Single photon emitters play a central role in the rapidly developing field of quantum technologies. Therefor new single photon sources are highly sought after. Understanding their properties is essential for their applications in integrated quantum technologies. Defect centers in hexagonal boron nitride (hBN) have become prominent candidates as single photon sources during the last years due to their highly favorable properties, like bright emission, narrow linewidth, and high photostability at even at room-temperature. Several recent studies have shown a spectral dependency on the excitation wavelength of fluorescence behavior of these emitters1,2. In general, both the intensity and second order autocorrelation function, as well as the emission spectrum, vary with the excitation wavelength. By tuning the excitation over a broad range inside the visible spectrum and performing measurements regarding the quantum nature as well as the spectral decomposition of the emission light, we gain further insight to the characteristic properties and energy level schemes of these defect centers. Especially interesting for the energetic investigation of individual emitters is the appearance of additional sharp emission lines at higher excitation frequencies. These lines can be interpreted as higher order excited states of the same quantum system. To verify the assumption of a single system as the origin of these additional states, spectral cross correlations between individual lines are measured in a free beam HBT setup. Further analysis of these excited states can be done by performing fluorescence life time measurements, as well as comparison between the emission rates in order to determine the efficiency of the different decay channels.
Efficient extraction of photons from quantum emitters is an important prerequisite for the use of such emitters in quantum optical applications as single photons sources or sensors. One way to achieve this is by coupling to a suited photonics structure, which guides away the emitter light. Here, we show the coupling of a single defect in hexagonal boron nitride (hBN) to a tapered optical fiber via a nanomanipulation technique [1]. Defects in hBN are capable of emitting single photons at room temperature while being photostable at the same time – two properties that make them ideal candidates for integration in single photon sources. The high control the manipulation technique provides avoids covering the whole nanofiber with emitters. We characterize the coupled system in terms of achievable count rates, saturation intensity, and spectral properties. Antibunching measurements are used to proof the single emitter nature of the defect. Our results pave the way for integration of single defects in hBN into photonic structure and their use as single photon sources in quantum optical applications such as quantum crypthography.
[1] A W Schell et al., ACS Photonics, 4, 761–767 (2017)
Among the quantum systems capable of emitting single photons, the class of recently discovered defects in hexagonal boron nitride (hBN) is especially interesting, as these defects offer much desired characteristics such as narrow emission lines and photostability. Like for any new class of quantum emitters, the first challenges to solve are the understanding of their photophysics as well as to find ways to facilitate integration in photonics structures. Here, we will show our investigation of the optical transition in hBN with different methods: Employing excitation with a short laser pulse the emission properties in case of linear and non-linear excitation can be compared [1]. The possibility to perform two-photon excitation makes this single photon emitter an interesting candidate as a biosensor. We further show the behaviour of defects in hBN when being excited with different wavelengths and deduce the consequences for its level scheme. Here, it is found that the quantum efficiency of the emitters varies strongly with excitation wavelength, a strong indication of a branched level system with different decay pathways.
[1] A W Schell et al., APL Photonics 1, 091302 (2016)
[2] A W Schell et al., arXiv:1706.08303 (2017)
Efficient extraction of photons from quantum emitters is an important prerequisite for the use of such emitters in quantum optical applications as single photons sources or sensors. One way to achieve this is by coupling to a suited photonics structure, which guides away the emitter light. Here, we show the coupling of a single defect in hexagonal boron nitride (hBN) to a tapered optical fiber via a nanomanipulation technique. Defects in hBN are capable of emitting single photons at room temperature while being photostable at the same time – two properties that make them ideal candidates for integration in single photon sources. The high control the manipulation technique provides avoids covering the whole nanofiber with emitters. We characterize the coupled system in terms of achievable count rates, saturation intensity, and spectral properties. Antibunching measurements are used to proof the single emitter nature of the defect. Our results pave the way for integration of single defects in hBN into photonic structure and their use as single photon sources in quantum optical applications such as quantum crypthography.
Funnelling the light emitted from quantum emitters like atoms, molecules, or defect centers into the guided mode of a single mode optical fiber is highly important for scaling up quantum optics experiments, since it provides the possibility to interconnect experiments at different locations and ensures high mode overlap of photons from different sources. Here, we present a photonic nanocavity on a tapered optical fiber. The cavities are formed by two Bragg mirrors fabricated by an ion beam [1]. Characterization in terms of transmission, reflection, and polarization are performed and compared with numerical simulations [2]. The quality factors of the fabricated devices can reach values over 300 while the mode volume is smaller than the cubic wavelength. Simulations indicate that a Purcell enhancement of 19.1 with 82 % coupling efficiency can be reached using this cavities. A comparison of cavities fabricated using a gallium beam is compared with cavities made using a helium beam giving insights about implantation of gallium in the ion beam milling fabrication of resonators. Using the knowledge from experiment and simulation, new designs for nanofiber Bragg grating cavities are developed and tested.
