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Diamond photonics is scaling up
Diamond photonics is scaling up Download PDF News & Views . Published: 14 September 2020 . QUANTUM OPTICS Diamond photonics is scaling up . Mehran Kianinia ? ORCID: orcid.org/0000-0003-4073-1492 1 & . Igor Aharonovich ? ORCID: orcid.org/0000-0003-4304-3935 1 , 2 ? . Nature Photonics ( 2020 ) Cite this article 4 Altmetric Metrics details Subjects . Optical properties of diamond . Integrated optics . Quantum optics . The integration of diamond waveguide arrays into an aluminium nitride photonic platform offers hope for the realization of scalable chips for quantum information processing. Download PDF The field of diamond photonics is undergoing a transformation. Over the past decade, the quantum community has invested an enormous effort to interface special defects in diamond known as colour centres with photonic resonators and waveguides 1 , 2 , 3 . These colour centres have long attracted attention as quantum emitters of light and optically accessible spin qubits. Creating such an interface is important because it marries control over the quantum properties (local density of states) of an emitter (colour centre) with the ability to capture and route the emitted photons with high efficiency. Both features are required to achieve an integrated quantum photonic circuit that can perform tasks in quantum information processing. Realizing this goal with diamond in a scalable manner on a chip has turned out to be challenging for numerous reasons. First, large single-crystal diamond wafers are still extremely rare. Polycrystalline material is available, but the increased absorption in the visible spectral range hinders operation in that range (the germanium–vacancy colour centre in diamond emits at 602 nm in the red) 4 . Second, nanofabrication of diamond is intricate as it is chemically inert and does not lend itself to easy processing, and isolation of suspended structures on a large, millimetre scale, is nearly impossible. Finally, interfacing diamond planar devices with photon collection architectures (that is, coupling the light into single-mode fibres or waveguides) is also a non-trivial task since diamond is a high-refractive-index material. Now, writing in Nature , Noel Wan and colleagues 5 elegantly address these challenges and demonstrate the first large-scale diamond photonic chip featuring 128 nearly identical spin qubits. The ingenuity of their approach was to build a hybrid system, which combines diamond waveguides that serve as hosts for the colour centres with an aluminium nitride (AlN) platform that serves as a photonic integrated circuit (PIC) and the ‘photonic bus’ 6 . Figure 1 shows the optical image of the integrated device. The broad transparency window of AlN, its availability in large wafer sizes and the established nanofabrication protocols makes it feasible to engineer the needed photonic chip 7 . Fig. 1: Optical image of the integrated device. Noel Wan, MIT a , AIN PIC with a socket for the diamond microchiplet. There are 8 channels on each side. b , Magnified image of the diamond microchiplet that is positioned in the AlN socket. Vertical spacing between each diamond channel (shown in red) is 3 μm. Full size image The tiny diamond slabs, which the authors refer to as ‘quantum microchiplets’, are picked and placed into sockets on the AIN PIC with nanometre resolution using piezo micromanipulators. The authors claim a placement success rate of 90% and a displacement error of sub-50 nm. The light emitted from the colour centres is collected through inverse tapered sections with efficiencies approaching 97%. The direct excitation of the emitters and the collection via the waveguide provides efficient resonant excitation collection, ~18?dB above background, without additional filtering or cross-polarization schemes. The team engineered 8-channel and 16-channel quantum microchiplets. Approximately 40% of the channels on the 8-channel microchiplets contained one or more suitable quantum emitters per waveguide. This probability is however reduced for the larger microchiplet as misalignment of the diamond and the AlN PIC becomes an issue. Importantly, however, there are currently no fundamental limitations to increase this value, as focused ion beam alignment techniques are becoming more and more accurate and automated. To introduce the colour centre spin qubits, focused ion implantation was employed. The silicon–vacancy and germanium–vacancy colour centres used in this work belong to the so-called group IV centres in diamond 8 . They are protected from charge fluctuations by their inversion symmetry, which makes them excellent candidates for scalable quantum photonics. The team went the extra mile and implemented a strain-tuning scheme using a capacitive actuator patterned onto the PIC. This was done to overcome the inhomogeneous spread in transition frequencies of the defects and to realize nearly coherent emitters with an excellent signal-to-noise ratio 2 . Upon application of a bias voltage, the resonance of a specific spin qubit can be slightly tuned by ~100 MHz, which should be sufficient to provide the spectral alignment needed to demonstrate qubit entanglement in the future. Interesting to note, that the ion implantation was done first, before the final geometry of the waveguides was defined and etched into the diamond. For the microchiplet, a qubit grid was registered, and identification of the fabricated quantum emitters relative to the alignment markers was achieved using a wide-field imaging technique. This fabrication sequence eliminates the need for targeted ion implantation into a fabricated nanostructure and eliminates the danger of device breakdown at the annealing step. These results are undoubtedly timely as the race towards a scalable working quantum photonic architecture heats up 9 . To date, only individual elements of a quantum photonic circuit have been demonstrated — that is, generation of indistinguishable photons from two consecutive photons or two separated colour centres. However, scaling these systems is a considerable challenge. These latest findings suggest that the hybrid approach could be a promising path forward. While the authors have not yet demonstrated clear generation of indistinguishable photons, they possess all the prerequisites to achieve this goal. The system is also amenable to the addition of microwave strip lines to realize quantum memories, harnessing the defects’ spin states. The introduction of beam splitters for reconfigurable entanglement is also feasible. Furthermore, future integration with on-chip superconducting single-photon detectors could bring new opportunities for realizing integrated quantum photonic chips. These futuristic, yet achievable milestones would have to be met before the demonstration of the heralded entanglement of a few qubits 10 and the realization of photonic quantum processing. Notably, the hybrid quantum photonic architecture approach is potentially amenable to other solid-state quantum emitters. The ingenious design of the AlN PIC with the ‘socket’ can be employed with quantum microchiplets fabricated from other materials — including (but not limited to) silicon carbide, hexagonal boron nitride or rare-earth ions in yttrium orthosilicate, which are all attracting increasing attention for scalable quantum photonics applications 11 . It seems scalable approaches for solid-state quantum photonics are finally becoming a reality, with a ‘best-of-both-worlds hybrid approach’ leading the race. References . 1. 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Cite this article . Kianinia, M., Aharonovich, I. Diamond photonics is scaling up. Nat. Photonics (2020). https://doi.org/10.1038/s41566-020-0695-9 Download citation Published : 14 September 2020 DOI : https://doi.org/10.1038/s41566-020-0695-9 Download PDF Associated Content . Nature Article Large-scale integration of artificial atoms in hybrid photonic circuits . Noel H. Wan . , Tsung-Ju Lu . , Kevin C. Chen . , Michael P. Walsh . , Matthew E. Trusheim . , Lorenzo De Santis . , Eric A. Bersin . , Isaac B. Harris . , Sara L. Mouradian . , Ian R. Christen . , Edward S. Bielejec . ?&? Dirk Englund .
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