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One of the most significant open inquiries in contemporary physics is: “Is gravity quantum?”
The other essential forces — electromagnetic, weak, and strong — have all been effectively articulated, but a complete and coherent quantum theory of gravity is still not available.
“Theoretical physicists have suggested numerous potential scenarios, spanning from gravity being fundamentally classical to entirely quantum, yet the discussion remains unsettled due to the absence of a definitive method to test gravity’s quantum characteristic in the laboratory,” states Dongchel Shin, a PhD student in the MIT Department of Mechanical Engineering (MechE). “The solution to this lies in developing mechanical systems that possess sufficient mass to experience gravity, while also being sufficiently quiet — quantum enough — to unveil how gravity interacts with them.”
Shin, who is also a MathWorks Fellow, investigates quantum and precision metrology platforms that explore fundamental physics and are intended to pave the way for prospective industrial technology. He is the principal author of a new paper that showcases the laser cooling of a centimeter-long torsional oscillator. The open-access paper, “Active laser cooling of a centimeter-scale torsional oscillator,” was recently published in the journal Optica.
Lasers have been consistently utilized to cool atomic gases since the 1980s, and have been employed in the linear motion of nanoscale mechanical oscillators since around 2010. This new paper marks the first instance of extending this technique to torsional oscillators, which are crucial to a global initiative aimed at studying gravity using these systems.
“Torsion pendulums have been traditional tools for gravity research since [Henry] Cavendish’s notable experiment in 1798. They’ve been employed to measure Newton’s gravitational constant, G, assess the inverse-square law, and investigate new gravitational phenomena,” clarifies Shin.
By utilizing lasers to eliminate nearly all thermal motion from atoms, scientists have developed ultracold atomic gases at micro- and nanokelvin temperatures over the past few decades. These systems now power the world’s most accurate clocks — optical lattice clocks — boasting timekeeping precision so exceptional that they would gain or lose less than a second over the universe’s lifespan.
“Historically, these two technologies evolved independently — one in gravitational physics, the other in atomic and optical physics,” remarks Shin. “In our research, we synthesize them. By applying laser cooling methods originally devised for atoms to a centimeter-scale torsional oscillator, we aim to connect the classical and quantum realms. This hybrid platform enables a new category of experiments — ones that could ultimately allow us to assess whether gravity requires a quantum theoretical framework.”
The new paper illustrates the laser cooling of a centimeter-scale torsional oscillator from room temperature to merely 10 millikelvins (1/1,000th of a kelvin) via a mirrored optical lever.
“An optical lever is a straightforward but potent measurement method: You direct a laser onto a mirror, and even minimal tilting of the mirror causes the reflected beam to noticeably shift on a detector. This amplifies small angular motions into easily quantifiable signals,” explains Shin, emphasizing that although the concept is simple, the team encountered challenges in practice. “The laser beam itself can fluctuate slightly due to air currents, vibrations, or imperfections in the optics. These fluctuations can misleadingly appear as mirror motion, restricting our capacity to accurately measure true physical signals.”
To address this issue, the team adopted the mirrored optical lever approach, which uses a second, mirrored version of the laser beam to cancel out the undesirable fluctuations.
“One beam interacts with the torsional oscillator, whereas the other reflects off a corner-cube mirror, counteracting any jitter without capturing the oscillator’s motion,” Shin states. “When the two beams converge at the detector, the genuine signal from the oscillator is preserved, while the false motion from [the] laser jitter is nullified.”
This method diminished noise by a factor of one thousand, enabling the researchers to detect motion with remarkable precision, nearly 10 times better than the oscillator’s own quantum zero-point fluctuations. “This level of sensitivity allowed us to cool the system to just 10 milli-kelvins using laser light,” Shin notes.
Shin asserts that this project is merely the beginning. “While we’ve attained quantum-limited precision below the zero-point motion of the oscillator, reaching the actual quantum ground state is our subsequent objective,” he states. “To accomplish this, we’ll need to further enhance the optical interaction — utilizing an optical cavity that amplifies angular signals or exploring optical trapping methods. These advancements could enable experiments where two such oscillators interact solely through gravity, allowing us to directly examine whether gravity is quantum or not.”
The paper’s other contributors from the Department of Mechanical Engineering encompass Vivishek Sudhir, assistant professor of mechanical engineering and the Class of 1957 Career Development Professor, and PhD candidate Dylan Fife. Additional co-authors are Tina Heyward and Rajesh Menon from the Department of Electrical and Computer Engineering at the University of Utah. Both Shin and Fife are members of Sudhir’s lab, the Quantum and Precision Measurements Group.
Shin states that one insight he has gained from this project is the vastness of the challenge the team is confronting. “Investigating quantum aspects of gravity experimentally requires not only a profound understanding of physics — relativity, quantum mechanics — but also necessitates practical expertise in system design, nanofabrication, optics, control, and electronics,” he mentions.
“Having a background in mechanical engineering, which encompasses both the theoretical and practical components of physical systems, has given me the right perspective to navigate and contribute meaningfully across these varied domains,” Shin expresses. “It has been incredibly gratifying to observe how this extensive training can assist in addressing one of the most fundamental inquiries in science.”
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