Conditional Motional Squeezing of an Optomechanical Oscillator Approaching the Quantum Regime

In the world of physics, there is a fundamental divide between the rules that govern the very small and the rules that govern the everyday world. At the scale of atoms and subatomic particles, objects behave in strange ways: they can exist in multiple states at once, and their properties are fuzzy, defined by probabilities rather than fixed values. This is the realm of quantum mechanics. In contrast, the objects we see and touch—chairs, cars, and people—follow the predictable laws of classical physics. For over a century, scientists have wondered where the line is drawn between these two worlds. Why do large objects not behave like quantum particles? To answer this, researchers are trying to push larger and larger objects into the quantum realm, forcing them to obey the strange rules of the very small. A key part of this effort involves "squeezing," a technique that reduces the natural fuzziness of an object's motion in one direction while allowing it to increase in another. If scientists can squeeze the motion of a large object enough, they can prepare it in a special quantum state, offering a new way to test how gravity and quantum mechanics might interact.

A team of researchers has now taken a significant step toward this goal by successfully squeezing the motion of a tiny, yet macroscopic, mechanical object. The object in question is a cantilever, a microscopic diving board made of gallium arsenide, weighing just 50 nanograms. While this mass is incredibly light to human senses, it is massive compared to the atoms usually studied in quantum experiments. The team managed to prepare this cantilever in a "classically squeezed" state, a condition where the uncertainty in its position is reduced to a level just barely above the absolute minimum allowed by quantum physics, known as the zero-point fluctuation. They achieved a minimum uncertainty of 1.07 times this fundamental limit. This result is three orders of magnitude better than previous attempts using similar methods, representing a massive leap in precision.

The experiment relied on a clever interplay between light and motion. The researchers placed the tiny cantilever inside a vacuum chamber and shone a laser at it. As the light bounced off the cantilever, it carried away information about the object's position. By measuring this light very precisely, the team could track the cantilever's movement in real-time. However, simply watching the object isn't enough to squeeze it; the act of measuring also pushes on the object, adding a jolt of noise known as "back-action." To overcome this, the researchers used a technique called conditional state preparation. They fed the measurement data into a computer filter that predicted the cantilever's position based on its past behavior. By using this prediction to understand the object's state more clearly than the raw measurement alone would allow, they could effectively "squeeze" the noise out of one aspect of its motion.

A critical part of their success was the speed of their measurements relative to the speed of the cantilever's vibration. For squeezing to work, the rate at which they gathered information had to be fast enough to keep up with the object's natural oscillations. In this experiment, the measurement rate was about 54% of the cantilever's resonance frequency, a condition that allowed them to break the usual balance where energy is shared equally between position and momentum. Instead, they managed to suppress the uncertainty in the position measurement significantly, while the uncertainty in the momentum measurement increased, a trade-off that is a hallmark of squeezed states. The final result showed that the position uncertainty was reduced to just 1.07 times the zero-point level, while the momentum uncertainty rose to nearly eight times that level, clearly demonstrating that the energy was no longer shared equally.

Despite this impressive achievement, the researchers are careful to note that they have not yet fully reached the "quantum squeezed" regime. Their state is described as "classically squeezed" because, while it violates the classical rule of equal energy distribution, it does not yet dip below the fundamental quantum noise floor in a way that would confirm a fully quantum mechanical state. The experiment was limited by various sources of noise, including electronic interference and the difficulty of keeping the laser perfectly stable. The team identified that non-stationary noise, which changes unpredictably over time, prevented them from filtering the data in real-time to create a definitive quantum state. However, their work proves that the protocol is viable. By showing that a measurement-based approach can squeeze a macroscopic object so effectively, they have paved the way for future experiments. With further improvements to reduce noise and increase measurement efficiency, the team believes it will soon be possible to prepare even larger objects, potentially including the massive mirrors used in gravitational wave detectors, in true quantum squeezed states. This progress brings scientists closer to understanding how the quantum world transitions into the classical world we experience every day.