The ability to control the state of individual atoms is a new challenge for science and technology in the 21st century. Currently, experiments on individual quantum systems such as trapped ions, single molecules, quantum dots, superconducting qubits and photons are crucial for the rapidly growing field of quantum information processing and communication. In general, solid state systems are preferable for scaling and the choice of material plays a crucial role; for example, in classical electronic devices continual performance enhancement and miniaturization is strongly linked to the success of silicon-based technology. For quantum applications, diamond has the potential to become the material of choice, because its large bandgap enables the control of optically active impurities and higher operation temperature. This focus issue collates original research contributions from some of the leading groups in the field as a showcase for the very latest developments in diamond-based quantum technologies.
The ability to control the state of individual atoms is a new challenge for science and technology in the 21st century. Currently, experiments on individual quantum systems such as trapped ions, single molecules, quantum dots, superconducting qubits and photons are crucial for the rapidly growing field of quantum information processing and communication. In general, solid state systems are preferable for scaling and the choice of material plays a crucial role; for example, in classical electronic devices continual performance enhancement and miniaturization is strongly linked to the success of silicon-based technology. For quantum applications, diamond has the potential to become the material of choice, because its large bandgap enables the control of optically active impurities and higher operation temperature. This focus issue collates original research contributions from some of the leading groups in the field as a showcase for the very latest developments in diamond-based quantum technologies.
Integrated photonic devices in diamond have tremendous potential for many quantum applications, including long-distance quantum communication, quantum information processing, and quantum sensing. These devices benefit from diamond's combination of exceptional thermal, optical, and mechanical properties. Its wide electronic bandgap makes diamond an ideal host for a variety of optical active spin qubits that are key building blocks for quantum technologies. In landmark experiments, diamond spin qubits have enabled demonstrations of remote entanglement, memory-enhanced quantum communication, and multi-qubit spin registers with fault-tolerant quantum error correction, leading to the realization of multinode quantum networks. These advances put diamond at the forefront of solid-state material platforms for quantum information processing. Recent developments in diamond nanofabrication techniques provide a promising route to further scaling of these landmark experiments towards real-life quantum technologies. In this paper, we focus on the recent progress in creating integrated diamond quantum photonic devices, with particular emphasis on spin-photon interfaces, cavity optomechanical devices, and spin-phonon transduction. Finally, we discuss prospects and remaining challenges for the use of diamond in scalable quantum technologies.
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.
The recently emerged field of quantum technology is aiming to employ quantum coherence and entanglement for realization of next generation sensors, standards, imaging systems, secure communication and computers.Although applications of quantum technologies cover a broad spectrum and their development stage varies from early demonstrations to commercially available devices, many challenges have been identified.The first includes tailoring materials for quantum technologies, which is playing an essential role in technological applications of quantum science.Solid state systems allowing long coherence time are essential for quantum computation and quantum sensing.Ultrapure materials with tailored isotopic content are important for reaching long coherence time of spin qubits (Awschalom et al., 2013).The ability to place single dopants with high precision and form spin qubits at desired location is essential for both quantum computing and quantum sensing applications (McCallum et al., 2012;Smith et al., 2019).The performance of different types of qubits is usually benchmarked in terms of controllability and coherence time.Usually solid-state systems allow fast control, but exhibit fast decoherence owing to complex environment.It is therefore important to search for new types of qubits combining isolation from environment and access via fast coherent control and readout.Novel approaches combining different quantum systems into build hybrid quantum devices for optimal performance is a promising avenue.Examples of such hybrid approaches are spin systems coupled to superconducting qubits (Kubo et al., 2011) and hybrid nuclear-electronic qubits (Morley et al., 2013).Optimal protocols for quantum technologies requires extension of coherence time of qubits beyond the coherence time of the isolated quantum system.The efficient protection toolkit includes dynamical decoupling (Yang et al., 2011) and quantum error correction (Terhal, 2015) based techniques.Although general principles of qubits protection were developed and tested experimentally in different model environments, it is essential to adapt them to realistic environmental noise.In the field of quantum sensing, it is also essential to combine protections against noise with non-reduced sensing performance.Experimental imperfections can be addressed using optimal control tools (Glaser et al., 2015).The development of efficient quantum algorithms is another growing field belonging to quantum software area.On one hand, it is essential to find problems where a quantum computer can outperform classical computers.On the other hand, it is essential to develop an application scenario for a limited number of qubits (Montanaro, 2016).In addition, in order to develop new algorithms, future work must include discoveries of application scenarios of already known algorithms.Application of quantum Fourier transform for sensing is a promising example of such new applications (Vorobyov et al., 2021).Signal processing is another field of quantum software that is becoming essential in applications.Advanced signal analysis protocols, like quantum compressed sensing, can be employed to quantify entanglement in large quantum devices via efficient quantum tomography (Riofrío et al., 2017).
