There's a handful of people working on a marijuana breathalyzer right now. Some have been at it for a few years, and others, like some researchers at Washington State University, are just starting. Per Oregon Live:
Hill said he and WSU doctoral student Jessica Tufariello are working on a handheld device that uses a technique called ion mobility spectrometry to detect THC in someone's breath.
Right now, officers and prosecutors rely on blood tests to determine how much active THC is present in a driver's blood. Those test results aren't immediately available to patrol officers who suspect someone is driving high.
There was a lot of media buzz last week when the research was announced out of Washington, with even many marijuana media outlets claiming that its going to be the future of how marijuana DUIIs are determined. However, I doubt this research, or any research involving a marijuana breathalyzer, will ever be used by officers in the field. Marijuana breathalyzers are built on junk science. Yes, they may eventually be able to detect if a person has marijuana in their system. However, they won't be able to tell what level of active THC is in a person's system, or how long ago they consumed marijuana, or most importantly if the person is impaired or not. Marijuana does not affect the system like alcohol does.