Physicists induce motionless quantum state in largest object yet
“Stationary” has very different meanings at quantum and real-world scales – an object that looks perfectly still to us is actually made up of atoms that are buzzing and bouncing around. Now, scientists have managed to slow down the atoms almost to a complete stop in the largest macro-scale object yet.
The temperature of a given object is directly tied to the motion of its atoms – basically, the hotter something is, the more its atoms jiggle around. By extension, there’s a point where the object is so cold that its atoms come to a complete standstill, a temperature known as absolute zero (-273.15 °C, -459.67 °F).
Scientists have been able to chill atoms and groups of atoms to a fraction above absolute zero for decades now, inducing what’s called the motional ground state. This is a great starting point to then create exotic states of matter, such as supersolids, or fluids that seem to have negative mass.
Understandably, it’s much harder to do with larger objects, because they’re made up of more atoms which are all interacting with their surroundings. But now, a large international team of scientists has broken the record for largest object to be induced into a motional ground state (or extremely closely to one, anyway).
Most of the time, these experiments are done with clouds of millions of atoms, but the new test was performed on a 10-kg (22-lb) object that contains almost an octillion atoms. Strangely enough, that “object” isn’t just one thing itself but the combined motion of four different objects, with a mass of 40 kg (88 lb) each.
The researchers conducted the experiment at LIGO, a huge facility famous for detecting gravitational waves as they wash over Earth. It does this by beaming lasers down two 4-km (2.5-mile) tunnels, and bouncing them back with mirrors – and those mirrors were the objects that the new study cooled to a motional ground state.
Cooling atoms is simple in principle – you just have to counter their motion with an equal and opposite force. But that requires measuring their motion extremely precisely, and complicating things even further, the very act of measuring them can exert a new force on them.
Intriguingly, the new study used this to the team’s advantage. The photons of light in LIGO’s lasers exert tiny bumps on the mirrors as they bounce off, and these disturbances can be measured in later photons. Since the beams are constant, the scientists have plenty of data about the motions of the atoms in the mirrors – meaning they can then design the perfect counteracting forces.
To do so, the researchers attached electromagnets to the back of each mirror, which reduced their collective motion almost to the motional ground state. The mirrors moved less than one-thousandth the width of a proton, essentially cooling down to a crisp 77 nanokelvins – a hair above absolute zero.
“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,” says Vivishek Sudhir, director of the project. “So, it’s remarkable that you can cool something so much heavier, to the same temperature.”
The team says that this breakthrough could enable new quantum experiments on the macro scale.
“Nobody has ever observed how gravity acts on massive quantum states,” says Sudhir. “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 research as published in the journal Science.
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