Probing phase transitions in a soft matter system using a single spin quantum sensor.

Phase transitions in soft matter systems reveal some of the interesting structural phenomena at the levels of individual entities constituting those systems. The relevant energy scales in soft matter systems are comparable to thermal energy (k(B)T similar to 10(-21) J). This permits one to observe interesting structural dynamics even at ambient conditions. However, at the nanoscale most experimental probes currently being used to study these systems have been either plagued by low sensitivity or are invasive at molecular scales. Nitrogen-vacancy (NV) centers in diamond is emerging as a robust quantum probe for precision metrology of physical quantities (e.g. magnetic field, electric field, temperature, and stress). Here, we demonstrate by using NV sensors to probe spin-fluctuations and temperature simultaneously to obtain information about controlled phase changes in a soft matter material as a function of temperature. The soft matter system chosen for the study is a standard liquid crystalline (LC) material which shows distinct phases close to room temperature. Individual NV centers at depths of a few nm are used as a probe to detect magnetic signals emanating from a few molecular layers of sample on the surface of the diamond. The organization and collective dynamics of LC molecules in nanoscopic volumes are discussed. Our study aims to extend the areas of application of quantum sensing using NV centers to probe the soft matter systems, particularly those exhibiting mesophases and interesting interfacial properties

Dynamical sensitivity control of a single-spin quantum sensor.

The Nitrogen-Vacancy (NV) defect in diamond is a unique quantum system that offers precision sensing of nanoscale physical quantities at room temperature beyond the current state-of-the-art. The benchmark parameters for nanoscale magnetometry applications are sensitivity, spectral resolution, and dynamic range. Under realistic conditions the NV sensors controlled by conventional sensing schemes suffer from limitations of these parameters. Here we experimentally show a new method called dynamical sensitivity control (DYSCO) that boost the benchmark parameters and thus extends the practical applicability of the NV spin for nanoscale sensing. In contrast to conventional dynamical decoupling schemes, where π pulse trains toggle the spin precession abruptly, the DYSCO method allows for a smooth, analog modulation of the quantum probe's sensitivity. Our method decouples frequency selectivity and spectral resolution unconstrained over the bandwidth (1.85 MHz-392 Hz in our experiments). Using DYSCO we demonstrate high-accuracy NV magnetometry without |2π| ambiguities, an enhancement of the dynamic range by a factor of 4 · 103, and interrogation times exceeding 2 ms in off-the-shelf diamond. In a broader perspective the DYSCO method provides a handle on the inherent dynamics of quantum systems offering decisive advantages for NV centre based applications notably in quantum information and single molecule NMR/MRI

Laser-induced heating in a high-density ensemble of nitrogen-vacancy centers in diamond and its effects on quantum sensing.

Among quantum sensors, the single nitrogen-vacancy (NV) defect in diamond has the highest sensitivity-to-size factor. For instance, a single-NV spin when used as a magnetometer could achieve sensitivities of the order of nT/root Hz, while the dimensions of the sensor are merely atomic in size. The sensitivity is limited only by the photon shot noise. One method to boost the magnetometer sensitivity to pico-tesla scales is to use many NV defect centers as an ensemble sensor [Phys. Rev. X 5, 041001 (2015)]. However, during the absorption-emission (fluorescence) optical cycles, the NV centers transfer a portion of the irradiation laser energy into phonons and heat the diamond matrix. This results in unintended fluorescence decrease, spin resonance lines shifts, and fluctuations. Hence, the advantages gained by packing a high density of NV centers are significantly reduced. Here we investigate the heat generation of ensemble NV centers in micrometer-sized diamond under 532 nm laser irradiation and its effects pertaining to sensing applications. These investigations help us to find strategies that mitigate the detrimental effects of heating and yet permits the use of ensemble NV defects for improved metrology applications

Enhancing fluorescence excitation and collection from the nitrogen-vacancy center in diamond through a micro-concave mirror.

We experimentally demonstrate a simple and robust optical fiber based method to achieve simultaneously efficient excitation and fluorescence collection from Nitrogen-Vacancy (NV) defects containing micro-crystalline diamond. We fabricate a suitable micro-concave mirror that focuses scattered excitation laser light into the diamond located at the focal point of the mirror. At the same instance, the mirror also couples the fluorescence light exiting out of the diamond crystal in the opposite direction of the optical fiber back into the optical fiber within its light acceptance cone. This part of fluorescence would have been otherwise lost from reaching the detector. Our proof-of-principle demonstration achieves a 25 times improvement in fluorescence collection compared to the case of not using any mirrors. The increase in light collection favors getting high signal-to-noise ratio optically detected magnetic resonance signals and hence offers a practical advantage in fiber-based NV quantum sensors. Additionally, we compacted the NV sensor system by replacing some bulky optical elements in the optical path with a I x 2 fiber optical coupler in our optical system. This reduces the complexity of the system and provides portability and robustness needed for applications like magnetic endoscopy and remote-magnetic sensing. Published by AIP Publishing