Qudi is a general, modular, multi-operating system suite written in Python 3 for controlling laboratory experiments. It provides a structured environment by separating functionality into hardware abstraction, experiment logic and user interface layers. The core feature set comprises a graphical user interface, live data visualization, distributed execution over networks, rapid prototyping via Jupyter notebooks, configuration management, and data recording. Currently, the included modules are focused on confocal microscopy, quantum optics and quantum information experiments, but an expansion into other fields is possible and encouraged. Qudi is available from https://github.com/Ulm-IQO/ qudi and is freely useable under the GNU General Public Licence.

Detecting and controlling nuclear spin nano-ensembles is crucial for the further development of nuclear magnetic resonance (NMR) spectroscopy and for the emerging solid state quantum technology.Here we present the fabrication of a ≈ 1 nanometre thick diamond layer consisting of 13 C nuclear spins doped with Nitrogen-Vacancy centres (NV) embedded in a spin-free 12 C crystal matrix. A single NV in the vicinity of the layer is used for polarization of the 13 C spins and the readout of their magnetization. We demonstrate a method for coherent control of few tens of nuclear spins by using radio frequency pulses and show the basic coherent control experiments -Rabi oscillations, Ramsey spectroscopy and Hahn echo, though any NMR pulse sequence can be implemented. The results shown present a first steps towards the realization of a nuclear spin based quantum simulator. *

Single-qubit sensing in a physically rotating reference frame with defect centers in diamond.

Due to its superior coherent and optical properties at room temperature, the nitrogen-vacancy (NV) center in diamond has become a promising quantum probe for nanoscale quantum sensing. However, the application of NV containing nanodiamonds to quantum sensing suffers from their relatively poor spin coherence times. Here we demonstrate energy efficient protection of NV spin coherence in nanodiamonds using concatenated continuous dynamical decoupling, which exhibits excellent performance with less stringent microwave power requirement. When applied to nanodiamonds in living cells we are able to extend the spin coherence time by an order of magnitude to the T1-limit of up to 30µs. Further analysis demonstrates concomitant improvements of sensing performance which shows that our results provide an important step towards in vivo quantum sensing using NV centers in nanodiamond.Introduction.-Nitrogen-Vacancy (NV) centers in diamond exhibit stable fluorescence and have a spin triplet ground state, which can be coherently manipulated by microwave fields [1]. Observation of spin-dependent fluorescence provides an efficient way to readout the spin state of NVs. The energy splitting of the NV spin depends on physical parameters, such as magnetic field [2-4], electric field [5, 6], temperature [7-10] and pressure [11,12]. A variety of quantum sensing protocols for precise measurement of these physical parameters in different scenarios have been developed [13][14][15][16][17][18][19][20][21][22]. These protocols are all based on determining the NV spin energy splitting, which is why the measurement sensitivity is limited by the NV spin coherence time.Spin coherence in bulk diamond is mainly affected by surrounding electronic impurities (P1 centers) and nuclear spins (natural abundance of 13 C isotope). The spin reservoir can be eliminated by using isotopically engineered high-purity type IIa diamond [23]. In order to mitigate the influence of any residual impurities, pulsed dynamical decoupling has been widely exploited to prolong spin coherence time [24][25][26]. Its excellent performance when applied to NVs in bulk diamond is a result of the quasi-static characteristics of the spin reservoir in bulk diamond and the high available microwave power. Unfortunately, these two factors may not be satisfied for NV centers in nanodiamonds, which are required for sensing applications in vivo. NVs contained within nanodiamonds typically exhibit poor spin coherence time, which has been attributed to nanodiamond surface spin noise and electric charge noise that include prominent high frequency components. Preserving the coherence of NVs in nanodiamonds becomes even more problematic

Sensing, localising and identifying individual nuclear spins or frequency components of a signal in the presence of a noisy environments requires the development of robust and selective methods of dynamical decoupling. An important challenge that remains to be addressed in this context are spurious higher order resonances in current dynamical decoupling sequences as they can lead to the misidentification of nuclei or of different frequency components of external signals. Here we overcome this challenge with engineered quantum sensing sequences that achieve both, enhanced robustness and the simultaneous suppression of higher order harmonic resonances. We demonstrate experimentally the principle using a single nitrogen-vacancy center spin sensor which we apply to the unambiguous detection of external protons. Introduction.-The detection of single nuclear spins represents an important yet challenging step towards single molecule magnetic resonance spectroscopy and imaging [1] which holds the promise for the observation of individual protein structures without the need for crystallization of large ensembles. The achievement of this goal may have significant impact on structural biology and medical imaging and may also provide a new tool for the investigation of nuclear spin dynamics in non-trivial quantum biological processes [2,3]. Recently, research has made impressive progress in single nuclear spin detection using various physical platforms [4][5][6][7][8], particularly in the highly challenging task of detecting single spins in external molecules [9][10][11][12][13][14]. Among those physical platforms, a sensor based on individual negatively charged nitrogen-vacancy (NV) centers in diamond [15,16] has demonstrated appealing prospects for applications in biology and medicine, due to its biocompatibility, nano-scale size and long coherence times under ambient conditions [17,18]. NV centers, shallowly implanted to within a few nanometers below the diamond surface, enable strong coupling between NV sensors and target nuclei, thereby promoting the detection sensitivity from a relative large ensemble of nuclei [9,10] to single nuclear spin sensitivity in small clusters of nuclear spins [11][12][13][14]. Besides the interest in single molecule magnetic resonance spectroscopy and imaging, single nuclear spin addressing also has an important role to play in the precise coherent control of nuclear spin qubits, where it may facilitate the realisation of quantum memories with long coherence times and nuclear spin based quantum information processors [19][20][21][22].One ultimate goal of single molecule magnetic resonance spectroscopy using a single spin sensor is to detect a single nuclear spin and further infer the structure of a single molecule