Zap iodine molecules with laser light and they will split into two quantum “cat states.” (Image: SLAC National Accelerator Laboratory)

The paradox of Schrödinger’s cat—in which a quantum cat is both alive and dead at the same time until we check to see which state it’s in—is arguably the most famous example of the bizarre counter-intuitive nature of the quantum world. Now, Stanford physicists have exploited this feature weirdness to make highly detailed movies of the inner machinery of simple iodine molecules.


In a new paper posted to the arXiv, and accepted for publication by Physical Review Letters, the physicists report they’ve used X-ray laser light to to capture details as small as the width of an atom, lasting just 30 millionths of a billionth of a second.

For decades after it was first proposed, Schrödinger’s cat was just a morbid thought experiment designed to illustrate the absurd implications of quantum mechanics. But in 2005 physicists at the National Institute of Standards and Technology successfully created an actual “cat state” in the laboratory out of six atoms all in simultaneous “spin up”and “spin down” states. Think of it as spinning clockwise and counter-clockwise at the same time. Since then, other physicists have created their own large cat states with photons.


Image: SLAC National Accelerator Laboratory

For this new experiment, the Stanford scientists zapped a two-atom molecule of iodine with an optical green laser. Absorbing that sudden burst of excess energy causes the molecule to split into two. One version is in an excited state, and the other is not. Both states existing simultaneously. Actually, if you take any group of molecules and zap them with a laser, you’ll see the same phenomenon. But you won’t be able to see what’s going on very clearly.



That’s the significance of this latest work. Once they had their cat state, the Stanford team followed up with a second burst of X-ray light. That light scattered off both versions of this excited-and-not-excited molecule, and then recombined to create what was essentially an X-ray hologram of concentric rings. It took a bit of additional clever processing to refine the details, but they ended up with a series of snapshots of the molecule at various points in time. And they were able to string those snapshots together to create a stop-motion movie.

Based on millions of individual images, the final movie captures all the possible ways an iodine molecule behaves when zapped with an x-ray laser.


“We see it start to vibrate, with the two atoms veering toward and away from each other like they were joined by a spring,” co-author Phil Bucksbaum, of Stanford University and SLAC National Accelerator Laboratory, said in a statement. “At the same time, we see the bond between the atoms break, and the atoms fly off into the void. Simultaneously, we see them still connected, but handing out for awhile at some distance from each other before moving back in.”

Finally the vibrations taper off. The entire process takes mere trillionths of a second.


Image: J.M. Glownia et al/Physical Review Letters.

Even better, the technique can be applied retroactively—say, to data collected by past experiments in quantum biology. This emerging field seeks to identify quantum effects in living systems, such as the small molecules involved in photosynthesis, migratory bird navigation, or vision.


[arXiv]