Materials for Quantum Technology

Quantum technologies are poised to move the foundational principles of quantum physics to the forefront of applications. This roadmap identifies some of the key challenges and provides insights on material innovations underlying a range of exciting quantum technology frontiers. Over the past decades, hardware platforms enabling different quantum technologies have reached varying levels of maturity. This has allowed for first proof-of-principle demonstrations of quantum supremacy, for example quantum computers surpassing their classical counterparts, quantum communication with reliable security guaranteed by laws of quantum mechanics, and quantum sensors uniting the advantages of high sensitivity, high spatial resolution, and small footprints. In all cases, however, advancing these technologies to the next level of applications in relevant environments requires further development and innovations in the underlying materials. From a wealth of hardware platforms, we select representative and promising material systems in currently investigated quantum technologies. These include both the inherent quantum bit systems and materials playing supportive or enabling roles, and cover trapped ions, neutral atom arrays, rare earth ion systems, donors in silicon, color centers and defects in wide-band gap materials, two-dimensional materials and superconducting materials for single-photon detectors. Advancing these materials frontiers will require innovations from a diverse community of scientific expertise, and hence this roadmap will be of interest to a broad spectrum of disciplines.

Quantum technology grown out of quantum information theory, including quantum communication, quantum computation and quantum sensing, not only provides powerful research tools for numerous fields, but also is expected to go to civilian use in the future. Solid-state spin-active defects are one of promising platforms for quantum technology, and the host materials include three-dimensional diamond and silicon carbide, and the emerging two-dimensional hexagonal boron nitride (hBN) and transition-metal dichalcogenides. In this review, we will focus on the spin defects in hBN, and summarize theoretical and experimental progresses made in understanding properties of these spin defects. In particular, the combination of theoretical prediction and experimental verification is highlighted. We also discuss the future advantages and challenges of solid-state spins in hBN on the path towards quantum information applications.

In recent years, hole-spin qubits based on semiconductor quantum dots have advanced at a rapid pace. We first review the main potential advantages of these hole-spin qubits with respect to their electron-spin counterparts and give a general theoretical framework describing them. The basic features of spin–orbit coupling and hyperfine interaction in the valence band are discussed, together with consequences on coherence and spin manipulation. In the second part of the article, we provide a survey of experimental realizations, which spans a relatively broad spectrum of devices based on GaAs, Si and Si/Ge heterostructures. We conclude with a brief outlook.

Lithium niobate (LNO) is a well established material for surface acoustic wave (SAW) devices including resonators, delay lines and filters. Recently, multi-layer substrates based on LNO thin films have become commercially available. Here, we present a systematic low-temperature study of the performance of SAW devices fabricated on LNO-on-insulator and LNO-on-Silicon substrates and compare them to bulk LNO devices. Our study aims at assessing the performance of these substrates for quantum acoustics, i.e. the integration with superconducting circuits operating in the quantum regime. To this end, we design SAW resonators with a target frequency of and perform experiments at millikelvin temperatures and microwave power levels corresponding to single photons or phonons. The devices are investigated regarding their internal quality factors as a function of the excitation power and temperature, which allows us to characterize and quantify losses and identify the dominating loss mechanism. For the measured devices, fitting the experimental data shows that the quality factors are limited by the coupling of the resonator to a bath of two-level-systems. Our results suggest that SAW devices on thin film LNO on silicon have comparable performance to devices on bulk LNO and are viable for use in SAW-based quantum acoustic devices.

Thermodynamic probes can be used to deduce microscopic internal dynamics of nanoscale quantum systems. Several direct entropy measurement protocols based on charge transport measurements have been proposed and experimentally applied to single-electron devices. To date, these methods have relied on (quasi-)equilibrium conditions between the nanoscale quantum system and its environment, which constitutes only a small subset of the experimental conditions available. In this paper, we establish a thermodynamic analysis method based on stochastic thermodynamics, that is valid far from equilibrium conditions, is applicable to a broad range of single-electron devices and allows us to find the difference in entropy between the charge states of the nanodevice, as well as a characteristic of any selection rules governing electron transfers. We apply this non-equilibrium entropy measurement protocol to a single-molecule device in which the internal dynamics can be described by a two-site Hubbard model.

