When a quantum system is at its lowest possible energy, it is said that it is in its ground state. This state has now almost been achieved for a human-scale object, the largest yet. Physicists have cooled the 10-kilogram (22-pound) optomechanical oscillator formed by the mirrors in the LIGO gravitational wave observatory to such a degree it was close to being in its motional quantum ground state.
As reported in Science, the oscillator is 10 trillion times more massive than the previous record-holder for the heaviest object prepared close to its motional ground state. This breakthrough will certainly be a way to dramatically improve the sensitivity of LIGO and detect gravitational waves in even more detail. But it might have another important application; it could allow ways to study large-scale quantum phenomena. This could provide ways to test gravity like never before.
"Nobody has ever observed how gravity acts on massive quantum states," project director Vivishek Sudhir, assistant professor of mechanical engineering at MIT, said in a statement. "We've demonstrated how to prepare kilogram-scale objects in quantum states. This finally opens the door to an experimental study of how gravity might affect large quantum objects, something hitherto only dreamed of."
The oscillator is not a single tangible object but it combined motions of the four 40-kilogram (88-pound) mirrors employed by LIGO. When they are considered all together, as a single system, physicists can reduce the whole arrangement to about one octillion of atoms (1026 atoms) weighing roughly 10 kilos (22 pounds).
One of the four Advanced LIGO 40-kg mirrors used in the LIGO detector. Image Credit: Caltech/MIT/LIGO Lab
"LIGO is designed to measure the joint motion of the four 40-kilogram mirrors," Sudhir added. "It turns out you can map the joint motion of these masses mathematically, and think of them as the motion of a single 10-kilogram object."
The predicted temperature for this collection of atoms to be in the ground state is just 11 nanokelvins. That is just 11 billionths of a degree over absolute zero. The oscillator was brought down to a temperature of 77 nanokelvins. The object was so still that it didn’t move more than one-thousandth the size of a proton.
"This is comparable to the temperature atomic physicists cool their atoms to get to their ground state, and that's with a small cloud of maybe a million atoms, weighing picograms," Sudhir explained. "So, it's remarkable that you can cool something so much heavier, to the same temperature."
The mirror technology has already been employed to better understand the effect of quantum mechanics in the macroscopic world. Last year, the team recorded the quantum fluctuation of the mirror due to the impossible tiny kicks delivered by lasers shining on it.