Magnetic wideband VHF localized field probe using magnon polaritons

TL;DR

The paper tackles ultra-low magnetic field sensing in the VHF range with a CMP-based probe that couples a YIG sphere to a copper cavity. By operating in the strong coupling regime, the authors realize a resonant heterodyne readout where a pump tone at $\omega_2$ generates a detectable sideband near $\omega_1$ in response to an ac field at $\omega_b$, enabling wideband, low-noise measurements. Tuning the bias field $H_0$ and the sphere position broadens the sensing bandwidth to about 148–225 MHz while preserving sub-pT sensitivity, achieving over 100 dB dynamic range at room temperature. The results point to compact, potentially on-chip implementations with planar resonators and to applications as tunable transducers or signal-processing units for quantum circuits.

Abstract

We present here an optimisation and demonstration of a wide band instrument capable of measuring localised and directionally alternated magnetic fields below pT in the very high frequency (VHF) range. We take advantage of the magnon-photon hybridization between a yttrium iron garnet (YIG) sphere and a copper resonant cavity to employ a resonant heterodyne detection scheme. The measurement is near instantaneous due to the strong coupling attained between magnons and photons.In this work measurements are reported showing a significant widening of the measurement bandwidth, obtained by tuning the YIG Larmor frequency with a bias magnetic field and adjusting the magnon-photon coupling strength. Minimum sensitivity in the sub pT regime is demonstrated in the range 150 -- 225 MHz at room temperature and expected to go to fT in cryogenic temperatures. Dynamic range is estimated to be above 100 dB. The sensitivity is found to be independent on size, being ready to in-chip miniaturization. Such device can be an important building block to quantum circuits, such as baluns, transducers or signal processing units.

Figures (3)

• Figure 1: VHF field band probe. a) Hybridization modes $\omega_{1,2}$ with linewidths $\gamma_{1,2}$ separated by $\omega_b$. Adding an ac field at $\omega_b$ generates a sideband $\xi_1$ at $\omega_1$. b) Experimental scheme: the probed field is parallel to the bias field. The microwave pump is filtered to remove its noise. Readout of the power at $\omega_1$ is done by a spectrum analyser after amplification. c) Transmission plot of the waveguide filter showing the 52 dB attenuation at $\omega_1$. d) Sensitivity measurement showing the voltage read at the spectrum analyser at $\omega_1$ for increasing values of signal fields. A linear fit provides the transduction coefficient of $17.1\pm 2I$ V/mT, and sensitivity is obtained from the of noise level measurement of 20 nV
• Figure 2: Effect of changing the coupling constants in the hybridization modes. Decreasing the coupling constant also reduces the effectivity of the high pass filter and $A_p^2$. The slope on the left is the roll-off of the waveguide high-pass filter.
• Figure 3: Summary of the sensitivity measurements for different probing frequencies (coupling constants). The square symbol is the extended cable measurements.