The unique correlations in quantum systems, i.e. superposition and entanglement, enable quantum technologies to outperform classical technologies. Quantum networks is a quantum technology in fast development with many promising applications. An essential building block in a quantum network is the interface between matter nodes and quantum channels. A color center in diamond coupled to a micro-cavity is a promising route to achieve an efficient interface. In this thesis we investigate the coupling of a single germanium vacancy (GeV) center to a Fabry-Pérot optical micro-cavity at room temperature. The Fabry-Pérot optical micro-cavity was formed by a macroscopic flat mirror and a fiber-based concave microscopic mirror. The concave mirror was created on the end-facet of a fiber with short high power laser pulses from a CO2-laser. High qulity dielectric coating allowed us to achieve a finesse of 11,000 with a∼1 µm thin diamond membrane incorporated in the cavity. We achieved a cavity length of only a few µm and a small mode volume V = 2.7µm, which was essential for good coupling between the GeV center and the optical cavity mode. By studying the same GeV center in a confocal microscope setup, i.e. outside of the micro-cavity, and in the micro-cavity setup, we were able to measure an improved optical coherence of the GeV emission in the cavity. We determined an enhancement of the spectral density by a factor of ∼15. Furthermore, we determined the emission efficiency into the cavity mode β and compared it with the theoretical modelling. From the theoretical modelling, we predict the expected Purcell factor Fp = 200 for the zero phonon line in future experiments at cryogenic temperatures. Going to cryogenic temperatures is necessary to achieve efficient spin-photon interfaces. This work provides valuable information for realizing efficient spin-photon interfaces, which is critical for the practical implementation of a quantum network.
The unique correlations in quantum systems, i.e. superposition and entanglement, enable quantum technologies to outperform classical technologies. Quantum networks is a quantum technology in fast development with many promising applications. An essential building block in a quantum network is the interface between matter nodes and quantum channels. A color center in diamond coupled to a micro-cavity is a promising route to achieve an efficient interface. In this thesis we investigate the coupling of a single germanium vacancy (GeV) center to a Fabry-Pérot optical micro-cavity at room temperature. The Fabry-Pérot optical micro-cavity was formed by a macroscopic flat mirror and a fiber-based concave microscopic mirror. The concave mirror was created on the end-facet of a fiber with short high power laser pulses from a CO2-laser. High qulity dielectric coating allowed us to achieve a finesse of 11,000 with a∼1 µm thin diamond membrane incorporated in the cavity. We achieved a cavity length of only a few µm and a small mode volume V = 2.7µm, which was essential for good coupling between the GeV center and the optical cavity mode. By studying the same GeV center in a confocal microscope setup, i.e. outside of the micro-cavity, and in the micro-cavity setup, we were able to measure an improved optical coherence of the GeV emission in the cavity. We determined an enhancement of the spectral density by a factor of ∼15. Furthermore, we determined the emission efficiency into the cavity mode β and compared it with the theoretical modelling. From the theoretical modelling, we predict the expected Purcell factor Fp = 200 for the zero phonon line in future experiments at cryogenic temperatures. Going to cryogenic temperatures is necessary to achieve efficient spin-photon interfaces. This work provides valuable information for realizing efficient spin-photon interfaces, which is critical for the practical implementation of a quantum network.
