I wonder, why has it taken this long to be able to try enabling this advance? I would think, real fuel injection for jets and not just the same afterburner stuff we've had for decades, would be already in the bag.
First, inventing an afterburner is a cinch compared to a fuel injector, especially one with "active" controls that precisely sense the measure of fuel needed at any given moment.
But the biggest reason for the delay is because the military customer has never asked for it -- until now, that is.
Engine companies still like to make money, and that means they do what the customer tells them to do. Until very recently, that meant finding ever-increasing ways to make military jet engines more powerful. The embodiment of that trend is the Pratt & Whitney F135, the 40,000lb-thrust monster powering the F-35 Joint Strike Fighter.
Efficiency hasn't been completely ignored in the process, but nor has it been embraced.
Back in the late-1980s, the air force started up a program called Integrated High Performance Turbine Engine Technology (IHPTET), with a major goal to improve fuel efficiency by a wide margin. Many study and design contracts followed, but when the JSF program decided to go primarily with the F135, which is just an improved version of the F-22's F119, the IHPTET community was forced to wait a few more years.
Then comes along the next-generation long range strike concept, and, voila, the military suddenly needs both a super-powerful and a super-efficient kind of engine. Although the IHPTET program no longer exists, it has provided the baseline for continued research on improving engine efficiency. IHPTET's successor is another program with an equally awkward name: Versatile Affordable Advanced Turbine Engines (VAATE). Under this new funding account, a new program exists called Project Advent (an acronym for Advanced Versatile Engine Technology), and this is the likely demonstration vehicle for the active combustion control concept.
Hope that answers your question, Neil.