Engineering and understanding microwave resonators is essential for the efficient electron-spin control and readout in electron-spin-resonance (ESR) spectroscopy. This work, focused on superconducting planar microresonators, presents finite-element simulations modeling the spin-photon coupling rate, in quantitative agreement with experimental data. Three resonator designs are optimized to address spectroscopy applications in $\ensuremath{\mu}$m-thick samples, with one exhibiting notably good magnetic field homogeneity at microwave frequencies.
Engineering and understanding microwave resonators is essential for the efficient electron-spin control and readout in electron-spin-resonance (ESR) spectroscopy. This work, focused on superconducting planar microresonators, presents finite-element simulations modeling the spin-photon coupling rate, in quantitative agreement with experimental data. Three resonator designs are optimized to address spectroscopy applications in \ensuremathμm-thick samples, with one exhibiting notably good magnetic field homogeneity at microwave frequencies.
Superconducting resonators are widely used in fields spanning from quantum computing to electron spin resonance (ESR) spectroscopy. With the goal of realizing superconducting resonators, a broad variety and combination of superconducting materials, substrates and fabrication processes have been used and thoroughly reported in the literature. High temperature superconductors such as YBCO and low temperature superconductors such as Nb, NbN, NbTiN and Al are the major actors in the domain. In this work, we investigate the possibility to extend the family of suitable low temperature superconductors for the realization of planar superconducting microwave resonators for future ESR applications. In particular, this study focuses on NbTi, a widely used material to realize superconducting cables but not investigated for planar resonating structures at GHz frequencies. A 150 nm thick film of NbTi is sputtered and patterned on top of an Al2O3 substrate. For devices resonating around 6.8 GHz quality factors greater than 10,000 are observed at 3 K and in magnetic fields up to 250 mT.
Electron spin resonance (ESR) is a powerful spectroscopy method which allows to identify and characterize paramagnetic species. It usually relies on detecting microwave radiation absorbed or emitted by the spins into a microwave resonator tuned to their Larmor precession frequency. Because of the weak spin-microwave coupling, conventional ESR spectroscopy has a low sensitivity. Recent experiments have demonstrated that superconducting quantum circuits have the potential to enhance the spin detection sensitivity to single spin detection; by namely using high quality factor small mode volume resonator. However, these demonstrations have been realized in very restrictive conditions, using well-known spin systems. To probe samples coming from chemistry or biology, quantum-circuit enabled spectrometers have to be adapted to be robust to losses. They have typically two origins: the dielectric losses introduced by the samples, mostly aqueous, and losses coming from the strong magnetic field required to tune the electronic spins to the resonator frequency. This PhD work centered on implementing a resonator that would achieve the same ESR sensitivity while being robust to dielectric losses and strong magnetic fields. Using NbTiN films deposited on sapphire, microwave resonators were shown to have quality factors above Q=105 in magnetic field, even when samples with dielectric losses above tan d > 0.05 were introduced. A study of the resonator intrinsic losses showed marked drops at multiple specific magnetic fields: a signature of coupling to spurious systems. Over the course of the PhD, mitigation nanofabrication techniques for these contaminants were developed, such as hard mask etching and annealing, to remove all but one of this contaminants. Finally, test of the spectrometer on a benchmark species (BDPA, α,γ-Bisdiphenylene-β-phenylallyl, a known ESR marker) were realized.
In one branch of quantum information processing, high cooperativity between spins and a resonator is essential. Planar superconducting resonators present an opportunity for miniaturization and fully integrated circuitry, but unfortunately their cooperativity is a fraction of that of their bulk counterparts. This study designs a planar microwave resonator to achieve homogeneous magnetic field, and utilizes the anisotropic g-factor tensor of erbium spins in yttrium orthosilicate for maximum spin-photon coupling. Cooperativity on par with that of a three-dimensional cavity is achieved, marking an exciting step toward integrated circuits for spin quantum information processing.
Hybrid quantum systems with high-Tc superconducting resonators
Superconducting microwave resonators are crucial elements of microwave circuits, offering a wide range of potential applications in modern science and technology. While conventional low-T[Formula: see text] superconductors are mainly employed, high-T[Formula: see text] cuprates could offer enhanced temperature and magnetic field operating ranges. Here, we report the realization of [Formula: see text] superconducting coplanar waveguide resonators, and demonstrate a continuous evolution from a lossy undercoupled regime, to a lossless overcoupled regime by adjusting the device geometry, in good agreement with circuit model theory. A high-quality factor resonator was then used to perform electron spin resonance measurements on a molecular spin ensemble across a temperature range spanning two decades. We observe spin-cavity hybridization indicating coherent coupling between the microwave field and the spins in a highly cooperative regime. The temperature dependence of the Rabi splitting and the spin relaxation time point toward an antiferromagnetic coupling of the spins below 2 K. Our findings indicate that high-Tc superconducting resonators hold great promise for the development of functional circuits. Additionally, they suggest novel approaches for achieving hybrid quantum systems based on high-T[Formula: see text] superconductors and for conducting electron spin resonance measurements over a wide range of magnetic fields and temperatures.
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 or high electron mobility transistor coupled with NbTi or YBa2Cu3O7 (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/Hz1/2 (−139 dBc/Hz), the best spin sensitivity is about 1×1010 spins/Hz1/2, and the best concentration sensitivity is about 3×1018 spins/Hz1/2m3. 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.
Planar superconducting microwave transmission line resonators can be operated at multiple harmonic resonance frequencies. This allows covering wide spectral regimes with high sensitivity, as it is desired, e.g., for cryogenic microwave spectroscopy. A common complication of such experiments is the presence of undesired “spurious” additional resonances, which are due to standing waves within the resonator substrate or housing box. Identifying the nature of individual resonances (“designed” vs “spurious”) can become challenging for higher frequencies or if elements with unknown material properties are included, as is common for microwave spectroscopy. Here, we discuss various experimental strategies to distinguish designed and spurious modes in coplanar superconducting resonators that are operated in a broad frequency range up to 20 GHz. These strategies include tracking resonance evolution as a function of temperature, magnetic field, and microwave power. We also demonstrate that local modification of the resonator, by applying minute amounts of dielectric or electron spin resonance-active materials, leads to characteristic signatures in the various resonance modes, depending on the local strength of the electric or magnetic microwave fields.