Hands-on quantum sensing with NV - centers in diamond J. L. Sánchez Toural * , V. Marzoa * , R. Bernardo-Gavito * , J. L. Pau ** and D. Granados *1 * IMDEA-Nanociencia, Calle Faraday 9, 28049, Madrid, Spain ** Departamento de Física Aplicada, Universidad Autónoma de Madrid, 28049, Madrid, Spain 1 I. ABSTRACT The physical properties of diamond crystals, such as color or electrical conductivity, can be controlled via impurities. In particular, when doped with nitrogen and under certain conditions, optically active nitrogen-vacany centers (NV ) can be induced. The center is an outstanding quantum spin system that en- ables, under ambient conditions, optical initialization, readout, and coherent microwave control with applications in sensing and quantum information. Under optical and radio frequency excitation, the Zeeman splitting of the degenerate states allows the quantitative mea- surement of external magnetic fields with high sensitivity. This study provides a pedagogical introduction to the prop- erties of the NV centers as well as a step-by-step process to develop and test, a simple magnetic quantum sensor based on color centers with large potential for the development of highly compact multi-sensor systems. Keywords: Quantum sensing. Diamond. Magnetometry at room temperature. Color centers. NV centers. Microwaves. II. I NTRODUCTION The name diamond comes from the Greek adamantem which means invincible. Diamond is an electrical insulator whose strong covalent bonds make it a material with extraor- dinary hardness, broadband optical transparency and extremely high thermal conductivity. In addition, it can withstand large electric fields and, when doped, behaves like a semiconductor. Diamonds are associated with the idea of perfection. How- ever they are rarely perfect and lattice irregularities or impu- rities are very common. Artificial diamonds can be produced artificially. The nature and density of impurities can be controlled during or after their growth. This alters their physical properties, such as color or electrical conductivity. Optically active defects are called color centers or NV centers. In the NV color center, a nitrogen atom substitutes a carbon atom and a vacancy, in one of four adjacent positions, replaces another carbon atom. In this configuration, one electron is 1 corresponding author: D. Granados (daniel.granados@imdea.org) unpaired and remains trapped inside the vacancy. The center is charged negatively when it captures an additional electron, usually from a nitrogen atom donor in the lattice. The spin state of the two electrons quantum system, can be controlled using microwave pulses and optically addressed by measuring the photoluminescence [1]. All this, together with the long coherence time of the quantum state, and possibility of working at room temperature, makes them an ideal physical platform for the development of a magnetic sensor, with unprecedented performance. The spin orientation of the two electrons trapped inside the center is aligned with the axis of symmetry (the line joining the vacancy and the nitrogen). Under 532 nm laser light illumination pulse, the center fluoresces emitting a photon in the red spectrum. Exciting with microwaves, the fluorescence changes in such a way that it is possible to determine the external magnetic field [2]. Current technologies providing a high magnetic sensitivity such as optical pumped magnetometry (OPM) [3], supercon- ducting quantum interference devices (SQUID) [4], micro- electromechanical systems (MEMS) [5] or magnetic resonance force microscopy (MRFM) [6] are highly successful technolo- gies that have made possible the measurement of the magnetic field generated by neuronal activity with great precision. These systems have been previously described and compared [7]. A SQUID is based on superconducting loops containing Josephson junctions [9]. It is a very sensitive magnetometer used to measure extremely small magnetic fields, sensitive enough to measure fields as low as 5 × 10 -14 T with a noise equivalent field of about 3 fT · Hz -1/2 . For the sake of comparison, it is important to notice that a common small neodymium magnet produces a magnetic field of about 10 -2 T , and neural activity in animals produces magnetic fields between 10 -9 T and 10 -6 T . Spin exchange relaxation- free (SERF) magnetometers measure magnetic fields by using lasers to detect the interaction of the magnetic field with alkali metal atoms in a vapor. They are potentially more sensitive and do not require cryogenic refrigeration but are orders of magnitude larger in size ( 1 cm 3 ) and must be operated in a near-zero magnetic field. SQUID requires cryogenics, OPM reduces the sensitivity when the device reduces its dimension [10] due to the atomic collisions that alter the spin. These technologies show lim- itations for miniaturization, since either they require special conditions and bulky instrumentation or their temporal reso- lution decreases when trying to reduce their dimensions. The quantum state of a color center in diamond can be