A set of four mirrors used by the Laser Interferometer Gravitational-Wave Observatory (LIGO) to detect ripples in space-time have been cooled down so much that they are nearly at their minimum-energy state. The mirrors mark the largest objects ever to be brought so near to this frigid quantum state, a fraction above absolute zero.
At a quantum scale, temperature and motion are one and the same: the more a particle is vibrating, the hotter it is. Those packets of vibration, also called phonons, must be removed to bring an object into its ground state. So far, this has only been achieved with objects with masses of tiny fractions of a gram.
Now, Chris Whittle at the Massachusetts Institute of Technology (MIT) and his colleagues have cooled a system with an effective mass of 10 kilograms from room temperature down to 77 nanokelvin, marking a huge leap in the mass of a system that can be brought near its ground state. The full system consists of four mirrors, each weighing 40 kilograms, but together they vibrate as if they were a single 10-kilogram object.
The team did this by using one of LIGO’s many feedback systems, in which a beam of light is shone at a mirror to measure its vibration, and then an electromagnetic field is applied to slow that motion. “It’s kind of like a child swinging on a swing: you push against their motion to bring them to a stop,” says Whittle.
Because the vibrations the researchers wanted to remove were so tiny, they needed to measure them extremely precisely to apply the right push, which is one reason they used LIGO’s extraordinarily exact system for this work. By using this loop, they reduced the average number of phonons in the system at a given time from about 10 trillion to just under 11.
The goal of this work is to help explain why we don’t generally see macroscopic objects in quantum states, which some physicists have suggested may be due to the effects of gravity.
“If you want to test that, you need two things: you need a large enough object that you can measure gravity’s effect on it, and you need to realise this object in a quantum state,” says Vivishek Sudhir at MIT, a member of the research team. Using these kinds of quantum states may also allow scientific instruments like LIGO to achieve higher precision, but that is far in the future, he says.
Journal reference: Science, DOI: 10.1126/science.abh2634
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