A quantumnetwork would allow the distribution of a quantum state over many spatially separated quantum nodes which individually possess the ability to generate, process and store quantum information. Connecting these nodes through quantum communication channels would enable sending quantum information over arbitrarily long distances, secret key distribution with guaranteed secure communication and distributed quantum computing. An appealing platform for implementation of quantum networks is the nitrogen-vacancy (NV) center in diamond. An NV center is an optically active defect in diamond created by the substitution of two adjacent carbon atoms in the diamond crystal matrix by a nitrogen atom and a neighboring vacancy. The NV center fits remarkably well in the described quantum network framework. Its electronic spin can be optically initialized, read out in a single shot, coherently manipulated and coupled to the nearby carbon-13 nuclear spins. These properties represent necessary ingredients of a multi-qubit quantum node. The possibility of the entanglement between the state of the NV center spin and a photon establishes a photonic quantum link which can enable the entanglement of the quantum nodes, forming a quantum network. First building blocks of the proposed quantum networks based on NV centers were demonstrated by performing entanglement between two distant NV centers separated by more than a kilometer. However, the current success rate of the entangling protocols is greatly reduced due to the low emission probability of the NV center photons into the resonant zero phonon line (ZPL) and the inefficient photon extraction from the diamond. This is the central problem which prevents promoting our experiments beyond proof-of-principle demonstration towards practical implementation of the proposed quantumnetworks. The goal of this thesis is tackling these issues by coupling NV centers to optical cavities which would greatly increase the rate of generation and collection of the ZPL photons through themechanism of Purcell enhancement. The design and the fabrication of the components constituting a diamond-basedFabry-Perot microcavity, as used in this thesis, are described in Chapter 4. For large enhancement of the NV center resonant emission, a low cavity mode volume is necessary. This is achieved by inserting a micrometers thin diamond slab into the cavity architecture; we present the fabrication details of such samples. Chapter 5 describes the fabrication of an integrated platform for microwave signal delivery to the NV centers within a diamond membrane in the cavity architecture. Microwave striplines and arrays of unique markers are embedded in the mirror onto which the diamond is bonded. We investigate the mirror optical properties post fabrication and find that the fabrication method preserves the mirror optical performance. We describe the diamond bonding method and demonstrate addressing of the NV center spin. In Chapter 6 we probe the properties of the cavity with the embedded diamond membrane through measurements of the cavity finesse. We investigate the cavity finesse dependence on length, mode structure and temperature. We further explore the operation at cryogenic temperatures by probing the effect of cryostation vibration on the cavity linewidth. Finally, in Chapter 7 we discuss ways of improving the existing experimental capabilities, outline the first steps for demonstrating enhancement of the NV center resonant emission and suggest future experiments that can be performed with this system. We conclude that coupling of the NV centers to the cavities developed in this research could lead to an increase of remote entanglement success rates by more than three orders of magnitude.
With their ability to process and transfer quantum information, large-scale entanglement-based quantum networks could be at the heart of a new age of quantum information, enabling fundamentally new applications such as distributed quantum computation, quantum communication, and quantum enhanced sensing. Due to their long spin coherence, controllable local qubit registers and optically active spins, color centers in diamond are prime candidates for nodes of such a network, and have enabled some of the most advanced quantum network demonstrations to date. These demonstrations include the distribution of a 3-node GHZ state across a quantum network, entanglement swapping, entanglement distillation, and memory-enhanced quantum communication. To move beyond current proof-of-principle networks, a further increase of entanglement generation rates is crucial. This thesis presents theoretical and experimental work on enhancing the spin-photon interface of color centers in diamond to achieve this goal, making use of the Purcell effect. The discussed work follows two main directions: embedding of color centers in open, tuneable Fabry-Perot micro-cavities, and in all-diamond photonic crystal cavities. First, we describe theoretical and experimental progress towards cavity-enhanced quantum networks based on nitrogen-vacancy (NV) centers in diamond. Due to their first order sensitivity to electric fields, we choose to embed NV centers in microns-thin diamond membranes that can be integrated into open fiber-based micro-cavities. We develop analytical methods to optimize the design of such open cavity systems for maximum Purcell enhancement of embedded color centers. We demonstrate a method to fabricate optically coherent NV centers in microns-thin diamond membranes, and use such structures to demonstrate the resonant excitation and detection of coherent, Purcell enhanced NV emission. A theoretical model in excellent agreement with our results suggests our system can improve entanglement rates between distant NV centers by two orders of magnitude with near-term improvements to the setup. Second, we describe progress towards an efficient spin-photon interface of group-IV color centers in diamond by coupling them to photonic nanostructures, allowing for large-scale integration. Due to their first-order insensitivity to electric fields, group-IV color centers can be brought in close proximity to surfaces (~ 100 nm), allowing for sub-wavelength mode volumes and thus very high Purcell factors. We numerically optimize photonic crystal cavity designs to maximize the Purcell enhancement of embedded emitters, test the robustness of our designs to real-world fabrication imperfections, and devise a method to efficiently interface nanophotonic structures. We then proceed to fabricate all-diamond photonic crystal cavities, making use of a dry etching technique that is selective to different crystallographic directions, and characterize the resulting optical quality factors. Finally, we marry the developed fabrication methods to fabricate microns-sized diamond platelets that can be transferred from a holding frame in a controlled fashion. This capability could be crucial for the realization of hybrid photonic circuits that we expect to be at the heart of future large-scale quantum networks.
