China has sent an “unbreakable” code from a satellite to the Earth, marking the first time space-to-ground quantum key distribution technology has been realized, Chinese media has reported.

China launched the world’s first quantum satellite last August, to help establish “hack proof” communications, a development the Pentagon has called a “notable advance”.

The official Xinhua news agency said the latest experiment was published in the journal Nature on Thursday, where reviewers called it a “milestone”.

The satellite sent quantum keys to ground stations in China between 645 km (400 miles) and 1,200 km (745 miles) away at a transmission rate up to 20 orders of magnitude more efficient than an optical fiber.

Xinhua cited Pan Jianwei, lead scientist on the experiment from the state-run Chinese Academy of Sciences, as saying. “That, for instance, can meet the demand of making an absolute safe phone call or transmitting a large amount of bank data,” Pan said.

Any attempt to eavesdrop on the quantum channel would introduce detectable disturbances to the system, Pan said. “Once intercepted or measured, the quantum state of the key will change, and the information being intercepted will self-destruct,” Xinhua said.

China still lags behind the United States and Russia in space technology, although President Xi Jinping has prioritized advancing its space program, citing national security and defense.

China insists its space program is for peaceful purposes, but the U.S. Defense Department has highlighted its increasing space capabilities, saying it was pursuing activities aimed at preventing adversaries from using space-based assets in a crisis.