A pendulum weighing a few grains of sand swings on a hair-thin glass fiber. Its motions are so faint that the thermal jostling of air molecules seems like a hurricane. Yet such a pendulum is the key to witnessing quantum wonders in the world of large objects.
An ordinary suspension suffers from structural noise: microscopic shifts in the fiber's material create a low-frequency hum that disturbs thermal peace. This noise, like the murmur of a crowd, masks the quiet quantum whisper. The researchers used gravitational dilution — the weaker the restoring force, the lower the losses. But structural noise remains a hurdle: its 1/f spectrum (noise that grows at low frequencies) requires stronger feedback to suppress thermal fluctuations. The authors calculated that with 10% detection losses, the required ratio of feedback force to thermal noise rises by 50%, and the required suspension gain Greq reaches 1.49.
To cross this threshold, the physicists crafted a 7 mg pendulum on a stepped fused silica fiber. The steps reduce losses, and silica is one of the 'quietest' materials. The oscillation frequency was 2.63 Hz, and the energy decay rate was only 361 nanohertz. This yielded a quality factor Q = 7.3×10⁶ — a measure of how long the pendulum swings without a push. The product ω₀Γ (frequency times damping) turned out to be record-breakingly small, equivalent to a suspension gain Gq ≈ 2.5 compared to a monolithic device. This comfortably exceeds the required threshold.
Now such a pendulum is ready for quantum entanglement — a state where two objects are linked by an invisible thread, and the tremor of one instantly echoes in the other. Gravity, usually a nuisance in quantum experiments, here became an ally, weakening the connection to the thermal world. The next step is to make two pendulums 'feel' each other without classical contact.
The quality factor of this pendulum is so high that if it were swinging in a vacuum without disturbances, its amplitude would halve only after several months of continuous oscillation.