Superconducting microwave oscillators as detectors for ESR spectroscopy

TL;DR

This paper introduces superconducting microwave oscillators integrated with feedback mechanisms for ESR spectroscopy, demonstrating improved sensitivity and noise performance at cryogenic temperatures, with potential advantages over traditional methods.

Contribution

It presents the design, fabrication, and application of superconducting resonator-based oscillators for ESR, a novel approach for enhancing sensor performance in this field.

Findings

Oscillators operate at 0.6 and 1.7 GHz frequencies.

Achieved lowest frequency noise of about 9 mHz/Hz$^{1/2}$.

Spin sensitivity of approximately $1\times10^{10}$ spins/Hz$^{1/2}$.

Abstract

Microwave superconducting resonators are extensively studied in fields such as quantum computing and electron spin resonance (ESR) spectroscopy. However, the integration of superconducting resonators with feedback mechanisms to create ultra-low noise oscillators is a relatively unexplored area, and the application of such oscillators in ESR spectroscopy has not yet been demonstrated. In this work, we report the design, fabrication, and application of microwave oscillators based on superconducting resonators for ESR spectroscopy, illustrating an alternative way for the improvement of the performance of oscillator based ESR sensors. Specifically, ESR spectra are obtained by measuring the oscillator's frequency shift induced by the ESR effect as a function of the applied static magnetic field. The oscillators are composed of a single heterojunction bipolar transistor (HBT) or high electron mobility transistor (HEMT) coupled with NbTi or YBa$_2$Cu$_3$O$_7$ (YBCO) superconducting resonators. The fabricated oscillators operate at frequencies of 0.6 and 1.7 GHz and temperatures up to 80 K (for YBCO resonators) and 8 K (for NbTi resonators). The lowest measured frequency noise is about 9 mHz/Hz$^{1/2}$ (-139 dBc/Hz), the best spin sensitivity is about $1\times10^{10}$ spins/Hz$^{1/2}$, and the best concentration sensitivity is about $3\times10^{18}$ spins/Hz$^{1/2}$m$^3$. The approach proposed in this work should allow for significantly better spin and concentration sensitivities compared to those achievable with normal conductors, up to operating frequencies, magnetic fields, and temperatures where superconductors exhibit substantially lower effective microwave resistance than normal conductors.