This system represents a small molecule in a nuclear magnetic resonance (NMR) machine. In a second draft paper to be published today on arXiv, Google is collaborating with a large group of NMR experts to explore this use.
From computers to molecules
NMR is based on the fact that the nucleus of every atom has a quantum property called spin. When nuclei are located close to each other, for example when they are in the same molecule, these spins can influence each other. NMR uses magnetic fields and photons to manipulate these spins and can be used to determine structural details, such as how far apart two given atoms are. But as molecules get larger, these spin networks can spread over long distances and become increasingly difficult to model. Thus, NMR was limited to the study of interactions of relatively close spins.
For this work, however, the researchers figured out how to use an NMR machine to create the physical equivalent of a quantum echo in a molecule. The work involved synthesizing a molecule with a specific isotope of carbon (carbon-13) at a known location in the molecule. This isotope could be used as a source of a signal propagating through the network of spins formed by the atoms of the molecule.
“The OTOC experiment is based on a many-body echo in which the polarization initially localized at the target spin migrates through the spin network before a Hamiltonian-engineered time reversal is refocused to the original state,” the team wrote. “This refocusing is sensitive to perturbations in the distant rotations of the butterfly, allowing us to measure the extent to which polarization propagates through the spin network.”
Naturally, something so complex needed a memorable nickname. The team came up with the TARDIS, or time-accurate reversal of dipolar interactions. Although the name reflects the “timeless order” aspect of OTOC, it is simply a set of control pulses sent to an NMR sample that triggers a perturbation of the molecule's network of nuclear spins. The second set of pulses then reflects the echo back to the source.
