Scientists Are Close to Uncovering Why Large Objects Don’t Exhibit Quantum Behavior

They almost quantum-cooled a 22-lbs optomechanical oscillator to its ground state.
Chris Young

An international group of researchers used new techniques to quantum cool oscillators on the mirrors of the Laser Interferometer Gravitational-Wave Observatory (LIGO), in a bid to reach their quantum ground state, a report from Ars Techninca explains.

The team, who published their findings in the journal Science, say the LIGO experiment may lead to a better understanding of the quantum states of human-sized objects, and not just those of the subatomic world.

Investigating the effect of gravity on massive quantum systems

Objects within the quantum realm behave very differently from everyday objects we can see with our eyes.

Phenomena such as quantum entanglement, which sees the state of separate particles connected no matter how far apart they are, sound like witchcraft if they're described in the context of human-sized objects — Albert Einstein himself dubbed the phenomena "spooky action at a distance."

In their experiments, the team of international researchers set out to discover if it's possible to get an everyday object to behave like a quantum object — they write that their "approach will enable the possibility of probing gravity on massive quantum systems."

In an interview with InverseVivishek Sudhir, a co-author on the paper and assistant professor of mechanical engineering at MIT, explained that the team set out to test the theory that gravity may be responsible for the fact that large items don't display quantum behavior.

"One way to test this theory is by an experiment where one realizes a quantum state of an object that is also massive enough that the effect of gravity on it can be measured," Sudhir told Inverse.

Recent advances have allowed scientists to place increasingly large objects in a quantum state by limiting the object's interactions with their environment using small oscillators and other equipment, and cooling them to reduce thermal disruption — as is the case with quantum computers, which are supercooled to stabilize qubits and reduce errors.

Increasingly close to ground state

In their new study, the researchers report that they got close to putting the 10-kg optomechanical oscillator used in the LIGO gravitational-wave observatory in their quantum ground state.

LIGO uses two large mirrors — each of which weighs 40 kg (88.2 lbs) — on opposite ends of long tunnels to allow laser light to bounce back and forth in order to allow scientists to measure any influence from a passing gravitational wave. 

"Using the suspended mirrors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) that form a 10-kg optomechanical oscillator, [the team] demonstrate the ability to cool such a large-scale object to nearly the motional ground state," the researchers wrote in their paper.

The scientists explain that reaching this ground state would increase the sensitivity of the machine, allowing researchers to "extend studies of quantum mechanics to large-scale objects."

The team cooled the 10-kg mechanical oscillator from room temperature to 77 nanokelvin, putting it in an average phonon occupation of 10.8. While the oscillator would have to be emptied of phonons to reach its quantum ground state, the researchers say their cooling experiment suppressed quantum back-action noise by 11 orders of magnitude.

Revealing the mysteries of quantum mechanics

For the overall LIGO experiment, two large observatories were built in the United States — one in Washington and the other in Louisiana — both of which detect gravitational waves using laser interferometry.

As NASA explains, "each LIGO observatory has two "arms" that are each more than 2 miles (4 kilometers) long. A passing gravitational wave causes the length of the arms to change slightly. The observatory uses lasers, mirrors, and extremely sensitive instruments to detect these tiny changes."

Animation of gravitational waves being detected.
Source: NASA

The most celebrated work to date from the team at the Laser Interferometer Gravitational-Wave Observatory saw them detect gravitational waves (which were 1.3 billion years old) for the first time in 2015, 100 years after they were first predicted by Albert Einstein.

Last year, a team from MIT measured the effects of quantum fluctuations on LIGO's 40-kg mirrors on the macroscopic level.

The latest experiment paves the way for the scientific community to compile a theory on the mysterious behaviors of the quantum world when compared to the one we see with our human eyes.

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