How atoms jump

It was the early twentieth century, and hydrogen, simplest of atoms, had sparked an upheaval. Physicists observing its radiation noted that it released energy at discrete frequencies rather than along a smooth spectrum. This led Niels Bohr, a founder of quantum mechanics, to suggest that electrons “jumped” instantaneously and at random times between fixed energy states. Erwin Schrödinger vigorously disputed “the nightmare that physical events consist in continual sequences of little fits and jerks.” Seven decades later, jumps were observed.

“But the question remained whether these were instantaneous or not,” says Michel Devoret, a professor of applied physics at Yale. “If you could resolve the jumps temporally, what would you see?” Devoret and eight other researchers did just this. They revealed that quantum jumps don’t occur instantaneously or randomly, and it’s even possible to catch and reverse the process as it unfolds.

Using ultra-high-speed monitoring equipment, they tracked the flicker of light created prior to the moment when a superconducting circuit designed to mimic an atom’s behavior jumped between energy levels. By experimental design, when the flickering lapsed into darkness, “this was an advance warning signal that a jump was about to happen,” Devoret says. “As this lull went on, the probability of a jump became more and more likely.” Four microseconds after the flickering stopped, a jump occurred about half the time.

The team was able to freeze and image the quantum system right at midpoint during the jump. Rapidly repeating this process provided a picture “in some abstract term” of an atom’s jump from one state to another. And irradiating the system with the right energy pulse during its jump would stop the process and turn it back. Devoret compared it to a game of ping-pong: “If you put a racket in front of the ball, you reverse its path,” he says—though he was careful to add that the image “can only imperfectly describe the physics.” He paused. “I’ve seen worse.”