Giant observatory moved to an almost pure quantum state

An international team of scientists led by physicists at the Massachusetts Institute of Technology has managed to translate a huge object into an almost pure quantum state by suppressing the motion created by the environment. This object was the resonator system gravitational-wave observatory LIGO.

Quantum system can reach the ground state in which thermal vibrations (phonons) of atoms are eliminated by cooling to almost absolute zero. So far, however, this has only been done for tiny nanometer-sized objects, such as clouds of millions of atoms weighing a few picograms. Making the transition to a pure quantum state for macroscopic objects, whose size is comparable to the size of a human, is technically difficult.

Giant observatory moved to an almost pure quantum state

In the new experiment, scientists used suspended mirrors of the gravitational-wave observatory with a giant laser interferometer (LIGO). The Advanced LIGO observatory consists of a pair of Michelson interferometers, each of which is a channel (shoulder) four kilometers long with a vacuum inside. Quartz mirrors, each weighing up to 40 kilograms, form Fabry-Perot optical resonators. When a gravitational wave passes through LIGO, the distance between the mirrors-pendulums changes (one arm shortens, the other lengthens), which is registered by the detectors as fluctuations in the power of optical radiation. The joint motion of each pair of mirrors and two oscillators as a whole can be mathematically considered as one object, namely a mechanical oscillator with a mass of ten kilograms.

The measurement process itself can inadvertently set the mirror in motion – this phenomenon is called quantum measurement inverse action. Photons reflecting off the mirror to gather information about its motion transfer momentum to the mirror, creating an error in the subsequent measurement. To compensate for this effect, scientists created a control circuit that applied an electrostatic force to the mirrors using gold electrodes at 400 volts. As a result of suppressing the reverse effect, the mirrors were shifted by no more than one thousandth of the proton size (less than 10 to the minus 20th power of a meter).

The remaining vibrational energy corresponded to 77 nanokelvin, which is very close to the calculated temperature of the oscillator’s ground state of 10 nanokelvin. Thus, the scientists managed to increase the mass of the object, whose state was brought almost to pure quantum. According to the researchers, the transformation of the LIGO observatory into a massive quantum system opens up new possibilities to accurately measure the effects of gravity.