Developing an inductive spectrometer for electron paramagnetic resonance detection and imaging at the micron scale using superconducting quantum circuits.

Introduction

Electron paramagnetic resonance (EPR) is a powerful spectroscopy method that allows for the identification of paramagnetic species and quantification of their interactions with their environment.

Sensitivity Limitations

Due to the weak spin-microwave coupling, conventional EPR spectroscopy has a low sensitivity, which limits its use to samples of macroscopic size. Recent experiments have demonstrated that superconducting quantum circuits have the potential to drastically enhance spin detection sensitivity, down to the detection of approximately 10 spins within 5 fL.

Current Limitations

However, these demonstrations have so far been conducted using well-known model spin systems and in restrictive conditions, including:

• Very narrow spin and detector linewidths
• Extremely low microwave losses
• Low static magnetic fields

These conditions are incompatible with typical EPR spectroscopy operations, such as probing aqueous or non-crystalline samples, applying strong magnetic fields, or studying species with short coherence lifetimes or spin-spin interactions that require large excitation bandwidths.

Proposed Solution

The restrictive conditions of these proof-of-concept experiments are not a prerequisite for achieving high-sensitivity EPR detection. Using recent advances in the fabrication process and the design of quantum circuits, I propose to lift these restrictions and build a quantum-circuit-based EPR spectrometer capable of probing a wide range of spin species and detecting, characterizing, and imaging EPR signals in micron-sized samples.

Goals

We will meet this goal by:

1. Developing a resilient high-sensitivity spectrometer capable of probing spins with short coherence times and characterizing spin-spin interactions.
2. Implementing imaging techniques with sub-micron resolution.
3. Benchmarking our spectrometer for typical volume-limited applications.

Research Implications

Our EPR spectrometer will open interesting research paths in biology, chemistry, or condensed matter, for instance, by allowing the detection of EPR signals in single cells, micro-protein crystals, or from organic semiconductors.