TD
T.M. Dolné
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The realization of large-scale Quantum Networks has the potential to enable applications such as distributed and blind quantum computing. In turn, these concepts can drive innovation in many high-impact areas of society such as data encryption, pharmaceutical research and energy-grid optimization. The main technological challenge in the realization of such networks lies in the development of their fundamental building blocks: the Quantum Nodes. Quantum Nodes need to be coherent quantum systems that can (i) generate remote entanglement through photons and (ii) store quantum information on nearby quantum memories. Historically, the Nitrogen-Vacancy center in diamond has been regarded as the most promising early-stage candidate system for the realization of small-scale quantum networks. The large-scale deployment of a Quantum internet with Nitrogen-Vacancy center is currently however unfeasible due to the fundamentally inefficient photonic interface of the Nitrogen-Vacancy center and its incompatibility with integrated photonics. This study focuses on an emergent quantum system that has the potential to improve on the Nitrogen-Vacancy center in terms of both these fundamental limits: the Tin-Vacancy center. The Tin-Vacancy center consists of a single tin atom at an interstitial position between two missing carbon atoms in the diamond lattice. This configuration leads to a spin-1/2 system with optically addressable local energy states within the diamond bandgap. In state-of-the-art experiments, capabilities such as spin control, optical coherence and single-photon generation have been demonstrated in an isolated fashion. The main goal of this work is to (i) show the integration of all these capabilities for two tin-vacancy centers simultaneously in a single setup and (ii) demonstrate the first reported indistinguishability of photons generated by two distinct tin-vacancy centers. The realization of these goals marks a significant step towards entanglement generation with tin-vacancy centers, which is an essential first hurdle for realizing demonstrating its use-case as a Quantum Node. One capability that is required for entanglement generation is coherent control of the spin state of a tin-vacancy center. This capability was demonstrated for two emitters in two different strain regimes. The influence of strain on the efficiency of microwave spin control was measured through the π-pulse durations of (0.378±0.002) μs and (0.68±0.01) μs for the emitterswith higher- and lower strain respectively. Crucially for entanglement generation, the π-pulse fidelity of 0.980±0.002 for the higher strain emitter vastly outperforms the 0.871±0.006 fidelity of the lower strain emitter. The lower strain emitter did however show a higher decoherence time of T2 = (316±3)μs compared to the higher strain emitter’s decoherence time of T2 = (47±2)μs. Both these timescales suffice for entanglement generation and can be extended through dynamical decoupling. Another key ingredient for entanglement protocols is the generation of single photons through coherent optical π-pulses. In this work, a 1.5ns optical π-pulse was calibrated and demonstrated to produce coherent single photon through a measurement of correlations in a Hanbury-Brown-Twiss interferometer. The measured correlations showed a normalized autocorrelation of g (2)(0) = 0.06±0.03, which is sufficiently low to prove the single-photon nature of the detection events. Finally, the setup was used to measure detection correlations in an experiment involving simultaneous excitation of two tin-vacancy centers, where the collected coherent single photons of both tin-vacancy centers are directed to a Hanbury-Brown-Twiss interferometer. The measured correlations of the detection events reveal a dip in the number of coincidences with a visibility of V = 0.55±0.05. Correcting this value for known noise sources from the measured correlations of the single photons from both emitters leads to an indistinguishability of η = 0.68±0.07. This measurement is the first experimental observation of indistinguishability between photons generated from distinct tin-vacancy centers. The remaining distinguishability is shown to largely originate from spectral diffusion, which is mitigated through time filtering to get a maximum visibility of V = 0.87 ± 0.08. Such a visibility poses a limit on the maximum achievable entanglement fidelity of F = 0.94±0.04, which is far above the required fidelity of 0.5 for demonstrating non-classical behavior. It is thus concluded that the results in this thesis provide evidence for the potential to generate entanglement between two tin-vacancy centers with the current setup configuration. Next steps to realize this potential involve the addition of a second optical pulse source, improvement in fiber-waveguide coupling and reduction of the required magnetic field for frequency tuning.
