Google has a useful quantum algorithm that outperforms a supercomputer

This system is a small molecule in a nuclear magnetic resonance (NMR) device. In a second draft paper published on arXiv later today, Google collaborated with a broad group of NMR experts to explore this use.

From computers to molecules

NMR is based on the fact that the nucleus of each atom has a quantum property called spin. When nuclei are 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 infer structural details, such as the distance between two given atoms. But as molecules grow larger, these spin networks can extend over greater distances and become increasingly complicated to model. NMR has therefore been limited to focusing on relatively close spin interactions.

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 the molecule with a specific isotope of carbon (carbon-13) at a known location on the molecule. This isotope could be used as the source of a signal propagating through the spin network formed by the atoms of the molecule.

“The OTOC experiment is based on an many-body echo, in which the polarization initially localized on a target spin migrates through the spin network, before a time reversal of Hamiltonian design refocuses on the initial state,” the team wrote. “This refocusing is sensitive to disturbances on the distant spins of the butterflies, which makes it possible to measure the extent of polarization propagation through the spin network.”

Naturally, something this complicated needed a catchy nickname. The team developed TARDIS, or Time-Accurate Reversal of Dipolar InteractionS. Although this name reflects the “out of temporal order” aspect of OTOC, it is simply a set of control pulses sent to the NMR sample that triggers a disruption of the molecule’s nuclear spin network. A second set of pulses then sends an echo back to the source.