Recent progress in hybrid diamond photonics for quantum information processing and sensing

Abstract

Point defects in diamond, particularly nitrogen-vacancy (NV) centers, have emerged as powerful tools for a broad range of quantum technologies. These defects are promising candidates for quantum information science, serving as deterministic single-photon sources and solid-state quantum memories. They have also been employed as nanoscale quantum sensors to detect various physical quantities, including magnetic fields, electric fields, and temperature, owing to their long spin coherence time at room temperature. Development of these diamond-based quantum technologies has been rapidly boosted by a recent quantum leap in nanofabrication technologies for high-quality single-crystal diamond. Incorporating these color centers into diamond nanostructures with mature integrated photonics provides a promising route to build scalable and practical systems for quantum applications. This review discusses recent progress and challenges in the hybrid integration of diamond color centers on cutting-edge photonic platforms.

Nitrogen-vacancy centers in diamond have emerged as powerful tools for a broad range of quantum technologies. This review discusses recent progress and challenges in the hybrid integration of diamond color centers on cutting-edge photonic platforms.

Introduction

Diamond has been considered a promising material for a wide variety of quantum technologies such as quantum sensing, quantum communication, and quantum computation^1–8: it can host various types of point defects (nitrogen-vacancy centers (NV), silicon-vacancy centers (SiV), tin-vacancy centers (SnV) and so on) with excellent quantum properties and long spin coherence time. Optically active spin qubits in diamond have shown great potential as solid-state quantum emitters and memories⁹, as exemplified by the demonstrations of room-temperature single-photon generation¹⁰ and laboratory-scale quantum network using multiple separate point defects¹¹. NV centers are also exploited for sensitive solid-state quantum sensors¹² to detect magnetic field^13–15, electric field¹⁶, strain, and temperature^17,18. For example, sensing magnetic fields based on a single NV center facilitates the detection of chemical and biomedical information^19–21 and fundamental spin dynamics with atomic scale^22–24. In addition, the use of an ensemble of NV centers can enhance the magnetic field sensitivities up to the order of picoteslas¹⁵, which is advantageous for medical applications demanding the detection of weak magnetic fields. Solid-state quantum sensors based on diamond are also beneficial for constructing compact and practical devices by combining diamond chips with optical fibers and complementary metal–oxide–semiconductor (CMOS) architectures of silicon photonics.

To unlock the potential of diamond-based quantum technologies, the implementation of micro/nanostructures in diamonds is crucial for the efficient use of NV photons^25–29. For quantum information processing and quantum sensing, the number of photons in the zero-phonon line (ZPL) and the total number of photons are, respectively, of great importance for practical applications. In quantum information processing, a reduction in coherent photon efficiency leads to errors of computations and low entanglement rates^30–32. For quantum sensing, the field sensitivity is degraded by a decrease in collected amount of NV photons³. Owing to recent advancements in diamond nanofabrication technologies, a wide variety of nanophotonic structures have been successfully demonstrated to date³³. However, the scalability is limited by the lack of the technology to fabricate wafer-scale diamond photonic integrated circuits. Also, diamond photonics is not fully developed in terms of quality, functionality, and compatibility with electronics compared to well-developed photonic platforms, including Si, SiN, LiNbO₃ and AlN. The attractive and alternative approach is the hybrid integration of diamond color centers on these sophisticated photonic platforms^34–36. For instance, Si photonics will be a key enabling technology to build large-scale and highly functional photonic circuits^37,38 owing to mature CMOS technology. SiN photonics is also compatible with CMOS fabrication³⁹ and offers ultra-low photonic loss^40,41. Meanwhile, the current nanofabrication technology for diamond photonics is inherently incompatible with existing hybrid integration approaches. Thus, the open question is how to combine diamond photonics with other state-of-the-art photonic technologies for future scalable and functionalized quantum applications.

This contribution reviews the hybrid integration of diamond color centers into cutting-edge photonics platforms. We first provide a brief overview of NV’s creation and diamond etching technology. We then summarize the advances in hybrid integration of diamond on a photonic chip for quantum information processing and quantum sensing. Moreover, we also discuss the related technologies including cryo-CMOS and compact diamond quantum sensors. We then discuss the future challenges and opportunities of diamond quantum photonics with heterogeneous photonics chips.

Creation of color centers in diamond

This section briefly introduces color centers in diamond and the ways to create them.

Color centers in diamond

NV centers and group-IV color centers in diamond exhibit different structural symmetries. These differences result in distinct spin properties and influence their applications in quantum technologies. Detailed information regarding the physics of color centers in diamond is provided in these articles^42–45. An NV center is created when a nitrogen atom replaces a carbon atom in the diamond lattice adjacent to a lattice vacancy, resulting in C_3v symmetry (Fig. 1a). Thanks to their high radiative quantum efficiency even at room temperature, as well as the short decay time of their excited state, NV centers can operate as excellent solid-state single-photon emitters (Fig. 1b)^10,46. In addition, the efficient spin-photon interface of NV centers makes them well-suited as nodes in a long-distance quantum network^47–51. Recent work has successfully demonstrated three-node entanglement-based quantum network that consists of three spatially independent quantum nodes based on NV centers, labeled Alice, Bob, and Charlie (Fig. 1c)⁷. Through the development of phase and frequency stabilization, they have demonstrated two quantum network protocols without postselection: the distribution of genuine multipartite Greenberger-Horne-Zeilinger (GHZ) entangled states across the three nodes and entanglement swapping via an intermediary quantum node. Its long spin coherence is also advantageous to build a quantum memory. By combining dynamical decoupling of an electron spin with selective phase-controlled driving of nuclear spins, a ten-qubit quantum register consisting of the electron spin of an NV center and nine nuclear spins in diamond has been demonstrated (Fig. 1d)⁵². The protection of an arbitrary single-qubit state has been demonstrated for over 75 s—the longest reported for a single solid-state qubit—and two-qubit entanglement has been preserved for over 10 s.

Fig. 1. Quantum technologies based on NV centers in diamond.

a Crystal structure of an nitrogen-vacancy (NV) center, showing C_3v symmetry. b Single photon source based on NV centers located in the i-layer of p-i-n junction to be electrically driven. Reproduced with permission from Springer Nature. Copyright 2012¹⁰. c Entanglement generation among remote NV centers, mediated by the polarization states of flying photons. Reproduced with permission from AAAS. Copyright 2021¹¹. d Quantum register constructed from ten qubits: electron and nuclear spins of NV center and surrounding eight nuclear spins of ¹³C isotope. Reproduced from ref. ⁵² under the terms of the Creative Commons Attribution License (CC BY 4.0).

