2023-10-24 18:45:59
At very small subatomic scales, empty space is actually filled with a faint “fog” of quantum “noise.”
This fog has been causing interference in the measurements of LIGO, a gravitational wave observatory, limiting its sensitivity.
Now, scientists on the LIGO team have made a significant advance in a quantum technology that allows them to overcome this limit imposed by “noise” and measure wrinkles in space-time in the entire range of gravitational frequencies detected by LIGO.
“Now that we have surpassed this quantum limit, we can do a lot more astronomy,” emphasizes Lee McCuller, from the California Institute of Technology (Caltech) in the United States and a member of the LIGO (Laser Interferometer Gravitational-wave Observatory) scientific team.
Each of the facilities that make up LIGO, located in the United States, consists of two 4-kilometer-long arms connected in an “L” shape. Laser beams travel down each arm, collide with giant suspended mirrors and return to the starting point. As gravitational waves sweep across the Earth, they cause LIGO’s arms to stretch and compress, knocking the laser beams out of sync. This causes characteristic interferences in the light of the two laser beams, revealing the presence of gravitational waves. However, the quantum noise hidden inside the vacuum tubes through which LIGO’s laser beams circulate can alter the timing of the photons and invalidate measurements.
In 2015, LIGO made history by making the first direct detection of gravitational waves, which basically appear as wrinkles in space-time. On that occasion, gravitational waves were generated by a pair of colliding and merging black holes. Since then, LIGO and its sister observatory in Europe, Virgo, have detected gravitational waves from dozens of black hole mergers as well as collisions between neutron stars.
LIGO’s success lies in its ability to measure the stretching and compression of the fabric of space-time at scales 10,000 trillion times smaller than the thickness of a human hair.
The quantum limit that has been hampering LIGO is determined by the fact that the laws of quantum physics roughly dictate that particles like photons will appear and disappear randomly in empty space, creating a kind of fog of quantum noise. which brings a level of uncertainty to LIGO’s laser measurements.
Quantum compression of light is a method to avoid quantum noise or, more specifically, to move it from one place to another with the aim of making more precise measurements where it does not appear.
The term “compression” here refers to the fact that light can be compressed so that it is more precise (less affected by quantum noise fog) in a certain aspect, such as its frequency, but in turn becomes more uncertain in another aspect, such as its power. This limitation is based on a fundamental law of quantum mechanics called the uncertainty principle, which states that both the position and momentum of objects (or the frequency and power of light) cannot be known at the same time.
Since 2019, LIGO’s twin detectors have been compressing light in such a way that they have improved their sensitivity in the upper frequency range of the gravitational waves they detect. But in return the measurements have been less precise in the low frequencies.
Now, however, LIGO’s new frequency-dependent optical cavities (long tubes the length of three football fields) allow light to be compressed in different ways, depending on the frequency of gravitational waves that is of interest at the time. , thus reducing noise across LIGO’s entire frequency range.
The image shows the new light compression technology in operation in a LIGO vacuum chamber. The photograph was taken at a time when the light compression system was acting and green light was being pumped out. (Image: Georgia Mansell/LIGO Hanford Observatory. CC BY-NC-ND 3.0)
Recent progress at LIGO means that detectors can now probe a greater volume of the universe and are expected to detect around 60 percent more mergers between high-mass objects than before. Having overcome the aforementioned quantum limit in this way has extraordinarily increased LIGO’s capacity to study exotic events that emit gravitational waves.
“We can’t control nature, but we can control our detectors,” summarizes Lisa Barsotti, a researcher at the Massachusetts Institute of Technology (MIT) in the United States and a member of the LIGO scientific team. She oversaw the development of new technology for LIGO.
Barsotti, McCuller and their colleagues present in the academic journal Physical Review Their report is titled “Broadband quantum enhancement of the LIGO detectors with frequency-dependent squeezing.” (Source: NCYT from Amazings)
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