Large-scale quantum networks will enable entirely new applications of quantum information science in fields such as quantum communication, distributed quantum computing, sensing, and metrology. To build nodes of such networks, diamond color defects are one of the promising candidates. Their excellent optical properties, fast spin-qubit control, and long spin coherence times make them well-suited for quantum information processing and quantum memory applications. Additionally, recent advances in the heterogeneous integration of diamond nanophotonic structures with photonic integrated circuits have made these systems more efficient and well-suited for scalable quantum processor architectures. In this comprehensive review, we discuss the optical and spin properties of these systems, recent progress in the building blocks of quantum networks, and demonstrations of metropolitan-scale quantum networks, as well as the challenges associated with these systems at both the fundamental and experimental levels, along with potential solutions.

Ein Quantenelektrometer auf Basis eines Zinn‐Defekts in Diamant ermöglicht einen neuen Blick in die verborgene Dynamik von Ladungsprozessen im Festkörper. Es zeigt mit atomarer Präzision, wie einzelne Elektronen im Kristall elektrische Felder formen und wie diese sich verändern.

The concept of quantum tokens dates back alongside quantum cryptography to Stephen Wiesner's seminal work in 1983[1]. Already this initial work proposes society-relevant applications such as secure quantum banknotes, which can be exchanged between a bank and a customer. This quantum currency is based on various physical states that can be easily verified but is protected from being copied by the fundamental quantum laws. Four decades later, these ideas have flourished in the field of quantum information, and the concept of quantum banknotes has not only adopted many varying names, such as quantum money, quantum coins, quantum-digital payments, and quantum tokens, but also reached its first experimental demonstrations. In this perspective article, we discuss the current state-of-the-art of quantum tokens in the field of quantum information, as well as their future perspectives. We present a number of physical realizations of quantum tokens with integrated quantum memories and their applicability scenarios in detail. Finally, we discuss how quantum tokens fit into the information security ecosystem and consider their relationship to post-quantum cryptography.

Abstract Spin-active colour centres in 4H silicon carbide are promising candidates as building blocks for quantum information applications. To increase the photon count rate of the emitters at low temperatures, the colour centres must be integrated into nanophotonic structures and characterised under cryogenic conditions. Here, we design and fabricate waveguide structures attached with an efficient Dinosaur photonic crystal reflector at one side. The devices show broadband reflection over a range of 60 THz with a peak reflectance above 80 %. Additionally, colour centres were integrated into these structures and characterised at cryogenic conditions. The emission was collected by a tapered-waveguide-tapered-fibre interface. Although the spectral stability of the emitters must be further improved for high excitation powers, the saturation intensity in standard PLE measurements is I s,LT = (103.8 ± 4.2) kcps. The count rate can be further improved to about 125 kcps with a charge-resonance check measurement scheme. To highlight the relevance of our devices, we theoretically show that these count rates enable optical single-shot readout with a fidelity exceeding 98%.

Quantum Physical Unclonable Functions (QPUFs) offer a physically grounded approach to secure authentication, extending the capabilities of classical PUFs. This review covers their theoretical foundations and key implementation challenges - such as quantum memories and Haar-randomness -, and distinguishes QPUFs from Quantum Readout PUFs (QR-PUFs), more experimentally accessible yet less robust against quantum-capable adversaries. A co-citation-based selection method is employed to trace the evolution of QPUF architectures, from early QR-PUFs to more recent Hybrid PUFs (HPUFs). This method further supports a discussion on the role of information-theoretic analysis in mitigating inconsistencies in QPUF responses, underscoring the deep connection between secret-key generation and authentication. Despite notable advances, achieving practical and robust QPUF-based authentication remains an open challenge.

In the rapidly evolving field of quantum technology, the precise and detailed description of quantum components is not just a necessity but the foundation for advancing research, development, and applications. Optically accessible quantum memories are key building blocks for devices such as quantum repeaters and two-factor authentication. The memory we describe here is based on a Group-IV-vacancy color center (SiV-/SnV-) coupled to a highly efficient cavity. It leverages state-dependent reflection from the cavity and implements high-fidelity fractional single qubit gates via a train of optical pi/8 pulses. We also describe its operation under microwave control, further extending our analysis. Our primary contribution in this work is the integration of this device model into a standardized software framework for quantum memory architectures.

