Optical quantum technology needs efficient sources for non-classical light. Solid-state emitters provide excellent mode purity, high brightness, and often also stable operation up to room temperature. At the same time the spin of individual impurities can be entangled with emitted photons. Nano-photonic structures can dramatically enhance the photon emission efficiency and thus the yield of quantum information processing tasks involving photons. One example is a node of a quantum repeater network. In this presentation we address the issue of enhanced photon collection from optically active defects in the solid-state such as diamond [1] or two-dimensional material [2]. We briefly introduce the emitters and then describe recent experiments where we couple them to dielectric/plasmonic antennas [3] and to SiO2/Si light collecting structures [4]. References [1] “Fiber-Coupled Diamond Micro-Waveguides toward an Efficient Quantum Interface for Spin Defect Centers”, M. Fujiwara, O. Neitzke, T. Schröder, A. W. Schell, J. Wolters, J. Zheng, S. Mouradian, M. Almoktar, S. Takeuchi, D. Englund, and O. Benson, ACS Omega 2, 7194-7202 (2017) [2] “Photodynamics of quantum emitters in hexagonal boron nitride revealed by low-temperature spectroscopy“, B. Sontheimer, M. Braun, N. Nikolay, N. Sadzak, I. Aharonovich, and Oliver Benson, Phys. Rev B 96, 121202(R) (2017). [3] “Accurate placement of single nano particles on opaque conductive structures“, N. Nikolay, N. Sadzak, A. Dohms, B. Lubotzky, H. Abudayyeh, R. Rapaport, and O. Benson, Appl. Phys. Lett, accepted (2018); arXiv:1807.10605 [4] “Fine-tuning of whispering gallery modes in on-chip silica microdisk resonators within a full spectral range“, R. Henze, C. Pyrlik, A. Thies, J.M. Ward, A. Wicht, O. Benson, Appl. Phys. Lett. 102, 041104 (2013).
Optical quantum technology needs efficient sources for non-classical light. Solid-state emitters provide excellent mode purity, high brightness, and often also stable operation up to room temperature. At the same time the spin of individual impurities can be entangled with emitted photons. Nano-photonic structures can dramatically enhance the photon emission efficiency and thus the yield of quantum information processing tasks involving photons. One example is a node of a quantum repeater network. In this presentation we address the issue of enhanced photon collection from optically active defects in the solid-state such as diamond [1] or two-dimensional material [2]. We briefly introduce the emitters and then describe recent experiments where we couple them to dielectric/plasmonic antennas [3] and to SiO2/Si light collecting structures [4]. References [1] “Fiber-Coupled Diamond Micro-Waveguides toward an Efficient Quantum Interface for Spin Defect Centers”, M. Fujiwara, O. Neitzke, T. Schröder, A. W. Schell, J. Wolters, J. Zheng, S. Mouradian, M. Almoktar, S. Takeuchi, D. Englund, and O. Benson, ACS Omega 2, 7194-7202 (2017) [2] “Photodynamics of quantum emitters in hexagonal boron nitride revealed by low-temperature spectroscopy“, B. Sontheimer, M. Braun, N. Nikolay, N. Sadzak, I. Aharonovich, and Oliver Benson, Phys. Rev B 96, 121202(R) (2017). [3] “Accurate placement of single nano particles on opaque conductive structures“, N. Nikolay, N. Sadzak, A. Dohms, B. Lubotzky, H. Abudayyeh, R. Rapaport, and O. Benson, Appl. Phys. Lett, accepted (2018); arXiv:1807.10605 [4] “Fine-tuning of whispering gallery modes in on-chip silica microdisk resonators within a full spectral range“, R. Henze, C. Pyrlik, A. Thies, J.M. Ward, A. Wicht, O. Benson, Appl. Phys. Lett. 102, 041104 (2013).
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.
