Quantum sensors emerged among quantum technologies as the ones with promising potential applications in the near future. This perspective reviews two leading quantum sensing platforms and their advancements toward biological applications: quantum light sources and color centers in diamonds. Quantum light, including squeezed states and N00N states, allows enhanced phase measurements by surpassing the classical shot noise limits. This advantage can be exploited in several contexts, enabling improved resolution and sensitivity, which are particularly valuable in biological contexts where traditional high-intensity illumination could damage or alter delicate samples. In parallel, color centers in diamonds, specifically nitrogen-vacancy and silicon-vacancy centers, also emerged as promising for sensing applications due to their high sensitivity and biocompatibility. These sensors enable detailed intracellular measurements, such as temperature detection, and show potential for measuring magnetic fields of biological origin. Despite these advancements, significant challenges remain in translating these technologies from a controlled laboratory environment to practical, widely applicable devices for diverse biological applications. Overcoming these challenges is crucial for unlocking the full potential of quantum sensors in the biological field.

Nitrogen-Vacancy (NV) center in nanodiamonds offers unique advantages for thermometry applied in biological samples. Small size and biocompatibility make nanodiamonds promising as nanoscale thermometers. The energy level structure of NV centers enables optical initialization and readout of the spin state, particularly in their triplet ground state, where the energy gap (Dgs) between |ms=0⟩ and |ms=±1⟩ is sensitive to environmental conditions. Variations in temperature, magnetic, or electric fields induce changes in the energy structure of the system, allowing for precise reconstruction of applied field values and facilitating the use of NV centers as sensors. The purpose of this work is to characterize the response of a set of nanodiamonds to a change in temperature: knowing the value of the coefficient that links the change in temperature to a change in Dgs makes possible to use nanodiamonds inside cells as thermometers. Moreover, some techniques that can reduce the uncertainty related to the estimation of this coupling constant are investigated.

The nanoscale fabrication of μm-spaced single-photon emitter arrays is crucial for the development of integrated photonic chips. We report on the fabrication and systematic characterization of germanium-vacancy (GeV) color centers arrays in diamond obtained upon ion implantation at the nanoscale. Ge2+ ion implantations at 35 keV and 70 keV energies were carried out using a focused ion beam (FIB) equipped with a liquid metal alloy ion source. The arrays of emitters are subsequently aligned to ø300 nm nanopillar waveguiding structures, fabricated using a combination of electron-beam lithography and plasma etching. The photon collection efficiency and photoluminescence (PL) signal-to-background ratio increased by a factor 8 with respect to the unstructured sample. The photophysical properties of the GeV emitters fabricated by this approach were unaltered with respect to those found in unprocessed diamond. The efficiency of the overall manufacturing process to fabricate individual GeV centers was assessed. Up to 33

In recent decades, nanodiamonds (NDs) have emerged as innovative nanotools for weak magnetic fields and small temperature variation sensing, especially in biological systems. At the basis of the use of NDs as quantum sensors are nitrogen-vacancy center lattice defects, whose electronic structures are influenced by the surrounding environment and can be probed by the optically detected magnetic resonance technique. Ideally, limiting the NDs' size as much as possible is important to ensure higher biocompatibility and provide higher spatial resolution. However, size reduction typically worsens the NDs' sensing properties. This study endeavors to obtain sub-100 nm NDs suitable to be used as quantum sensors. Thermal processing and surface oxidations were performed to purify NDs and control their surface chemistry and size. Ion irradiation techniques were also employed to increase the concentration of the nitrogen-vacancy centers. The impact of these processes was explored in terms of surface chemistry (diffuse reflectance infrared Fourier transform spectroscopy), structural and optical properties (Raman and photoluminescence spectroscopy), dimension variation (atomic force microscopy measurements), and optically detected magnetic resonance temperature sensitivity. Our results demonstrate how surface optimization and defect density enhancement can reduce the detrimental impact of size reduction, opening to the possibility of minimally invasive high-performance sensing of physical quantities in biological environments with nanoscale spatial resolution.

We present a novel approach for the optical activation of the negatively-charged silicon vacancy ( VSi−) center in ion irradiated silicon carbide (SiC) via ns-pulsed laser annealing in the 234–2180 mJ cm− 2 energy density range. The laser annealing process is investigated under 355 nm and 532 nm wavelengths at pulse energy densities below the melting threshold and validated by means of Raman spectroscopy and photoluminescence mapping. The combined effect of ns pulsed laser annealing and subsequent thermal treatment is also assessed. The results offer a promising resource for the development of integrated photonic SiC devices and could be extended to a potentially wide range of applications involving other classes of solid-state quantum emitters.

