Origin of the sideband anomaly in diamond spin–cavity experiments

Determine the physical mechanism responsible for the sideband anomaly observed in sideband spectroscopy of substitutional nitrogen P1 and nitrogen‑vacancy NV⁻ defects in diamond, namely that both the left and right sidebands are simultaneously blue‑shifted relative to the cavity resonance, despite linear spin–cavity models predicting opposite shifts and the cavity’s Kerr nonlinearity being too weak in the present experiment to account for the effect.

Background

In the linear theory developed in the paper, the left and right sidebands are related by a symmetry in the detuning plane, implying that one sideband should be red‑shifted if the other is blue‑shifted. Experimentally, however, both sidebands are found on the same (blue) side of the cavity resonance, which the authors term the sideband anomaly.

The authors show that Kerr nonlinearity can, in principle, break the linear symmetry and produce such an anomaly. Nonetheless, they argue that Kerr nonlinearity in their setup is too weak to explain the observed effect, leaving its physical origin unresolved.