Ensembles of nitrogen-vacancy (NV) centres in diamond are a leading platform for practical quantum sensors. Reproducible and scalable fabrication of NV-ensembles with desired properties is crucial, as is an understanding of how those properties influence performance. This work addresses these issues by characterising nitrogen-doped diamond produced by the chemical vapour deposition (CVD) method across a range of synthesis conditions. This is shown to produce material with widely differing absorption characteristics, which is linked to the level of defects other than substitutional nitrogen (N_S) and NV. In such material, the achievable concentration of NV⁻ ([NV⁻]) is found to be influenced by the as-grown properties. At the 10–20 ppm level for [N_S], the production of CVD-grown material with strain levels sufficient not to limit achievable device sensitivity is demonstrated and a favourable product of [NV⁻] and is obtained. Additionally, reproducible properties over a batch of 23 samples from a single synthesis run are achieved, which appears promising for the scalability efforts underway in this area of research.

In recent years, machine and quantum learning have gained considerable momentum sustained by growth in computational power and data availability and have shown exceptional aptness for solving recognition- and classification-type problems, as well as problems that require complex, strategic planning. In this work, we discuss and analyze the role machine and quantum learning are playing in the development of diamond-based quantum technologies. This matters as diamond and its optically addressable spin defects are becoming prime hardware candidates for solid state-based applications in quantum information, computing and metrology. Through a selected number of demonstrations, we show that machine and quantum learning are leading to both practical and fundamental improvements in measurement speed and accuracy. This is crucial for quantum applications, especially for those where coherence time and signal-to-noise ratio are scarce resources. We summarize some of the most prominent machine and quantum learning approaches that have been conducive to the presented advances and discuss their potential, as well as their limits, for proposed and future quantum applications.

Photonic processors are pivotal for both quantum and classical information processing tasks using light. In particular, linear optical quantum information processing requires both large-scale and low-loss programmable photonic processors. In this paper, we report the demonstration of the largest universal quantum photonic processor to date: a low-loss 12-mode fully tunable linear interferometer with all-to-all mode coupling based on stoichiometric silicon nitride waveguides.

We systematically study the performance of compact lumped element planar microwave Nb₇₀Ti₃₀N (NbTiN) resonators operating at 5 GHz in external in-plane magnetic fields up to 440 mT, a broad temperature regime from 2.2 K up to 13 K, as well as mK temperatures. For comparison, the resonators have been fabricated on thermally oxidized and pristine, (001) oriented silicon substrates. When operating the resonators in the multi-photon regime at T = 2.2 K, we find internal quality factors Q_int ≃ 2 × 10⁵ for NbTiN resonators grown on pristine Si substrates. In addition, we investigate the Q-factors of the resonators on pristine Si substrates at millikelvin temperatures to assess their applicability for quantum applications. We find Q_int ≃ 2 × 10⁵ in the single photon regime and Q_int ≃ 5 × 10⁵ in the high power regime at T = 7 mK. From the excellent performance of our resonators over a broad temperature and magnetic field range, we conclude that NbTiN deposited on Si (100) substrates, where the surface oxide has been removed, constitutes a promising material platform for electron spin resonance and ferromagnetic resonance experiments using superconducting planar microwave resonators.

Colour centres in hexagonal boron nitride (hBN) have emerged as intriguing contenders for integrated quantum photonics. In this work, we present a detailed photophysical analysis of hBN single emitters emitting at the blue spectral range. The emitters are fabricated by different electron beam irradiation and annealing conditions and exhibit narrow-band luminescence centred at 436 nm. Photon statistics as well as rigorous photodynamics analysis unveils potential level structure of the emitters, which suggests lack of a metastable state, supported by a theoretical analysis. The potential defect can have an electronic structure with fully occupied defect state in the lower half of the hBN band gap and empty defect state in the upper half of the band gap. Overall, our results are important to understand the photophysical properties of the emerging family of blue quantum emitters in hBN as potential sources for scalable quantum photonic applications.

