Single photons and individual quantum systems are at the heart of recent developments in quantum technologies and are about to enable a variety of novel applications in sensing, communication, and computing. Photonic devices are the key to control interactions between quantum systems and light as well as to simultaneously engineer the properties of photons. For scalable quantum technologies, the employed quantum systems are solid-state based, thus placing the field of quantum photonics at the intersection of physics, nanotechnology, and material sciences. This special issue features 14 contributions and addresses recent advances in several material platforms.
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Quantum technologies using solid-state based quantum systems have seen tremendous advances in recent years. These advances have been triggered by a multitude of factors, including optimized materials and nanofabrication processes for quantum applications as well as novel designs for photonic components and completely new approaches to apply solid state quantum system. The special issue includes papers addressing quantum photonics based on color centers in diamond, defects in hexagonal boron nitride, quantum dots as well as rare earth ions in a solid matrix and thus addresses most relevant material systems.
Diamond, hosting more than 500 different color centers, is one of the leading material systems for quantum sensing and quantum information processing. Here, photonic devices interface phonons and spins and aid to efficiently read-out color center-based sensors. Challenges for diamond-based devices still arise from the controlled generation of color centers (addressed in this issue in ), as well as potential instabilities of color centers especially close to surfaces (investigated in ). Moreover, dedicated nanofabrication processes for diamond devices are still under investigation. The special issue features 4 manuscripts addressing this field, in particular the fabrication of thin membranes , dense nanopillars arrays  and pyramidal diamond tips the latter via diamond growth . Employing diamond growth might mitigate potential damage during top-down nanofabrication (investigated in Radtke et al ). In addition to those technological advances, Zhang et al.  present a new method for diamond-based thermometry and Xu et al.  introduce a new concept to quantum information processing with closely spaced color centers.
Semiconductor quantum dots are versatile single photon sources which can harness mature fabrication processes for semiconductors. The role of phonons in these systems  as well as the spectral tuning of a quantum dot embedded in a photonic structure  are featured in this special issue. Wei et al. moreover report highly polarized single photon emission from epitaxial quantum dots in photonic crystal cavities .
Rare-earth-doped ceramics and approaches to manufacture waveguides in this material that are potentially interesting for quantum memory applications  extend the range of potential materials, while defects in boron nitride are used to test an optimized, modular system for single photon generation . Sun et al. finally present a metallo-dielectric antenna structure that is applicable to many of the quantum systems presented here .
In conclusion, this issue summarizes recent advances in the broad field of quantum nanophotonics, bringing together contributions from different material platforms and different applications. We hope that it will stimulate novel ideas and concepts. We thank the authors for their contributions as well as the reviewers and the editorial staff at Optical Materials Express for their efficient work.
1. S. Trofimov, S. Tarelkin, S. Bolshedvorskii, V. Bormashov, S. Troshchiev, A. Golovanov, N. Luparev, D. Prikhodko, K. Boldyrev, S. Terentiev, A. Akimov, N. Kargin, N. Kukin, A. Gusev, A. Shemukhin, Y. Balakshin, S. Buga, and V. Blank, “Spatially controlled fabrication of single NV centers in IIa HPHT diamond,” Opt. Mater. Express 10(1), 198–207 (2020). [CrossRef]
2. J. Aspinall, S. Adekanye, I. Brown, A. Dhawan, and J. Smith, “Instabilities in nanodiamond nitrogen-vacancy centre single photon sources under prolonged pulsed excitation,” Opt. Mater. Express 10(2), 332–341 (2020). [CrossRef]
3. A. Trycz, B. Regan, M. Kianinia, K. Bray, M. Toth, and I. Aharonovich, “Bottom up engineering of single crystal diamond membranes with germanium vacancy color centers,” Opt. Mater. Express 9(12), 4708–4715 (2019). [CrossRef]
4. A. Schmidt, B. Naydenov, F. Jelezko, J. Reithmaier, and C. Popov, “Fabrication of highly dense arrays of nanocrystalline diamond nanopillars with integrated silicon-vacancy color centers during the growth,” Opt. Mater. Express 9(12), 4545–4555 (2019). [CrossRef]
5. M. Batzer, B. Shields, E. Neu, C. Widmann, C. Giese, C. Nebel, and P. Maletinsky, “Single crystal diamond pyramids for applications in nanoscale quantum sensing,” Opt. Mater. Express 10(2), 492–500 (2020). [CrossRef]
6. M. Radtke, L. Render, R. Nelz, and E. Neu, “Plasma treatments and photonic nanostructures for shallow nitrogen vacancy centers in diamond,” Opt. Mater. Express 9(12), 4716–4733 (2019). [CrossRef]
7. S. Zhang, S. Li, B. Du, Y. Dong, Y. Zheng, H. Lin, B. Zhao, W. Zhu, G. Wang, X. Chen, G. Guo, and F. Sun, “Thermal-demagnetization-enhanced hybrid fiber-based thermometer coupled with nitrogen-vacancy centers,” Opt. Mater. Express 9(12), 4634–4643 (2019). [CrossRef]
8. Z. Xu, Z. Yin, Q. Han, and T. Li, “Quantum information processing with closely-spaced diamond color centers in strain and magnetic fields [Invited],” Opt. Mater. Express 9(12), 4654–4668 (2019). [CrossRef]
9. E. Denning, J. Iles-Smith, N. Gregersen, and J. Mork, “Phonon effects in quantum dot single-photon sources,” Opt. Mater. Express 10(1), 222–239 (2020). [CrossRef]
10. M. Schmidt, M. Helversen, S. Fischbach, A. Kaganskiy, R. Schmidt, A. Schliwa, T. Heindel, S. Rodt, and S. Reitzenstein, “Deterministically fabricated spectrally-tunable quantum dot based single-photon source,” Opt. Mater. Express 10(1), 76–87 (2020). [CrossRef]
11. Y. Wei, T. Zhao, B. Yao, R. Su, Y. Yu, J. Liu, and X. Wang, “Bright and highly-polarized single-photon sources in visible based on droplet-epitaxial GaAs quantum dots in photonic crystal cavities,” Opt. Mater. Express 10(1), 170–177 (2020). [CrossRef]
12. H. Zhang, J. Yang, J. Brown, S. Gray, T. Ketcham, B. Aitken, and D. Nolan, “La3+ and Er3+ co-doped Y2O3 transparent ceramics with a tunable refractive index and long coherence lifetime,” Opt. Mater. Express 10(1), 99–104 (2020). [CrossRef]
13. H. Lim, K. Jeong, D. Lee, and K. Hong, “Modular system for fluorescence-based single photon generation using a retro-reflector,” Opt. Mater. Express 9(12), 4644–4653 (2019). [CrossRef]
14. Y. Sun, V. Yaroshenko, A. Chebykin, E. Ageev, S. Makarov, and D. Zuev, “Metal-dielectric nanoantenna for radiation control of a single-photon emitter,” Opt. Mater. Express 10(1), 29–35 (2020). [CrossRef]