We introduce a fully automated microscope, localize more than 16,000 solid-state quantum emitters, and characterize almost 12,000 of them. Using a neural network, we select the most suitable ones and deterministically fabricate nanostructures around them.

The coherence characteristics of a tin-vacancy (SnV) color center in diamond are investigated through optical means, including linewidth broadening effects and coherent population trapping (CPT) between the ground-state orbital levels. Spectral analysis is required as due to the large spin-orbit splitting of the orbital ground states, thermalization between the ground states occurs at rates that are impractical to measure directly in the time domain. First, by implementing a temperature-dependent linewidth broadening measurement, including the challenging-to-measure D transition, phononic coupling coefficients are determined. These measurements are performed on an emitter with a lifetime-limited linewidth and atomlike properties, making the measurement representative for high-quality SnVs. Next, a CPT-type experiment is carried out to independently analyze thermal decoherence processes at 4 K. The spectral information is transformed into its conjugate variable time, providing picosecond resolution and revealing an orbital depolarization timescale of ∼30ps. Consequences of the investigated dynamics are then used to estimate spin dephasing times limited by thermal effects.

Quantum photonic devices require efficient access to solid-state spin qubits serving as quantum memories or single-photon sources and nonlinearities. Additionally, they demand nonlinear and electro-optic components for frequency converters and phase shifters. The serial production of quantum devices needs the on-chip integration of all these photonic components. These requirements motivate the hetero-integration of materials with complementary properties. Diamond color centers provide access to spin registers, long coherence times, and suitability for quantum networking. In contrast, aluminum gallium nitride-on-aluminum nitride (AlGaN/AlN) photonics features second-order nonlinear and first-order electro-optic effects. Here, we demonstrate the accurate integration of diamond waveguides containing nitrogen-vacancy (NV) centers into AlGaN/AlN waveguides using a pick-and-place method. Both waveguides possess tapered sections for adiabatic mode transfer. We model the tapered diamond-AlGaN/AlN interface with finite element simulations, implementing fabrication constraints and placement uncertainties. Measured coupling efficiencies coincide closely with simulation model predictions.

Abstract The function of nucleic acids is governed not only by their structure but also by their dynamics. At the molecular scale, transitions between functional structural states are superimposed on rapid thermal fluctuations, resulting in an intricate interplay that is challenging to resolve experimentally, particularly at the single-molecule level. Here, we introduce a novel approach for unraveling sub-microsecond dynamics in oligonucleotides, enabling direct observation of fluctuations in single DNA molecules. By immobilizing nucleic acids vertically on graphene and exploiting distance-dependent graphene energy transfer of fluorescent molecules attached to the DNA, we relate fluctuations in fluorescence intensity to biomolecular dynamics. We show that ionic strength modulates the fluctuations and that structural defects in DNA, such as nucleotide gaps or mismatches, alter the measured dynamics. The experimental findings are complemented by atomistic molecular dynamics simulations and kinetic Monte Carlo simulations, establishing a direct link between theoretical predictions of structure and dynamics and experimentally accessible fluctuation timescales. Overall, our findings advance the understanding of how thermal fluctuations affect oligonucleotides and are modulated by both external and internal stimuli.

The concept of laser intracavity absorption spectroscopy is extended to magnetometry with nitrogen-vacancy color centers in diamond. We demonstrate a 475-fold enhancement of spin contrast and measure a 180-fold improvement of magnetic sensitivity.

