We report a direct demonstration of quantum-enhanced sensing in the Fourier domain by comparing single- and two-photon interference in a fiber-based interferometer under strictly identical noise conditions. The simultaneous acquisition of both signals provides a common-mode reference that enables a fair and unambiguous benchmark of quantum advantage. Spectral analysis of the interferometric outputs reveals that quantum correlations do not increase the amplitude of the modulation peak, but instead lower the associated noise floor, resulting in the expected 3 dB improvement in signal-to-noise ratio. This enhancement persists in the sub-shot-noise regime, where the classical signal becomes buried in the spectral background while the two-photon contribution remains resolvable. These observations establish Fourier-domain quantum super-sensitivity as an operational and broadly applicable resource for precision interferometric sensing.

Long-wave infrared (LWIR) wavelengths are expected to provide a better robustness against meteorological perturbations as well as an increased covertness for hard target FMCW LiDAR. However, the implementation and modulation of LWIR sources and detectors constitutes an important challenge. With a custom quantum cascade detector, we demonstrate for the first time a QCL-based FMCW ranging system up to 50 m on an outdoor static target, with a < 2 % relative precision. It relies on the linear optical frequency modulation of the QCL, up to 8.4 GHz in 65 µs with 0.03 % of nonlinearities. Furthermore, through a novel method of self-mixing interferometry, we exploit the laser perturbations caused by optical feedback to perform FMCW ranging with fewer components. Similar results are obtained with both architectures, opening the way to low-complexity LWIR FMCW systems, where the low maturity of optical isolators and components makes it a critical asset.

This paper presents progress in the development of crystalline waveguides to enable efficient and integrated quantum memories for long-distance quantum communication. The primary aim is to tackle the difficulties presented by the weak light-matter interaction strength observed in rare-earth ion (REI) doped crystals, specifically in Yttrium Orthosilicate (YSO), which limits optical storage efficiencies. In contrast to other approaches for photonic integration, our method utilizes traditional lithography and dry-etching techniques. This approach maintains the beneficial properties of bulk crystals while offering design adaptability. We refined processes such as bonding, lapping, polishing, and dry-etching to achieve the optimal YSO waveguide structure, as shown in our simulations. This study paves the way for completely integrated quantum repeaters and also improves the integration of other quantum technologies based on REI-doped crystals, including quantum computing, single-photon generation, RF spectral analysis, and microwave-to-optical quantum transduction.

We present a comprehensive study of spin relaxation dynamics at cryogenic temperatures in a rare-earth-doped crystal used for quantum memory applications: ^171Yb:Y_2SiO_5. Spin relaxation is indeed a major limiting factor for both the efficiency and storage time of quantum memory protocols based on atomic frequency combs in rare-earth materials. The relaxation dynamics among the four ground-state hyperfine levels were simultaneously investigated by optically perturbing the spin population distribution and monitoring its return to thermal equilibrium through optical absorption spectroscopy. By applying different types of perturbations, we were also able to distinguish between two types of relaxation processes, induced by spin-phonon and spin-spin interactions. Below 1 K, we observed that the re-thermalization of the Yb^3+ ion population takes several hours, driven solely by direct phonon absorption or emission. However, the effective lifetime of individual spin states is much shorter - on the order of several seconds in low-doped (2 ppm) samples and of milliseconds in 10 ppm samples - due to spin-spin interactions. These findings provide valuable guidelines for optimizing doping levels and operating temperatures in rare-earth-doped crystals for quantum applications. Notably, they suggest that atomic frequency combs with lifetimes of several hours could be realized using ^171Yb:Y_2SiO_5 crystals with slightly less than 2 ppm doping and operating near 1 K.

Recent developments in quantum technologies involving atomic systems highlight the need for complex optical waveforms at visible wavelengths, where available wideband modulation devices able to handle high optical power are hard to obtain. A promising solution uses sum frequency generation (SFG) with a telecom wavelength laser and a pump laser to create wideband optical waveforms in the visible. However, during this frequency conversion, additional photons from parasitic nonlinear processes can be generated, which may disrupt the atomic systems afterwards. This study presents a qualitative and quantitative analysis of the parasitic noise produced during the SFG process, aiming to produce optical waveforms for programming and controlling solid-state quantum memories.

