In pursuit of a global quantum key distribution (QKD) network, a service based on untrusted nodes on geostationary satellites could offer wide coverage, continuous operation, and enhanced security compared to the trusted node alternative. Although this scenario has been studied for entanglement-based protocols, such an approach would require large-area telescopes both on the ground and in space. In this work, we analyze the performance of two QKD protocols well adapted to this scenario, namely twin-field (TF) and mode-pairing (MP) QKD, which exhibit high resilience to high-loss channels. Leveraging an in-depth simulation of communication channels corrected with adaptive optics, we assess the expected secret key rates for both protocols in a configuration involving two 50 cm telescopes on board the satellite and ground-based telescopes ranging from 20 cm to 1 m in aperture. Our results show that, in the best case and considering realistic detectors, it is possible to achieve secret key rates on the order of a few 100 bit s-1 for both TF and MP-QKD. We show, notably, that secret key generation is potentially feasible even with 20 cm ground telescopes, highlighting the high scalability potential of such a configuration.

We present results of our study devoted to the development of a time correction algorithm needed to precisely synchronize a free-running Rubidium atomic clock with the Coordinated Universal Time (UTC). This R&D is performed in view of the Hyper-Kamiokande (HK) experiment currently under construction in Japan, which requires a synchronization with UTC and between its different experimental sites with a precision better than 100 ns. We use a Global Navigation Satellite System (GNSS) receiver to compare a PPS and a 10 MHz signal, generated by a free-running Rubidium clock, to the Global Positioning System (GPS) Time signal. We use these comparisons to correct the time series (time stamps) provided by the Rubidium clock signal. We fit the difference between Rubidium and GPS Time with polynomial functions of time over a certain integration time window to extract a correction of the Rubidium time stamps in offline or online mode. In online mode, the latest fit results are used for the correction until a new comparison to GPS Time becomes available. We show that with an integration time window of around 104 seconds, we can correct the time stamps drift, caused by the frequency random walk noise and the deterministic frequency drift of the free running Rubidium clock, so that the time difference with respect to GPS Time stays within a +/- 5 ns range in both offline or online correction mode. Presented results could be of interest for other experiments in the field of neutrino physics and multi-messenger astrophysics.

Future quantum communication infrastructures will rely on both terrestrial and space-based links integrating high-performance optical systems engineered for this purpose. In space-based downlinks in particular, the loss budget and the variations in the signal propagation due to atmospheric turbulence effects impose a careful optimization of the coupling of light in single-mode fibers required for interfacing with the receiving stations and the ground networks. In this work, we perform a comprehensive study of the role of adaptive optics (AO) in this optimization, focusing on realistic baseline configurations of prepare-and-measure quantum key distribution (QKD), with both discrete and continuous-variable encoding, and including finite-size effects. Our analysis uses existing experimental turbulence datasets at both day and night time to model the coupled signal statistics following a wavefront distortion correction with AO, and allows us to estimate the secret key rate for a range of critical parameters, such as turbulence strength, satellite altitude and ground telescope diameter. The results we derive illustrate the interest of adopting advanced AO techniques in several practical configurations.

Free-space optical satellite-earth links often operate at the SNR limit. We show that under low SNR conditions it is more favorable to operate at higher speeds with lower order modulation formats. This follows from the Shannon capacity limit formula where the transmission bandwidth has a linear impact, whereas the SNR has a logarithmic effect. In practice, bandwidth limitations from the hardware need to be considered making capacity maximization a joint optimization. Here we experimentally substantiate these findings by e.g., comparing two 128 Gbit/s signals - encoded either as DP 64 GBd 2 PAM or a DP 32 GBd 4 PAM signal. It is found that the 64 GBd 2 PAM performs better for the same bit-error rate. To facilitate highest speed operation we employ a packaged plasmonic modulator with a 3 dB bandwidth > 110 GHz. The plasmonic modulator also enabled us to send up to 160 GBd 2 PAM signals, achieving to the best of our knowledge the highest symbol rate in any free-space optical communication link. Reaching an achievable information rate of 276 Gbit/s. Even higher rates of 424 Gbit/s were achieved by employing a DP 128 GBd 4 PAM signal. This shows that once hardware bandwidth limitations come in place it is more favorable to increase the modulation complexity. Furthermore, we have shown that plasmonic organic hybrid modulators can withstand space radiation with only minor degradations. Lastly, the conditions during the experiment have been investigated and have been shown to constitute a worst-case scenario for earth-GEO feeder links.

Future quantum networks will be composed of both terrestrial links for metropolitan and continent-scale connections and space-based links for global coverage and infrastructure resilience. However, the propagation of quantum signals through the atmosphere is severely impacted by the effects of turbulence. This is even more the case for entanglement-based quantum communication protocols requiring two free-space channels to be considered simultaneously. In this work, we assess the advantage of turbulence mitigation by adaptive optics, in particular during daytime link operation, so as to increase the coupling of the received signal into an optical fiber. We show in particular that this improves the performance of entanglement-based quantum key distribution by up to a few hundred bits per second when compared with the uncorrected scenario

SummaryT-REFIMEVE is a new project funding the infrastructure REFIMEVE, in which one of the objectives is the deployment of a White Rabbit network at the French national scale. Here we report on our current status of the deployment of a White Rabbit network disseminating UTC(OP) to five academic users over a maximum distance of 40 km. We report loop-back measurements and compare results obtained with time transfer and frequency transfer, and discuss the results.

Plasmonic modulators have been assessed for operation up to 200 GBaud in a turbulent 53 km free-space-optical link. They are shown to withstand space radiation and large temperature ranges making them ideal for space applications.