Thermodynamic probes can be used to deduce microscopic internal dynamics of nanoscale quantum systems. Several direct entropy measurement protocols based on charge transport measurements have been proposed and experimentally applied to single-electron devices. To date, these methods have relied on (quasi-)equilibrium conditions between the nanoscale quantum system and its environment, which constitutes only a small subset of the experimental conditions available. In this paper, we establish a thermodynamic analysis method based on stochastic thermodynamics, that is valid far from equilibrium conditions, is applicable to a broad range of single-electron devices and allows us to find the difference in entropy between the charge states of the nanodevice, as well as a characteristic of any selection rules governing electron transfers. We apply this non-equilibrium entropy measurement protocol to a single-molecule device in which the internal dynamics can be described by a two-site Hubbard model.

The mutual interplay between electron transport and magnetism has attracted considerable attention in recent years, primarily motivated by strategies to manipulate magnetic degrees of freedom electrically, such as spin–orbit torques and domain wall motion. Within this field the topological Hall effect, which originates from scalar spin chirality, is an example of inter-band quantum coherence induced by real-space inhomogeneous magnetic textures, and its magnitude depends on the winding number and chiral spin features that establish the total topological charge of the system. Remarkably, in the two decades since its discovery, there has been no research on the quantum correction to the topological Hall effect. Here we will show that, unlike the ordinary Hall effect, the inhomogeneous magnetization arising from the spin texture will give additional scattering terms in the kinetic equation, which result in a quantum correction to the topological Hall resistivity. We focus on two-dimensional systems, where weak localization is strongest, and determine the complicated gradient corrections to the Cooperon and kinetic equation. Whereas the weak localization correction to the topological Hall effect is not large in currently known materials, we show that it is experimentally observable in dilute magnetic semiconductors. Our theoretical results will stimulate experiments on the topological Hall effect and fill the theoretical knowledge gap on weak localization corrections to transverse transport.

Using the state of valley-polarization of electrons in solids is a promising new paradigm for information storage and processing. The central challenge in utilizing valley-polarization for this purpose is to develop methods for manipulating and reading out the final valley state. Here, we demonstrate optical detection of valley-polarized electrons in diamond. It is achieved by capturing images of electroluminescence from nitrogen-vacancy centers at the surface of a diamond sample that are excited by electrons drifting and diffusing through the sample. Monte Carlo simulations are performed to interpret the resulting experimental diffusion patterns. Our results give insight into the drift-diffusion of valley-polarized electrons in diamond and yield a way of analyzing the valley-polarization of ensembles of electrons.

Lithium niobate (LNO) is a well established material for surface acoustic wave (SAW) devices including resonators, delay lines and filters. Recently, multi-layer substrates based on LNO thin films have become commercially available. Here, we present a systematic low-temperature study of the performance of SAW devices fabricated on LNO-on-insulator and LNO-on-Silicon substrates and compare them to bulk LNO devices. Our study aims at assessing the performance of these substrates for quantum acoustics, i.e. the integration with superconducting circuits operating in the quantum regime. To this end, we design SAW resonators with a target frequency of and perform experiments at millikelvin temperatures and microwave power levels corresponding to single photons or phonons. The devices are investigated regarding their internal quality factors as a function of the excitation power and temperature, which allows us to characterize and quantify losses and identify the dominating loss mechanism. For the measured devices, fitting the experimental data shows that the quality factors are limited by the coupling of the resonator to a bath of two-level-systems. Our results suggest that SAW devices on thin film LNO on silicon have comparable performance to devices on bulk LNO and are viable for use in SAW-based quantum acoustic devices.

Up until now, ground state cooling using optomechanical interaction is realized in the regime where optical dissipation is higher than mechanical dissipation. Here, we demonstrate that optomechanical ground state cooling in a continuous optomechanical system is possible by using backward Brillouin scattering while mechanical dissipation exceeds optical dissipation which is the common case in optical waveguides. The cooling is achieved in an anti-Stokes backward Brillouin process by modulating the intensity of the optomechanical coupling via a pulsed pump to suppress heating processes in the strong coupling regime. With such dynamic modulation, a significant cooling factor can be achieved, which can be several orders of magnitude lower than for the steady-state case. This modulation scheme can also be applied to Brillouin cooling generated by forward intermodal Brillouin scattering.

