Let us suppose to be playing heads-or-tails, and we want to know whether the coin we are using is fair or not: wouldn’t it be great to get a good grasp of it just at the first toss, instead of tossing it multiple times and counting the “heads” and “tails” events?

If we had a quantum coin, instead of a normal one, this seemingly impossible task would become reality thanks to the first experimental implementation of a novel quantum measurement protocol called “Protective Measurement”. The experiment, recently published on Nature Physics [doi: 10.1038/nphys4223], was realized in the INRIM laboratories in collaboration with researchers of Polytechnic of Milan, University of Tel Aviv and University of Bristol.

In this protocol, the particle (our quantum coin) under test weakly interacts with the measurement device, and at the same time it is protected from disturbance arising from the measurement process, extracting with good precision the parameter to be estimated (the “fairness” of our coin) while keeping the particle substantially unperturbed.

Keeping our heads-or-tails parallel, usually we would test the fairness of our coin by tossing it, stopping the flipping coin with our hands (strong interaction, disturbing our system) and reading our heads or tails result; by repeating this sequence multiple times, an average of heads and tails outcomes would tell us whether our coin is fair or not.

On the contrary, a Protective Measurement equivalent would be tossing the coin in air once and having a device that gradually makes it flip more and more slowly until, if the coin is fair, we see it stopping exactly in the vertical position.

It’s like obtaining an average value from a single event instead of a whole ensemble, something in line of principle unachievable.

In the end, such measurement protocol constitutes a disruptive paradigm shift in the framework of quantum mechanics, and the advantages it presents could have significant repercussions both for fundamental science and for several applications, e.g. quantum metrology and other quantum technologies.