Intracellular temperature is a crucial parameter that influences metabolic processes, enzymatic activity, and cellular signaling. Advances in nanoscopic thermometry using nitrogen-vacancy (NV) centers in diamond provide unprecedented precision in mapping temperature variations within living cells. NV centers, atomic-scale defects in the diamond lattice, exhibit quantum properties sensitive to temperature, enabling non-invasive and high-resolution thermal measurements. These properties, combined with the biocompatibility and stability of nanodiamonds, make them ideal for probing cellular thermodynamics in real time. By integrating NV-based thermometry with multi-electrode array (MEA) techniques, it is possible to correlate intracellular temperature variations with changes in cellular metabolism and electrophysiological activity. This combination offers unique insights into how metabolic processes influence cellular function, particularly under varying physiological and pathological conditions. For example, mapping temperature heterogeneity within cellular microenvironments can shed light on mitochondrial thermogenesis or metabolic alterations in disease states. This approach also holds promise for studying the role of intracellular temperature in the development and progression of neurodegenerative diseases, where metabolic dysfunction is a peculiar indicator. Another intriguing application could involve the functionalization of nanodiamonds to target specific organelles, enabling even more precise investigations of localized thermal dynamics in the future. For now, current advancements in NV-based thermometry already demonstrate its robustness as a tool for exploring the dynamic interplay between intracellular temperature and metabolism. These insights provide a foundation for advancing our understanding of cellular energetics and for developing innovative approaches in diagnostics and therapeutics.

The nitrogen vacancy (NV) center in diamond is an intriguing electronic spin system with applications in quantum radiometry, sensing, and computation. In those experiments, a bias magnetic field is commonly applied along the NV symmetry axis to eliminate the triplet ground-state manifold's degeneracy (S = 1). In this configuration, the eigenvectors of the NV spin's projection along its axis are called strong-axial-field states. Conversely, in some experiments, a weak magnetic field is applied orthogonally to the NV symmetry axis, leading to eigenstates that are balanced linear superpositions of strong-axial-field states, referred to as dressed states. The latter are sensitive to environmental magnetic noise at the second order, allowing to perform magnetic field protected measurements while providing increased coherence times. However, if a small axial magnetic field is added in this regime, the linear superposition of strong-axial-field states becomes unbalanced. This paper presents a comprehensive study of free induction decay (FID) measurements performed on an NV center ensemble in the presence of strain and weak orthogonal magnetic field, as a function of a small magnetic field applied along the NV symmetry axis. The simultaneous detection of dressed states and unbalanced superpositions of strong-axial-field states in a single FID measurement is shown, gaining insight into coherence time, nuclear spin, and the interplay between temperature and magnetic field sensitivity. The discussion concludes by describing how the simultaneous presence of magnetically sensitive and insensitive states opens up appealing possibilities for both sensing and quantum computation applications.

Nitrogen Vacancy centers in diamond interact with local magnetic and electric fields, temperature, strain, and pressure. Their ease of operation and exceptional performance has led to the emergence of a first generation of commercial NV-based quantum sensors, as scanning-probe systems, giving them wide recognition as the quantum technology with the most imminent market potential. In recent years, there is an effort to advance the TRL of those quantum technologies through several European initiatives.

The recent demonstration of optically active telecom emitters makes silicon a compelling candidate for solid state quantum photonic platforms. Particularly fabrication of the G center has been demonstrated in carbon-rich silicon upon conventional thermal annealing. However, the high-yield controlled fabrication of these emitters at the wafer-scale still requires the identification of a suitable thermodynamic pathway enabling its activation following ion implantation. Here we demonstrate the efficient activation of G centers in high-purity silicon substrates upon ns pulsed laser annealing. The proposed method enables the non-invasive, localized activation of G centers by the supply of short non-stationary pulses, thus overcoming the limitations of conventional rapid thermal annealing related to the structural metastability of the emitters. A finite-element analysis highlights the strong non-stationarity of the technique, offering radically different defect-engineering capabilities with respect to conventional longer thermal treatments, paving the way to the direct and controlled fabrication of emitters embedded in integrated photonic circuits and waveguides.

We report on a robust method for reconstructing multi-mode optical fields. In particular, our method correctly identifies the number and types of modes as well as each mode’s energy, which we demonstrated experimentally. Our method uses multidetector trees and both high-order autocorrelation functions of detected photons (the Glauber function) and high-order autocorrelation functions of no-photon detection. Here we show successful reconstruction for classical and nonclassical multimode fields that contain up to 4 modes whose types are not disclosed to the reconstruction algorithm. This method is significantly more successful than the previously reported one, based on high-order Glauber functions only.

