Impurity spins in crystal matrices are promising components in quantum technologies, particularly if they can maintain their spin properties when close to surfaces and material interfaces. Here, we investigate an attractive candidate for microwave-domain applications, the spins of group-VI ^{125}Te^{+} donors implanted into natural Si at depths as shallow as 20 nm. We show that surface band bending can be used to ionize such near-surface Te to spin-active Te^{+} state, and that optical illumination can be used further to control the Te donor charge state. We examine spin activation yield, spin linewidth, and relaxation (T_{1}) and coherence times (T_{2}) and show how a zero-field 3.5 GHz "clock transition" extends spin coherence times to over 1 ms, which is about an order of magnitude longer than other near-surface spin systems.

Rare-earth doped crystals have long coherence times and the potential to provide quantum interfaces between microwave and optical photons. Such applications benefit from a high cooperativity between the spin ensemble and a microwave cavity -- this motivates an increase in the rare earth ion concentration which in turn impacts the spin coherence lifetime. We measure spin dynamics of two rare-earth spin species, $^{145}$Nd and Yb doped into Y$_{2}$SiO$_{5}$, coupled to a planar microwave resonator in the high cooperativity regime, in the temperature range 1.2 K to 14 mK. We identify relevant decoherence mechanisms including instantaneous diffusion arising from resonant spins and temperature-dependent spectral diffusion from impurity electron and nuclear spins in the environment. We explore two methods to mitigate the effects of spectral diffusion in the Yb system in the low-temperature limit, first, using magnetic fields of up to 1 T to suppress impurity spin dynamics and, second, using transitions with low effective g-factors to reduce sensitivity to such dynamics. Finally, we demonstrate how the `clock transition' present in the $^{171}$Yb system at zero field can be used to increase coherence times up to $T_{2} = 6(1)$ ms.

Impurity spins in crystal matrices are promising components in quantum technologies, particularly if they can maintain their spin properties when close to surfaces and material interfaces. Here, we investigate an attractive candidate for microwave-domain applications, the spins of group-VI ^{125}Te^{+} donors implanted into natural Si at depths as shallow as 20 nm. We show that surface band bending can be used to ionize such near-surface Te to spin-active Te^{+} state, and that optical illumination can be used further to control the Te donor charge state. We examine spin activation yield, spin linewidth, and relaxation (T_{1}) and coherence times (T_{2}) and show how a zero-field 3.5 GHz "clock transition" extends spin coherence times to over 1 ms, which is about an order of magnitude longer than other near-surface spin systems.

Impurity spins in crystal matrices are promising components in quantum technologies, particularly if they can maintain their spin properties when close to surfaces and material interfaces. Here, we investigate an attractive candidate for microwave-domain applications, the spins of group-VI Impurity spins in crystal matrices are promising components in quantum technologies, particularly if they can maintain their spin properties when close to surfaces and material interfaces. Here, we investigate an attractive candidate for microwave-domain applications, the spins of group-VI $^{125}$Te$^+$ donors implanted into natural Si at depths as shallow as 20~nm. We show that surface band-bending can be used to ionise such near-surface Te to spin-active Te$^+$ state, and that optical illumination can be used further to control the Te donor charge state. We examine spin activation yield, spin linewidth, relaxation ($T_1$) and coherence times (\ttwo) and show how a zero-field 3.5~GHz `clock transition' extends spin coherence times to over 1~ms, which is about an order of magnitude longer than other near-surface spin systems.

Mantas Šimėnas, ∗ James O’Sullivan, ∗ Oscar W. Kennedy, ∗ Sen Lin, Sarah Fearn, Christoph W. Zollitsch, Gavin Dold, Tobias Schmitt, Peter Schüffelgen, Ren-Bao Liu, and John J. L. Morton 5, † London Centre for Nanotechnology, UCL, 17-19 Gordon Street, London, WC1H 0AH, UK Department of Physics, Centre for Quantum coherence and The Hong Kong Institute of Quantum Information Science and Technology, The Chinese University of Hong Kong, Hong Kong, China Department of Materials, Imperial College London, London SW7 2BX, UK Institute for Semiconductor Nanoelectronics, Peter Grünberg Institute 9, Forschungszentrum Jülich and RWTH Aachen University, Germany Department of Electrical and Electronic Engineering, UCL, Malet Place, London, WC1E 7JE, UK

