Physicists at Harvard have built a radio receiver out of building blocks the size of two atoms. It is, almost certainly, the tiniest radio receiver in the world.

And since it's a radio, it can play whatever you want to send its way, including Christmas music, as this video by the Harvard team that designed it makes clear:

Electrical engineering professor Marko Loncar and graduate student Linbo Shao applied basic radio engineering principles to a very small-scale machine.

As Leah Burrows, spokeswoman for Harvard's John A. Paulson School of Engineering and Applied Sciences, explains:



"Radios have five basic components: a power source, a receiver, a transducer to convert the high-frequency electromagnetic signal in the air to a low-frequency current, a tuner, and a speaker or headphones to convert the current to sound."



With those five components as a starting point, let's consider the inner workings of the tiny radio, where there is a diamond crystal made of carbon atoms.

The researchers replace some of those carbon atoms with nitrogen atoms, and leave a hole next to each one. That nitrogen atom/hole pair, called a nitrogen-vacancy center, basically creates the first two parts of the radio: the power source and the receiver.

A green laser pointed at the nitrogen-vacancy center excites the electrons in the diamond. That's the power.

When a radio wave hits those excited electrons around the nitrogen-vacancy center, it's converted into red light. That's the receiver. It's also one of the reasons nitrogen-vacancy centers are so compelling as a building block for tiny machines — they are natural light emitters.

An electromagnet near the receiver can change the frequency to which the receiver is sensitive. That's the tuner.

But at that point, your "radio" is just a glowing red light. It still hasn't made any sounds.

For the last step, a common device called a photodiode converts the red light back to an electrical current, and a speaker or pair of headphones grabs that current and broadcasts it as sound.

And voila: Christmas music, if that's what you choose to play through your tiny radio.

The team published its work in the journal Physical Review Applied. Loncar and Shao are not the first to use nitrogen-vacancy centers for small-scale engineering. Other research groups are working on harnessing the natural light-emitting ability of the diamond imperfections to create quantum computers.

One benefit of a radio so small you can barely see it is that the machine works at extreme temperatures. The Harvard team tested its radio at up to 662 degrees Fahrenheit.

"Diamonds have these unique properties," Loncar said in a Harvard announcement of the findings. "This radio would be able to operate in space, in harsh environments and even the human body, as diamonds are biocompatible."



Copyright 2018 NPR. To see more, visit http://www.npr.org/.

STEVE INSKEEP, HOST:

Some other news now - a reminder that, even in unsettled times, people continue with discoveries and inventions. NPR's Rebecca Hersher has been listening to the world's tiniest radio receiver.

REBECCA HERSHER, BYLINE: This song is just about two minutes long, so let's see if we can understand how they built the radio that's playing it before the song ends.

(SOUNDBITE OF SONG, "IT'S THE MOST WONDERFUL TIME OF THE YEAR")

ANDY WILLIAMS: (Singing) It's the most wonderful time of the year.

HERSHER: Here is Marko Loncar, the lead researcher on the project.

MARKO LONCAR: It's pretty cool because it's diamond radio.

HERSHER: The diamond inside the radio has little imperfections, where pairs of carbon atoms are missing.

LONCAR: And we replace one carbon atom with nitrogen.

HERSHER: And next they each nitrogen atom, there's a little hole full of electrons. And that hole has a special property. When you shine a green laser at the hole, it glows red.

LONCAR: Which is pretty cool. So imagine having an engagement ring that, you know, you can shine a laser at and glows in the dark.

HERSHER: And the more excited the electrons in the diamond hole get, the brighter the red light. So here's how this little radio works - radio waves are broadcast from somewhere - say, your local NPR station. They pass through the diamond and cause the red glow to change, convert that light to audio and there you go. It's a radio receiver.

LONCAR: So, you know, we are scientists, so first we played the "Star Wars" theme. We kind of like that.

HERSHER: OK, time out from the two-minute Christmas song so we can hear some of that.

(SOUNDBITE OF JOHN WILLIAMS' "STAR WARS MAIN THEME")

HERSHER: And we're back.

(SOUNDBITE OF SONG, "IT'S THE MOST WONDERFUL TIME OF THE YEAR")

WILLIAMS: (Singing) It's the most...

HERSHER: So they transmit the Christmas or "Star Wars" song as radio waves, and the diamond picks it up.

LONCAR: I played it for my son. He has very high standards apparently (laughter), so he said, well, it's crackling a bit, but it's OK. So it is true that, you know, it's not super hi-fi yet.

HERSHER: Loncar thinks that's a problem with how they amplify the sound, among other things. As for what these might be used for in the future, maybe communicating in space. These tiny diamond radios are really tough. They can withstand temperatures of up to 620 degrees. And that was just about two minutes.

Rebecca Hersher, NPR News. Transcript provided by NPR, Copyright NPR.