NV center is also considered as a leading candidate for a quantum sensor that are able to accurately detect various physical quantities including magnetic field, temperature, strain, and electric fields⁴. In particular, it has shown considerable promise as a magnetometer for a wide range of applications, owing to its long spin coherence time. The principle of magnetic sensing relies on the energy level structure of the electrons, as illustrated in Fig. 2a. The electron energy of the NV⁻ center can be controlled by a green laser (typically 532 nm) and microwave irradiation. When the spin state m_s = 0 transitions to m_s = ±1 via microwave irradiation, the red fluorescence intensity decreases due to the population transfer to the non-radiative singlet state. Figure 2b shows an optically detected magnetic resonance (ODMR) spectrum, where optical and microwave methods are combined to probe the spin states of defects, such as NV centers, through changes in fluorescence intensity. Under an external magnetic field, the energy level of m_s = ±1 splits owing to the Zeeman effect, resulting in splitting the dip in Fig. 2b. This sensitive shift in dips caused by subtle changes in the magnetic field enables NV centers to function as highly sensitive magnetometers. Moreover, NV’s long spin coherence time—even at room temperature—makes it an ideal platform for quantum sensing devices, in contrast to other quantum sensor technologies such as superconducting quantum interference devices (SQUIDs) and optically pumped magnetometers (OPMs).When single NV center is embedded in the edge of a diamond scanning probe⁵³ or diamond nanobeam⁵⁴, this serves as an atomic-scale quantum sensor to explore condensed matter physics. For instance, probe-based sensing has been employed for studying atomically thin crystals of the 2D van der Waals magnets (Fig. 2c)^55,56. The nanobeam-based approach has been utilized for exploring the property of magnons (Fig. 2d)^23,57. Applications of NV centers for condensed matter physics is deeply discussed in ref. ²². When NV centers are shallowly implanted in diamonds, they can function as magnetic resonance imaging (MRI, Fig. 2e)⁵⁸ and nuclear magnetic resonance (NMR, Fig. 2f)⁵⁹ at nanoscale for proteins, molecules, and DNA (see also ref. ²¹). Robust nature of diamond also allows us to explore condensed matter physics at megabar pressures^60–62. Moreover, magnetic field sensitivities of the order of picoteslas have been achieved by employing an ensemble of NV centers (with a typical density of $~$10¹⁷ cm^-3)^15,63, which is beneficial for biological and medical applications that involve detecting weak magnetic fields. To date, the best field sensitivity is reported to be 210 fT/$\sqrt{\mathrm{Hz}}$ (0.5 nT/$\sqrt{\mathrm{Hz}}$) using ensemble NV centers⁶³ (a single NV center⁶⁴). Recent studies have demonstrated the magnetic imaging of living cells at nanoscale^65,66, magnetic detection of action potentials in living large squid (Fig. 2g)⁶⁷ and muscle fibres⁶⁸, and magnetocardiography of living rats at a millimeter-scale (Fig. 2h)⁶⁹. Furthermore, NV centers in diamonds are also being studied for other fascinating applications, including the development of gyroscope⁷⁰ and investigation of geological samples⁷¹.

Fig. 2. NV centers for quantum sensing applications.

a Optically ground and excited energy levels of the triplet electron spin in the NV center. Spin-selective decay via the singlet states occurs when the system is optically pumped, leading to the decreased red fluorescence intensity for magnetic-field-sensitive $m_s$ = ±1 states. b Zero-bias optically detected magnetic resonance (ODMR) spectrum of the NV center as a response to optical pumping and frequency-swept microwave irradiation. c Diamond scanning probe equipped with the single NV center for detecting magnetic fields produced by 2D van der Waals magnets. Reproduced with permission from AAAS. Copyright 2019⁵⁵. d Detection of magnons using the NV center embedded in a diamond nanobeam structure. Reproduced with permission from AAAS. Copyright 2021⁵⁷. e Subnanometer-scale MRI setup enabled by a shallowly implanted single NV center. Reproduced with permission from Springer Nature. Copyright 2014⁵⁸. f NMR-based detection of protein molecules using a shallow single NV center. Reproduced with permission from AAAS. Copyright 2016⁵⁹. g Noninvasive detection of neuron activity in a living marine worm via magnetometry using an ensemble of NV centers. Reproduced with permission from ref. ⁶⁷. h Magnetocardiography using the bulk diamond chip containing an ensemble of NV centers as a millimeter-scale sensor. Reproduced from ref. ⁶⁹ under the terms of the Creative Commons Attribution License (CC BY 4.0).

For group-IV color centers, the group-IV atom is located at an interstitial lattice site between two vacancies of diamonds. Surprisingly, Debye–Waller factor (i.e., the fraction of radiative emission occurring within the ZPL) is 70% for SiV centers, significantly higher than that of NV centers (~3%) and thus suitable for quantum communications and quantum photonic computing. The inversion-symmetric D_3d structure of the defect (Fig. 3a) also results in no permanent electric dipole, making the optical transition insensitive to electric field fluctuations and thereby protecting the optical coherence. Recent work has demonstrated a two-node quantum network composed of multi-qubit registers based on SiV centers in diamond nanocavities⁷², which has been further integrated with a telecommunication fiber network and periodically-poled lithium niobate devices in a more recent study (Fig. 3b)⁷³. Each SiV is localized in a nanophotonic cavity within an individually operated cryostat held at temperatures below 200 mK in two separate laboratories. Long-lived nuclear spin qubits are employed to provide second-long entanglement storage and integrated error detection. By integrating efficient bidirectional quantum frequency conversion of photonic communication qubits from visible (737 nm) to telecom wavelength (1350 nm), the entanglement of two nuclear spin memories has been achieved through 40 km spools of low-loss fiber and a 35-km long fiber loop deployed in the Boston area urban environment. However, one of the drawbacks of SiV centers is the operation temperature (below 1 K). To this end, other group-IV vacancies operating at higher temperature, such as SnV^74,75 and lead vacancy (PbV) centers^76,77 are being intensively studied recently (see also ref. ⁴³).

Fig. 3. SiV centers in diamond for quantum information technologies.

a Crystal structure of a silicon-vacancy (SiV) center, showing D_3d symmetry. b Remote entanglement generation between the SiV centers embedded in photonic crystal cavity systems, mediated by time-bin photon states and the efficient spin-photon interface. Reproduced from ref. ⁷³ under the terms of the Creative Commons Attribution License (CC BY 4.0).