Rare-earth doped crystals have proven to be of considerable interest for quantum information processing, with demonstrations of quantum memories and the potential for coherent microwave–optical conversion as part of a quantum network. By interfacing these rare-earth based technologies with superconducting devices, they can complement existing technologies — which form the basis of many state-of-the-art quantum computers — forming a hybrid quantum information processing architecture exhibiting the advantages of fast superconducting processors alongside the extended coherence time and frequency conversion available from rare-earths. This thesis demonstrates the suitability of a widely used rare-earth doped crystal, yttrium orthosilicate (Y2SiO5 or YSO) doped with 145Nd or with 171Yb, as a substrate for fabrication of superconducting resonators. Designs for these resonators are presented with simulations of their electromagnetic modes, and their fabrication detailed. These devices are cooled to milliKelvin temperatures in a dilution refrigerator in order to investigate properties of the device and its coupling to electron spin transitions within the substrate where a coupling rate of the order gens/2π =1.2–7.1 MHz is observed. Dielectric loss due to two-level systems is measured with a loss-tangent tan δ = 4*10−6, and a coupling between resonator and spins is observed to have a high cooperativity C = 8–250. The superconducting resonator is then used to perform pulsed spectroscopy of electron spin resonance (ESR) transitions in the high-cooperativity regime, measuring two-pulse coherence times of T2 = (409 ± 14) μs for 200 ppm 145Nd:Y2SiO5 and T2 = (1170 ± 90) μs for 50 ppm 171Yb:Y2SiO5. Chirped pulses are used to invert spin magnetisation and drive arbitrary rotations, enabling the measurement of relaxation dynamics and Rabi oscillations. Dynamical decoupling sequences are explored as a method for extending coherence time. Experiments exploiting coherence-enhancing 'near-ZEFOZ' transitions in 171Yb are proposed, and continuous-wave measurements are used to demonstrate the working principle of such a scheme.

The quantum spin properties of nitrogen-vacancy defects in diamond enable diverse applications in quantum computing and communications 1 . However, fluorescent nanodiamonds also have attractive properties for in vitro biosensing, including brightness 2 , low cost 3 and selective manipulation of their emission 4 . Nanoparticle-based biosensors are essential for the early detection of disease, but they often lack the required sensitivity. Here we investigate fluorescent nanodiamonds as an ultrasensitive label for in vitro diagnostics, using a microwave field to modulate emission intensity 5 and frequency-domain analysis 6 to separate the signal from background autofluorescence 7 , which typically limits sensitivity. Focusing on the widely used, low-cost lateral flow format as an exemplar, we achieve a detection limit of 8.2 × 10 −19 molar for a biotin–avidin model, 10 5 times more sensitive than that obtained using gold nanoparticles. Single-copy detection of HIV-1 RNA can be achieved with the addition of a 10-minute isothermal amplification step, and is further demonstrated using a clinical plasma sample with an extraction step. This ultrasensitive quantum diagnostics platform is applicable to numerous diagnostic test formats and diseases, and has the potential to transform early diagnosis of disease for the benefit of patients and populations.

We show experimentally and describe theoretically how a conventional magnetic resonance Hahn echo sequence can lead to a self-stimulated pulse echo train when an inhomogeneously broadened spin ensemble is coupled to a resonator. Effective strong coupling between the subsystems assures that the first Hahn echo can act as a refocusing pulse on the spins, leading to self-stimulated secondary echoes. Within the framework of mean field theory, we show that this process can continue multiple times leading to a train of echoes. We introduce an analytical model that explains the shape of the first echo and numerical results that account well for the experimentally observed shape and strength of the echo train and provides insights into the collective effects involved.

Yttrium orthosilicate (Y$_2$SiO$_5$, or YSO) has proved to be a convenient host for rare-earth ions used in demonstrations of microwave quantum memories and optical memories with microwave interfaces, and shows promise for coherent microwave--optical conversion owing to its favourable optical and spin properties. The strong coupling required by such microwave applications could be achieved using superconducting resonators patterned directly on Y$_2$SiO$_5$, and hence we investigate here the use of Y$_2$SiO$_5$ as an alternative to sapphire or silicon substrates for superconducting hybrid device fabrication. A NbN resonator with frequency 6.008 GHz and low power quality factor $Q \approx 400000$ was fabricated on a Y$_2$SiO$_5$ substrate doped with isotopically enriched Nd$^{145}$. Measurements of dielectric loss yield a loss-tangent $\tan\delta = 4 \times 10^{-6}$, comparable to sapphire. Electron spin resonance (ESR) measurements performed using the resonator show the characteristic angular dependence expected from the anisotropic Nd$^{145}$ spin, and the coupling strength between resonator and electron spins is in the high cooperativity regime ($C = 30$). These results demonstrate Y$_2$SiO$_5$ as an excellent substrate for low-loss, high-Q microwave resonators, especially in applications for coupling to optically-accessible rare earth spins.

Superconducting resonators interfaced with paramagnetic spin ensembles are used to increase the sensitivity of electron spin resonance experiments and are key elements of microwave quantum memories. Certain spin systems that are promising for such quantum memories possess 'sweet spots' at particular combinations of magnetic fields and frequencies, where spin coherence times or linewidths become particularly favorable. In order to be able to couple high-Q superconducting resonators to such specific spin transitions, it is necessary to be able to tune the resonator frequency under a constant magnetic field amplitude. Here, we demonstrate a high quality, magnetic field resilient superconducting resonator, using a 3D vector magnet to continuously tune its resonance frequency by adjusting the orientation of the magnetic field. The resonator maintains a quality factor of $> 10^5$ up to magnetic fields of 2.6 T, applied predominantly in the plane of the superconductor. We achieve a continuous tuning of up to 30 MHz by rotating the magnetic field vector, introducing a component of 5 mT perpendicular to the superconductor.