To the human eye, most stationary objects appear to be just that: still and completely at rest. However, if we were given a quantum lens, allowing us to see objects on the scale of individual atoms, what was an apple sitting idly on our desk would appear as a collection of vibrating particles, very moving.
In recent decades, physicists have found ways to supercool objects so that their atoms are either nearly stationary or in their “emotional ground state.” To date, physicists have fought small objects, such as clouds of millions of atoms, or nanogram-scale objects, in such pure quantum states.
Now, for the first time, scientists at MIT and elsewhere have cooled a large human-scale object until it approaches its ground state of motion. The object is not tangible in the sense of being situated in one place but is the combined movement of four separate objects, each of which weighs around 40 kilograms. The “object” the researchers cooled has an estimated mass of about 10 kilograms and comprises about 1×1026, or nearly 1 octillion, of atoms.
The researchers took advantage of the Laser Interferometer’s Gravitational-Wave Observatory (LIGO) ability to measure mass motion with extreme precision and supercool the collective motion of the masses to 77 nanokelvins, just shy of the object’s predicted ground state of 10 nanokelvins.
Their results, appearing today in Science, represent the largest object that cools down to its ground state of motion. Scientists say they now have the opportunity to observe the effect of gravity on a massive quantum object.
“No one has ever observed how gravity acts in massive quantum states,” says Vivishek Sudhir, an assistant professor of mechanical engineering at MIT, who led the project. “We have shown 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 that until now has only been dreamed of.”
The study authors are members of the LIGO Laboratory and include lead author and graduate student Chris Whittle, postdoc Evan Hall, research scientist Sheila Dwyer, Dean of the School of Science, and Astrophysics Professor Curtis and Kathleen Marble, Nergis Mavalvala , and assistant professor of mechanical engineering Vivishek Sudhir.
All objects incorporate some type of motion as a result of the many interactions that atoms have with each other and from outside influences. All this random movement is reflected in the temperature of an object. When an object cools near zero temperature, it still has residual quantum motion, a state called an “emotional ground state.”
To stop an object in its tracks, an equal and opposite force can be exerted on it. (Think stopping a baseball in mid-flight with the force of your glove.) If scientists can accurately measure the magnitude and direction of an atom’s motions, they can apply counteracting forces to lower its temperature, a technique known as feedback cooling.
Physicists have applied feedback cooling through various means, including laser light, to bring individual atoms and ultralight objects to their basic quantum states, and have attempted to supercool progressively larger objects to study quantum effects in larger systems and traditionally classics.
“The fact that something has a temperature is a reflection of the idea that it interacts with the things around it,” says Sudhir. “And it is more difficult to isolate the largest objects from all the things that happen around them.”
To cool the atoms in a large object to the near-ground state, one would first have to measure their motion with extreme precision, to know the degree of recoil required to stop this motion. Few instruments in the world can achieve such precision. LIGO, as it happens, can.
The gravitational wave detection observatory comprises twin interferometers at separate locations in the US Each interferometer has two long tunnels connected in an L-shape and extending 4 kilometers in either direction. At each end of each tunnel is a 40-kilogram mirror suspended by thin fibers, swinging like a pendulum in response to any disturbance, like an incoming gravitational wave. A laser at the tunnel nexus is split and sent down each tunnel, then reflected back to its source. The timing of the return lasers tells scientists precisely how much each mirror moved, accurate to 1 / 10,000th the width of a proton.
Sudhir and his colleagues wondered if they could use LIGO’s motion measurement precision to first measure the motion of large objects on a human scale, then apply a counter force, opposite to what they are measuring, to bring the objects back to their ground state.
Reacting against the action
The object they intended to cool is not an individual mirror, but the combined movement of the four LIGO mirrors.
“LIGO is designed to measure the joint motion of the four 40-kilogram mirrors,” explains Sudhir. “It turns out that you can mathematically map the joint motion of these masses and think of them as the motion of a single 10-kilogram object.”
By measuring the motion of atoms and other quantum effects, Sudhir says, the very act of measuring can randomly kick the mirror and set it in motion, a quantum effect called “measurement feedback.” As individual photons from a laser bounce off a mirror to gather information about their motion, the impulse of the photon recoils in the mirror. Sudhir and his colleagues realized that if mirrors are continuously measured, as in LIGO, the random recoil of past photons can be observed in the information carried by subsequent photons.
Armed with a full record of both quantum and classical disturbances in each mirror, the researchers applied an equal and opposite force with electromagnets attached to the back of each mirror. The effect caused the collective motion to almost stop, leaving the mirrors with so little energy that they did not move more than 10-20 meters, less than one-thousandth the size of a proton.
The team then compared the object’s remaining energy, or motion, with temperature, and found that the object was sitting at 77 nanokelvins, very close to its ground state of motion, which they predict to be 10 nanokelvins.
“This is comparable to the temperature that atomic physicists cool their atoms to their ground state, and that’s with a small cloud of maybe a million atoms, weighing picograms,” says Sudhir. “So it is remarkable that something much heavier can be cooled to the same temperature.”
“Preparing something in the ground state is often the first step to putting it in exciting or exotic quantum states,” says Whittle. “So this work is exciting because it could allow us to study some of these other states, on a massive scale that has never been done before.”
This research was supported, in part, by the National Science Foundation.
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