Abstract Temperature is one of the most relevant parameters for the regulation of intracellular processes. Measuring localized subcellular temperature gradients is fundamental for a deeper understanding of cell function, such as the genesis of action potentials, and cell metabolism. Notwithstanding several proposed techniques, at the moment detection of temperature fluctuations at the subcellular level still represents an ongoing challenge. Here, for the first time, temperature variations (1 °C) associated with potentiation and inhibition of neuronal firing is detected, by exploiting a nanoscale thermometer based on optically detected magnetic resonance in nanodiamonds. The results demonstrate that nitrogen‐vacancy centers in nanodiamonds provide a tool for assessing various levels of neuronal spiking activity, since they are suitable for monitoring different temperature variations, respectively, associated with the spontaneous firing of hippocampal neurons, the disinhibition of GABAergic transmission and the silencing of the network. Conjugated with the high sensitivity of this technique (in perspective sensitive to < 0.1 °C variations), nanodiamonds pave the way to a systematic study of the generation of localized temperature gradients under physiological and pathological conditions. Furthermore, they prompt further studies explaining in detail the physiological mechanism originating this effect.

The recent demonstration of optically active telecom emitters in silicon has paved the way for realizing industrial-scale silicon-based solid-state quantum photonic platforms. The scientific community has been pursuing the implementation of novel single-photon devices for quantum technology applications by introducing extrinsic impurities inside the silicon lattice upon ion implantation. Here we report the optical characterization through single-photon microscopy of intrinsic W centers in high-purity silicon substrates upon carbon implantation and subsequent rapid thermal annealing. The photoluminescence investigation of their emission properties at cryogenic temperatures allowed us to identify the effects of the post-implantation thermal treatment in the formation of telecom quantum emitters based on interstitial silicon clusters upon the introduction of an extrinsic atomic species.

We report on a systematic optical and structural investigation of the MgV color center in diamond. The results show unique tunable properties of the center making it appealing for its utilization in quantum information processing.

The authors report on the characterization at the single‐defect level of germanium‐vacancy (GeV) centers in diamond produced upon Ge − ion implantation and different subsequent annealing processes, with a specific focus on the effect of high‐pressure‐high‐temperature (HPHT) processing on their quantum‐optical properties. Different post‐implantation annealing conditions are explored for the optimal activation of GeV centers, namely, 900 °C 2 h, 1000 °C 10 h, 1500 °C 1 h under high vacuum, and 2000 °C 15 min at 6 GPa pressure. A systematic analysis of the relevant emission properties, including the emission intensity in saturation regime and the excited state radiative lifetime, is performed on the basis of a set of ion‐implanted samples, with the scope of identifying the most suitable conditions for the creation of GeV centers with optimal quantum‐optical emission properties. The main performance parameter adopted here to describe the excitation efficiency of GeV centers as single‐photon emitters is the ratio between the saturation optical excitation power and the emission intensity at saturation. The results show an up to eightfold emission efficiency increase in HPHT‐treated samples with respect to conventional annealing in vacuum conditions, suggesting a suitable thermodynamic pathway toward the repeatable fabrication of ultra‐bright GeV centers for single‐photon generation purposes.

A novel technique is introduced for the reconstruction of multimode optical fields, based on simultaneously exploiting both the generalized Glauber's K-th-order correlation function g(K)$g<^>{(K)}$ and a recently proposed anti-correlation function (dubbed theta(K)$\theta <^>{(K)}$) which is resilient to Poissonian noise. It is experimentally demonstrated that this method yields mode reconstructions with higher fidelity with respect to those obtained with reconstruction methods based only on g(K)$g<^>{(K)}$'s, even requiring less "a priori" information. The reliability and versatility of this technique make it suitable for a widespread use in real applications of optical quantum measurement, from quantum information to quantum metrology, especially when one needs to characterize ensembles of single-photon emitters in the presence of background noise (due, for example, to residual excitation laser, stray light or unwanted fluorescence).

Temperature is one of the most relevant parameters for the regulation of intracellular processes. Measuring localized subcellular temperature gradients is fundamental for a deeper understanding of cell function, such as the genesis of action potentials, and cell metabolism. In this work I will review our latest progresses in NV-based thermometry ultimately leading to the first localized temperature increase detection in a firing neuronal network with precision under 0.1 K.

