ShareAlways closer to absolute zero
NIST physicists have cooled a mechanical object to a temperature below the so-called “quantum limit.”
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 in a paper recently published in the journal Nature*, researchers at the National Institute of Standards and Technology (NIST) in the USA claim their new technique could allow to make stuff colder than previously thought possible.
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, lead author of the paper J. D. Teufel said, meaning a little bit of heat gets added even as you remove energy over all. 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. They had a hunch that colder was possible, if they could eliminate the “kicks” from the packets of light.
To do this, they “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.
Now that it's proven to work, Teufel says the technique can be refined to get objects even colder. It opens the door to building instruments of unprecedented sensitivity – for example for measuring the deformations induced by the passage of gravitational waves – and to understanding quantum mechanics — one of physics's most mysterious branches — better than ever before.
*J. B. Clark et al. “Sideband cooling beyond the quantum backaction limit with squeezed light”. Nature Volume 541 (January 2017): 191–195. DOI: 10.1038/nature20604
See also the Washington Post article.
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, lead author of the paper J. D. Teufel said, meaning a little bit of heat gets added even as you remove energy over all. 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. They had a hunch that colder was possible, if they could eliminate the “kicks” from the packets of light.
To do this, they “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.
Now that it's proven to work, Teufel says the technique can be refined to get objects even colder. It opens the door to building instruments of unprecedented sensitivity – for example for measuring the deformations induced by the passage of gravitational waves – and to understanding quantum mechanics — one of physics's most mysterious branches — better than ever before.
*J. B. Clark et al. “Sideband cooling beyond the quantum backaction limit with squeezed light”. Nature Volume 541 (January 2017): 191–195. DOI: 10.1038/nature20604
See also the Washington Post article.