Measuring a Quintillion Photons at the Speed of Light

An individual could travel around the world more than seven times in a single second if moving at the speed of light. That’s pretty fast.

A beam of light used in physics professors Michael Ware and Justin Peatross’s quantum-physics experiment can contain 1,000,000,000,000,000,000 photons worth of energy. That’s a lot of photons.

Measuring the number of photons in a beam of light and other similar tasks are incomprehensible to most people, but Ware makes his living studying them. And he loves it.

“I’m just fundamentally interested in how the world works,” Ware said. “It’s fundamental physics with an emphasis on fun.”

It might not sound like fun to everyone, but it’s impressive nonetheless. And yet, Ware remains humble about it.

“We sit in dark rooms and count photons,” Ware said.

Ware and Peatross have recently been counting photons to understand how they scatter when interacting with electron wave packets—a common light interaction.

“You can think of a [photon] as a particle of light, although that’ll give you some bad intuition,” Ware said.

In addition to acting like a particle, a photon also exhibits wave-like properties. Essentially, it’s the basic unit of light.

But what about an electron wave packet?

An electron’s position and motion in space and time can be described using a wave function, a quantum-mechanical model. The wave functions are distributions of probability, Ware explained.

“There is a 100 percent probability that the electron is somewhere [within the wave packet], but it’s not at a specific place when it’s just being an electron,” Ware said. “Quantum mechanics says [a wave packet] just simply represents the probability.”

When a photon hits an electron wave packet, the photon scatters.

“And so the question is, If you have this big, diffuse electron, does the photon just go right through, or does it still scatter?” Ware said.

John Corson, a graduate student who worked with Ware and Peatross several years ago, aimed to find the answer to this question theoretically. Corson suggested the size of the wave packet doesn’t affect how strongly it scatters light.

“If you have an electron that’s tiny and you have a photon that comes along and scatters off of it, it’ll scatter with a certain strength,” Ware said. “And if that electron expands so you don’t know where it is—the wave packet is big and diffuse—a photon will scatter with the same strength, which is not really intuitive if you think about it.”

When Ware asked other high-intensity laser physicists this question, they often responded that the scattering should decrease as the wave packet size increased.

“Most people in the field would have said, ‘Well, as the electron wave packet gets larger, then it will scatter less. It won’t be able to scatter photons as well,’” Ware said.

Their answers can be explained by thinking of a small, compact ball of powder and one that has been thrown up in the air and dispersed. If something was fired at each of those, which is most likely to scatter?

“Your intuition would say you should be able to scatter better by hitting the small, dense thing rather than this big, huge diffused thing,” Ware said. “Turns out that’s not how electrons behave. They’re just as good to scatter when they’re tiny as when they’re big.”

Peatross proposed an experiment to verify this theory. He and Ware let an electron wave packet expand in a chamber. They then periodically hit it with a laser pulse, measuring the scattering of the photon.

“This is a hard measurement to make because we want to detect one photon in the presence of a billion billion photons around,” Ware said.

Not only were they detecting one photon out of a quintillion, they also had an insanely short amount of time to do so.

“So light is fast, but it’s not infinitely fast,” Ware said. “It’s still within what we can measure.”

It took 2 ½ nanoseconds for a single photon to travel to the detector at the top of the chamber where Ware and Peatross conducted their experiment. For reference, a single nanosecond is to one second as one second is to thirty-one years.

“The signal we want would come out, and then three nanoseconds later, all heck would break loose, and all these photons would come through,” Ware said.

Ware and Peatross confirmed through this experiment that the strength of the scatter was the same no matter the size of the wave packet, despite intuition suggesting otherwise.

“So this isn’t going to be something that’s going to build a better device, but what it does is it helps us understand how quantum mechanics works,” Ware said. “I think it’s an interesting result in the sense it demonstrates that the intuition most people have is wrong.

Professor Ware and Professor Peatross equally divided their research between the two of them. Unfortunately, Professor Peatross was unavailable for an interview.

By James Collard Posted on