Ion implantation

The most common way to create color centers in diamond is ion implantation: accelerating ions (e.g., nitrogen and silicon) of an element into diamond. In this process, vacancies are introduced at the same time and subsequent annealing creates defect centers. This method enables us to control the depth of defect centers from the top surface, which can be calculated via stopping and range of ions in matter (SRIM) simulations. To create defects in the middle of diamond nanostructures (close to the diamond surface) for single photon sources (quantum sensors), the nitrogen ions are typically implemented into a pure single-crystal diamond (i.e., a type-IIa diamond) with energies of ~100 (6) keV, which results in mean implantation depths of ~110 (10) nm⁷⁸. The subsequent annealing procedure activates the migration of vacancies to implanted nitrogen impurities, leading to the formation of NV defects. We note that single NV center close to the diamond surface serves as a sensitive and atomic-scale quantum sensor for a broad spectrum of research field ranging from chemistry, biology^19–21 and condensed matter physics^22–24

The advantage of this approach is that we can precisely control the position of point defects when combined by mask structure. For instance, NV center was created at the center of a photonic crystal cavity by combining nitrogen ion implantation and silicon masks, resulting in the horizontal positional accuracy of 50 nm which is limited by the aperture size of the mask⁷⁹. Another promising approach is based on a focused ion beam, which can create single color center without mask structures^80–83. For both approaches, the horizontal positional derivation (i.e., the depth spread due to implantation) is better than 30 nm. Meanwhile, these approaches have a relatively low yield; the reported yield for creating NV (SiV) centers using ion implantation (focused ion beam) is approximately 25%⁷⁹ (20%)⁸².

Electron irradiation

An ensemble of NV centers is powerful to enhance the sensitivity of quantum sensing. Ensembles of NV centers in diamond can be produced through electron irradiation. Starting from a diamond sample with a high density of nitrogen impurities (i.e., a type-Ib diamond), NV defects can be efficiently created through high energy irradiation of electrons and subsequent annealing of the sample. The concentration of NV centers is 1.8 ppm (3.2 × 10¹⁷cm⁻³) with a total fluence of 5 × 10¹⁷ electrons cm⁻² and an irradiation power of 2.0 MeV⁶⁹. We note that there is a trade-off between the spin coherence time and the NV density, since the nearby unconverted nitrogen and created NV centers are dominant sources of decoherence. More detailed discussion can be found the following ref. ³.

Laser writing

Laser processing was shown to be an effective approach to introduce color centers in diamond. This approach induces vacancies into diamonds without residual damage, allowing us to bind with existing substitutional nitrogen impurities in the lattice to create high-quality single NV centers^84,85. Recently, by employing femtosecond laser processing to not only create vacancies in the crystal but also anneal the diamond, the creation of NV centers with in-plane position derivation of ~30 nm and near-unity yield (96%) has been demonstrated⁸⁶. However, this approach suffers from the relatively poor depth positioning accuracy of the resultant NV centers which was limited to a few hundred nanometers. This would be improved by using delta doping of nitrogen impurities^87–91, where a thin nitrogen-doped layer is formed by adding nitrogen gas during the diamond growth process. This approach can create a slab of shallow NV within a depth precision of few nanometers.

Breakthrough in diamond nanotechnology

To accelerate advances in diamond-based quantum technologies, it is important to implement micro/nanostructures in diamonds. However, the heteroepitaxial growth of high-quality single-crystal diamond thin films on different materials is challenging due to the current diamond growth technology⁹². Therefore, in this section, we will summarize the advances since the 2010s in the nanofabrication based on single-crystal diamond. The growth of single-crystal diamonds are introduced elsewhere⁹³.

Thin-film fabrication based on etching

It is desirable to prepare diamond thin films on other substrates, following silicon-on-insulator platforms. To this end, a thinning-membrane approach was proposed. In 2011, a 280-nm-thick diamond membrane was fabricated on silicon oxide by thinning a 5-µm-thick type IIa single-crystal diamond membrane⁹⁴. During the diamond plate thinning by using oxygen plasma, the membrane was mounted on a 2 mm-thick SiO₂ substrate thermally grown on a silicon wafer (Fig. 4). By following the standard nanofabrication procedure, a ring resonator with a Q-factor of ~4000 was reported. Up to now, various photonic structures were demonstrated based on this approach, including a waveguide-coupled ring resonator^95–98, one dimensional PhC^99,100, and two dimensional PhC^101,102. The negative aspect of this approach, however, is the non-uniform thickness of the membrane (so-called “wedging”) due to non-uniform thickness of the starting, mechanically polished material, which hinders scalable fabrication of diamond PhC cavities.

Fig. 4. Top-down dry etching process of diamond via thinning-membrane approach.

The thick diamond plate is dry etched down to a few hundreds of nm thickness, followed by mask and resist layer deposition and lithography. Reproduced with permission from Springer Nature. Copyright 2011⁹⁴.

Angled etching

Another nanofabrication approach is based on “angled etching”, which can fabricate suspended nanobeam structures with triangular cross-sectional profile by applying ions of oxygen plasma to diamond at an oblique angle (Fig. 5a). The first demonstration of angled etching for diamond was performed by placing a single-crystal diamond chip in a perforated metal structure designed for use in conjunction with an inductively coupled plasma reactive ion etching (ICP-RIE) (so-called “Faraday cage”)¹⁰³. The trajectories of ions were modified by the Faraday cage in the chamber, resulting in the undercut of nanostructures in single-crystal diamond with triangular cross sections. Since then, this approach has been employed to fabricate a wide variety of structures such as racetrack resonators (Fig. 5b)¹⁰⁴, one-dimensional PhC nanobeams^79,104, nanocone structures¹⁰⁵, and photonic wire waveguide^106–108. It is notable that angled etching has also been utilized to fabricate nanostructures in other single-crystal materials such as silicon carbide^109,110, quartz¹¹¹, and gallium nitride¹¹².

Fig. 5. Detailed description of angled etching.

a Angled-etching procedure following: (i) mask formation, (ii) vertical etching of diamond, (iii) angled etching via anisotropic plasma injection with an oblique angle, (iv) removal of the mask. Adapted with permission from ref. ¹⁰³. Copyright 2012 American Chemical Society. b Racetrack resonator as an example of the angled-etched diamond structure. Reproduced with permission from Springer Nature. Copyright 2014¹⁰⁴.

However, photonic devices fabricated with Faraday cage exhibit non-uniformity across the substrate due to position-dependent variations in ion trajectories^113,114. An alternative approach for reproducibility is reactive ion beam angled etching (RIBAE)¹¹⁵. In this method, the ion beam generated in an ion gun is collimated and accelerated towards the chamber through a series of grids. The ion beam can be made uniform over an area of diameter as large as 4-inch, enabling improved undercut etch uniformity over the entire area of a bulk diamond substrate when tilting the sample stage. RIBAE has been used to fabricate photonic devices including diamond mirrors for high power laser¹¹⁶, dispersion engineered waveguides for supercontinuum generation¹⁰⁸, and one-dimensional PhC cavities for quantum network^73,117,118. Nevertheless, the drawback of angle etching is that achievable nanostructures are limited: angled etching is not compatible with the fabrication of specific waveguide structures and two-dimensional PhCs because of the resulting characteristic triangular cross-sectional profile.

