Nitrogen-vacancy (NV) centers feature outstanding properties such as a spin coherence time of up to 1 s as well as a level structure offering the possibility to initialize, coherently manipulate, and optically read-out the spin degree of freedom of the ground state. However, only about 3% of their photon emission is channeled into the zero phonon line (ZPL), limiting both the rate of indistinguishable single photons and the signal-to-noise ratio (SNR) of coherent spin-photon interfaces. We here report on the enhancement of the SNR of the optical spin read-out achieved by tuning the mode of a two-dimensional photonic crystal (PhC) cavity into resonance with the NV-ZPL. PhC cavities are fabricated by focused ion beam milling in thin reactive ion etched ultrapure single crystal diamond membranes featuring modes with Q-factors of up to 8250 at mode volumes below one cubic wavelength. NV centers are produced in the cavities in a controlled fashion by a high resolution atomic force microscope implantation technique. On cavity resonance, we observe a lifetime shortening from 9.0 ns to 8.0 ns as well as an enhancement of the ZPL emission by almost one order of magnitude. Although on resonance the collection efficiency of ZPL photons and the spin-dependent fluorescence contrast are reduced, the SNR of the optical spin read-out is almost tripled for the cavity-coupled NV centers.
Nitrogen-vacancy (NV) centers feature outstanding properties such as a spin coherence time of up to 1 s as well as a level structure offering the possibility to initialize, coherently manipulate, and optically read-out the spin degree of freedom of the ground state. However, only about 3% of their photon emission is channeled into the zero phonon line (ZPL), limiting both the rate of indistinguishable single photons and the signal-to-noise ratio (SNR) of coherent spin-photon interfaces. We here report on the enhancement of the SNR of the optical spin read-out achieved by tuning the mode of a two-dimensional photonic crystal (PhC) cavity into resonance with the NV-ZPL. PhC cavities are fabricated by focused ion beam milling in thin reactive ion etched ultrapure single crystal diamond membranes featuring modes with Q-factors of up to 8250 at mode volumes below one cubic wavelength. NV centers are produced in the cavities in a controlled fashion by a high resolution atomic force microscope implantation technique. On cavity resonance, we observe a lifetime shortening from 9.0 ns to 8.0 ns as well as an enhancement of the ZPL emission by almost one order of magnitude. Although on resonance the collection efficiency of ZPL photons and the spin-dependent fluorescence contrast are reduced, the SNR of the optical spin read-out is almost tripled for the cavity-coupled NV centers.
The negatively charged nitrogen vacancy centre in diamond is known for its coherent spin properties and optical interface, and thus is regarded a promising candidate for quantum information applications [1]. Realisation of an efficient spin-photon interface with the NV centre is made challenging however by the fact that, in bulk diamond, only 3-4% of spontaneously emitted photons occur in the zero phonon line (ZPL). Placing NV centre in an optical cavity is being explored by several groups [2][3][4] as an effective way to selectively enhance the coherent emission of NVs and thereby increase the efficiency of the coherent spin-photon coupling. Previous work reported successful coupling of the NV in nano-diamond to an open access micro-cavity and observed enhanced ZPL emission [5]. However the NV centres in nano-diamond suffer from broadened zero phonon transition and poor spin coherence. By fabricating NV centres in a ~micrometre thick membrane of high purity single crystal material we can take advantage of the tunability of open access cavities, and at the same time, provide close-to-bulk crystal environment to maintain the coherent spin properties of the NV centres. Here we report our work on the tunable cavity coupling of the ZPL of a NV centre in a 1.2micrometre-thick diamond membrane at 4K. The diamond membrane is fabricated from a 0.5mm-thick E6 CVD diamond plate where ion implantation is carried out on both surfaces to create NV centres at the depth of around 70nm. The plate is then machined into 30micrometre-thick slices, and thinned by ICP-RIE with a combination of Ar/Cl[6] and pure oxygen plasma etching recipes. The open cavity consists of a concave mirror (99.99% reflectivity) deposited on a template fabricated using Focused Ion Beam (FIB) milling[7] and a planar mirror (99.8% reflectivity) which supports the membrane. For bare cavities with mirror radii of curvature (RoC) of 12micrometre, we measured a finesse of F~2000 and mode volume as small as 0.75micrometre^3. In-situ tuning of the cavity resonance is achieved with piezoelectric actuators. When mounted in our bath cryostat the cavity modes have dominant Lorentzian line profiles which indicate a passive stability of the cavity length of better than 0.15nm. No active locking is currently deployed. With the presence of a diamond membrane inside the cavities, the measured finesse and mode volume of a cavity with 12micrometre RoC are found to be around 300 and 3 micrometre^3, respectively. We attribute the reduction in finesse to scattering at the membrane-air and membrane-mirror interfaces. On coupling to the ZPL of a target NV centre, we record a factor of 4 increase in the saturated intensity of ZPL fluorescence compared to that measured from the same NV centre in absence of the concave mirror. This result is consistent with the calculated Purcell factor of 16 combined with a relatively low efficiency of light extraction (estimated to be around 19%) from the cavity due to the scattering losses.
We present the controlled creation of single nitrogen-vacancy (NV) centers via ion implantation at the center of a photonic crystal cavity which is fabricated in an ultrapure, single crystal diamond membrane. High-resolution placement of NV centers is achieved using collimation of a 5 keV-nitrogen ion beam through a pierced tip of an atomic force microscope. We demonstrate coupling of the implanted NV centers' broad band fluorescence to a cavity mode and observe Purcell enhancement of the spontaneous emission. The results are in good agreement with a master equation model for the cavity coupling.
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
Towards building large-scale integrated photonic systems for quantum information processing, spatial and spectral alignment of single quantum systems to photonic nanocavities is required. Here, we demonstrate spatially targeted implantation of nitrogen vacancy (NV) centers into the mode maximum of 2-d diamond photonic crystal cavities with quality factors up to 8000, achieving an average of 1:1 0:2 NVs per cavity. Nearly all NV-cavity systems have significant emission intensity enhancement, reaching a cavity-fed spectrally selective intensity enhancement, Fint, of up to 93. Although spatial NV-cavity overlap is nearly guaranteed within about 40 nm, spectral tuning of the NV’s zero-phonon-line (ZPL) is still necessary after fabrication. To demonstrate spectral control, we temperature tune a cavity into an NV ZPL, yielding FZPL int ~ 5 at cryogenic temperatures.
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.
Quantum computation and information processing is an exciting multi-disciplinary field, whose objective is to harness the power of superposition and entanglement of quantum states to establish a new regime of computation for Mankind. This work demonstrates our efforts, in both materials and device engineering, towards building an efficient spin-photon interface for a promising solid-state spin qubit, namely the nitrogen-vacancy (NV) centre in diamond. Improvement in optical coherence of NV centre is realised via the novel laser-writing fabrication technique and quantified using high-resolution spectroscopy. A new metric concerning the charge-state stability of the NV centre is introduced to better translate the measured NV quality to entanglement generation efficiency. To address the low branching ratio of NV emission into the zero-phonon line (ZPL) transition, a hybrid open-cavity device incorporating low-dimensional micro-structured NV-containing diamond membrane is fabricated and characterised. In addition to the more commonly quoted emission enhancement factor, the collection efficiency of the cavity device is also quantified via numerical simulations. Combining all observed improvements in key metrics under the framework of double-heralding entanglement schemes, a two order of magnitude increase in entanglement generate rate can be expected. The established simulation methodology is widely applicable in the design and analysis of similar devices with an emitter-in-cavity setup.