Single photon sources based on single emitters such as 2D materials and impurities in diamond are of great interest for many application fields including quantum communication, quantum metrology and quantum sensing. In photometry and radiometry, a single photon source, having the ability to emit only one photon in a well-defined time and frequency domain, represents a promising candidate to realize a photon number-based primary standard for quantum radiometry. At the Korea Research Institute of Standards and Science (KRISS), we have realized three kinds of single photon sources based on various single emitters at room temperature: silicon vacancy in diamond, defects in gallium nitride (GaN), and vacancy in hexagonal boron nitride (hBN). We found common factors related with the relaxation times of the internal states that indirectly affect the photon number stability. We observed a high photon number stability in the GaN emitter due to faster relaxation times compared with the hBN emitter, which on the other hand produced high rates of photons per second (> 106). Moreover, we demonstrate repeatable radiant flux measurements of a bright hBN single photon emitter for a wide radiant flux range from a few tens of femtowatts to one picowatt.
Single photon sources based on single emitters such as 2D materials and impurities in diamond are of great interest for many application fields including quantum communication, quantum metrology and quantum sensing. In photometry and radiometry, a single photon source, having the ability to emit only one photon in a well-defined time and frequency domain, represents a promising candidate to realize a photon number-based primary standard for quantum radiometry. At the Korea Research Institute of Standards and Science (KRISS), we have realized three kinds of single photon sources based on various single emitters at room temperature: silicon vacancy in diamond, defects in gallium nitride (GaN), and vacancy in hexagonal boron nitride (hBN). We found common factors related with the relaxation times of the internal states that indirectly affect the photon number stability. We observed a high photon number stability in the GaN emitter due to faster relaxation times compared with the hBN emitter, which on the other hand produced high rates of photons per second (> 106). Moreover, we demonstrate repeatable radiant flux measurements of a bright hBN single photon emitter for a wide radiant flux range from a few tens of femtowatts to one picowatt.
High-purity single-photon sources (SPS) that can operate at room temperature are highly desirable for a myriad of applications, including quantum photonics and quantum key distribution. In this work, we realize an ultra-bright solid-state SPS based on an atomic defect in hexagonal boron nitride (hBN) integrated with a solid immersion lens (SIL). The SIL increases the source efficiency by a factor of six, and the integrated system is capable of producing over ten million single photons per second at room temperature. Our results are promising for practical applications of SPS in quantum communication protocols.
High-purity single photon sources (SPS) that can operate at room temperature are highly desirable for a myriad of applications, including quantum photonics and quantum key distribution. In this work, we realise an ultra-bright solid-state SPS based on an atomic defect in hexagonal boron nitride (hBN) integrated with a solid immersion lens (SIL). The SIL increases the source efficiency by a factor of six, and the integrated system is capable of producing over ten million single photons per second at room temperature. Our results are promising for practical applications of SPS in quantum communication protocols.
Single photons are the key ingredient for many photonic quantum technologies including quantum key distribution and measurement-based quantum computing. However, it remains difficult to create devices with the appropriate specifications for use in non-laboratory environments. The optical microcavity platform provides an attractive route towards a room temperature single photon source device. Our ultra-small focused ion beam (FIB) milled open-access cavities offer enhancement of the spontaneous emission rate, tunability of the emission spectrum and increased light collection. The embedment of solid-state emitters within these cavities enables us to create a robust room temperature single photon source device, with the potential for high efficiencies and single photon purities. Defects such as the nitrogen-vacancy (NV) centre in diamond have been shown to be stable room-temperature sources of single photons. There are new single emitters emerging in two-dimensional materials such as hexagonal boron nitride (hBN). Here we present developments in room-temperature coupling of single defects to open-access microcavities of a planar-hemispherical geometry with mode volumes down to λ3. We report enhancements in the spectral density of photons into a single cavity mode, combined with improved single photon purities. It will be shown that the NV-cavity system provides a ~3% single photon emission efficiency with purities of up to 94%. The hBN-cavity system provides count rates >1Mcts/s into a single cavity mode with purities up to 96%. With these high single photon purities, such devices would be robust against photon number splitting attacks making them attractive for applications in quantum cryptography.