Optical heterodyne generation systems for communications and spectroscopy applications require laser sources with high-frequency stability and significantly reduced phase noise. In this paper, we present an optimized configuration employing an Optical Frequency Locked Loop (OFLL) based on an unbalanced fiber interferometer. We demonstrate frequency stabilization and frequency tuning of two Integrated Tunable Laser Assembly (ITLA) telecom lasers by combining analog and digital feedback loops, we show that significant optical phase noise reduction can be achieved simultaneously with frequency stabilization. Additionally, we demonstrate that the same interferometric setup can effectively measure optical phase noise and the optical frequency modulation transfer function, for which we provide measurements for one ITLA laser. Finally, we present results on the stabilization of the fiber interferometer itself using an optical reference, aiming to further enhance the long-term stability of the frequency-locked lasers. The simplicity, robustness, and effectiveness of the proposed approach highlight its potential for high-frequency communication systems and advanced spectroscopic applications.

The constant evolution of wireless communication systems, where increasingly greater capacity of data volume management is required, has led many researchers to explore the millimeter-wave terahertz bands (0.1-10 THz). The generation of data signals at such frequencies often rely on optical techniques, among which the heterodyning of two lasers offers among other advantages a wide frequency tunability of the high frequency carrier. The frequency stability of the carrier is among the key parameters for the performances of a communication system. We report here on a scheme where two narrow-linewidth laser sources are stabilized on a single fiber interferometer, following a double optical frequency locked loop (OFLL) architecture. We show that with this simple setup involving only low-frequency electronics, the frequency stability of the heterodyne signal is improved from a few tens of MHz to below 100 kHz, while it can be tuned from a few tens of MHz up to a few THz.

171Yb3+-doped Y2SiO5 (YSO) crystals are a promising platform for optical quantum memories in longdistance quantum communications. The relevance of this material lies in 171Yb long optical and spin coherence times, along with a large hyperfine splitting, enabling long quantum storage over large bandwidths. Mechanisms affecting the optical decoherence are, however, not precisely known, especially since low-temperature measurements have so far focused on the 2 to 4 K range. In this work, we performed two- and three-pulse photon echoes and spectral hole burning to determine optical homogeneous linewidths in two 171Yb:YSO crystals doped at 2 and 10 ppm. Experiments were performed in the 40 mK to 18 K temperature range, leading to linewidths between 320 Hz, among the narrowest reported for rare-earth ions, and several MHz. Our results show that above similar to 6 K, the homogeneous linewidth I'h is mainly due to an elastic two-phonon process which results in a slow broadening with temperature, with I'h reaching only 25 kHz at 10 K. At lower temperatures, interactions with 89Y nuclear spin flips, paramagnetic defects or impurities, and also Yb-Yb interactions for the higher concentrated crystal are likely the main limiting factor to Ph. In particular, we conclude that the direct effect of a spin and optical excited state lifetime is a minor contribution to optical decoherence in the whole temperature range that is studied. Our results indicate possible paths and regimes for further decreasing homogeneous linewidths or maintaining narrow lines at higher 171Yb concentration.

Rare earth doped crystals are a promising platform for long-distance quantum communications, where they can perform quantum light storage. Among different systems, ${}^{171}$Yb${}^{3+}$-doped Y${}_{2}$SiO${}_{5}$ (YSO) crystals stand out because of long optical and spin coherence times, and large hyperfine splitting, enabling long-time storage over large bandwidths. Here, the authors study the mechanisms affecting the optical decoherence in a broad range of temperatures (40 mK to 18 K) using high-resolution spectroscopic techniques, and identify ways to further improve the crystal's properties.

Optical manipulation of quantum systems requires stable laser sources able to produce complex waveforms over a large frequency range. In the visible region, such waveforms can be generated using an acousto-optic modulator driven by an arbitrary waveform generator, but these suffer from a limited tuning range typically of a few tens of MHz. Visible-range electro-optic modulators are an alternative option offering a larger modulation bandwidth, however they have limited output power which drastically restricts the scalability of quantum applications. There is currently no architecture able to perform phase-stabilized waveforms over several GHz in the visible or near infrared region while providing sufficient optical power for quantum applications. Here we propose and develop a modulation and frequency conversion set-up able to deliver optical waveforms over a large frequency range, with a high spurious extinction ratio, scalable to the entire visible/near infrared region with high optical power. The optical waveforms are first generated at telecom wavelength and then converted to the emitter wavelength through a sum frequency generation process. By adapting the pump laser frequency, the optical waveforms can be tuned to interact with a broad range of optical quantum emitters or qubits such as alkali atoms, trapped ions, rare earth ions, or fluorescent defects in solid-state matrices. Using this architecture, we were able to detect and study a single erbium ion in a nanoparticle. We also generated high bandwidth signals at 606 nm, which would enable frequency multiplexing of on-demand read-out Pr3+:Y2SiO5 quantum memories.