Currently, nanodiamonds (ND) are showing promising perspectives in many research fields due to their inertness, fluorescence, biocompatibility and the possibility to easily functionalize their surface termination. These appealing properties are finding ever-growing interest especially in biomedicine, with potential applications in drug delivery and optical biosensing. The presented work is focused on the enhancement of the Nitrogen-Vacancy (NV) centers fluorescence properties by combining MeV proton irradiation with the surface termination control. To this scope, ion irradiation processes were carried out by exploring a wide range of fluences (10 14 - 10 17 cm -2 ) to create new NV centers, thus allowing to define the conditions maximizing their creation and the emission yield. The collected data were described by a mathematical model predicting the efficiency of the NV center formation as a function of the fluence. This model can be applied not only for the case study described in the present work (MeV proton in ND), but more in general for different ion implantation processes willing to create new NV centers in diamond. Thermal oxidations at different temperatures and times were also carried out on ND to study the evolution of surface chemical groups (DRIFT analysis), as well as their influence on optical properties (Raman / photoluminescence spectroscopy). Besides the general NV centers fluorescence, also NV - /NV 0 ratio was evaluated at the different processing steps (both in terms of oxidation and irradiation treatments) and the results were interpreted based on DRIFT data. Optically-Detected-Magnetic-Resonance (ODMR) spectra were also acquired to evaluate how treatment conditions influenced the sensitivity of the technique. Finally, surface functionalization with hyaluronic acid conjugates was performed to allow proper stability and dispersibility of ND in solution and to confer targeting ability towards cancer cells. In-vitro tests of cellular uptake and viability were conducted with oxidized and proton-irradiated ND to evaluate their potential as fluorescent probes and drug delivery systems.

The Second Quantum Revolution facilitates the engineering of new classes of sensors, communication technologies, and computers with unprecedented capabilities. Supply chains for quantum technologies are emerging, some focused on commercially available components for enabling technologies and/or quantum-technologies research infrastructures, others with already higher technology-readiness levels, near to the market. In 2018, the European Commission has launched its large-scale and long-term Quantum Flagship research initiative to support and foster the creation and development of a competitive European quantum technologies industry, as well as the consolidation and expansion of leadership and excellence in European quantum technology research. One of the measures to achieve an accelerated development and uptake has been identified by the Quantum Flagship in its Strategic Research Agenda: The promotion of coordinated, dedicated standardisation and certification efforts. Standardisation is indeed of paramount importance to facilitate the growth of new technologies, and the development of efficient and effective supply chains. The harmonisation of technologies, methodologies, and interfaces enables interoperable products, innovation, and competition, all leading to structuring and hence growth of markets. As quantum technologies mature, the time has come to start thinking about further standardisation needs. This article presents insights on standardisation for quantum technologies from the perspective of the CEN-CENELEC Focus Group on Quantum Technologies (FGQT), which was established in June 2020 to coordinate and support the development of standards relevant for European industry and research.

We report a systematic photoluminescence (PL) investigation of the spectral emission properties of individual optical defects fabricated in diamond upon ion implantation and annealing. Three spectral lines at 620 nm, 631 nm, and 647 nm are identified and attributed to the SnV center due to their occurrence in the PL spectra of the very same single-photon emitting defects. We show that the relative occurrence of the three spectral features can be modified by oxidizing the sample surface following thermal annealing. We finally report the relevant emission properties of each class of individual emitters, including the excited state emission lifetime and the emission intensity saturation parameters.

The growing interest in understanding the complex mechanisms that regulate biological processes has prompted the study and the improvement of quantum sensors, potentially capable of detecting a wide class of physical quantities of biological interest. Among the various sensors proposed for biological applications, nitrogen-vacancy (NV) centers in artificial diamond have emerged as a truly promising solution primarily thanks to their excellent bio-compatibility. Such NV sensors can be synthesized of nanometer size. The nanodiamonds can be inserted inside cells and, if properly functionalized, they can be targeted to organelles, such as mitochondria, or ion channels. In addition to the advantages regarding their chemical and structural composition, the NV sensors have distinguished themselves thanks to their sensitivity respect different physical quantity, such as magnetic and electric fields, temperatures and pressures variations. Although the sensitivity achieved by the NV quantum sensors is not yet sufficient to detect the very weak electromagnetic fields generated by biological processes, the thermal variation generated at the cellular level seem at the moment a more attractive field of application. Temperature is an important parameter for the regulation of intracellular processes and its detection is fundamental for a more complete understanding of them. Cellular activity and metabolism can affect the local temperature in cells and pathological conditions such as cancer, Parkinson and Alzherimer's disease can alter it. In this sense, local temperature monitoring within cells is also important for clinical application. Here we will present our experimental setup dedicated to local temperature measurement in neuronal cell cultures. The measurement technique is based on optically detected magnetic resonance (ODMR) with the NV centers in the nanodiamonds, suitably engineered to be sensitive and at the same time bio-compatible. We will demonstrate a proof of principle experiment in which we measure the local temperature variation in cultured hippocampal neurons. The temperature sensitivity is 3 K/Hz l/2 . In addition we will show how the nanodiamonds with a size of around 200 nm are internalized by the neurons.