# Slowing down light in an opaque crystal for a whole minute

I just read about a team of physicists at the University of Darmstadt, Germany, that managed to completely slow down a beam of light that traveled through an opaque crystal (article here).

How is it possible for a beam of light come to a complete stop? In the article they mentioned that they fired a laser at the crystal causing the atoms to go into a quantum superposition. How does this affect the stopping of the light? Also if the uncertainty principle applies to photons (which I do not know if it does), how does this not violate the uncertainty principle if the photons aren't moving?

• They did not bring it to a halt. I have heard people describing transmission of light through a medium being slow because electrons absorb and re-emit the photons...I don't know the answer, but if at all it helps kindle a thought... – mehfoos Jul 26 '13 at 1:11
• Dear Reds - I added the optics and optical-materials tags. This question is about advanced optical materials science. In a very real sense (see physics.stackexchange.com/a/72293/26076) light is not slowed down at all. It is being "relayed" though a sequence of absorptions-re-emission cycles, but the light part of the wave is still travelling at speed $c$ in between these events. Beyond this explanation, you need a scientist who understands the physics of the optical materials in question to answer your question fully. – Selene Routley Jul 26 '13 at 4:05

I'm not an expert, but it seems to me that they used a magnetic field to

Step 1: Turn the field off so that the light energy 'stores itself' into a lattice of cold $^{87}\text{Rb}$ atoms.

Step 2: Turn the field on so that the stored energy becomes light again.

So it's not light itself that's stopping or slowing down, but they're just 'temporarily storing' the light energy in atoms. They successfully stored it for 16 seconds, not a minute, but it's still considered 'minute-scale'.

References: Dudin, Y. O., L. Li, and A. Kuzmich. "Light storage on the time scale of a minute." Physical Review A 87.3 (2013): 031801.

This does not directly answer your question, but I figured it would be useful information.

In light storage (both cold and hot) experiments, light and matter fields are coupled. A new quasi-particle called a "Dark state polariton" (Flieshhaur and Lukin) describes the conversion of the state of the system from light to a spin wave which is generated by ground state coherences (typically between hyperfine levels). There are many ways to skin this cat. The general algorithm is to prepare an atomic pure state by optical pumping, generate a spin wave during which you store light (called writing) and before the system dephases, send in a "read" laser to recover the stored light pulse. Reference 3 is a particularly beautiful experiment.

Here are a few seminal manuscripts that you may find useful.

Quantum memory for photons: I. Dark state polaritons

Dark-State Polaritons in Electromagnetically Induced Transparency

Storage of light in atomic vapor

Matthew D. Eisaman Thesis (Lukin's group)