Sources of single photons are integral to a number of applications in the emerging field of quantum technologies. For any practical implementation of single-photon sources, a variety of different properties and an overall comprehensive characterization of the sources are required. Color centers in hBN and group IV color centers in diamond are promising candidate sources due to their favorable properties and rich internal level structure. Both of these candidates, however, also present some serious challenges. Although bright and spectrally narrow, defects in hBN suffer from emission instabilities. Furthermore, limited knowledge about their atomic composition leads to behaviour, which is often unpredictable. Meanwhile, Group IV centers generally exhibit low extraction efficiencies due to total internal reflection inside the high-index diamond host material. In this thesis, several large-scale series of experiments are conducted to investigate both types of emitters in a confocal fluorescence microscope at room temperature. The excitation characteristics of a large number of single-photon emitters in hBN from a narrow spectral window are investigated using a fully automated procedure. The results are compared to theoretical predictions for several carbon defects from the literature to gain further knowledge of the atomic nature of the underlying defects. Based on this approach, the CB, CN, CBVN, and CNVB centers are confidently ruled out as possible origins of the observed emission. Some uncertainty remains about the assignment of the carbon trimer C2CN as well as carbon-based donor-acceptor pairs. In a separate series of experiments, emission instabilities of hBN emitters from the same spectral region are studied. The detected fluctuations in brightness were conclusively attributed to spectral diffusion of the emission. Furthermore, optical stabilization of these effects via short-wavelength irradiation favors an explanation of the observed behavior via nearby charge carriers and the resulting Stark effect. Tin-vacancies, as representatives of group IV centers, are studied in a novel type of diamond waveguide structure for light collection enhancement. A center with sharp zero-phonon emission around 619 nm, a g(2)(0) value of 0.32, and simultaneous stable count rate of 3.4 Mcps was found. By restricting the collection to the zero-phonon line, g(2)(0) = 0.1 could be achieved. In a widespread characterization of the sample, it was found that most structures contain more than one negatively charged SnV center, which leaves an opportunity to improve the implantation parameters. In additional experiments on single tin-vacancy centers, a possible transition to a higher excited state as well as the charge-state transitions of the defects were probed. The results of these measurements contribute to the growing body of research on tin-vacancies and other group IV centers.
Solid-state single-photon sources are the central blocks for many scalable quantum applications including quantum computing, quantum key distributions, and quantum simulations. Over the years, diverse quantum emitters have been studied. This thesis endeavored investigations into single-photon sources in diamond, silicon carbide and gallium nitride in three directions: Firstly, identification of the single-photon sources that have never been reported before and fabrication technique for scalable single germanium vacancy centers in diamond. Telecom wavelength single photon sources have been found in both gallium nitride and silicon carbide. More importantly, those emitters work even at room temperature and are promising for quantum key distribution applications in the future. Secondly, the spin properties study of the single silicon vacancy in diamond: the silicon vacancy optical transition has been coherently controlled in the work. The optical Rabi oscillations in the time domain have been observed. The ultrafast spin control of the silicon vacancy has been also achieved. Thirdly, quantum applications of the divacancy spins in silicon carbide: nano-scale thermometry based on the quantum control of the divacancy spin in silicon carbide has been introduced. Especially, we found a self-protected effect that the transverse strain can protect the spin from the local magnetic field and enables longer coherence time. Finally, some possible directions have been discussed.
Room temperature solid-state emitters provide a promising source of single photons for use in quantum technologies. The emitters studied in this thesis are the nitrogen-vacancy (NV) centre in diamond, and defects in two-dimensional hexagonal boron nitride (hBN). In this thesis, I present investigations into the coupling of these defects to open-access microcavities at room temperature. The optical microcavity serves to confine the electric field down to mode volumes on the order of λ3, increasing the spontaneous emission rate of these emitters and channelling this emission into well defined cavity modes. The general properties of both emitters will be presented, including examples of single photon emission, emission spectra, saturation behaviour and photo-stability. A few examples highlighting the long-term photo-stability of the NV centre under pulsed excitation will be displayed. This will highlight the suitability of the NV centre for a practical device. Following the preparation and characterisation of each emitter, single photon emission into the cavity modes is verified, reaching single photon purities greater than 97%. Enhancements in the spectral density of photons were observed for both the NV centre (~25×) and defect in hBN (~3×), along with decreases in the excited state lifetimes. While the NV centre provides a stable source of single photons, the cavity enhancement is limited by its broad emission spectrum. Defects in hBN present near-ideal optical properties in the absence of the cavity, and were expected to deliver superior results within the cavity. However, the poor photo-stability of these defects did not allow complete characterisation of the cavity coupled emission. FDTD simulations were combined with changes in the excited state lifetime to infer the quantum efficiency of these defects. The results of this thesis form the foundation for the realisation of a robust room temperature single photon source.