Quasi-isotropic etching

Alternative and fascinating approach to produce air-suspended photonic structures is “quasi-isotropic etching”. This technique relies on a zero bias oxygen plasma after standard vertical RIE. Figure 6a illustrates the procedure of the quasi-isotropic etching¹¹⁹. (i) First, polished bulk single crystal diamond chips are cleaned in boiling piranha and coated with thick hard mask (e.g., silicon nitride based on PECVD or LPCVD). The samples are then coated with an electron beam resist (e.g., ZEP 520 A and HSQ). (ii) Nanostructures are patterned in the resist by performing electron beam lithography and subsequent development. (iii) Patterns are transferred to the hard mask by standard dry etching. (iv) Patterns are transferred to the diamond by anisotropic oxygen-plasma-based ICPRIE etching. (v) The sidewalls of etched diamond are protected with a conformal layer deposited by atomic layer deposition of aluminum oxide or PECVD of silicon nitride. (vi) A short ICPRIE etch removes top protective layer, leaving only the sides covered. (vii) A zero-bias oxygen ICPRIE plasma undercuts the nanostructures. The elevated temperature and pressure enhance the etch rate of diamond. (viii) The sample is soaked in HF to remove the remaining hard mask and protective layer. Figure 6b, c display the SEM images of disk resonator¹¹⁹ and one-dimensional PhC¹²⁰ fabricated by quasi-isotropic etching, respectively. To date, nanostructures such as disk resonators^119,121, photonic waveguides¹²², one-dimensional PhC nanobeams^123–126, phononic crystal¹²⁷, two-dimensional PhC cavities¹²⁸, and nanobeam quantum sensor²⁴ have been successfully fabricated by using the quasi-isotropic etching. Nevertheless, the relatively complicated etching procedure causes fabrication imperfections such as rough side walls of etched surfaces and non-flat bottom surface, which results in the experimentally limited Q factors < ~10⁴ at visible wavelength.

Fig. 6. Quasi-isotropic etching.

a Fabrication steps: (i) resist deposition, (ii) resist patterning, (iii) mask patterning, (iv) vertical etching, (v) mask layer deposition on side walls, (vi) selective removal of bottom masks, (vii) undercut by zero-bias oxygen plasma etching, and (viii) removal of whole masks. b Scanning microscope image (SEM) of a disk resonator fabricated by quasi-isotropic etching. Adapted with permission from ref. ¹¹⁹. Copyright 2015 American Chemical Society. c Photonic crystal structure as another example. EBL electron beam lithography, ICPRIE inductively coupled plasma reactive ion etching, PECVD plasma-enhanced chemical vapor deposition. Reproduced from ref. ¹²⁰ under the terms of the Creative Commons Attribution License (CC BY 4.0).

Smart cut approach

Both angled and quasi-isotropic etching are the most common approaches in diamond nanophotonics to date. However, they generally suffer from several challenges as discussed above. In addition, typical Q factors of the nanocavities demonstrated by these methods (~10⁴) are much lower than those simulated (~10⁶) due to fabrication imperfections stemming from complicated etching procedures. An ion slicing (or so called “smart cut”) technique is promising to produce large-area and uniform thin membrane from single-crystal diamond for high-quality diamond nanophotonics. Smart cut technique is based on ion implantation, which creates a graphitized layer near the top surface of diamond that can be easily removed by electrochemically etching. Historically, the proof-of-concept demonstration for ion slicing was experimentally shown in 1992¹²⁹. Since then, several groups demonstrated the fabrication of nanostructures by using the diamond membrane ion-sliced based on H⁺ or He⁺ ion^130–132. However, the critical issues were the ion damage in these films and residual built-in strain induced by ion implantation¹³³. Recently, X. Guo et al. proposed the novel approach of fabricating high-quality diamond membrane by combining smart-cut technique with subsequent regrowth of diamond layer^134,135. Figure 7a shows the basic flow of preparing high-quality diamond thin membranes. First, He⁺ implantation with subsequent annealing is preformed to form the graphitized layer (dark gray underneath the top diamond membrane). Color centers are created via either ion implantation. Diamond membrane is undercut through electrochemical etching in DI water. Next, the air-suspended membrane is transferred to another substrate by utilizing transfer printing. Finally, the damaged layer is removed by oxygen-based dry etching. By leveraging this high-quality diamond membrane, S. Ding et al. demonstrated one- and two-dimensional diamond PhC cavities with Q factors of 1.8 × 10⁵ and 1.6 × 10⁵, respectively¹³⁶. Figure 7b shows the process flow of the diamond nanofabrication, which is very similar to the standard semiconductor process. Figure 7c, d display the SEM images of one- and two-dimensional diamond PhC cavities based on the smart cut approach. We note that Q-factors demonstrated by their group are the highest for visible PhC cavities of any material. This technology will be expected to establish a new paradigm in diamond nanophotonics. This approach may face challenges such as poor adhesion and strain of diamond membranes during the hybrid integration on chip. We consider that chemical surface treatment and post-annealing could effectively enhance adhesion and relieve strain^137,138. It is also noteworthy that thin-film diamond membranes based on the smart-cut approach or oxygen plasma etching present significant challenges for scalable production due to their time-consuming and scale-limited processes. A promising solution to address this issue is inspired by the mechanical exfoliation method for thin-film polycrystalline diamond membranes^139,140. A recent study demonstrated that edge-exposed exfoliation using adhesive tape enables mass production of large-area (2-inch wafer), ultrathin (sub-micrometer thickness), ultraflat (sub-nanometer surface roughness), ultraflexible (360° bendable), and transferrable diamond membranes¹⁴¹.

Fig. 7. Diamond nanofabrication based on smart cut approach.

a Membrane fabrication process via graphitized layer formation, crystal overgrowth with optional ion implantations, electrochemical etching, flip and transfer of the suspended membrane, and removal of the damaged layer by the dry etching stopped at the regrown layer. Reproduced from ref. ¹³⁴ under the terms of the Creative Commons Attribution License (CC BY 4.0). b Description of fabrication process, including: (1) initial condition of a sample, in which a diamond membrane is placed on top of a SiO₂ substrate, (2) formation of metal frame via liftoff process, (3) deposition of mask and resist layer, (4) transferring of lithography pattern to the mask layer, (5) top-down etching of the diamond membrane along the mask, and (6) undercut by wet etching process. c One-dimensional and d two-dimensional photonic crystal cavities with inversion symmetry against height direction to achieve high quality factor, enabled by the undercut. Reproduced from ref. ¹³⁶ under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (CC BY-NC-ND 4.0).