Quantum light is a key resource for the development of quantum-enhanced technologies such as secure communications, quantum networks, distributed quantum computing, metrology, etc. The development of these technologies requires light sources emitting single photons in a pure quantum state as well as efficient photon-photon gates. Such sources and gates can be obtained making use of the single-photon sensitivity of an atomic transition. We study artificial atoms in the form of semiconductor quantum dots to develop building blocks for optical quantum technologies. We use the tools of opto-electronics and nanotechnology to fabricate close to ideal atom-photon interfaces, where a single artificial atom interacts with a single mode of the optical field. We show that cavity quantum electrodynamics allows to largely isolate the artificial atoms from all sources of decoherence such as charge noise and crystal vibrations. We obtain bright solid-state sources of single photons with very high quantum purity, not only in the frequency basis, but for the first time also in the photon number basis. Finally, we have made progresses toward the development of efficient two-photon gates, with devices performing as nonlinear switches at the single-photon level. Some references: High-performances quantum light sources Somaschi et al., Nature Photonics 10, 340 (2016) Grange et al, Phys. Rev. Lett. 118, 253602 (2017) Senellart, Solomon and White, Nature Nanotechnology 12, 1026 (2017) J. C. Loredo, C. Anton, et. al, arXiv:1810.05170 Scaling-up optical quantum computing Loredo et al., Optica 3, 433 (2016) Loredo et al., Phys. Rev. Lett. 118, 130503 (2017) Toward efficient photon-photon gates Giesz et al., Nat. Comm 7, 11986 (2016) De Santis et al, Nature Nanotechnology 12, 663 (2017)
Luminescent lattice defects (so called “color centers”) in diamond provide a robust, solid-state architecture for exploring quantum optics, with many facilitating direct access to highly coherent electron and nuclear spins. The past two decades have seen tremendous advances in our ability to prepare and control the quantum states of these atom-like systems. For instance, demonstrations that leverage the diamond nitrogen vacancy (NV–) center as a spin-photon interface have included the generation of indistinguishable photons, a quantum bit memory exceeding 1 second, entanglement of two distance solid-state qubits, and deterministic long distance quantum teleportation. While diamond color centers are an attractive platform for scalable quantum information processing, the diamond solid-state matrix, by its very nature, inhibits efficient photon collection into either free-space or fiber-optical systems. Due to its large refractive index, emitted photons are inevitably trapped within the diamond crystal by total internal reflection, limiting count rates and overall scalability of information processing via quantum optical protocols. To this end, there has been significant progress towards integrating individual diamond color centers within well-defined radiation channels: namely, those provided by monolithic diamond nanophotonic networks. Currently, on-chip low loss (~ few db/cm) diamond waveguide networks are available, as well as key passive photonic elements like micron scale ring and disk resonators and photonic crystal cavities, with high optical quality (Q) factors. Specifically, we recently demonstrated a scalable ‘angled-etching’ nanofabrication method for realizing nanophotonic networks starting from bulk single-crystal diamond substrates (MJ Burek et al., Nano Letters 2012, MJ Burek et al., Nat Comm. 2014). Angled-etching employs anisotropic oxygen-based plasma etching at an oblique angle to the substrate surface, resulting in suspended optical structures with triangular cross-sections. Using this approach, we demonstrated high Q-factor (> 10^5) optical nanocavities fabricated in bulk single-crystal diamond, operating over a wide wavelength range (visible to telecom). Beyond isolated photonic devices, we have further developed free-standing angled-etched waveguides which efficiently route photons between diamond optical nanocavities, while maintaining physical support through attachment to the bulk substrate. Although on-chip diamond nanophotonics realized by angled-etching continue to mature, efficient off-chip optical coupling schemes which provide near seamless transition of photons on-chip into commercial optical fibers remain an outstanding challenge. This is especially pertinent in applications involving single photons, such as quantum optics with diamond color centers. Herein, we demonstrate a high efficiency fiber-optical interface with aforementioned on-chip diamond nanophotonic networks, achieving > 90% power coupling at both telecom and visible wavelengths. The coupling scheme utilizes a single mode optical fiber, with one end fabricated into a conical taper, to adiabatically transition guided light between on-chip tapered diamond waveguides and off-chip optical fiber networks. Additionally, we develop techniques to robustly pigtail single-mode fibers with diamond waveguide couplers, yielding a packaged technology that may potentially enable deployment in harsh environments, including vacuum, sub-Kelvin cryogenics, or even liquid/biological environments. With these newly developed optical integration schemes, single-crystal diamond is now a viable integrated nanophotonics platform, servicing a range of applications, from non-linear optics and chemical sensing, to quantum science and cavity optomechanics.