The burgeoning field of nanophotonics has grown to be a major research area, primarily because of the ability to control and manipulate single quantum systems (emitters) and single photons on demand. For many years, studying nanophotonic phenomena was limited to traditional semiconductors (including silicon and GaAs) and experiments were carried out predominantly at cryogenic temperatures. In the last decade, however, diamond has emerged as a new contender to study photonic phenomena at the nanoscale. Offering a plethora of quantum emitters that are optically active at room temperature and ambient conditions, diamond has been exploited to demonstrate super‐resolution microscopy and realize entanglement, Purcell enhancement, and other quantum and classical nanophotonic effects. Elucidating the importance of diamond as a material, this progress report highlights the recent achievements in the field of diamond nanophotonics, and conveys a roadmap for future experiments and technological advancements.
In the last few years quantum information has played an increasingly central role in the research activities of many scientists within wide ranging areas of physics, mathematics and computer science as it known to be more efficient than its classical counterpart. The impact and advantages of quantum information protocols emerge in numerous situations. In the case of cryptography, quantum dynamics guarantees secure protocols, and in quantum computation, factorization of large numbers, intractable with classical algorithms, can be solved much faster with a quantum computer. It is now widely believed that quantum information will play a leading role in future technologies. Together with the ongoing development of more efficient schemes to solve both new and old tasks in information science, a great deal of interest has been devoted to selecting suitable physical systems where one could realize these ideas. In particular, the quest for large scale integrability has stimulated an increasing interest in the field of solid state physics. Nanoelectronics seems the natural arena to realize physical implementation of quantum hardware. Qubits made out of solid-state devices, such as spins/charges in quantum dots or superconducting nanocircuits, may offer a greater advantage in this respect because fabrication techniques allow for scalability to a large number of coupled qubits. Recent experimental breakthroughs in semiconducting and superconducting nanostructures constitute the first important steps towards the realization of a solid-state quantum computer. In addition the interest in solid state quantum computation has stimulated a large body of research aimed at understanding the properties of entanglement in solid state systems. This Focus issue of New Journal of Physics gathers together contributions from leading experts in the area of solid state quantum information with the aim of providing a panorama of the most exciting scientific questions currently being investigated in the field. Focus on Solid State Quantum Information Contents Quasiparticle entanglement: redfinition of the vacuum and reduced density matrix approach P Samuelsson, E Sukhorukov and M Buttiker Pseudospin quantum computation in semiconductor nanostructures V W Scarola, K Park and S Das Sarma Superconducting qubit network with controllable nearest-neighbour coupling M Wallquist, J Lantz, V S Shumeiko and G Wendin Weak coupling Josephson junction as a current probe: effect of dissipation on escape dynamics J M Kivioja, T E Nieminen, J Claudon, O Buisson, F Hekking and J P Pekola The single Cooper-pair box as a charge qubit K Bladh, T Duty, D Gunnarsson and P Delsing Quantum state transfer in arrays of flux qubits A O Lyakhov and C Bruder Spin filling of a quantum dot derived from excited-state spectroscopy L H Willems van Beveren, R Hanson, I T Vink, F H L Koppens, L P Kouwenhoven and L M K Vandersypen Clauser–Horne inequality for the full counting statistics F Taddei, R Fazio and E Prada Recent advances in exciton based quantum information processing in quantum dot nanostructures H J Krenner, S Stufler, M Sabathil, E C Clark, P Ester, M Bichler, G Abstreiter and J J Finley Spatially highly resolved study of AFM scanning tip-quantum dot local interaction S Kicin, A Pioda, T Ihn, M Sigrist, A Fuhrer, K Ensslin, M Reinwald and W Wegscheider Transfer of entanglement from electrons to photons by optical selection rules M Titov, B Trauzette, B Michealis and C W J Beenakker Non-Abelian Chern-Simons models with discrete gauge groups on a lattice B Doucot and L B Ioffe Landau–Zener transitions in qubits controlled by electromagnetic fields Martijn Wubs, Keiji Saito, Sigmund Kohler, Yosuke Kayanuma and Peter Hänggi Phase-slip flux qubits J E Mooij and C J P M Harmans Mediated tunable coupling of flux qubits Alec Maassen van den Brink, A J Berkley and M Yalowsky Divergent beams of nonlocally entangled electrons emitted from hybrid normal-superconducting structures Elsa Prada and Fernando Sols Decoherence from ensembles of two-level fluctuators Josef Schriefl, Yuriy Makhlin, Alexander Shnirman and Gerd Schön Semiconductor quantum dots for electron spin qubits W G van der Wiel, M Stopa, T Kodera, T Hatano and S Tarucha Rosario Fazio, NEST-INFM and Scuola Normale Superiore, Pisa, Italy