The proposed membrane approach is also highly compatible with a miniaturized Fabry-Pérot microcavity, which is based on an optical fiber and a mirror and has been employed for various quantum emitters including semiconductor quantum dots^142,143, point defects in diamond^144,145 and 2D materials^146,147, and organic molecules¹⁴⁸. This type of cavity offers the advantage of in situ spatial and spectral tuning, combined with high Q-factors and excellent mode matching to a propagating Gaussian beam, albeit at the cost of a large mode volume (typically ~125$\times {\left(\frac{\lambda }{n}\right)}^3$). This approach has enabled Purcell enhancement and precise position control of quantum dot-based single photon sources along with near-unity efficient photon collection¹⁴⁹. D.Riedel, et al. exploited this approach to realize Purcell-enhanced single photon emission from an NV center in a thin single crystal diamond membrane (Fig. 8)¹⁴⁴. The Fabry-Pérot cavity consists of a plane bottom mirror and a concave top mirror, both of which are distributed Bragg reflectors with reflectivity >99.99%. The curved top mirror is fabricated by creating a concave depression in a silica substrate with laser ablation of a high-power CO₂ laser followed by mirror coating¹⁴³. The 200 nm-thick diamond membrane was prepared by using smart-cut approach. These membranes are subsequently transferred to a planar mirror that is placed on three-axis nanopositioners by using a micromanipulator. They reported Purcell factor for the ZPL of ~30 together with the high Q-factor of 58,500. In addition, S. Häußler et al. demonstrated Purcell enhancement of SiV⁻ centers in a thin single crystal diamond membrane by using the same manner¹⁴⁵. They observe cavity-coupled fluorescence from an ensemble of SiV⁻ centers with an enhancement factor of ~1.9.

Fig. 8. Diamond quantum photonics based on a Fabry-Pérot microcavity.

The Fabry-Pérot cavity consists of a plane bottom mirror and a concave top mirror, both of which are distributed Bragg reflectors with reflectivity >99.99%. The curved top mirror is fabricated by creating a concave depression in a silica substrate with laser ablation of a high-power CO₂ laser followed by mirror coating. The 200 nm-thick diamond membranes are prepared by using smart-cut approach and subsequently transferred to a planar mirror that is placed on three-axis nanopositioners by using a micromanipulator. DBR: distributed Bragg reflector. Reproduced from ref. ¹⁴⁴ under the terms of the Creative Commons Attribution License (CC BY 4.0).

Summary of photonic structures

Table 1 summarizes diamond photonic structures with their purposes and features.

Table 1.

Summary of photonic structures with their purposes and features

Photonic Structure	Purpose	Feature
Waveguide	Guiding light in a photonic chip	Propagation loss: 0.34 dB/cm-196
Photonic crystal	Light-matter interaction for quantum emitters	Q-factor: 1.8 × 105 (1D), 1.6 × 105 (2D)136 Mode volume: $0.5\times {\left(\frac{\lambda }{n}\right)}^3$ (1D), $2.18\times {\left(\frac{\lambda }{n}\right)}^3$ (2D)136
Ring resonator	Light-matter interaction for quantum emitters and nonlinear photonics	Q-factor: 1.5 × 105 at telecom104 Mode volume: $2.3\times {\left(\frac{\lambda }{n}\right)}^3$104
Microdisk resonator	Light-matter interaction	Q-factor: >105 at telecom119 Mode volume: 12.6 $\times {\left(\frac{\lambda }{n}\right)}^3$119

Hybrid integration for quantum information science

As reviewed in the previous section, the scalability of integrated diamond photonics is limited by the lack of the technology of fabricating wafer-scale diamond photonic integrated circuits. Thus, the next stage to realize scalable diamond quantum photonics is the hybrid integration of color centers in diamond nanostructures onto a photonic chip based on other materials particularly well-developed photonic materials, including Si, SiN, LiNbO₃ and AlN. In this section, we review the hybrid integration of diamond color centers on such photonic chips.

Pick-and-Place

A potential solution for hybrid integration of diamond nanostructures on chips is the use of pick-and-place techniques³⁴, which offer a viable route to combine color centers with state-of-the-art integrated photonics. Thus far, the hybrid integration of a diamond micro-waveguide containing a single color center onto a SiN and LN waveguide has been demonstrated using a micromanipulator^150,151. This approach has also been employed for the hybrid integration of various types of quantum emitters (e.g., III–V semiconductor quantum dots, 2D materials, and carbon nanotubes)³⁶.

The integration of multiple quantum emitters on a single photonic chip is critical for large-scale quantum information processing. Meanwhile, integrating multiple quantum emitters presents several challenges, including inhomogeneities of each emitter, poor device yield, and complex device requirements. Wan et al., solved these issues by the hybrid integration of ‘quantum microchiplets (QMCs)’—arrays of nanobeams which contain highly coherent quantum emitters—on a photonic chip¹⁵² (Fig. 9). A 128-channel, defect-free diamond array of GeV and SiV centers was successfully integrated on the same AlN-based photonic chip. The integration inside SEM system provided the positional accuracy of their process to be 38 ± 16 nm. Spectral inhomogeneities of each color center in the device were also compensated in situ by strain wavelength tuning.

Fig. 9. Large-scale quantum system assembled by pick-and-place integration of diamond microstructures.

Quantum microchiplets (QMCs) are placed on a photonic integrated circuit (PIC) which is composed of AlN waveguides as optical interfaces and electrodes to control the optical transitions. The chip and QMCs were separately fabricated, followed by an assembly process based on pick-and-place inside SEM system. Reproduced with permission from Springer Nature. Copyright 2020¹⁵².

Nanodiamonds are also promising hosts for quantum emitters and quantum sensors that provide significant versatility in types of integration due to their small size and ease of production^153–155. In particular, their small size is advantageous for achieving an efficient emitter-cavity interface^156,157. To scalably fuse nanodiamonds with integrated photonics, several groups have tackled the hybrid integration of nanodiamonds on chip^158–160. Among them, K. G. Fehler et al. demonstrated the hybrid integration of SiV⁻ center in nanodiamond onto SiN photonic crystal cavities^161,162. They employed atomic force microscope nanomanipulation to attain control of spatial and dipole alignment. They achieved a Purcell enhancement of more than 4 on individual optical transitions of SiV⁻ under on-chip optical excitation. In addition, P. P. J. Schrinner et al. demonstrated the integration of NV centers in diamond on low autofluorescent Ta₂O₅-on-insulator waveguides¹⁶³. They succeeded in achieving both anti-bunching and optically detected magnetic resonance (ODMR). Moreover, H. C. Weng, et al. demonstrated heterogeneous integration of NV centers in nanodiamond with SiN photonics from a standard 180 nm CMOS foundry process¹⁶⁴. Nanodiamonds in solution are deposited onto the chip, where predefined sites form a regular array on the waveguide. The solvent is then allowed to evaporate, leaving the nanodiamonds in place. Further research into controlling the position, dipole orientation, and quantity of point defects in diamond is essential for scalable quantum photonic applications.