Quantum light and in particular single photons have become essential resources for a growing number of quantum applications including quantum computing, quantum key distribution and quantum metrology. Solid-state atomlike systems such as semiconductor quantum dots and color defects in crystals have become the hallmark of highly pure single photon emitters in the past two decades. A particular interest has been developed in nanocrystal quantum dots (NQDs) and color centers in diamond as potential compact room-temperature emitters. There are however several challenges that inhibit the use of such sources in current technologies including low photon extraction efficiency, low emission rates and relatively low single photon purities. In this work we will review our efforts in overcoming these technical difficulties using several complementary methods including designing several nanoantenna devices that enhance the directionality and emission rate of the nanoemitter. In addition, we developed several temporal heralding techniques to overcome the hurdle of low single photon purity in NQDs in an effort to produce a highly pure, bright and efficient single photon source on-chip.
Very recently, the interest for quantum technologies by the scientific community and industry has strongly increased. Different types of implementations have been proposed as a practical implementation for a quantum bit. In particular, quantum photonics is a strong candidate for such applications. We are interested in using single photons and single spins in diamond as a solid state host matrix (nanodiamonds or membranes). Integration of nanosources of light is currently a major bottleneck preventing the realisation of all-photonic chips for quantum technologies and nanophotonics applications. Ideally, one needs optical circuitry, on-chip photodetection and on-chip generation of quantum states of light (single photons, entangled photons…). Our recent work on a new platform for quantum photonics using integrated optics can offer an easier and robust way to create compact quantum circuits that can be on chip and scalable. In this context, the coupling between waveguides and single photon emitters is critical. The goal of our research is to efficiently couple single photon emitters with a new platform made of optical glass waveguides. These waveguides are based on the so-called ion-exchange glass technology and is know in the photonics industry for quite some time but has never been used in the context of quantum technologies. Efficient light-matter interface is of primordial importance in this system. To achieve this goal, several paths are undertaken such as the use of dielectric and plasmonic structuration in order to increase the light interaction with the waveguide or to develop fabrication techniques to insert the emitters directly inside the guide (for nanodiamonds). We will show what is our current state of the art for placing single emitters at the right place on our optical waveguides made of ion-exchange in glass and in particular what can be done to improve our first promising results in order to get near unity coupling between the optical bus and single photon emitters. We will show first results with semiconductor nanocrystals (NCs) but also using nitrogen-vacancy and silicon-vacancy defects in nanodiamond. The very first step in of our approach consists in the design of the structured waveguide using electromagnetic FDTD. We demonstrated that it is possible to achieve more than 90% coupling. In practice, before using coloured centres in diamond, we started working with CdSe/ZnS semiconductor nanocrystals. So far, we use straight waveguides defined to be single modes at the nominal wavelength of the emission line of the nanoemitters. The positioning of nanoemitters is still a challenge to be achieved. We developed an original technique based on photopolymerisation of light where the nanocrystals are grafted into a light sensitive polymer and can be placed at adequate positions.
Single-photon sources that are bright, pure, and interference-ready are essential for quantum communication and photonic quantum information processing, but many solid-state platforms still rely on bulky optical excitation, careful alignment, and post-selection to achieve useful linewidth, stability, and brightness. Scalable quantum photonics instead requires turnkey quantum-light engines that can be triggered on demand, stabilized against environmental noise, and efficiently interfaced with fibers or photonic circuits. This review surveys recent progress in electronic and photonic integration of single quantum emitters in two-dimensional materials, focusing on localized excitonic emitters in transition metal dichalcogenides and defect-based color centers in hexagonal boron nitride. On the electronic side, we discuss electrical injection, fast modulation, electrostatic stabilization, and Stark tunability as routes to suppress blinking, spectral wandering, and charge-noise-induced broadening. On the photonic side, we review waveguide and resonator platforms that funnel emission into well-defined optical modes and, in some cases, enhance radiative rates through the Purcell effect. We connect these integration strategies to key source metrics, including single-photon purity, brightness, spectral stability, and photon indistinguishability. We conclude that the next stage of progress will depend on co-designed electronic and photonic architectures that jointly optimize on-demand operation, stabilization, tunability, and packaging-compatible optical interfacing.