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Abstract Owing to its remarkable optical properties and its ability to be controlled by microwave fields at room temperature, nitrogen-vacancy (NV) centre in diamond has been a promising platform for various nanotechnological applications, e.g. quantum sensing and magnetic resonance spectroscopy. The qubit associated with the NV centre ground state spin has been the key element for the realisation of such applications. Its coherence time determines the application performances, e.g. the sensitivity of the NV-qubit based quantum sensor. Due to unwanted and unavoidable interactions with noisy environment, the qubit undergoes decoherence that shortens its coherence time and hence reduces its application performance. Therefore, the investigation and mitigation of the decoherence of the NV qubit are of the utmost importance. Here, we simulate the qubit decoherence using Ornstein-Uhlenbeck process that is essential for designing a set of modulating control pulses to mitigate the effect.

Efficient initialization and manipulation of quantum states is important for numerous applications and it usually requires the ability to perform high fidelity and robust swapping of the populations of quantum states. Stimulated Raman adiabatic passage (STIRAP) has been known to perform efficient and robust inversion of the ground states populations of a three-level system. However, its performance is sensitive to the initial state of the system. In this contribution we demonstrate that a slight modification of STIRAP, where we introduce a non-zero single-photon detuning, allows for efficient and robust population swapping for any initial state. The results of our work could be useful for efficient and robust state preparation, dynamical decoupling and design of quantum gates in ground state qubits via two-photon interactions.

We investigate the rephasing efficiency of sequences of phased pulses for spin echoes and light storage by electromagnetically induced transparency (EIT). We derive a simple theoretical model and show that the rephasing efficiency is very sensitive to the phases of the imperfect rephasing pulses. The obtained efficiency differs substantially for spin echoes and EIT light storage, which is due to the spatially retarded coherence phases after EIT light storage. Similar behavior is also expected for other light-storage protocols with spatial retardation or for rephasing of collective quantum states with an unknown or undefined phase, e.g., as relevant in single-photon storage. We confirm the predictions of our theoretical model by experiments in a Pr\\(^{3+}\\):Y\\(_{2}\\)SiO\\(_{5}\\) crystal.

We introduce a method to rotate arbitrarily the excitation profile of universal broadband composite pulse sequences for robust high-fidelity population inversion. These pulses compensate deviations in any experimental parameter (e.g. pulse amplitude, pulse duration, detuning from resonance, Stark shifts, unwanted frequency chirp, etc.) and are applicable with any pulse shape. The rotation allows to achieve higher order robustness to any combination of pulse area and detuning errors at no additional cost. The latter can be particularly useful, e.g., when detuning errors are due to Stark shifts that are correlated with the power of the applied field. We demonstrate the efficiency and universality of these composite pulses by experimental implementation for rephasing of atomic coherences in a \\(\\text{Pr}^{3+}\\text{:}\\text{Y}_2\\text{SiO}_5\\:\\) crystal.

We propose a scheme for sensing of an oscillating field in systems with large inhomogeneous broadening and driving field variation by applying sequences of phased, adiabatic, chirped pulses. The latter act as a double filter for dynamical decoupling, where the adiabatic changes of the mixing angle during the pulses rectify the signal and partially remove frequency noise. The sudden changes between the pulses act as instantaneous \\(\\pi\\) pulses in the adiabatic basis for additional noise suppression. We also use the pulses' phases to correct for other errors, e.g., due to non-adiabatic couplings. Our technique improves significantly the coherence time in comparison to standard XY8 dynamical decoupling in realistic simulations in NV centers with large inhomogeneous broadening and is suitable for experimental implementations with substantial driving field inhomogeneity. Beyond the theoretical proposal, we also present proof-of-principle experimental results for quantum sensing of an oscillating field in NV centers in diamond, demonstrating superior performance compared to the standard technique.

Over the years, an enormous effort has been made to establish nitrogen vacancy (NV) centers in diamond as easily accessible and precise magnetic field sensors. However, most of their sensing protocols rely on the application of bias magnetic fields, preventing their usage in zero- or low-field experiments. We overcome this limitation by exploiting the full spin \\(S=1\\) nature of the NV center, allowing us to detect nuclear spin signals at zero- and low-field with a linearly polarized microwave field. As conventional dynamical decoupling protocols fail in this regime, we develop new robust pulse sequences and optimized pulse pairs, which allow us to sense temperature and weak AC magnetic fields and achieve an efficient decoupling from environmental noise. Our work allows for much broader and simpler applications of NV centers as magnetic field sensors in the zero- and low-field regime and can be further extended to three-level systems in ions and atoms.

We propose a scheme for mixed dynamical decoupling (MDD), where we combine continuous dynamical decoupling with robust sequences of phased pulses. Specifically, we use two fields for decoupling, where the first continuous driving field creates dressed states that are robust to environmental noise. Then, a second field implements a robust sequence of phased pulses to perform inversions of the dressed qubits, thus achieving robustness to amplitude fluctuations of both fields. We show that MDD outperforms standard concatenated continuous dynamical decoupling in realistic numerical simulations for dynamical decoupling in NV centers in diamond. Finally, we also demonstrate how our technique can be utilized for improved sensing.

We introduce universally robust sequences for dynamical decoupling, which simultaneously compensate pulse imperfections and the detrimental effect of a dephasing environment to an arbitrary order, work with any pulse shape, and improve performance for any initial condition. Moreover, the number of pulses in a sequence grows only linearly with the order of error compensation. Our sequences outperform the state-of-the-art robust sequences for dynamical decoupling. Beyond the theoretical proposal, we also present convincing experimental data for dynamical decoupling of atomic coherences in a solid-state optical memory.

We experimentally demonstrate composite stimulated Raman adiabatic passage (CSTIRAP), which combines the concepts of composite pulse sequences and adiabatic passage. The technique is applied for population transfer in a rare-earth doped solid. We compare the performance of CSTIRAP with conventional single and repeated STIRAP, either in the resonant or the highly detuned regime. In the latter case, CSTIRAP improves the peak transfer efficiency and robustness, boosting the transfer efficiency substantially compared to repeated STIRAP. We also propose and demonstrate a universal version of CSTIRAP, which shows improved performance compared to the originally proposed composite version. Our findings pave the way towards new STIRAP applications, which require repeated excitation cycles, e.g., for momentum transfer in atom optics, or dynamical decoupling to invert arbitrary superposition states in quantum memories.

We introduce a method to suppress unwanted transition channels, even without knowing their couplings, and achieve perfect population transfer in multistate quantum systems by the application of composite pulse sequences. Unwanted transition paths may be present due to imperfect light polarization, stray electromagnetic fields, misalignment of quantization axis, spatial inhomogeneity of trapping fields, off-resonant couplings, etc. Compensation of simultaneous deviations in polarization, pulse area, and detuning is demonstrated. The accuracy, the flexibility and the robustness of this technique make it suitable for high-fidelity applications in quantum optics and quantum information processing.