Electron Spin Resonance (ESR) is an essential technique for characterising materials with unpaired electrons. Improving the sensitivity of ESR measurements is a major research goal to bring the benefits of such characterisation to ever smaller or more dilute samples.
This has been given added prominence by the rise in solid state quantum information processing technology with the concurrent investigation and manipulation of materials and surfaces in solid state devices at ever diminishing length scales.
We establish a testbed system for the development of high-sensitivity ESR techniques for small samples at millikelvin temperatures. Our system is centred on a high Q niobium nitride planar superconducting resonator designed to have a concentrated mode volume to couple to a small amount of paramagnetic material, with resilience to magnetic fields of up to 400 mT.
In our first set of measurements on our resonator `chip' we demonstrate high-cooperativity coupling between an organic radical microcrystal containing 10^{12} spins in a pico-litre volume, and our resonator mode, at 65 mK. Conventional ESR spectrometers by contrast ordinarily measure sample volumes a million times larger. We detect a saturation recovery relaxation rate via the dispersive frequency shift of the resonator. Techniques such as these could be suitable for reading out the quantum state of the spin ensemble in quantum information memory protocols.
The second set of measurements presented here demonstrate the capability of the pulsed ESR spectrometer developed in this thesis.
We use the custom-built framework to characterise a sample of rare earth-doped crystalline solid, potentially of interest as a quantum information storage medium, over 10 to 400 mK.
This experiment also reaches the high-cooperativity regime, mediated by the high Q superconducting resonator.
ESR measurements with even further enhanced coupling might eventually allow for pulsed ESR interrogation of very few spins and provide insights into the surface chemistry of, for example, material defects in superconducting quantum processors.
As such, the framework is of interest for developing on-chip ESR techniques of the kind that could, in the future, enhance our understanding of the materials of solid state quantum devices.Open Acces
