Solid-state quantum sensors are attracting wide interest because of their exceptional sensitivity at room temperature. In particular, the spin properties of individual nitrogen vacancy (NV) color centers in diamond make it an outstanding nanoscale se
nsor of magnetic fields, electric fields, and temperature, under ambient conditions. Recent work on ensemble NV-based magnetometers, inertial sensors, and clocks have employed $N$ unentangled color centers to realize a factor of up to $sqrt{N}$ improvement in sensitivity. However, to realize fully this signal enhancement, new techniques are required to excite efficiently and to collect fluorescence from large NV ensembles. Here, we introduce a light-trapping diamond waveguide (LTDW) geometry that enables both high fluorescence collection ($sim20%$) and efficient pump absorption achieving an effective path length exceeding $1$ meter in a millimeter-sized device. The LTDW enables in excess of $2%$ conversion efficiency of pump photons into optically detected magnetic resonance (ODMR) fluorescence, a textit{three orders of magnitude} improvement over previous single-pass geometries. This dramatic enhancement of ODMR signal enables broadband measurements of magnetic field and temperature at less than $1$ Hz, a frequency range inaccessible by dynamical decoupling techniques. We demonstrate $sim 1~mbox{nT}/sqrt{mbox{Hz}}$ magnetic field sensitivity for $0.1$ Hz to $10$ Hz and a thermal sensitivity of $sim 400 ~mumbox{K}/sqrt{mbox{Hz}}$ and estimate a spin projection limit at $sim 0.36$ fT/$sqrt{mbox{Hz}}$ and $sim 139~mbox{pK}/sqrt{mbox{Hz}}$, respectively.
Single photons are fundamental elements for quantum information technologies such as quantum cryptography, quantum information storage and optical quantum computing. Colour centres in diamond have proven to be stable single photon sources and thus es
sential components for reliable and integrated quantum information technology. A key requirement for such applications is a large photon flux and a high efficiency. Paying tribute to various attempts to maximise the single photon flux we show that collection efficiencies of photons from colour centres can be increased with a rather simple experimental setup. To do so we spin-coated nanodiamonds containing single nitrogen-vacancy colour centres on the flat surface of a ZrO2 solid immersion lens. We found stable single photon count rates of up to 853 kcts/s at saturation under continuous wave excitation while having excess to more than 100 defect centres with count rates from 400 kcts/s to 500 kcts/s. For a blinking defect centre we found count rates up to 2.4 Mcts/s for time intervals of several ten seconds. It seems to be a general feature that very high rates are accompanied by a blinking behaviour. The overall collection efficiency of our setup of up to 4.2% is the highest yet reported for N-V defect centres in diamond. Under pulsed excitation of a stable emitter of 10 MHz, 2.2% of all pulses caused a click on the detector adding to 221 kcts/s thus opening the way towards diamond based on-demand single photon sources for quantum applications.
