“Absolute zero” isn't just cold — it's still. It is the point at which the motion of the atoms that make up an object stops completely, when the object has no energy left to give.
The rules of physics say it's impossible to cool an object to absolute zero, to remove all thermal energy until its atoms come to a standstill. But researchers at the National Institute of Standards and Technology are getting really close. In a paper published Wednesday in the journal Nature, they describe using a laser to make a microscopic aluminum drum colder than anything like it has been cooled before. In doing so, they defied the quantum limit for supercooling mechanical objects.
The new technique will allow physicists to make stuff colder than previously thought possible, said lead author John Teufel, a physicist at the NIST facility in Boulder. It opens the door to building instruments of unprecedented sensitivity, and to understanding quantum mechanics — one of physics's most mysterious branches — better than ever before.
Gives you chills, doesn't it? (Sorry, guys, I had to.)
Recall that everything in the universe is in motion. Not just on broad scales — planets orbiting suns, galaxies turning — but on the smallest ones. Even the most seemingly impassive object buzzes with internal activity. The atoms that make up a piece of aluminum, like the one in Teufel's experiment, are always bumping into and bouncing off one another, jumping, spinning, spreading out and pressing together. This is what physicists are referring to when they talk about temperature: not some abstract idea of an object's “warmth” but a measure of the thermal motion of its atoms.
Physicists are interested in supercooling for two reasons. First, if you can remove the thermal energy from an object, it becomes much more sensitive to outside perturbations. Researchers like those at LIGO — the lab that detected gravitational waves last year — want their instruments to be as cool as possible so they can be sure that any tiny fluctuations are a result of vast cosmic forces, and not just boring thermal motion.
In addition, eliminating the distraction of an object's thermal motion allows scientists to finally see the motion that results from quantum energy, which is much more interesting. It will give insight into the forces that dictate how the universe functions at atomic and subatomic scales.
Researchers have managed to cool individual atoms and even a quantum gas until they near or sink below absolute zero. But supercooling larger, solid objects — which will be essential to building better instruments and understanding quantum mechanics at a macroscopic level — has proved harder.
The best technique for removing thermal energy from objects is called sideband cooling. It uses an array of lasers to slow the atoms down. This may seem counterintuitive — we're used to light warming things up, like the sun — but in sideband cooling, the carefully calibrated angle and frequency of the light allows photons to snatch energy from the atoms as they interact.
“If you shine exactly the right light in the right way you can make sure the light is always pushing against the motion of the atoms,” Teufel said. (For an in-depth explanation of the process, see this great video from PBS.)
Scientists have been cooling atoms with lasers for several decades, but there was a limit to how cold they could get. Quantum mechanics tells us that's because of the way light works. Instead of flowing in a continuous stream, it travels in discrete packets, called quanta. Each packet “gives a little kick” as it arrives, Teufel said, meaning a little bit of heat gets added even as you remove energy over all. It's like trying to keep a leaf suspended in the air with several sputtering hoses — every time the stream falters, the leaf drifts.
Using sideband cooling, researchers at NIST had previously cooled their quantum drum — a microscopic aluminum membrane that vibrates like a drumhead — to its lowest energy “ground state.” At that point, the drum's thermal motion was one-third the amount of its quantum motion. Some thought this represented the “quantum limit” — the coldest temperatures that could be achieved according to the laws of quantum mechanics.
“The limit of how cold you can make things by shining light on them was the bottleneck that was keeping people from getting colder and colder,” Teufel said. “The question was, is it fundamental or could we actually get colder?”
He had a hunch that colder was possible, if scientists could eliminate the “kicks” from the packets of light.
To do this, Teufel and his colleagues “squeezed” their lasers, using a special kind of superconducting circuit to produce a light beam in which the quanta were forced to follow one another in orderly fashion. This didn't eliminate all of the “kicks” from the lasers, but it got rid of a lot. When the scientists tried again to cool their drum with squeezed light, they got it so that thermal motion was one-fifth the magnitude of quantum motion. That's a million times colder than room temperature, 10,000 times colder than the vacuum of space, and colder than any object like this has been before.
Now that it's proven to work, Teufel says the technique can be refined to get objects even colder — maybe even as cold as absolute zero.
“In principle if you had perfect squeezed light you could do perfect cooling,” he said. “No matter what we’re doing next with this research, this is now something we can keep in our bag of tricks to let us always start with a colder and quieter and better device that will help with whatever science we’re trying to do.”