Throw a stone into a pond, and ripples spread across the water. Throw two stones nearby, and the ripples intersect, creating a pattern of reinforced and canceled waves. That's interference. In the quantum world, waves of probability play the role of these ripples, and a particle's ability to produce interference is called coherence. But how do you measure this ability if any measurement itself destroys the pattern, as if you tried to photograph the ripples on water by hitting it with an oar?
The authors of the new study found a workaround. Instead of measuring coherence directly, they propose subjecting the particle to random momentum kicks, as if an invisible drummer were beating a rhythm on the probability waves. After such kicks, the interference pattern smears, and the particle loses some of its coherence. By how much the pattern degrades, you can infer how much coherence the particle originally had. This method is akin to assessing visual acuity not by reading an eye chart, but by how quickly a person starts squinting in slightly hazy air.
The researchers built a mathematical framework that quantifies coherence loss. They introduced two measures: one based on the concept of information entropy, which behaves like an ideal resource: it cannot be increased by free operations. The second measure is easier for experimentalists but less rigorous theoretically. To test their theory, they calculated how a Gaussian wave packet (a bell-shaped probability cloud) falling in a gravitational field would lose coherence. It turns out that even in such a simple scenario, coherence decays predictably, offering hope for experimental verification.
This approach resembles the work of a translator who doesn't listen to every word but watches how the meaning of a phrase changes under interference. If a quiet whisper is drowned out by noise, the original signal was weak. Similarly, here: the loss of coherence from random kicks reveals how ready the particle was to interfere. The practical benefit of this method lies in creating more reliable quantum devices, where coherence must be maintained like a candle flame in the wind.
In everyday life, we encounter coherence loss when listening to the radio in a car: as you drive away from the station, the music starts to hiss — random disturbances break the wave's coherence.
In Hannu Rajaniemi's novel 'The Quantum Thief,' coherence is used as a currency, and its loss threatens the disintegration of personality — much like in this work, where coherence acts as a resource.