Scientists Built A Sound Laser That Does Something Light Lasers Struggle To Accomplish

Scientists Levitated a Particle Smaller Than a Virus and Made It Fire Like a Laser

Lasers are everywhere. In your phone, your doctor’s office, the checkout line at the grocery store. But for all their uses, ordinary light-based lasers have a limitation: their output doesn’t naturally include certain properties that physicists need for the most sensitive measurements and advanced information processing.

Extracting those properties out of a light laser means running the beam through special crystals, and the resulting signal tends to be not very bright. Researchers at the University of Rochester and Rochester Institute of Technology have now demonstrated a new kind of laser built around sound waves, one that produces bright, coherent output while simultaneously doing something no sound-wave laser has ever done.

Published in Nature Communications, the work centers on a glass bead roughly a thousand times thinner than a human hair, levitated in mid-air by a focused laser beam inside a vacuum chamber. That bead acts as a laser for sound waves, known as phonons, while also generating a linked noise-redistributing property called “squeezing” across two separate channels at the same time. No phonon laser had achieved that combination before.

Why Light Lasers Fall Short

Squeezing is a technique that suppresses noise in one measurement variable by redistributing it into another, letting instruments detect extremely faint signals. Squeezing helped improve the sensitivity of detectors that first captured gravitational waves, the ripples in spacetime from colliding black holes. Producing squeezed light requires passing a laser beam through crystals with special optical properties, but those properties are inherently weak. Squeezed light sources are, as a result, not very bright.

In mechanical systems, the equivalent properties are far stronger and easier to harness. That difference is exactly what the Rochester team exploited.

How the Phonon Laser Works

At the center of the experiment sits a silica bead 100 nanometers in diameter, suspended by a 150-milliwatt laser beam inside a chamber pumped down to near-zero pressure. Because the trapping laser is linearly polarized, the invisible “bowl” holding the bead is slightly elongated in one direction. That asymmetry gives the bead two distinct natural wobble directions: one along the x-axis at 115,000 cycles per second, and one along the y-axis at 130,000 cycles per second.

On their own, those two wobble directions are entirely independent. To link them, the team periodically rocked the orientation of the trapping field back and forth at a rate equal to the sum of the two wobble frequencies. This drove energy into the system and forced the two directions to exchange energy and develop correlated behavior, similar in spirit to how certain crystals split a single photon into two lower-energy ones. Here, the process produced pairs of phonons in the two mechanical directions instead.

Left unchecked, that energy input would push the bead’s wobbles into runaway growth until the particle escaped the trap. To prevent that, the team applied a nonlinear braking force that grows stronger as the wobbles grow larger, stabilizing the system and allowing it to settle into a steady, laser-like state.

Each experimental run lasted 200 milliseconds and was repeated 100 times. For the first 30 milliseconds, only the braking force was active, letting the bead settle into a calm baseline. Then the coupling between the two wobble directions was switched on.

A Phonon Laser That Squeezes Noise

Below a certain coupling strength, both wobble directions showed ordinary random thermal noise, the kind of random jitter any warm object produces. Above that threshold, both changed sharply. Wobble amplitude in both directions jumped and stabilized at nearly identical levels, and the statistical signature of each channel shifted from random, heat-driven noise to the orderly pattern of a coherent, laser-like source. With an average of about 1.06 million phonons, the measured spread in phonon number came in far below what a purely thermal state would show, confirming that each channel on its own was already behaving sub-thermally, in a classical sense.

More significant results emerged when the team examined the two channels together. Without coupling, noise in the x and y wobble directions was independent and symmetric. Near the coupling threshold, that symmetry collapsed. Noise in the difference between the two channels was sharply suppressed while noise in their sum was amplified, a lopsided pattern with a squeezing ratio of 15.8 (plus or minus 0.8). Above the threshold, as both channels entered the full lasing state, the squeezing ratio settled at 5.2 (plus or minus 0.6), with noise in the amplitude difference holding at roughly half its original thermal value even as coupling strength continued to climb.

Simultaneous lasing and two-mode squeezing in a phonon system had no prior example in the scientific literature.

What Comes Next for the Phonon Laser

Right now, the device operates in what physicists call the classical regime, meaning the phonon numbers are large enough that quantum effects do not yet govern its behavior. Still, the researchers describe their results as “a necessary stepping stone in the longer-term development of a quantum mechanical extension.” A quantum version of the device could, in principle, produce mechanically entangled states, a resource central to quantum computing and quantum sensing that is currently difficult to generate with sufficient brightness.

A glass bead the size of a large virus, floating in near-vacuum, simultaneously acting as a laser and a correlated noise-reduction source appears to be a relatively direct mechanical approach to achieving something that has proven harder to do in optical systems.

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