The quantum spin properties of nitrogen-vacancy defects in diamond have diverse applications including quantum computing and communications 1 , but 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 vital for early disease detection, however, often lack the required sensitivity. Here we investigated fluorescent nanodiamonds as an ultra-sensitive 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.We focused on the common, low-cost lateral flow format as an exemplar, achieving detection limits of 8.2 × 10 −19 M for a biotin-avidin model, 10 5 -fold more sensitive than gold nanoparticles; and a use-case demonstration of single-copy detection of HIV-1 RNA with a short 10-minute isothermal amplification step, including a pilot using a clinical plasma sample with an extraction step. This ultra-sensitive quantum-diagnostics platform is applicable to numerous diagnostic test formats and diseases with the potential to transform early diagnosis, benefiting patients and populations.Rapid point-of-care tests have transformed access to disease testing in a variety of community settings, including clinics, pharmacies and the home 30 . Among the most common tests worldwide are paper microfluidic lateral flow assays (LFAs), with 276 million sold in 2017 for malaria alone 31 . LFAs satisfy many of the REASSURED criteria 32 for diagnostics, however, despite widespread use they are still limited by inadequate sensitivity to detect the low levels of biomarkers necessary for early disease detection.Fluorescent markers can be highly sensitive, but are practically limited by background fluorescence from the sample, substrate, or readout technique. In the case of nitrocellulose substrates used in LFAs, there is a significant background autofluorescence 7 , which inherently limits sensitivity. Various methods have been reported to reduce this effect, such as membrane modification to reduce background fluorescence 33 , exciting in the nearinfrared range and using upconverting nanoparticles 34 , and time-gated detection using longpersistent phosphors 35 to separate background fluorescence, which has a shorter lifetime.These methods have shown ∼10-fold improvements in sensitivity over gold nanoparticles, limited by relatively low brightness.Here we show the use of FNDs as a fluorescent label in an LFA format as a demonstrator of their first use for in vitro diagnostics, taking advantage of their high brightness and selective modulation. The use of a narrowband resonator allows for the lowpower generation of microwave-frequency electromagnetic fields, suitable for a point-ofcare device, to efficiently separate the signal from the background in the frequency domain by lock-in 6 detection. We aimed, after characterisation, functionalisation,...
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.
Yttrium orthosilicate (Y2SiO5, 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 Y2SiO5, and hence we investigate here the use of Y2SiO5 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 ≈ 400 000 was fabricated on a Y2SiO5 substrate doped with isotopically enriched 145 Nd. Measurements of dielectric loss yield a loss-tangent tan δ = 4 × 10 −6 , comparable to sapphire. Electron spin resonance (ESR) measurements performed using the resonator show the characteristic angular dependence expected from the anisotropic 145 Nd spin, and the coupling strength between resonator and electron spins is in the high cooperativity regime (C = 30). These results demonstrate Y2SiO5 as an excellent substrate for low-loss, high-Q microwave resonators, especially in applications for coupling to optically-accessible rare earth spins.
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