Solid-state immersion lenses have the potential to become a key component in photonic quantum technologies, as they enable efficient collection of quantum light. Although they are conceptually simple and powerful at the same time, they are not yet widely used today. This is a consequence of the challenges in the controlled processing of the typically ultra-hard crystalline materials such as diamond in combination with the deterministic positioning of the lens relative to the position of the quantum emitter. Here we present a novel patterning approach for the fabrication of solid-state immersion lenses using focused ion beam milling, which eliminates the need for post-processing. The hemispherical lens shape is defined by a radially varying ion flux applied in a continuous Archimedean spiral beam path. In line with the prediction from continuum modeling, we observe periodic surface corrugations at the side walls of the lenses indicating that the ion flux is locally perfectly homogenized by our approach. Microstructural investigations reveal a damage layer of only 10 - 15 nm above an otherwise intact diamond crystal. This corresponds to the expected beam damage obtained from atomistic binary collision modeling, and proves that redeposition is indeed negligible. To demonstrate their performance, lenses with nitrogen vacancy centers are optically examined, revealing a sixfold enhancement in light collection efficiency in combination with reduced background noise.

Optically addressable solid-state spin defects have emerged as powerful multimodal quantum sensors, with nitrogen-vacancy (NV) centers in bulk diamond providing benchmark quantum control and sensitivity under ambient conditions. Embedding such defects in nanodiamonds (NDs) extends these capabilities to mobile probes capable of accessing complex biological and nanoscale environments. Reduced dimensions, however, introduce constraints beyond volumetric spin impurities, notably enhanced lattice strain and surface-induced noise sources, which shorten NV spin relaxation times (T1 and T2) and destabilize the NV charge state, as well as resulting in pronounced particle-to-particle variability in NDs typically produced by top-down approaches. These effects complicate both sensing performance and the quantitative interpretation of multimodal signals in realistic environments. This article provides a structured perspective on the physical mechanisms by which material properties constrain NV behavior in NDs, together with mitigation strategies that shape the robust use of these mobile quantum sensors for biosensing and nanoscale science.

DNA‐based nanoscale architectures provide an attractive bottom‐up alternative to lithographic approaches for photonic device fabrication, offering molecular precision, intrinsic scalability, and biocompatibility. Here, we exploit these advantages to construct one‐dimensional photonic wires using DNA origami with up to nine precisely positioned organic dyes. Excitons are injected at both ends by fluorescence resonance energy transfer (FRET) and diffuse along the wire, where their mutual encounters enhance single‐photon emission through singlet–singlet annihilation. Using picosecond time‐resolved photon antibunching (psTRAB) and simulations, we directly infer exciton dynamics on the level of single structures, reaching a quantitative understanding when taking into account spectral crosstalk arising from direct acceptor excitation as well as the underlying donor photophysics. We also identify a photostabilization mechanism driven by diffusive separation of excitons. Our results establish a design framework for DNA‐based excitonic elements and highlight the potential of DNA nanotechnology for nanoscale photonic structures.

Hexagonal boron nitride (hBN) is gaining increasing attention in the field of biomolecule characterization due to its compatibility with single-molecule fluorescence imaging and real-time tracking. Embedding fluorescent molecules within hBN layers offers potential for molecular-resolution sensing devices, since these probes are highly sensitive to their surroundings. Yet, the effect of hBN surfaces on the fluorophore properties remains largely unexplored. Here, we monitor the photophysical properties of ATTO647N-ssDNA on hBN surfaces and elucidate the effects of the environment and substrate. We demonstrate that the presence of hBN increases the photobleaching time and changes intermittency dynamics. By combining van der Waals stacking and FDTD simulations, we subsequently engineer hBN optical cavities to modulate the emission from individual molecules, showing that the brightness can be tuned by a factor of 4. Our findings shed light on light-matter interactions in hybrid nanostructures, which can enable single-molecule imaging and biosensing at high spatial and temporal resolution.

