Fast quantum squeezing of a nanomechanical oscillator with an inverted potential

In the microscopic world, the rules of physics behave differently than they do in our daily lives. One of the most fundamental rules is that there is a limit to how precisely we can know the state of an object. This is not a flaw in our measuring tools, but a built-in feature of nature itself, often called quantum uncertainty. Imagine trying to measure the position and speed of a tiny particle at the same time; the more precisely you pin down one, the fuzzier the other becomes. This uncertainty creates a background "noise" or jitter that exists even in a perfect vacuum, known as vacuum fluctuations. For scientists trying to build ultra-sensitive sensors or powerful quantum computers, this noise is a major obstacle. To get around it, researchers use a technique called squeezing. Instead of trying to eliminate the noise entirely, they reshape it. They take the uncertainty from one property, such as position, and push it into a different property, like momentum, that is less important for the task at hand. This allows them to see the signal they are looking for much more clearly, effectively creating a state of matter that is quieter than nature usually allows.

For decades, achieving this kind of control over mechanical objects has been incredibly difficult, especially for anything larger than a single atom. Most successful experiments have required freezing objects to temperatures near absolute zero and using complex electronic circuits to stabilize them. However, a team of researchers at ETH Zurich has found a way to squeeze the motion of a tiny, solid object at room temperature, without the need for extreme cold. They worked with a single nanoparticle, a cluster of silica spheres roughly one hundred and twenty nanometers in diameter, which is about a thousand times thinner than a human hair. This particle was not glued down or tethered to a surface; instead, it was held in mid-air by a beam of laser light, a technique known as optical levitation. By trapping the particle in a vacuum chamber and cooling its motion using the same laser light, the researchers brought the particle's spinning motion, or libration, down to its lowest possible energy state, known as the quantum ground state.

The breakthrough came when the scientists decided to change the rules of the trap holding the particle. Normally, the laser light creates a stable bowl-like shape that keeps the particle centered. If the particle moves away from the center, the light pushes it back, much like a marble sitting at the bottom of a bowl. The researchers, however, wanted to do something different. They rapidly switched the orientation of the laser's polarization, effectively flipping the bowl upside down. In this new, inverted shape, the center is no longer a safe resting place but a precarious peak. If you place a marble on the very top of a hill, even the tiniest nudge will cause it to roll away, and the further it rolls, the faster it accelerates. This is what the researchers created for their nanoparticle: an inverted potential where the particle is perched on a peak of instability.

When the particle was placed in this inverted potential, its motion began to change in a dramatic and predictable way. Instead of wobbling gently back and forth, the fluctuations in its position and speed began to grow exponentially. One aspect of its motion was stretched out and amplified, while the perpendicular aspect was compressed and squeezed. This process happened with incredible speed. Within just two hundred and fifty nanoseconds—a time so short it is less than one-fifth of a single oscillation cycle—the researchers had squeezed the particle's motion by eleven decibels below the level of vacuum fluctuations. This is a significant reduction in noise, meaning the particle was in a state of motion that was far more precise than what is normally possible. The experiment showed that by using this inverted potential, they could generate quantum squeezing much faster than the rate at which the environment would normally destroy the delicate quantum state.

The team also observed how long this squeezed state could survive. Once the particle was returned to a stable trap, the squeezed state began to fade, eventually returning to a normal, noisy state after about thirty oscillation periods. The researchers determined that this decay was caused by the very light used to cool and measure the particle, which introduced a small amount of random noise. They calculated that if they could remove this specific source of noise, the squeezed state could last for roughly two hundred and forty oscillation periods, nearly ten times longer than what they observed. This suggests that the current limitation is not a fundamental law of physics, but rather a technical hurdle that can be overcome with better experimental design.

This work is significant because it demonstrates a new way to control the quantum behavior of massive objects without the need for cryogenic refrigeration. Previous methods for achieving such strong squeezing relied on complex electronic setups in freezing environments. By using light to create an unstable potential, the researchers showed that room-temperature quantum control is possible. The ability to generate these squeezed states so quickly and at room temperature opens the door for new types of sensors that could detect incredibly weak forces or particles. It also provides a platform for testing the boundaries of quantum mechanics, allowing scientists to see how large an object can be while still behaving according to quantum rules. The experiment proves that by manipulating the shape of the energy landscape, researchers can accelerate the generation of quantum states, offering a powerful new tool for the future of quantum sensing and metrology.