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arXiv:2210.12426 · Advanced Quantum Technologies

Purcell and Collection Efficiency Enhancement of Single NV‐ Center Emission Coupled to an Asymmetric Tamm Structure

Nitesh Singh, Rajesh V. Nair

Published Feb 7, 2023• 9 citations
arXivHTMLPDF

Abstract

Abstract The resonant modes associated with engineered photonic structures of different spatial‐dimension are essential to obtain bright on‐demand single photon sources for quantum technologies. Negatively‐charged nitrogen‐vacancy (NV‐) center in diamond is proposed as an excellent single‐photon source at room temperature. The possible optical readout of spin states in diamond makes it a good candidate for the spin‐photon interface. However, poor light collection, feeble zero‐phonon line, low emission rate, and broad phonon‐induced emission limit the use of NV‐ centers in quantum technologies. Here, a feasible, easy‐to‐fabricate, asymmetric Tamm structure coupled with a single NV‐ center to enhance the emission rate of zero phonon line (ZPL) with better light collection efficiency is presented. The asymmetric Tamm structure shows dual resonant mode with one of the modes coinciding with NV‐ ZPL wavelength of 640 nm. The mode quality factor ( Q ‐factor) is 140, which is greater than the Q ‐factor of conventional Tamm structure and hence, improved field intensity localization. The authors achieved four times Purcell enhancement and five times enhanced light collection efficiency at 640 nm. The proposed asymmetric Tamm structure helps to generate bright single photons using NV‐ center and improves the efficacy of the spin‐photon interface.

