What happens to photonic molecules?

[unsaint32]

TL;DR Summary

In 2013, scientists from Harvard and MIT creates never-before-seen form of matter called photonic molecule.

My source is below:
https://scitechdaily.com/harvard-mit-scientists-create-never-seen-form-matter/


In short, using rubidium atoms, photon particles (as in laser form) are bound together to form a massless molecule.. or acting like a molecule, but with a bonding structure nonetheless. Here is my question... What happens to those photon molecules? Do they exist forever bonded? Or does the bond degrade over time?

 

[TeethWhitener]

That pop sci article is a mess.

What’s actually going on is that the researchers sent photons through a medium with properties such that the photons would interact in a very specific manner with the atoms in the medium. This creates a quasiparticle called a polariton that is quite difficult to describe using a familiar layman-level analogy. But when that quasiparticle encounters another photon, the new state is actually lower in energy than the two free photons + unperturbed medium. When this happens, we call it a bound state, which is analogous in certain ways to the bound state of an electron to an atom or two atoms in a molecule.

But to answer your question, the whole experiment is fantastically delicate, so it’s not like some magic was performed to make molecules of light which are then somehow stored on a shelf. Rydberg states are remarkably long-lived for excites atoms (on the order of seconds to hours, compared with nanoseconds for run-of-the-mill excited states), but it’s not like some stable state of matter has been made. Be careful with what you read in pop sci articles. It’s usually simplified beyond the point of containing anything resembling actual science.

 

[unsaint32]

So, if the molecules of light is not somehow stored on a shelf, are you saying that the bond disintegrates and the photon particles go their separate ways? If so, I wonder how long the molecules will stay bonded? Right away? A minute? Or longer?

 

[TeethWhitener]

So, if the molecules of light is not somehow stored on a shelf, are you saying that the bond disintegrates and the photon particles go their separate ways? If so, I wonder how long the molecules will stay bonded? Right away? A minute? Or longer?

This really isn’t the best way to think about this. I’ll try to put it in as familiar terms as possible, but it’ll probably still be lacking.

When a photon enters the medium, it causes a disturbance, kind of like an eddy in the wake of a boat passing through water (but note that this still isn’t a great picture of what’s going on). That disturbance can interact with another photon to create a more complicated but lower energy disturbance. This new disturbance is the “photonic molecule” that the pop sci article refers to.

As for lifetime, the Nature paper off which the article in OP is based gives a polarization entanglement concurrence time (probably as close to what you’re asking as possible) of around 0.2 microseconds. After that point, the system decoheres—basically interactions with the environment erase the disturbance.

 

[unsaint32]

I appreciate your thorough answers. Do you think photon molecule based logic gate is possible as some scientists suggest? How can something that can only exist for 0.2 microseconds be used in a physical world?

 

[TeethWhitener]

I appreciate your thorough answers. Do you think photon molecule based logic gate is possible as some scientists suggest? How can something that can only exist for 0.2 microseconds be used in a physical world?

I have no idea how one would apply these bound photon states to problems in the real world. I know the general class of Rydberg matter has seen some interest from the quantum computing community, so maybe there are applications there.

As for the state only existing for 0.2 $\mu s$, computer RAM currently routinely operates in the GHz range on a single core. This is less than 0.001 $\mu s$. The state these researchers made is actually quite long-lived in the world of AMO physics.