The coherent excitation of an optically active spin system is one of the key elements in the engineering of a spin-photon interface. Using the novel SUPER scheme, we coherently control the main optical transition of a tin-vacancy color center in diamond with nonresonant ultrashort optical pulses. Furthermore, we implement a femtosecond control scheme using resonant pulses for achieving record short quantum gates applied to diamond color centers. We simulate the applicability of the SUPER scheme to spin qubits and experimentally investigate spin mixing. Finally, we propose a spin-spin entanglement scheme in a scenario where the excitation with broadband pulses is incompatible with spin-selective excitation. The employed ultrafast quantum gates open up a new regime of quantum control with solid-state color centers, enabling multi-gate operations and efficient spectral filtering of the excitation laser from deterministically prepared coherent photons.

The optical spectrum of a quantum system is jointly determined by the properties of the emitter and the driving field. All-optical spectral control can hence be a promising method to engineer the properties of single photon emitters for quantum technological applications. It was proposed that driving a two-level system with a periodic sequence of optical pi-pulses during the excited state lifetime shifts the emission and absorption maximum to an arbitrarily detuned pulse carrier frequency, enabling the mitigation of spectral diffusion in noisy emitters. In this article, we report on the first experimental observation of this effect. We implement the protocol on a solid-state emitter and reduce its inhomogeneously broadened optical linewidth close to the lifetime limit. By detuning the excitation laser, we are able to concentrate approximately half of the absorption to a freely selectable target frequency. Our approach is solely based on properties of coherently evolving quantum systems, rendering it applicable to a wide range of individual and ensembles of quantum emitters.

The tin-vacancy (SnV) center in diamond is a promising spin-photon interface for quantum networks, combining favorable optical properties with spin coherence above 1K. Unfolding the full potential requires cavity enhancement to increase photon-emitter coupling efficiency. Here, we demonstrate cavity-enhanced light-matter coupling of SnV centers in a fully tunable Fabry-Pérot microcavity operating at temperatures down to 1K with in-situ magnetic field control. We access the diamond-like regime of hybrid cavity modes through integration of low-roughness diamond membranes, where the field is concentrated inside the diamond and Purcell enhancement is maximized. Diamond-like modes deliver a more than two-fold increase in the effective Purcell factor over air-like modes, reaching C_0 = 4.1(1) compared to C_0 = 1.85(5) in the air-like case, while simultaneously relaxing mechanical stability requirements. Resonant probing reveals coherent cavity-emitter coupling with 96

Qubit coherence is an essential figure of merit for quantum information processing applications such as quantum computing, or quantum repeaters. Understanding the coherence properties of the underlying physical qubits that facilitate such applications is therefore are often limited by coupling to lattice phonons, which in turn constrains operation temperature. Here we study phonon induced electronic spin decoherence in group-IV vacancy centers in diamond. We begin by modeling the spin-phonon interaction and then employ the widely used Born-Markov approximation, highlighting its inconsistencies in this setting and its deviations from experimental observations. To close the gap between theoretical predictions and experimental results, we relax certain approximations, investigate their contributions to the predicted coherence times, and identify the dominant sources of discrepancy. We further demonstrate that experimentally measured electronic spin coherence dynamics are consistently captured within a non-Markovian framework, and we show how the magnetic field orientation influences the qubit coherence time.

Protective coatings of functional DNA nanostructures with materials like silica or cationic polymers have evolved as a simple, yet powerful strategy to improve their stability even under extreme conditions. While over time, various materials and protocols have been developed, the characterization and quality assessment of the coating is either time consuming, highly invasive, or lacks detailed insights on single nanostructures. Here, a cyanine dye-based molecular sensor is introduced to noninvasively probe the coating of DNA origami by either a cationic polymer or by silica, in real-time and on a single nanostructure level. The cyanine dye reports changes in its local environment upon coating via increased fluorescence lifetime induced by steric restriction and water exclusion. Exploiting the addressability of DNA origami and the reversibility of the molecular sensor, the coating layer is probed at selected positions and in degrading conditions. Finally, the molecular sensor is combined with DNA PAINT super-resolution imaging to investigate coating and structural integrity as well as preserved addressability of DNA nanostructures. The reported sensor presents a valuable tool to probe the coating of DNA nanodevices in complex biochemical environments in real-time and at the single nanostructure level and aids the development of novel stabilization strategies.