The past decade has seen an explosion of development in new devices, modalities, and architectures for advanced sensing, computation, and communication. This growth has been driven by the massive investments of large consumer-tech companies in data-hungry domains of customer interest. As these companies develop their products and seek a competitive edge, technologies which can provide advantage across application areas by making computation cheaper, or sensors more knowledgeable, or communication faster, are attracting more interest in and outside academia. Photonics, or the study of how to manipulate light, is a broad-based platform that inherently delivers on these promises. Although photonics has been a part of daily life since the invention of the laser, modern needs and aspirational designs which meet those needs far exceed the capabilities of the platforms which have traditionally constituted the photonics toolkit.\nThere are two specific areas where standard photonic materials are lacking - support for visible wavelengths and functionality for quantum optics. Currently, there are many implementations of systems which use visible light or quantum optics on the tabletop, but the drive to scale these technologies and deliver them to the consumer encounters a roadblock in their size and cost. Visible light sensors, for instance, could greatly benefit from conversion into an integrated photonics platform, where the optical components are printed onto a small substrate. However, the standard materials used for such systems (silicon and indium phosphide) absorb visible light, and other materials (like silicon nitride) do not have the attributes required for advanced functionality. More compellingly, there exist very few material platforms which can host the quantum defects required for next-generation technologies like quantum computation or secure communication.\nThe past decade has seen extensive research into an emerging material platform which can deliver on the ability to bring integrated visible photonics and quantum optics to the same chip - diamond. With a broad transparency region (from UV to the far infrared) and high refractive index, diamond makes an excellent material for run-of-the-mill photonic devices. More importantly, it harbors atom-like defects in its crystal lattice - the nitrogen-vacancy, silicon-vacancy, and germanium-vacancy centers, among many more - which provide a direct interface between the photons circulating within the diamond device and the quantum world. Taken together, these simple properties of diamond constitute a powerful approach towards implementation of advanced quantum technologies, such as repeaters for secure quantum communication or even, one day, computers.\nBefore that future arrives, diamond's capability must first be developed. Diamond has only been available in significant quantities for research use for about a decade, and as such, the processing technologies which are taken for granted in silicon or silicon nitride-based work must be redeveloped. Within the context of photonic devices, we discuss different fabrication strategies and their implications for device design. For visible light devices especially, we investigate methods to reduce sidewall roughness for etched structures.\nInterfacing with diamond photonic structures also brings its own unique set of challenges due to its high refractive index and very small material footprint. To this end, we develop a method termed "loaded tapered-fiber coupling" to optically access large, free-standing diamond devices. For size-limited diamond resonators and waveguides, we also develop auxiliary optical waveguides to act as spot-converters for visible wavelengths, an improvement on previous techniques for telecom-range wavelengths.\nFinally, by synthesizing these improvements in fabrication, we embark on a set of experiments in diamond microresonators. Nonlinear optics is an excellent testbed for such improvements, both for diamond's superlative properties in that regard and their high sensitivity to improvements in fabrication. First, we look at a process known as Raman lasing by constructing long path-length resonators.This technique involves the shifting of an input photon by 40 THz via the interaction with the diamond's crystal lattice.The first demonstration in a diamond integrated device was shown with a pump wavelength of 1600 nm, with an output that was tuned over 100 nm and a low threshold. Leveraging the developments in visible photonics for diamond, a second Raman laser was demonstrated, this time at near-visible (720 nm) wavelengths.This too had a Stokes output which was tunable to over 100 nm and a very low threshold. Raman-specific effects such as polarization conversion were also investigated.\nA cousin to the Raman effect, the Kerr nonlinearity plays a key role in so-called microresonator (or Kerr) frequency combs. We survey the possibilities and difficulties in implementation for a diamond platform, highlighting the competition between the Kerr and Raman nonlinearities. Although the outlook for a technologically relevant Kerr comb in diamond is ultimately pessimistic, avenues for improvement are highlighted. Namely, we investigate via simulation a related class of effects, known as supercontinuum generation.Taking advantage of subtleties of the Raman process in diamond, we can find a preferred geometry which enables generation of a broadband light spectrum natively in diamond.\nFinally, we look at alternative applications of diamond, particularly in high-power systems. Due to its excellent thermal conductivity and power-handling capacity, optics fashioned purely out of diamond can show significant advantages compared to other materials. We end with a discussion of the design principles and results stemming from this work.