[1] A W Schell et al. Sci. Rep. 5, 9619 (2015)
[2] H Takashima et al. Opt. Express 24, 15050-15058 (2016)
Recently, two-dimensional materials have gained much interest for various applications in nanophotonics and quantum optics, as they possess a strong luminescence and are able to host single quantum emitters. Excitation of quantum emitters via a two-photon process can be employed for high resolution imaging and has applications in quantum optics. Here, we present one- and two-photon excitation of single defects in hexagonal boron nitride (hBN) and analyse the properties of the emitted light [1]. We find clear antibunching signals that prove the single emitter character in both excitation cases. To gain further knowledge, we also obtain saturation curves. From a comparison of one- and two-photon case insights about the level structure of the defects can be obtained. These results will not only help the fundamental understanding of defects in hBN, but also help to introduce this class of emitters in optical imaging, as the defects in hBN are of small spatial extend, photostable and emit their fluorescence well in the wavelength region of the biological optical window.
[1] A. W. Schell et al. arXiv:1606.09364 (2016)
Single-photon sources based on solid-state emitters, like quantum dots, molecules or defect centers in diamond, are one of the key components for an integrated quantum technology. Here, we will show different strategies used in order to integrate single-photon emitters. Among others, we introduce an hybrid approach using photon emission from defect centers in diamond and laser-written photonic structures. Waveguides, microresonators, and optical antennas can be fabricated and oriented with respect to the single emitters. We describe our general approach before we specifically address the problem of efficient single-photon collection through optical antennas. We discuss the limitations of the method, its potential for scalability as well as its extension towards optical sensing applications.
KEYWORDS: Single photon, Luminescence, Near field, Fiber couplers, Nitrogen, Optical filters, Quantum information, Fiber lasers, Confocal microscopy, Near field optics
Further miniaturization of funcionalized quantum optical systems down to nm-dimensions and their integration
into fibre optical networks is a major challange for future implementations of quantum information, quantum
communication and quantum processing applications. Furthermore, scalability, long-term stability and room- as
well as liquid helium temperature operation are benchmarking properties of such systems.
In this paper, we present the realizations of fiber-coupled diamond-based single photon systems. First, an
alignment free, μm-scale single photon source consisting of a single nitrogen vacancy center facet coupled to
an optical fiber operating at room temperature is presented. Near-field coupling of the single nitrogen vacancy
center is realized by placing a pre-selected nanodiamond directly on the fiber facet in a bottom-up approach.
Its photon collection efficiency is comparable to a far-field collection via an air objective with a numerical
aperture of 0.82. As the system can be simultaneously excited and its photons be recollected through the
fiber, it can be used as a fiber-connected single quantum sensor that allows optical near-field probing on the
quantum level. Secondly single nanodiamonds that contain nitrogen vacancy defect centers, are near-field coupled
to a tapered fiber of 300 nanometer in diameter. This system provides a record-high number of 97 kcps single
photons from a single defect center into a single mode optical fiber. The entire system can be cooled to liquid
Helium temperatures and reheated without breaking. Furthermore, the system can be evanescently coupled to
various nanophotonic structures, e.g. microresonators. The system can also be applied for integrated quantum
transmission experiments and the realization of two-photon interference. It can be used as a quantum-randomnumber
generator as well as a probe for nano-magnetometry.
KEYWORDS: Single photon, Luminescence, Optical fibers, Quantum information, Quantum cryptography, Near field, Diamond, Near field optics, Confocal microscopy, Quantum communications
Miniaturization of quantum optical devices down to μm-dimensions and integration into fibre optical networks
is a major prerequisite for future implementations of quantum information communication and processing applications.
Also scalability, long-term stability and room-temperature operation are important properties of such
devices. Lately there have been major improvements in down-sizing logical structures and functionalizing optical
fibers. Here we present an alignment free, μm-scale single photon source consisting of a single quantum emitter
on an optical fiber operating at room temperature. It easily integrates into fiber optic networks for quantum
cryptography or quantum metrology applications. Near-field coupling of a single nitrogen-vacancy center is
achieved in a bottom-up approach by placing a pre-selected nanodiamond directly on the fiber facet. Its high
photon collection efficiency is equivalent to a far-field collection via an objective with a numerical aperture of 0.82.
Furthermore, simultaneous excitation and recollection through the fiber is possible introducing a fiber-connected
single emitter sensor that allows near-field probing with quantum mechanical properties.
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