Suppressing Cavity Frequency Noise Using a Kerr Nonlinearity

Superconducting microwave cavities are essential for applications like qubit readout and optomechanical sensing, where they function as high-precision frequency sensors. However, these devices are often plagued by low-frequency environmental noise—such as mechanical vibrations or magnetic flux fluctuations—that shifts the cavity's resonance frequency, degrading sensitivity and stability. While active feedback loops (like Pound-Drever-Hall locking) can mitigate this, they require complex external control electronics. This paper explores whether a cavity's own intrinsic nonlinear dynamics can be harnessed to provide passive, self-stabilizing feedback.

The researchers utilize a superconducting microwave cavity coupled to a transmon qubit, which introduces a Kerr nonlinearity—a phenomenon where the cavity's resonance frequency depends on the number of photons stored inside it. By applying a strong, fixed-frequency pump tone, the cavity is driven into a regime where the Kerr effect creates an intrinsic feedback loop. When the bare cavity frequency shifts due to noise, the intracavity photon number changes, which in turn triggers a Kerr-induced frequency shift that opposes the original noise. The team characterizes this "Kerr locking" using two-tone spectroscopy and evaluates the long-term stability of the system using Allan deviation analysis.

The experimental results show that Kerr locking effectively suppresses resonance-frequency fluctuations. By operating in a regime where the pump is blue-detuned relative to the dressed cavity resonance, the system achieves a stable, self-correcting state. The standard deviation of the cavity frequency was reduced from 23.2 MHz in the unlocked state to 297 kHz in the locked state. Furthermore, the Allan deviation analysis confirms that the locked cavity reaches the 1/f noise floor, representing a nearly two-order-of-magnitude improvement in frequency stability without the need for continuous, active external tracking.