In recent years, hole-spin qubits based on semiconductor quantum dots have advanced at a rapid pace. We first review the main potential advantages of these hole-spin qubits with respect to their electron-spin counterparts and give a general theoretical framework describing them. The basic features of spin–orbit coupling and hyperfine interaction in the valence band are discussed, together with consequences on coherence and spin manipulation. In the second part of the article, we provide a survey of experimental realizations, which spans a relatively broad spectrum of devices based on GaAs, Si and Si/Ge heterostructures. We conclude with a brief outlook.

Topological phenomena at the oxide interfaces attract the scientific community for the fertile ground of exotic physical properties and highly favorable applications in the area of high-density low-energy nonvolatile memory and spintronic devices. Synthesis of atomically controlled ultrathin high-quality films with superior interfaces and their characterization by high resolution experimental set up along with high output theoretical calculations matching with the experimental results make this field possible to explain some of the promising quantum phenomena and exotic phases. In this review, we highlight some of the interesting interface aspects in ferroic thin films and heterostructures including the topological Hall effect in magnetic skyrmions, strain dependent interlayer magnetic interactions, interlayer coupling mediated electron conduction, switching of noncollinear spin texture etc. Finally, a brief overview followed by the relevant aspects and future direction for understanding, improving, and optimizing the topological phenomena for next generation applications are discussed.

In recent years, machine and quantum learning have gained considerable momentum sustained by growth in computational power and data availability and have shown exceptional aptness for solving recognition- and classification-type problems, as well as problems that require complex, strategic planning. In this work, we discuss and analyze the role machine and quantum learning are playing in the development of diamond-based quantum technologies. This matters as diamond and its optically addressable spin defects are becoming prime hardware candidates for solid state-based applications in quantum information, computing and metrology. Through a selected number of demonstrations, we show that machine and quantum learning are leading to both practical and fundamental improvements in measurement speed and accuracy. This is crucial for quantum applications, especially for those where coherence time and signal-to-noise ratio are scarce resources. We summarize some of the most prominent machine and quantum learning approaches that have been conducive to the presented advances and discuss their potential, as well as their limits, for proposed and future quantum applications.

Quantum technologies are poised to move the foundational principles of quantum physics to the forefront of applications. This roadmap identifies some of the key challenges and provides insights on material innovations underlying a range of exciting quantum technology frontiers. Over the past decades, hardware platforms enabling different quantum technologies have reached varying levels of maturity. This has allowed for first proof-of-principle demonstrations of quantum supremacy, for example quantum computers surpassing their classical counterparts, quantum communication with reliable security guaranteed by laws of quantum mechanics, and quantum sensors uniting the advantages of high sensitivity, high spatial resolution, and small footprints. In all cases, however, advancing these technologies to the next level of applications in relevant environments requires further development and innovations in the underlying materials. From a wealth of hardware platforms, we select representative and promising material systems in currently investigated quantum technologies. These include both the inherent quantum bit systems and materials playing supportive or enabling roles, and cover trapped ions, neutral atom arrays, rare earth ion systems, donors in silicon, color centers and defects in wide-band gap materials, two-dimensional materials and superconducting materials for single-photon detectors. Advancing these materials frontiers will require innovations from a diverse community of scientific expertise, and hence this roadmap will be of interest to a broad spectrum of disciplines.

Photonic quantum technology is essentially based on the exchange of individual photons as information carriers. Therefore, the development of practical single-photon sources that emit single photons on-demand is a crucial contribution to advance this emerging technology and to promote its first real-world applications. In the last two decades, a large number of quantum light sources based on solid-state emitters have been developed on a laboratory scale. Corresponding structures today have almost ideal optical and quantum-optical properties. For practical applications, however, one crucial factor is usually missing, namely direct on-chip fiber coupling, which is essential, for example, for the direct integration of such quantum devices into fiber-based quantum networks. In fact, the development of fiber-coupled quantum light sources is still in its infancy, with very promising advances having been made in recent years. Against this background, this review article presents the current status of the development of fiber-coupled quantum light sources based on solid-state quantum emitters and discusses challenges, technological solutions and future prospects. Among other things, the numerical optimization of the fiber coupling efficiency, coupling methods, and important realizations of such quantum devices are presented and compared. Overall, this article provides an important overview of the state-of-the-art and the performance parameters of fiber-coupled quantum light sources that have been achieved so far. It is aimed equally at experts in the scientific field and at students and newcomers who want to get an overview of the current developments.