Hybrid GaP-on-diamond approach

We have introduced several recent nanofabrication technologies of diamond in “Breakthrough in diamond nanotechnology”. An alternative approach uses a thin waveguiding layer based on GaP (n = 3.3) that is formed on diamond surface (n = 2.4)^165,166. This approach allows for chip-scale integration of devices with uniform chip thickness and high yield, which is not easy to achieve by undercutting or thinning the diamond. M. Gould et al. demonstrated the chip-scale transmission measurements for three key components of a GaP-on-diamond integrated photonics platform: waveguide-coupled disk resonators, directional couplers, and grating couplers (Fig. 10)¹⁶⁷. They also presented proof-of-principle measurements demonstrating NV center emission coupled into selected devices. The potential total quantum efficiency of ZPL photon collection into the bus waveguide in this approach is approximately 33%. Furthermore, GaP exhibits a ${\chi }^{\left(2\right)}$ optical nonlinearity and thus allows for integrated electro-optic switches based on the linear electro-optic effect¹⁶⁸. The main disadvantage of GaP-on-diamond systems is a reduced interaction between NV centers and guided optical modes due to the relatively small overlap between a GaP photonic mode and NV center. The comparison between hybrid diamond integrated devices and other platforms is summarized in Table 2. We included semiconductor quantum dots (QDs)^169–179 and defects in 2D materials such as WSe₂¹⁸⁰ and hexagonal boron nitride (hBN)¹⁸¹ in this comparison.

Fig. 10. Hybrid GaP-on-diamond photonics.

Top: schematic views with overlaid finite-difference time-domain (FDTD) simulations. Bottom: SEM images of integrated GaP-on-diamond devices. a Waveguide-coupled disk resonators, b directional coupler, c grating out-coupler. Reproduced with permission from ref. ¹⁶⁷. Copyright 2016 Optical Society of America.

Table 2.

Comparison between hybrid diamond devices and other platforms

Refs.	Emitter	Structure	Hybrid integration	Cavity	CMOS-compatible demonstration	$g^{\left(2\right)}\left(0\right)$	Output	Emitter-chip coupling
150	NV	Diamond µ-WG on SiN WG	MM	N	N	0.07	Edge	43%
167	NV	GaP-on-diamond WG	EG of GaP	Y	N	―	Grating	33%
150	SiV	Diamond NB on LN WG	MM	N	N	0.63	Grating	Diamond–LN transmission: 92%
152	SiV and GeV	Diamond NB arrays on AlN WG	MM	N	N	0.05 (SiV) 0.06 (GeV)	Edge	55%
164	NV	Nanodiamond on SiN WG	MM	N	Y	0.48	Grating	9%
161	SiV	Nanodiamond on SiN PhC-connected WG	MM	Y	N	―	Grating	~4%
169	QD	GaAs NW in SiN WG	MM	N	N	0.07	Edge	12%
170	QD	GaAs NW in SiN WG	MM	N	N	0.13	Edge	12%
171	QD	InP NB on Si WG	MM	N	N	0.25	Grating	Collection: ~3%
172	QD	GaAs NB on SiN WG	MM	N	N	0.07	Edge	31%
173	QD	GaAs PhC on GaAs WG	TP	Y	N	0.23	Grating	63%
174	QD	GaAs PhC on Si WG	TP	Y	Y	0.30	Grating	~70%
175	QD	GaAs PhC on SiN WG	TP	Y	Y	0.1	Grating	53%
176	QD	InP PhC on Si WG	TP	Y	Y	0.20	Grating	~82%
177	QD	GaAs NB on SiN WG	WB	Y	N	0.13	Edge	72%
178	QD	GaAs NB on SiN WG	WB	N	N	0.11	Edge	3%
179	QD	GaAs diode arrays with Si WGs	WB	N	N	0.23	Edge	8%
180	defect	WSe2 on SiN WG	TP	N	N	0.47	Edge	NA
181	defect	hBN in SiN WG	TP	N	Y	0.22	Edge	12%

EG Epitaxial growth, LN Lithium niobate, MM Micromanipulation, NB Nanobeam, NW Nanowire, TP Transfer printing, WB Wafer bonding, WG Waveguide.

Cryo-CMOS technology

In current solid-state qubit implementations, the challenge is how to coexist the quantum chip in a dilution refrigerator and the room-temperature electronics especially fabricated by CMOS procedure. By elaborating the device design and addressing the heating and power issues, plenty of functionalities have been shown to operate at cryogenic temperature, including optical modulators¹⁸², programmable photonic circuits¹⁸³, control of gate-defined quantum dots^184–186, and large-scale superconducting quantum computing^187–189.

Color centers in diamond as quantum emitters and memories are also required to operate at cryogenic temperature. In addition, for most quantum applications, millions of physical qubits are required to encode thousands of logical qubits. To build a fully connected graph state to maximize our use of qubit resources, L. Li et al. introduced a modular quantum system-on-chip architecture that hosts thousands of individually addressable SnV spin qubits in two-dimensional arrays of QMCs (8 × 8) into a CMOS-based application-specific integrated circuit designed for cryogenic control¹⁹⁰. The system’s codesign with CMOS electronics is advantageous for the compact two-dimensional array of qubit arrangements, qubit inhomogeneous compensation, and system’s control elements.

For large-scale heterogeneous integration, they developed lock-and-release heterogeneous integration technology as illustrated in Fig. 11a. This procedure enables the parallel transfer of an 8 × 8 matrix of quantum memories to the central region (500 × 500 μm) of the CMOS chip socket that includes 1024 quantum channels in total. First, the diamond parent chip that was placed on a PDMS film attached to a glass slide was flipped and aligned with a locking structure. Subsequently, the QMCs were finely adjusted with two probes, which resulted in the QMC transfer yield of 100%. After alignment, the parent bulk diamond was vertically moved to lock the QMC. Finally, the diamond was horizontally moved and retracted to break the tethers between the QMC and the bulk diamond for release. Figure 11b, c display SEM images of a single central quantum channel and a single QMC region of the chip, respectively. The orange region indicates the individual CMOS backplane electrode region beneath the QMCs. A perturbative cavity design was implemented in each quantum channel as a dielectric antenna optimized for free-space collection, allowing for both a quality factor of 2000 and a collection efficiency of 96% with an NA of 0.9 in simulation. Figure 11d shows an optical microscope image of the 1024 quantum channels integrated into the CMOS control chip. For each quantum channel, we expect to have around three resonant quantum emitters on average at a certain optical frequency. They estimated the average tuning range of around 2 GHz within the applicable voltage range. The number of quantum channels in this design can be readily scaled by means of increased qubit density, larger active regions and optical networking across quantum system-on-chip modules.