DOI
10.1002/qute.202200142

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Abstract The resonant modes associated with engineered photonic structures of different spatial‐dimension are essential to obtain bright on‐demand single photon sources for quantum technologies. Negatively‐charged nitrogen‐vacancy (NV‐) center in diamond is proposed as an excellent single‐photon source at room temperature. The possible optical readout of spin states in diamond makes it a good candidate for the spin‐photon interface. However, poor light collection, feeble zero‐phonon line, low emission rate, and broad phonon‐induced emission limit the use of NV‐ centers in quantum technologies. Here, a feasible, easy‐to‐fabricate, asymmetric Tamm structure coupled with a single NV‐ center to enhance the emission rate of zero phonon line (ZPL) with better light collection efficiency is presented. The asymmetric Tamm structure shows dual resonant mode with one of the modes coinciding with NV‐ ZPL wavelength of 640 nm. The mode quality factor ( Q ‐factor) is 140, which is greater than the Q ‐factor of conventional Tamm structure and hence, improved field intensity localization. The authors achieved four times Purcell enhancement and five times enhanced light collection efficiency at 640 nm. The proposed asymmetric Tamm structure helps to generate bright single photons using NV‐ center and improves the efficacy of the spin‐photon interface.
The nitrogen-vacancy (NV) center in diamond has an optically addressable, highly coherent spin. However, an NV center even in high quality single-crystalline material is a very poor source of single photons: extraction out of the high-index diamond is inefficient, the emission of coherent photons represents just a few per cent of the total emission, and the decay time is large. In principle, all three problems can be addressed with a resonant microcavity, which significantly boosts the emission rate of coherent photons into the cavity mode based on the Purcell effect. In practice though, it has proved difficult to implement this concept: photonic engineering hinges on nano-fabrication yet it is notoriously difficult to process diamond without degrading the NV centers. \nIn this thesis, we present a microcavity scheme which employs minimally processed diamond membranes, thereby preserving the high quality of the starting material. The miniaturized plano-concave Fabry-Perot microcavity platform features full in situ spatial and spectral tunability. We demonstrate a clear change in the lifetime for multiple individual NV centers on tuning both the cavity frequency and anti-node position. The overall Purcell factor for the zero-phonon line (ZPL) of ~30 translates to an increase in the ZPL emission probability from ~3% to ~46%. \nFurthermore, we report the creation of a low-loss, broadband optical antenna giving highly directed output from a coherent single spin in the solid state. The device, the first crystalline solid-state realization of a dielectric antenna, is engineered for individual NV electronic spins in diamond. The photonic structure preserves the high spin coherence of single-crystalline diamond (T2 ~100µs). We demonstrate a directionality of close to 10 and single photon count rates approaching one MHz. The analysis of the angular emission pattern of our device suggests that 95% of the broadband NV fluorescence is channeled into a solid angle corresponding to a numerical aperture of 0.8. \nThe abovementioned approaches feature complementary benefits. The narrowband enhancement of the ZPL emission rate provided by the microcavity benefits applications in quantum information processing relying on coherent photons. With the prospect of integrating lifetime-limited emitters and achieving a high ZPL collection efficiency our results pave the way for much enhanced spin-photon and spin-spin entanglement rates. On the other hand, by channeling the major fraction of the broadband NV fluorescence into a narrow solid angle the dielectric optical antenna facilitates efficient spin readout. Our approach enables a near-unity collection efficiency which, upon mitigation of the known photon losses, renders it a potential key technology for quantum sensing applications. \n
The realisation of an efficient spin-photon interface using the negatively charged nitrogen-vacancy centre (NV-) in diamond requires the resonant coupling of the zero phonon line (ZPL) of the centre to an optical microcavity [1]. The cavity serves to encourage emission into the ZPL in preference to the phonon sidebands, and potentially also to increase the homogeneous linewidth via a reduction of the lifetime (the Purcell effect) thus assisting in the generation of indistinguishable photons. Cavity coupling thus has potential to improve by orders of magnitude the speed with which entanglement can be created between spatially remote NV- centres [2], and to enable other mechanisms for realising spin-photon interfaces via non-demolition measurements [3].
An efficient interface between a spin qubit and single photons is a key enabling system for quantum science and technology. We report on a coherently controlled diamond nitrogen-vacancy center electron spin qubit that is optically interfaced with an open microcavity. Through Purcell enhancement and an asymmetric cavity design, we achieve efficient collection of resonant photons, while on-chip microwave lines allow for spin qubit control at a 10 MHz Rabi frequency. With the microcavity tuned to resonance with the nitrogen-vacancy center’s optical transition, we use excited state lifetime measurements to determine a Purcell factor of 7.3 ± 1.6. Upon pulsed resonant excitation, we find a coherent photon detection probability of 0.5% per pulse. Although this result is limited by the finite excitation probability, it already presents an order of magnitude improvement over the solid immersion lens devices used in previous quantum network demonstrations. Furthermore, we use resonant optical pulses to initialize and read out the electron spin. By combining the efficient interface with spin qubit control, we generate two-qubit and three-qubit spin-photon states and measure heralded Z-basis correlations between the photonic time-bin qubits and the spin qubit. Current applications of NV centers in diamond as spin-photon interfaces for quantum networks are limited by low coherent photon emission. Here, the authors integrate a coherently controlled NV spin qubit with an open microcavity to achieve Purcell-enhanced emission and demonstrate spin-photon state generation.
We propose a novel source of non-classical light consisting of plasmonic aperture with single-crystal diamond containing a single Nitrogen-Vacancy (NV) color center. Theoretical calculations of optimal structures show that these devices can simultaneously enhance optical pumping by a factor of 7, spontaneous emission rates by Fp~50 (Purcell factor), and offer collection efficiencies up to 40%. These excitation and collection enhancements occur over a broad range of wavelengths (~30 nm), and are independently tunable with device geometry, across the excitation (~530 nm) and emission (~600-800 nm) spectrum of the NV center. Implementing this system with top-down techniques in bulk diamond crystals will provide a scalable architecture for a myriad of diamond NV center applications.
Recent demonstrations of entanglement between two remote Nitrogen-Vacancy centers, have opened the way for their use in distributed quantum networks. An efficient spin-photon interface will now be required to help realize this system as a technology. Here we demonstrate the tunable enhancement of the zero phonon line of a single nitrogen-vacancy colour centre in nanodiamond at cryogenic temperatures. A plano-hemispherical open cavity, fabricated using focused ion beam milling provides mode volumes as small as 1.25 cubic microns and quality factor Q ~ 3000. It will be shown how the open geometry and independently adjustable mirrors allows for precise placement of the emitter in the centre of the cavity mode, and crucially enables in-situ tuning of the cavity resonances. At optimal coupling, the signal from individual zero phonon line transitions is enhanced by a factor of 6.25 through the Purcell effect and the overall emission rate of the NV- centre is increased by 40% compared with that measured from the same centre in the absence of cavity field confinement. This Purcell enhancement is mapped out as a function of cavity mode volume. These results represent a proof of principle for a tunable cryogenic spin-photon interface. However by far the best NV optical and spin coherences are to be found in bulk material and efforts towards the production of diamond membranes are currently being made, with dimensions suitable for open-cavity coupling. Efforts towards this and preliminary results will also be discussed.
Negatively charged nitrogen-vacancy ($\mathrm{N}\text{\ensuremath{-}}$${V}^{\ensuremath{-}}$) centers in diamonds have rapidly developed due to their exceptional quantum optical and spin properties at room temperature. However, their applicability is limited by broad phonon-sideband emission with feeble coherent emission at the zero-phonon line (ZPL). This calls for the exploration of photonic cavities to enhance ZPL emission. We propose an easy-to-fabricate hybrid metal-dielectric nanocavity with low mode volume to enhance ZPL emission while suppressing phononic emission channels due to different $\mathrm{N}\text{\ensuremath{-}}$V charge states. We find a 400% enhancement in the emission rate and outcoupling of emitted light at the ZPL with better directivity by deterministic positioning of the $\mathrm{N}\text{\ensuremath{-}}$${V}^{\ensuremath{-}}$ inside the nanocavity, corroborated by numerical simulations. The pump-dependent measurements confirm better emission saturation intensity for the cavity-coupled $\mathrm{N}\text{\ensuremath{-}}$${V}^{\ensuremath{-}}$s. The proposed tunable nanocavity structure can be applied to a broad range of quantum emitters to control their emission so that their use in quantum technologies could be improved.