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The realization of large-scale Quantum Networks has the potential to enable applications such as distributed and blind quantum computing. In turn, these concepts can drive innovation in many high-impact areas of society such as data encryption, pharmaceutical research and energy-grid optimization. The main technological challenge in the realization of such networks lies in the development of their fundamental building blocks: the Quantum Nodes. Quantum Nodes need to be coherent quantum systems that can (i) generate remote entanglement through photons and (ii) store quantum information on nearby quantum memories. Historically, the Nitrogen-Vacancy center in diamond has been regarded as the most promising early-stage candidate system for the realization of small-scale quantum networks. The large-scale deployment of a Quantum internet with Nitrogen-Vacancy center is currently however unfeasible due to the fundamentally inefficient photonic interface of the Nitrogen-Vacancy center and its incompatibility with integrated photonics. This study focuses on an emergent quantum system that has the potential to improve on the Nitrogen-Vacancy center in terms of both these fundamental limits: the Tin-Vacancy center. The Tin-Vacancy center consists of a single tin atom at an interstitial position between two missing carbon atoms in the diamond lattice. This configuration leads to a spin-1/2 system with optically addressable local energy states within the diamond bandgap. In state-of-the-art experiments, capabilities such as spin control, optical coherence and single-photon generation have been demonstrated in an isolated fashion. The main goal of this work is to (i) show the integration of all these capabilities for two tin-vacancy centers simultaneously in a single setup and (ii) demonstrate the first reported indistinguishability of photons generated by two distinct tin-vacancy centers. The realization of these goals marks a significant step towards entanglement generation with tin-vacancy centers, which is an essential first hurdle for realizing demonstrating its use-case as a Quantum Node. One capability that is required for entanglement generation is coherent control of the spin state of a tin-vacancy center. This capability was demonstrated for two emitters in two different strain regimes. The influence of strain on the efficiency of microwave spin control was measured through the π-pulse durations of (0.378±0.002) μs and (0.68±0.01) μs for the emitterswith higher- and lower strain respectively. Crucially for entanglement generation, the π-pulse fidelity of 0.980±0.002 for the higher strain emitter vastly outperforms the 0.871±0.006 fidelity of the lower strain emitter. The lower strain emitter did however show a higher decoherence time of T2 = (316±3)μs compared to the higher strain emitter’s decoherence time of T2 = (47±2)μs. Both these timescales suffice for entanglement generation and can be extended through dynamical decoupling. Another key ingredient for entanglement protocols is the generation of single photons through coherent optical π-pulses. In this work, a 1.5ns optical π-pulse was calibrated and demonstrated to produce coherent single photon through a measurement of correlations in a Hanbury-Brown-Twiss interferometer. The measured correlations showed a normalized autocorrelation of g (2)(0) = 0.06±0.03, which is sufficiently low to prove the single-photon nature of the detection events. Finally, the setup was used to measure detection correlations in an experiment involving simultaneous excitation of two tin-vacancy centers, where the collected coherent single photons of both tin-vacancy centers are directed to a Hanbury-Brown-Twiss interferometer. The measured correlations of the detection events reveal a dip in the number of coincidences with a visibility of V = 0.55±0.05. Correcting this value for known noise sources from the measured correlations of the single photons from both emitters leads to an indistinguishability of η = 0.68±0.07. This measurement is the first experimental observation of indistinguishability between photons generated from distinct tin-vacancy centers. The remaining distinguishability is shown to largely originate from spectral diffusion, which is mitigated through time filtering to get a maximum visibility of V = 0.87 ± 0.08. Such a visibility poses a limit on the maximum achievable entanglement fidelity of F = 0.94±0.04, which is far above the required fidelity of 0.5 for demonstrating non-classical behavior. It is thus concluded that the results in this thesis provide evidence for the potential to generate entanglement between two tin-vacancy centers with the current setup configuration. Next steps to realize this potential involve the addition of a second optical pulse source, improvement in fiber-waveguide coupling and reduction of the required magnetic field for frequency tuning.