This thesis investigates the coupling of the fluorescence of nitrogen-vacancy (NV) centers in diamond to tunable optical microresonators at ambient conditions, in particular in the regime of Purcell enhancement. We use fiber-based, open-access Fabry-Pérot cavities optimized for high finesse and ultra-small mode volume. Different regimes of cavity enhancement are studied that are complementary to each other: A first experiment relies on a high-finesse cavity with dielectric mirrors. The scaling laws of Purcell enhancement are explicitly demonstrated by a large-range variation of both the cavity mode volume (V = 16 − 600 µm^3 ) and the quality factor (Q = 6 · 10^3 − 2 · 10^6). We detect an enhancement of the emission spectral density by up to a factor of 300. The full potential of this resonator can be exploited with emitters having a linewidth which is narrower than the resonance linewidth of the cavity. This concept holds promise for the implementation of wavelength-tunable, narrow-band single-photon sources as well as the generation of indistinguishable single-photons at ambient conditions. However, for broad-band emitters like the NV center at room temperature, the emission lifetime is not affected noticeably in this configuration. In order to directly observe lifetime changes and Purcell-enhanced single-photon emission, we manufacture fiber-based cavities with silver-coated mirrors having ultra-small mode volumes, as small as V = 1.0 λ^3 = 0.34 µm^3. We demonstrate cavity-enhanced fluorescence imaging, which allows to locate and analyze several single NV centers with one cavity. The Purcell effect is evidenced by an enhanced fluorescence collection of up to 1.6 · 10^6 photons per second from single-NV centers and a tunable variation of the emission lifetime corresponding to an effective Purcell factor of up to 2. We furthermore investigate a benefcial regime of optical confinement where the Fabry-Pérot cavity mode is combined with additional mode confinement by the diamond nanocrystal itself, enabling sub-λ^3 mode volumes. We perform simulations that predict effective Purcell factors of up to 11 for NV centers and of up to 63 for silicon-vacancy centers, revealing a great potential for bright single-photon sources and effcient spin readout at ambient conditions.
Color centers in diamond, among them the negatively charged germanium vacancy (GeV−), are promising candidates for many applications of quantum optics, such as a quantum network. For efficient implementation, the optical transitions need to be coupled to a single optical mode. Here, we demonstrate the transfer of a nanodiamond containing a single ingrown GeV− center with excellent optical properties to an open Fabry–Pérot microcavity by nanomanipulation utilizing an atomic force microscope. Coupling of the GeV− defect to the cavity mode is achieved, while the optical resonator maintains a high finesse of F=7700, and a 48-fold spectral density enhancement is observed. This article demonstrates the integration of a GeV− defect with a Fabry–Pérot microcavity under ambient conditions with the potential to extend the experiments to cryogenic temperatures toward an efficient spin-photon platform.
The Nitrogen-Vacancy (NV) centre in diamond possesses an optical read out of its spin state and shows great promise for applications in solid state quantum technologies. Its broad phonon assisted emission spectrum, with only 4% of emission from the zero-phonon line, is a major drawback to this. The optical microcavity will be essential in efficiently interfacing these centres with photonic networks. Here we present investigations into the coupling of NV centres in nanodiamond to tunable open-microcavities, both at room temperature and at cryogenic temperatures. These structures will be shown to achieve good photon confinement, with mode volumes down to 5λ3. Room temperature studies on ensembles of NVs will illustrate the tunable spectral and spatial overlap between the emitter and the cavity mode. It will be shown that small enhancements are possible in this regime. After the preparation and characterisation of single emitters, low temperature coupling to the narrowed zero-phonon line will be the central theme of this thesis. Single photon emission into the cavity mode is verified. We observe the enhancement of the light-matter interaction in this regime, by a 39% increase in the emission rate when cavity-coupled, with the dependence on mode volume also demonstrated. These results are important for the realisation of a spin-photon interface in scalable quantum networks.