New sensitivity record reached for electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy, also known as electron spin resonance spectroscopy, is a technique for analyzing paramagnetic species in a sample. For over 70 years, EPR spectroscopy has been a staple of diverse fields such as dosimetry, archaeology, biochemistry and materials science.

Despite its widespread use, the method has had limited sensitivity and requires many spins to produce a detectable signal. In Applied Physics Letters, an international team of researchers reports their attempt to increase the sensitivity of inductively detected EPR spectroscopy by means of quantum technologies based on superconducting circuits. By optimizing the superconducting resonator design, they demonstrate that sensitivity can be enhanced to the level of 65 spins detectable in a 1-second integration time — the best reported sensitivity ever achieved and a new state of the art.

In 2016, the same group demonstrated a sensitivity of 2000 spins detectable in 1-second integration time. According to author Patrice Bertet, they improved upon that previous work with ultralow-noise microwave detection and stronger coupling of the spins to the resonant circuit. Superconducting amplifiers enabled ultralow-noise amplification of the spin signals, down to the quantum limit. A stronger spin-microwave coupling came from superconducting microresonators that confined the drive and detection microwave field at the spin location.

An unexpected outcome of the work involved the Carr-Purcell-Meiboom-Gill pulse sequence, which further enhances sensitivity by using multiple refocusing pulses to generate several echoes per sequence. Experimentally, the sensitivity gain proved less than predicted, with the discrepancy traced to the presence of phase noise in the superconducting microresonators.

The team continues to pursue even greater spin sensitivity of EPR spectroscopy, with the ultimate goal of single-spin sensitivity, by further reducing the resonator dimensions.

Source: “Inductive-detection electron-spin resonance spectroscopy with 65 spins/√Hz sensitivity,” by S. Probst, A. Bienfait, P. Campagne-Ibarcq, J. J. Pla, B. Albanese, J. F. Da Silva Barbosa, T. Schenkel, D. Vion, D. Esteve, K. Mølmer, J. J. L. Morton, R. Heeres, and P. Bertet, Applied Physics Letters (2017). The article can be accessed at https://doi.org/10.1063/1.5002540 .