Fig. 11. Cryo-compatible CMOS application-specific integrated circuit (ASIC) with the parallel-integrated diamond microchiplets.

a Parallel integration of an $8\times 8$ array of diamond QMCs, during which each the QMC is aligned to a CMOS socket, mechanically locked to the socket, broken apart from the mother diamond substrate, and installed with the unity yield at the end. b SEM image of a free-space-coupled cavity antenna (scale bar, 1 $\mathrm{\mu }\mathrm{m}$), supported by c a diamond QMC on the CMOS socket (scale bar, $10\mathrm{\mu }\mathrm{m}$). d Optical microscope image of the whole QMC array on the CMOS ASIC. Reproduced with permission from Springer Nature. Copyright 2024¹⁹⁰.

Hybrid integration for quantum sensing

In this section, we review the hybrid integration of diamond color centers on chips for practical quantum sensing.

Pick-and-Place method

Among the pick-and-place techniques, transfer printing is promising as a back-end process for the hybrid integration of photonic and electronic components with desirable materials^{173,174,191–194}. However, the challenge is the incompatibility of current nanofabrication technology for diamond photonics with existing hybrid integration approaches. For instance, angled etching produces a diamond nanostructure with triangular cross section that is structurally difficult to place on other substrates by using conventional pick-and-place techniques. The bottom of the diamond nanostructure formed through quasi-isotropic etching is even non-flat depending on its width and shape¹⁹⁵. Thus, the development of hybrid integration techniques with near-unity yields, regardless of the sample structure, is essential for scalable diamond quantum photonics.

The authors and their peers proposed and demonstrated the deterministic hybrid integration of a diamond NV triangular nanobeam on chip¹⁰⁶. The proposed “pick-flip-and-place” transfer printing picks up and flips a suitable diamond nanostructure by using a film with weaker adhesion, transfers it to the other film with stronger adhesion, and places it on a photonics chip (Fig. 12a). The advantage of this approach is to provide a flat surface interface between a diamond nanostructure and a photonic chip, enabling us to integrate diamond nanostructures with arbitrary design on chip with near-unity success. Figure 12b shows an SEM image of the integrated nanobeam diamond structure. Notably, the proposed technique can be also applied to hybrid integration of quantum emitters described in the previous section.

Fig. 12. Transfer printing based on pick-flip-and-place technique.

a Operation flow: (i) picking up air-bridged structure using a PDMS film, (ii) re-picking up the structure from the flipped film, (iii) placing the structure onto an arbitrary substrate, and (iv) completed picture. b SEM image of the integrated nanobeam diamond structure, where flat interface is ensured between the structure and the substrate. Reproduced from ref. ¹⁰⁶ under the terms of the Creative Commons Attribution License (CC BY 4.0). c SEM image of the integrated diamond micro-ring resonator using the same method. d Optically detected magnetic resonance spectrum obtained from the ring resonator, reaching the theoretical limit of spin contrast (~25%). Reproduced with permission from ref. ¹⁹⁶ under the terms of the Creative Commons Attribution License (CC BY 4.0).

The same authors also addressed nanoscale quantum sensing with high field sensitivity by using on-chip diamond micro-ring resonators¹⁹⁶. They fabricated the ring resonator containing high-density NV centers by using pick-flip-and-place transfer printing, which enables the efficient use of photons by confining them in a nanoscale region (Fig. 12c). The ring resonator possesses a Q-factor of 2000 even containing high-density NV centers. The device yielded the magnetic sensitivity of 1.0 μT/$\sqrt{\text{Hz}}$ on a photonic chip with a measurement contrast of theoretical limit (~25%) as shown in the optically detected magnetic resonance spectrum of Fig. 12d. They also showed that the proposed on-chip approach can improve sensitivity via efficient light extraction with photonic waveguide coupling.

CMOS-compatible sensing

For practical applications of quantum sensing, it is desirable to make the whole sensor device compact. However, existing systems of diamond quantum sensing involve bulky and discrete off-the-shelf instruments that limit practical applications and scalability of the approach. For example, quantum sensing based on NV centers requires a number of functional components such as excitation laser source, an optical filter to cut off the laser, a microwave generator to manipulate NV spins, and a photodetector. D. Kim et al., demonstrated the integration of NV-based quantum sensors with CMOS electronics to realize a compact and scalable sensor device¹⁹⁷ (Fig. 13a, b). Using standard CMOS technology, they integrated the necessary components for quantum sensing (e.g., microwave antenna, optical filter based on metamaterials, photodetector and so on) in a 200 μm × 200 μm footprint. A 45° cut in the corner of the diamond guides green pump laser from outside to NV centers inside the diamond. This side excitation can suppress the intrusion of laser background into the photodetector located below the diamond. The green laser pump beam is further filtered out by optical filters based on a periodic metal–dielectric structure. An on-chip microwave generator and inductor allow us to manipulate the NV electron spin transitions. The demonstrated field sensitivity of magnetometry was 32 μT$/\sqrt{\mathrm{Hz}}$.

Fig. 13. Prototypes of a quantum sensing module with small footprints.

a Configuration of the CMOS-integrated quantum sensor chip. b Top view of the chip before the diamond slab is integrated. Reproduced with permission from Springer Nature. Copyright 2019¹⁹⁷. c Fiber-integrated probe system for biomedical applications. An exterior image (top) and the system configuration (bottom). Reproduced from ref. ²⁰⁰ under the terms of the Creative Commons Attribution License (CC BY 4.0). d Portable device integrated with a fiber optical input. The device picture (top) and the configuration (bottom). Reproduced from ref. ²⁰¹ under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (CC BY-NC-ND 4.0)

Compact device for practical applications

To construct compact devices, several groups combined optical fiber with diamond quantum sensing^198–202. A. Kuwahata et al. demonstrated a compact probe system integrated into a fiber-optics platform to detect the magnetic field generated by magnetic nanoparticles for diagnosis of breast cancer (Fig. 13c)²⁰⁰. For efficient lock-in detection of signals from the magnetic nanoparticles, AC magnetic field was generated by the excitation coil of several hundred microteslas, which results in magnetization of magnetic nanoparticles. The minimum detectable AC magnetic field was approximately 57.6 nT for one second measurement time. The device enabled us to detect the micromolar concentration of magnetic nanoparticles at distances of a few millimeters. A compact and portable nanodiamond-based measuring instrument was also recently demonstrated with the field sensitivity of 1.34 μT$/\sqrt{\mathrm{Hz}}$, operating on the USB 3.0 power supply of a laptop computer²⁰³. Its portability is achieved through low power consumption in both the optical fiber and microwave components. The optics employ a diamond corner cube that enhances the photodiode current by a factor of 2.1 compared to a planar diamond, while the microwave source consumes 20 dB less power thanks to a microwave resonator with a λ/4 open stub that strongly drives the NV center magnetically.

