For the first time, physicists have experimentally observed a first-order phase transition occur in a quantum system - verifying years of theoretical predictions.

Phase transitions are something that we see on a daily basis, when our ice melts into water, or steam evaporates from a boiling kettle. While these transitions are easy for us to observe, phase transitions also happen on the very tiny, quantum scale, where they play an important role in physics. But, up until now, no one had ever witnessed one experimentally.

In this case, a team of researchers from the Institute of Science and Technology, Austria, has observed a quantum system flip from opaque to transparent and back again, a phase transition called a photon-blockade breakdown that was only predicted for the first time two years ago.

"We have observed this random switching between opaque and transparent for the first time and in agreement with theoretical predictions," said lead researcher Johannes Fink.

Now that we've finally seen this type of quantum phase transition happening, scientists will finally be able to study them in more detail, and hopefully get better control over quantum systems in the future.

So what is a photon-blockade breakdown?

Phase transitions are controlled by the changing of one parameter - in the case of melting ice, that parameter is temperature. Once a system gets above 0 degrees Celsius (32 degrees Fahrenheit) then the ice melts.

A photon-blockade breakdown transition is different - temperature doesn't play a role, but the amount of power pumped into the system does.