UCL Quantum Science and Technology Institute


Quantum technology know-how improves sensitivity for ESR spectroscopy

30 November 2020

UCLQ-led research demonstrates that new ESR setup with a cryogenic preamplifier shortens ESR experiments by 50 times at cryogenic temperature.

Plots display pulsed ESR spectra of Bos Taurus complex I obtained after one-hour long measurements with (left) and without (right) the preamplifier.

Electron spin resonance (ESR) spectroscopy is a method used to manipulate and study unpaired electron spins and their environment using microwaves and strong magnetic fields. It is used in various fields of science and industry ranging from quantum technologies to understanding the properties and behaviours of proteins and biomaterials.

Typically, ESR signals are very weak and many hours or even days are required to make measurements. To improve spectrometer sensitivity, the signals are amplified before reaching the detector.

In currently available commercial ESR spectrometers, the first signal amplifier is situated at room temperature, while the sample is frequently placed at cryogenic temperature. This is far from ideal, as room temperature thermal noise significantly degrades the sensitivity.

Research by a multidisciplinary team from UCLQ, Imperial College London, ETH Zürich, and Bielefeld University, published in Journal of Magnetic Resonance, details a new set-up for ESR spectroscopy equipping an X-band ESR probehead with an ultra low-noise cryogenic microwave preamplifier.

Taking inspiration from frequently implemented set-ups used in quantum technology applications and NMR spectroscopy, corresponding author, UCLQ Research Fellow Dr Mantas Šimėnas, said: “To improve spectrometer sensitivity our solution is to put the signal amplifier as close to the sample as possible and then cool down the amplifier, ESR probehead and sample to low temperatures together.  Such very low-noise microwave amplifiers are currently frequently used in dilution fridge setups for quantum technology applications. We wanted to transfer this idea to conventional ESR experiments by equipping commercially available ESR probeheads with a low-noise microwave preamplifier, which can be cooled together with the sample.”

In conventional pulsed ESR experiments, researchers typically excite spin systems using very powerful and short microwave pulses thousands of times per second. The resulting spin response (so called spin echo), which researchers want to measure, is very weak and it is emitted almost immediately after the microwave pulses. This poses a challenge how to detect the spin signal and simultaneously protect the amplifier from the high energy pulses. To overcome these obstacles, the team had to introduce and accommodate an additional microwave circuit on the ESR probehead, which protects the amplifier from the high-power pulses, while maintaining typical functionalities of the probehead.

Researchers managed to shorten the ESR measurement time by about 50 times at low temperature. In addition the researchers found that an impressive 200 times reduction in measurement time could be obtained for some ESR experiments, which do not require a broad excitation bandwidth. The probehead performance was demonstrated on typical ESR samples of biological origin, supplied by the UCLQ team’s collaborators.

The reduction in measurement time means that experiments which would usually take days now can be done in tens of minutes. The team also anticipate that their upgrade will allow measurements of previously inaccessible samples that for example have much lower spin concentration or can only be produced in very low quantities.

This research was funded by UK Engineering and Physical Sciences Research Council, the European Research Council, Deutsche Forschungsgemeinschaft, and the Leverhulme Trust.


Image: Plots display pulsed ESR spectra of Bos Taurus complex I obtained after one-hour long measurements with (left) and without (right) the preamplifier. (Credit: M. Šimėnas et al. Journal of Magnetic Resonance (2020), doi: https://doi.org/10.1016/j.jmr.2020.106876)