M. Stürner et al. demonstrated a fiber-integrated compact diamond magnetometer (Fig. 13d)²⁰¹. The proposed portable device comprises all necessary components for quantum sensing including offset magnetic field, laser source, microwave generator, and signal processing unit. A single-mode fiber guides the green laser to optically initialize NV centers. deploys a balanced detection scheme built up by two photodetectors augmented close to the diamond. Microwave was sufficiently provided by a split-ring resonator formed by two inductively coupled transmission lines, which were terminated by two capacitive gaps. The bandwidth of the resonator is 23.48 ± 0.08 MHz, and the resonance frequency can be tuned in the range from 2.8 to 3.0 GHz by putting a metallized plate (resonator tuning stick) on the resonator structure. Surprisingly, this portable setup yields a sensitivity of ≈344 pT$/\sqrt{\mathrm{Hz}}$. The comparison between hybrid diamond sensing devices and other quantum devices is summarized in Table 3.

Table 3.

Comparison between hybrid diamond sensing devices and other quantum devices

Refs.	Platform	Operation temperature	Sensitivity	Device size
197	NV center	Room temperature	32 μT$/\sqrt{\mathrm{Hz}}$	200 μm × 200 μm
198	NV center	Room temperature	7 nT/$\sqrt{\mathrm{Hz}}$	1.1 × 0.7 × 0.7 m3
199	NV center	Room temperature	310 pT$/\sqrt{\mathrm{Hz}}$	0.44 × 0.55 × 0.6 m3
200	NV center	Room temperature	57.6 nT	Several centimeters
201	NV center	Room temperature	344 pT$/\sqrt{\mathrm{Hz}}$	35 × 110 × 120 mm³
202	NV center	Room temperature	44 pT$/\sqrt{\mathrm{Hz}}$	63 × 47 × 21 mm³
255	SQUID	Cryogenic temperature	A few fT$/\sqrt{\mathrm{Hz}}$	Several meters
256	OPM	Room temperature	10 fT$/\sqrt{\mathrm{Hz}}$	2 × 2 × 5 cm3

Discussion

The nanofabrication techniques of diamond reviewed above has accelerated the research of quantum technologies based on color centers in diamond. The hybrid integration techniques discussed in this review have also enabled the fusion of high-performance diamond quantum devices with cutting-edge photonic chips. Despite these progresses, there still remain fundamental challenges to construct more complex and larger-scale diamond quantum photonic devices. In this section, we will discuss the short-term and long-term challenges for quantum applications based on diamonds.

Challenges in quantum information processing

A short-term goal for this field is how to precisely control quantum emitters. We reviewed several ways to create color centers in diamonds in “Creation of color centers in diamond”. However, each approach has different challenges including position accuracy, yields, and dipole orientation. Especially, NV centers are required to be positioned at the exact center of photonic cavities to obtain efficient ZPL collection. We believe that the combination of smart-cut technique with delta-doped layer would solve this issue and enable the creation of high-quality photonic cavities hosting perfectly position-controlled NV centers. Another issue is broadening of the linewidth of NV centers caused by charge-state instability when they are positioned near the diamond surface²⁰⁴. This issue can be mitigated through chemical treatments such as surface passivation²⁰⁵, termination²⁰⁶, doping²⁰⁷, and the introduction of graphene²⁰⁸. Alternatively, the use of group IV-based color centers is promising, as they are insensitive to electric field fluctuations due to the absence of a permanent electric dipole, which arises from the inversion-symmetric D_3d structure of the defect. The frequency stabilization of SiV centers has also been reported through electromechanical strain control²⁰⁹.

A long-term goal of diamond-based quantum information processing is scalability, such as (i) the wafer-scale process of diamond nanofabrication and (ii) massive hybrid integration. The smart-cut approach with wafer-scale growth of diamond would be the most promising way to massively fabricate high-quality diamond nanostructures. Massive integration would be also possible by developing transfer printing technique with automated operation²¹⁰ and in parallel manner²¹¹. The other challenge of scalable diamond quantum photonics is to implement additional functions on chip for future quantum applications, such as electrical excitation^10,212, excitation through a waveguide^213–215, and efficient chip-to-fiber interface^{176,216–218}. Controlling the properties of NV centers is also crucial, including the manipulation of the charge state between neutral and negatively charged NV centers^219–227 and spectral tuning^228–232.

Challenges in quantum sensing

As introduced in “Creation of color centers in diamond”, quantum sensing has a wide range of applications, spanning from fundamental physics to medical fields. For a near-term goal in quantum sensing, improving factors such as extraction efficiency of NV emissions, spin coherence time, and photoluminescence contrast is crucial for any applications with higher sensitivity. Improvement of coherence time is addressed in various ways including decoupling techniques^233–236, surface treatments^237,238, and material synthesis²³⁹. Photoluminescence contrast could also be enhanced by perfectly aligning NV orientations^240,241 or inducing stimulated emission^242–245. We note that quantum defects in hBN have recently emerged as alternative solid-state quantum sensors in atomic scale^246,247. Investigation and development of such novel materials are also imperative for broadening the applications of quantum sensing.

One of the ultimate goals in quantum sensing is to construct compact sensor devices with excellent performance. To this end, a number of functional components are required to be integrated on a single photonic chip. Design of the device needs to be tailored to its intended applications, such as fundamental physics research, NMR and MRI measurements, and medical diagnosis. Diamond photonic nanostructures offer significant advantages for all of these applications, as they have the potential to enable highly sensitive quantum sensing²⁴⁸. Recent advancements in metasurfaces will also be incorporated to further miniaturize the device^249–251. Furthermore, future scalable and high-throughput fabrication of diamond quantum sensors might allow for monitoring electric vehicle batteries²⁵², sensing in microfluidics systems²⁵³, and detecting dark matters²⁵⁴. These increasing demands will accelerate the development of truly practical diamond quantum sensors.

Conclusion

A review of recent progress regarding the development of hybrid diamond quantum photonics platforms was presented. A brief explanation of the techniques for creating color centers in diamond, as well as the fabrication methods for diamond nanostructures, is provided. We reviewed the hybrid integration of diamond color centers on chips for quantum information processing and practical quantum sensing. The review concludes with a discussion of the challenges and perspectives in diamond quantum photonics. We emphasize that the advantage of hybrid integration lies in the ability to integrate diamond quantum components after their optimization and sophistication. Although numerous challenges persist, the hybrid integration approach, combined with recent advancements in diamond nanofabrication techniques, is poised to elevate diamond quantum technologies to the next level.

Author contributions

R.K. and K.T. researched data for the article. All authors contributed substantially to discussion of the content. All authors wrote the article. All authors reviewed and/or edited the manuscript before submission.

Peer review

Peer review information

Communications Engineering thanks Minghao Li and the other, anonymous, reviewers for their contribution to the peer review of this work. Primary Handling Editors: [Anastasiia Vasylchenkova, Miranda Vinay and Rosamund Daw]. Peer review reports are available.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s44172-025-00398-2.