# NV-Diamond

Optical sensing of magnetic fields, electric fields, rotations, and temperature can be achieved using negatively charged nitrogen-vacancy (NV) centers in diamond. Single NV centers and ensembles of NV centers can be detected with high spatial resolution and can be used as sensors with nm,μm, or mm resolution.

Optical sensing of magnetic fields, electric fields, rotations, and temperature can be achieved using negatively charged nitrogen-vacancy (NV) centers in diamond. Single NV centers and ensembles of NV centers can be detected with high spatial resolution and can be used as sensors with nm,µm, or mm resolution.
The activities in Mainz include several different setups.

## All-optical diamond-based magnetometry

Most commonly microwave fields are employed, to use the color center in diamonds as a sensor. However, there are cases where the use of strong MW fields proves to be detrimental to the sensing protocol and therefore can prohibit the employment of an NV-based sensor. An example is the detection of MFs generated by eddy currents in conductive materials in the context of magnetic induction tomography (MIT), a research application currently undertaken in our laboratory, where the presence of a conductive structure under examination will heavily affect the application of MW to the diamond. Another example is MF mapping of conductive, magnetic structures.

We employ a multi-pronged approach to this problem, the first is straight forward and was recently published in APL (Appl. Phys. Lett. 109, 053505).
It is based on magnetically sensitive features in the NV fluorescence. We demonstrated, that the ground-state level anti-crossing of the NV center at a background field of around 100\,mT can be used to measure small variations in magnetic fields. Research on this is still ongoing and investigates the optimization of the magnetic sensitivity as a function of diamond properties.

A second all-optical sensor experiment is currently being set-up. Here, we are trying to measure the ground state level splitting incorporating a lambda-scheme with the green pump light. While electro-magnetically induced transparency with diamonds has been demonstrated at cryogenic temperatures, the application-driven employment at room temprature could so far not be realized.

## Cavity-enhanced absorption sensor

Single negatively-charged nitrogen-vacancy (NV) centers and ensembles of NV centers in diamond can be used as nanoscale high spatial resolution sensors with a plethora of applications of magnetic and electric fields,
rotations, to strain and temperature. The majority of these sensors are based on the NV centers, photoluminescence (PL) and
suffer from low photon-detection efficiency and background
fluorescence. There have been attempts to increase the photon collection but still improvements must be made to reach the quantum projection
noise (PN) limited sensitivity associated with the
finite number of sensing spins. Compared to NV sensors based on PL detection, sensors based on detection of infrared (IR) absorption achieve high photon-detection efficiency and can reach a sensitivity closer to
the PN-limit when a cavity is used to enhance the detection-contrast even at room temperature.

## Single-NV setup

The other experiments according to NV color centers in diamond are performed using ensembles of NVs. In the single-NV setup we focus on only one color center to perform magnetic field sensing in a volume of several nm$^{3}$ around the NV. Sensitivities up to the single spin level have been shown.
Our Setup which is currently constructed aims for measuring single-molecule NMR of molecules on the diamond surface with a single shallow NV center. Therefore our goals with this setup is to extend the magnetic resonance techniques using NV-centers to Zero- and Ultralow-Field NMR (ZULF-NMR), an alternative technique where measurements are performed in the absence of a strong magnetic field, which is already used to investigate macroscopic NMR samples. ZULF-NMR allows us to perform single-molecule identification via analysis of J-coupling data.

## Annealing facility and singlet state search

This setup is basically used for two fundamentally different projects. A picture of the experimental apparatus is.
The experimental setup basically consists of a vacuum chamber with a vacuum up to $10^{-8}$ mbar with an oven inside consisting of a porolytic boron nitride heater plate surrounded by a heat shield.
This oven under high vacuum conditions can therefore be used for diamond annealing. A process where the diamond is heated to several hundreds of degree celcius (up to 1200). At these high temperatures the vacancies in the diamond start to diffuse and when they reach a substitutional Nitrogen atom they form a NV center.
The second project is to investigate the distance between the stable triplet and the metastable singlet ground state.
This energy difference can be calculated by measuring the thermal population of the singlet state via IR absorption.