^171Yb^3+-doped Y_2SiO_5 crystals are a promising platform for optical quantum memories in long-distance quantum communications. The relevance of this material lies in ^171Yb long optical and spin coherence times, along with a large hyperfine splitting, enabling long quantum storage over large bandwidths. Mechanisms affecting the optical decoherence are however not precisely known, especially since low-temperature measurements have so far focused on the 2 to 4 K range. In this work, we performed two- and three-pulse photon echoes and spectral hole burning to determine optical homogeneous linewidths in two 171 Yb:YSO crystals doped at 2 and 10 ppm. Experiments were performed in the 40 mK to 18 K temperature range, leading to linewidths between 320 Hz, among the narrowest reported for rare-earth ions, and several MHz. Our results show that above 6 K the homogeneous linewidth is mainly due to an elastic two-phonon process which results in a slow broadening with temperature, the homogeneous linewidth reaching only 25 kHz at 10 K. At lower temperatures, interactions with ^89Yb nuclear spin-flips, paramagnetic defects or impurities, and also Yb-Yb interactions for the higher concentrated crystal, are likely the main limiting factor to the homogeneous linewidth. In particular, we conclude that the direct effect of spin and optical excited state lifetime is a minor contribution to optical decoherence in the whole temperature range studied. Our results indicate possible paths and regimes for further decreasing the homogeneous linewidths or maintaining narrow lines at higher ^171Yb concentration.

$^{171}$Yb$^{3+}$-doped Y$_2$SiO$_5$ crystals are a promising platform for optical quantum memories in long-distance quantum communications. The relevance of this material lies in $^{171}$Yb long optical and spin coherence times, along with a large hyperfine splitting, enabling long quantum storage over large bandwidths. Mechanisms affecting the optical decoherence are however not precisely known, especially since low-temperature measurements have so far focused on the 2 to 4 K range. In this work, we performed two- and three-pulse photon echoes and spectral hole burning to determine optical homogeneous linewidths in two 171 Yb:YSO crystals doped at 2 and 10 ppm. Experiments were performed in the 40 mK to 18 K temperature range, leading to linewidths between 320 Hz, among the narrowest reported for rare-earth ions, and several MHz. Our results show that above 6 K the homogeneous linewidth is mainly due to an elastic two-phonon process which results in a slow broadening with temperature, the homogeneous linewidth reaching only 25 kHz at 10 K. At lower temperatures, interactions with $^{89}$Yb nuclear spin-flips, paramagnetic defects or impurities, and also Yb-Yb interactions for the higher concentrated crystal, are likely the main limiting factor to the homogeneous linewidth. In particular, we conclude that the direct effect of spin and optical excited state lifetime is a minor contribution to optical decoherence in the whole temperature range studied. Our results indicate possible paths and regimes for further decreasing the homogeneous linewidths or maintaining narrow lines at higher $^{171}$Yb concentration.

Manipulation of optically addressable quantum emitters need stable laser sources able to perform complex and high bandwidth waveforms from visible range to infrared. Such waveforms can be generated using an acoustic-optical modulator; however with a limited bandwidth. Here we present a laser source able to deliver high-bandwidth and low noise optical signals at multiple frequencies over a span of a few GHz (scalable to 100 GHz), with a high extinction ratio with respect to all other spurious optical signals. The central wavelengths can be tuned to match the resonance of different optical quantum emitters in the visible or IR.

We demonstrate, for the first time, Ramsey CPT spectroscopy with a miniature electro-optical bench associated to a dual-frequency generator based on combined optical injection locking and optical phase locking techniques Preliminary results show Ramsey CPT resonance with a contrast of 2% at the Cesium D2 line. Frequency difference locking loops lead to a contribution of Dick effect to fractional frequency stability lower than 1.7x10-13 at 1 s, in line with targeted clock stability of 5x10-13 at 1 s.

