Upper limit found for quantum world (article in PRL by Leiden PI Tjerk Oosterkamp et al.)


The small quantum world and our world of perception obey different laws of nature. Leiden physicists search for the border between both worlds. Now they set an upper limit in Physical Review Letters. Publication on February 24th.

The laws of nature in the domain of quanta do not apply to our everyday lives. We are used to assign an exact location and time to objects. But fundamental particles can only be described by probability distributions. As if the police writes out speeding tickets for driving 30 to 250 km/h somewhere between Paris and Berlin, with a probability peak for 140 km/h in Frankfurt.

Because the laws are completely different in both worlds, a clear boundary might exist between them. Then you would want to determine per object whether it obeys quantum or macroscopic laws, depending on size and mass. Still, it has been a mystery where this frontier lies exactly. Leiden physicist Tjerk Oosterkamp and his research group establish an upper limit, closing in on the answer.

‘We keep excluding values, so that we slowly close in on the boundary’s location,’ says Oosterkamp. ‘If we only have a small area left, we can better design our experiments to see what is happening at the edge of the quantum world.’

According to a certain quantum mechanical model, you can describe a particle’s position with a probability distribution that sometimes spontaneously ‘collapses’. In that case its position is indeed determined precisely, within a certain margin. This margin and how often the spontaneous collapse occurs, form the two parameters that physicists are after. If they find those, they have a complete formula to define a strict border between quantum and macro.

Now Oosterkamp has determined an upper limit on these parameters of 31 collapses per year per atomic mass unit with a margin on the location of 10 nanometers, to 1 collapse per 100 years with a margin of 1 micron. For their next measurement they expect fewer collapses, so they can define an even stricter upper limit.

The red line shows the upper limit for parameters rc (margin on location) and λ (frequency of wave function collapse per atomic mass unit). The black dotted line gives the expectation for Oosterkamp’s next experiment. The purple ball estimates the values based on the fact that electrons definitely behave quantum mechanically around large molecules. In the end, the goal is to found the values for both parameters. This completes the formula for the boundary between the quantum and macroscopic world. For a given size and mass, this formula tells which laws apply to the object.