https://phys.org/news/2018-11-griffith-precision-limit.html

The researchers combined three techniques—entanglement (a kind of quantum connection that can exist between the photons), passing the beams back and forth along the measurement path, and a specially-designed detection technique.

"Every time a photon passes through the sample, it makes a kind of mini-measurement. The total measurement is the combination of all of these mini-measurements," said Griffith's Dr. Sergei Slussarenko, who oversaw the experiment. "The more times the photons pass through, the more precise the measurement becomes.