We developed and implemented a miniature electro-optical bench for the stabilization of a dual-frequency laser beam for high-contrast CPT interrogation of cesium.Preliminary results of optical intensity and wavelength simultaneous stabilizations give respective noise reduction of 15 dB and 60 dB at low frequencies.These performances are in line with targeted clock stability of 5×10 -13 at 1 s.For longer time scales, a study of the optical power stability shows a drastic reduction of power fluctuations and highlights the temperature sensitivity of the optical components.

We present our latest results related to the development of solid-state dual-frequency lasers for the generation of stable, ultra-low phase noise millimeter-wave and terahertz signals. After presenting the architecture of such lasers delivering sub-Hz Lorentzian linewidth signals, and their stabilization setups based on electro-optical frequency division, we describe the specific setup that we have assembled to further study the origin of the optical and beatnote phase noise. It includes a specific laser setup and a custom-made optical phase noise test bench. Preliminary results are presented, and perspectives are discussed.

Rare earth ions are actively investigated as optically addressable spin systems for quantum technologies thanks to their long optical and spin coherence lifetimes. 171Yb3+, which has 1/2 electron and nuclear spins, recently raised interest for its simple hyperfine structure that moreover can result in long coherence lifetimes at zero magnetic field, an unusual property for paramagnetic rare earth ions. Here, we report on the optical inhomogeneous and homogeneous linewidths in 171Yb3+:Y2SiO5 (site 2) for different doping concentrations. While inhomogeneous linewidth is not correlated to 171Yb3+ concentration, the homogeneous one strongly decreases between 10 and 2 ppm doping level, reaching 255 Hz at 3 K. This is attributed to a slowing down of 171Yb3+ ground state spin flip-flops.

Single crystals of 171Yb3+:Y2SiO5 are promising systems for quantum technologies as an efficient optical to spin interface. This is due to the long optical and spin coherence lifetimes, or narrow homogeneous linewidths, observed in this material at zero magnetic field. Inhomogeneous linewidths are also of importance since they affect peak absorption, available bandwidth or spin–spin interactions. In this work, we investigate inhomogeneous linewidths in a series of 171Yb3+:Y2SiO5 single crystals grown by the Czochralski method with the purpose of identifying the broadening mechanisms in the regime of ultra-low doping, i.e. in the ppm range. In a series of samples obtained with varying doping level, growth atmosphere, and cut at different locations in a crystal boule, optical inhomogeneous linewidths between 0.6 and 1.3 GHz are observed. This study suggests that these variations are due to changes in dislocation densities. Spin inhomogeneous linewidths, recorded using electron paramagnetic resonance, did not show the differences between samples observed in the optical domain. They were otherwise found to vary as a function of magnetic field orientation, enabling determination of directions that minimize e.g. detrimental spin–spin interactions. These results suggest ways to reduce optical inhomogeneous linewidths in 171Yb3+:Y2SiO5, which is of interest for high efficiency quantum memories and optical to microwave quantum transducers.

Recent progress in the realization of high-quality optical resonators and waveguides along with the possibility to incorporate rare-earth ions, make lithium niobate a promising material to build integrated platforms for quantum information processing. 171Yb3+ is a particularly attractive system because it can show long coherence lifetimes at zero magnetic field thanks to transitions insensitive to magnetic field fluctuations. In this paper, we investigate the optical and spin properties of 171Yb3+ ions in LiNbO3 bulk crystals as well as implanted waveguides. Using hole-burning spectroscopy and optically detected magnetic resonance, we studied ground and excited state hyperfine structures and probed optical and spin spectral holes. Importantly, the hole linewidths suggest that part of the ions in the waveguides are in a similar environment as in the bulk sample. We furthermore characterized spin population relaxation and coherence lifetimes of 171Yb3+ ions in the bulk crystal at temperatures between 50 mK and 9 K. At low temperatures, T2 up to 9.5 ??s (34 kHz homogeneous linewidth) and spin relaxation rates as long as ???100 ms were measured. Our results show that 171Yb3+:LiNbO3 is a system that exhibits narrow optical homogeneous linewidths over a 50 GHz bandwidth together with an electron spin degree of freedom. This is of interest for a variety of applications in integrated quantum photonics such as quantum memories or quantum processors.