The tool doesn’t track people’s identities or locations but uses Bluetooth wireless signals to detect when two phones are within 6 feet (1.8 meters) of each other for at least 15 minutes, officials said. Newsom said it will be ready for use on Thursday.

“The process is private, anonymous and secure, and is one of the many tools in the state’s data-driven approach to help reduce the spread,” Newsom, a Democrat, said in a statement.

Sixteen other states, plus Guam and Washington, D.C., have already made available the system co-created by Apple and Google, though most residents of those places aren’t using it. The tool has been used on a pilot basis on University of California campuses to try to track virus cases.

The announcement came as coronavirus cases are exploding in California and more than 80 percent of the state’s residents are under orders not to leave their homes for at least the next three weeks except for essential purposes.

Newsom’s administration issued the stay-at-home rules shuttering restaurant dining, salons and playgrounds in Southern California and a large swath of the state’s Central Valley agricultural region after more than 85% of beds in intensive care units were occupied in those regions.

Five San Francisco Bay Area counties voluntarily joined the rules, saying they didn’t want to wait until their ICU capacity dropped too low to take action. Those restrictions will last until Jan. 4, a week longer than the state’s timeline.

Ten months into the pandemic, most of the state is now back to where it started with the stay-at-home rules.

But unlike in March, when the pandemic was in its infancy and California was the first state to impose such rules, fewer people are likely to obey them. Several law enforcement agencies have said they have no plans to enforce them and are counting on people to voluntarily wear masks and practice physical distancing.

California on Sunday announced more than 30,000 new coronavirus cases — the most ever in a single day. More than 9,700 people were hospitalized because of COVID-19, including more than 2,200 in intensive care. The state had just over 1,500 ICU beds available as of Sunday.

Californians will be able to activate the tool in their iPhone settings or on Android phones by downloading the CA Notify app from the Google Play store. Many residents will get a notification Thursday inviting them to participate.

The encounters are temporarily logged in a way that doesn’t reveal a person’s identity or geographic location.

If people who have activated the technology test positive for the virus, they get a verification code from the California Department of Public Health and can use that to send an anonymous alert to other participating phone users they might have exposed over the past 14 days.

The recent rise in infections began in October and is being blamed largely on people ignoring safety measures and socializing with others.