Nanohertz Pendulum toward Macroscopic Entanglement under Structural Damping

• URL: http://arxiv.org/abs/2608.02462v1
• Date: Mon, 03 Aug 2026 16:32:16 GMT
• Title: Nanohertz Pendulum toward Macroscopic Entanglement under Structural Damping
• Abstract summary: Pendulums are attractive for macroscopic quantum control because gravity dilution reduces mechanical loss.<n>A $1/f$ force-noise spectrum associated with structural damping produces a low-frequency tail that penalizes entanglement.<n>This tail raises the required back-action-to-thermal force-noise ratio by about $50%$, corresponding to a required suspension gain.
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• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: Pendulums are attractive for macroscopic quantum control because gravity dilution reduces mechanical loss, while the $1/f$ force-noise spectrum associated with structural damping allows nearly lossless trapping to suppress the thermal noise sampled at an upward-shifted resonance. The same $1/f$ spectrum, however, produces a low-frequency tail that penalizes entanglement. With $10\%$ detection loss, we find that this tail raises the required back-action-to-thermal force-noise ratio by about $50\%$, corresponding to a required suspension gain $G_{\rm req}=1.49$. To overcome this structural-noise penalty, we realize a $7$-mg pendulum suspended by a stepped fused-silica fiber, with an energy-decay rate $Γ/2π=361(39)$ nHz ($Q\equivω_0/Γ=7.3(8)\times10^6$) at $ω_0/2π=2.63$ Hz. The reduction in $ω_0Γ$ yields a measured gain $G_q\simeq2.5$ relative to the previous monolithic device, exceeding the requirement.

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