A levitated nanosphere shares quantum entanglement with light at room temperature

An experiment linked the motion of a 100-nanometre silica sphere to light leaving an optical cavity, even beyond the region where they interacted.

Abstract editorial illustration accompanying the article: A levitated nanosphere shares quantum entanglement with light at room temperature

Editorial illustration — not from the study.

The team trapped a 100-nanometre silica sphere in an optical tweezer inside an optical cavity. Light scattered from the sphere coupled its motion to two cavity fields: one cooled the motion, while another generated correlations between the sphere and light leaving the cavity.

Using heterodyne detection, the researchers reconstructed the optical and mechanical correlations and found that they exceeded a limit expected for separable, non-entangled systems. The correlations extended into the outgoing light, showing that the nonclassical relationship could be distributed beyond the interaction region.

What the experiment found

The experiment produced entanglement between the sphere’s center-of-mass motion and a propagating optical mode at room temperature. Entanglement was strongest when the selected optical bandwidth was about 40 kilohertz. The measured entanglement indicator reached 0.931 ± 0.015, with an additional systematic uncertainty of ±0.02; values below 1 indicate entanglement in the researchers’ normalization. The corresponding logarithmic negativity was 0.10 ± 0.03. The effect remained over a broad range of cavity detunings, or frequency offsets, set by the cavity linewidth.

Why the light link matters

The work demonstrates an interface between a stationary mechanical object and a quantum state carried away by light. Because the outgoing light can carry the nonclassical correlations beyond the interaction region, levitated optomechanical systems may provide a platform for continuous-variable quantum communication and for experiments probing quantum behavior in relatively large mechanical objects.

Evidence and caveats

This was a laboratory experiment using heterodyne measurements of light leaving an optical cavity. The researchers reconstructed the correlations directly from the detected data and used them to test whether the mechanical and optical systems could be separated into independent states. Their model showed the same overall pattern but predicted stronger entanglement than the direct reconstruction. The researchers attribute the difference mainly to slow fluctuations in system parameters, especially the relative phases of the optical fields, during data collection. These fluctuations reduce the entanglement that can be observed and used under the reported conditions. The optical mode was also selected with a simple rectangular frequency filter rather than an optimized one, and the study does not establish performance for a wider range of systems or applications.

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