeping an eye on very active Schrödinger’s cats

An illustration of Schrodinger's cats by Dr Marco Piani

A theory of multi-level quantum coherence has been developed in international research involving the University of Strathclyde.

The study is a significant step beyond the theory which allows the famous ‘Schrödinger’s cat’ to be in a quantum superposition of being dead and alive at the same time.

The team has demonstrated the use of its theory through an experiment on a four-level quantum system.

The study could help researchers understand and explore the coherent properties of quantum systems. The researchers hope the results and tools explained in their work could have far-reaching applications from the foundations of physics to new fields such as quantum biology.

The research has been published in the journal Physical Review X. 

Dr Marco Piani took part in the study at Strathclyde’s Department of Physics. He said that such complex superpositions presented a great opportunity, but also a big challenge for the researchers.

“Imagine a very active Schrödinger’s cat not only wanting to eat and play at the same time, but also wanting to climb a pole and poke a toy,” he said.

Properties

Beyond the increase in the number of possible ways to create superpositions, the team showed that the properties of such systems were fundamentally different to those of their two-state counterpart.

“Understanding coherence in multi-level systems, is much more complicated than distinguishing presence and absence of coherence, and requires a detailed study of the type and degree of coherence present in the system,” Dr Piani said.

The researchers developed the tools to tell the difference between a cat that is truly doing all four things at once, and one that does `only’ two at a time, while some other times choosing the other options.

The team experimentally demonstrated their results on a single particle of light, which in their experiment can be in any superposition of four distinct states.

The study was led at the University of Queensland and also involved the University of Nottingham.