2 Photon Quantum Interference In A Paint Drop

Light is scattered within a drop of paint so frequently that it would seem impossible to demonstrate quantum effects. But despite the thousands of possible paths the light can take, like a water molecule in a waterfall, researchers of the University of Twente have demonstrated that there are only two exits.

Depending on the light pattern that enters the paint, two photons always come out through the same exit, or through different ones, almost as though they avoid each other. Scientists at UT’s MESA+ Institute for Nanotechnology published the findings in Physical Review A.

The majority of the experiments showing that light sometimes behaves like a wave and sometimes like a particle, are as basic as possible. One physics textbook example is Young’s two slit experiment. The number of possible light paths is limited, but even at this level, the experiments strongly challenge our intuition.

Nevertheless, Tom Wolterink and colleagues at the University of Twente, prove that two photons falling on a drop of paint, can just leave through two possible exits.

According to the so-called Hong-Ou-Mandel effect, the photons come out through one of the two exits at the same time. Which one that will be, cannot be predicted beforehand.

It could be compared to sending two drunk people into a labyrinth over and over, and having them always leave through one door arm in arm.

The researchers did this by programming the light in a very clever way. It is possible to influence scattering and interference inside the paint, limiting the number of exits to just two, even when there are thousands of possible paths.

Programming the incident light in a slightly different way, results in the opposite. The number of exits is still two.

But if photon one leaves through one exit, photon two always leaves through the other. Which one will go through which exit is not known, but the photons always seem to avoid each other: a counterintuitive result again, because weak light pulses don’t ‘feel’ each other’s presence.

Study: Programmable two-photon quantum interference in 10^3 channels in opaque scattering media