These last years have seen a raising interest for ground to GEO satellites optical very high throughput links, i.e.GEOfeeder links, or GEO-FL.However, despite their potential, these applications have to overcome atmospheric turbulence, which requires the development of mitigation techniques, such as adaptive optics (AO).In the case of GEO-FL, AO performance is limited by the Point-Ahead Angle (PAA) induced anisoplanatism.We describe here how our feedback on our field experiments helped us to design ONERA's AO-compensated ground station, FEELINGS, and the status of said ground station in the fall of 2022.

SummaryWe report the performance of REFIMEVE, a national metrological network of optical fiber links, using the academic fiber network. It enables the coherent dissemination of time and/or frequency reference signals from LNE-SYRTE to around 15 labs and more than 30 in the future. We will show the latest extensions of the network in the Paris urban area and all over France, the architecture required for such al network, and the progress in terms of robustness and uptime. We will also present some applications to precision measurements, in particular ultra-high-resolution molecular spectroscopy in the mid-infrared spectral range.

Abstract Free-space optical (FSO) communication technologies constitute a solution to cope with the bandwidth demand of future satellite-ground networks. They may overcome the RF bottleneck and attain data rates in the order of Tbit/s with only a handful of ground stations. Here, we demonstrate single-carrier Tbit/s line-rate transmission over a free-space channel of 53.42 km between the Jungfraujoch mountain top (3700 m) in the Swiss Alps and the Zimmerwald Observatory (895 m) near the city of Bern, achieving net-rates of up to 0.94 Tbit/s. With this scenario a satellite-ground feeder link is mimicked under turbulent conditions. Despite adverse conditions high throughput was achieved by employing a full adaptive optics system to correct the distorted wavefront of the channel and by using polarization-multiplexed high-order complex modulation formats. It was found that adaptive optics does not distort the reception of coherent modulation formats. Also, we introduce constellation modulation – a new four-dimensional BPSK (4D-BPSK) modulation format as a technique to transmit high data rates under lowest SNR. This way we show 53 km FSO transmission of 13.3 Gbit/s and 210 Gbit/s with as little as 4.3 and 7.8 photons per bit, respectively, at a bit-error ratio of 1 ∙ 10−3. The experiments show that advanced coherent modulation coding in combination with full adaptive optical filtering are proper means to make next-generation Tbit/s satellite communications practical.

Optical feeder links (OFL) are expected to become part of future Very High Throughput Satellite (VHTS) systems in response to the growing demand for higher capacity and lower costs. H2020 VERTIGO (Very High Throughput Satellite Ground Optical Link) project was set to prove key optical communication technologies and to address: 1) Throughput increase with high spectral and power efficiencies. 2) Higher optical power generation and delivery. 3) Atmospheric turbulence mitigation by optical and digital processing. Transmit and receive optical communication models were developed in rack units for assessing, in laboratory and outdoor trials, their intrinsic performance, robustness against atmospheric turbulence and compatibility with other technologies. The models for 25 Gbps OOK/DPSK and RF analog modulation with optically pre-amplified direct or differential detection are reported with the achieved performance. An atmospheric channel emulator fed with time series established by simulations was used to mimic the propagation losses and fading of the optical signal coupled into the receiver. Both the downlink and uplink under weak or strong turbulence were emulated. For digital transmission experiments, the performance metrics include BER curves, detection sensitivity and power penalty. State-of-the-art sensitivities were achieved especially under 25 Gbps DPSK. For RF analog transmission, the performance metrics were constellation diagrams and Error Vector Magnitude (EVM) measured for various modulations from QPSK to 64-QAM. Are reported the results of optical transmission experiments first performed in the laboratory under static and dynamic propagation channels, then in the outdoor trial successfully carried out in July between Jungfraujoch and Zimmerwald in Switzerland.

We demonstrate the gain brought by adaptive optics for space-ground QKD links. Refined modeling of turbulence, adaptive optics and QKD ,including finite-size effects, shows improvement by several orders of magnitude of the secret key rate.

ONERA is currently developing an experimental Optical Ground Station (OGS), FEELINGS, dedicated firstly to GEO-feeder links to investigate GEO-feeder link optimization and related scientific and experimental issues, and to pave the way of future OGS, with high capacity and operability. Extension to LEO links is already included. We present the FEELINGS OGS design and current status. We discuss how critical scientific issues related to GEO-feeder links will be addressed through it.

HARMONI is the first light, adaptive optics assisted, integral field spectrograph for the European Southern Observatory’s Extremely Large Telescope (ELT). A work-horse instrument, it provides the ELT’s diffraction limited spectroscopic capability across the near-infrared wavelength range. HARMONI will exploit the ELT’s unique combination of exquisite spatial resolution and enormous collecting area, enabling transformational science. The design of the instrument is being finalized, and the plans for assembly, integration and testing are being detailed. We present an overview of the instrument’s capabilities from a user perspective, and provide a summary of the instrument’s design. We also include recent changes to the project, both technical and programmatic, that have resulted from red-flag actions. Finally, we outline some of the simulated HARMONI observations currently being analyzed.

A 1 Tbit/s 53km single channel free-space optical (FSO) link is demonstrated. High bandwidth, high order modulation formats and advanced adaptive optics are utilized. We show that the absence of a nonlinear-Shannon limit in combination with adaptive optics enables record data-transmission with low link failures.

Optical technologies are extremely competitive candidates to achieve very-high throughput links between ground and GEO satellites; however, their feasibility relies on the ability to mitigate channel impairments due to atmospheric turbulence. For that purpose, Adaptive Optics (AO) has already proved to be highly efficient on the downlink. However, for the uplink, anisoplanatism induced by point-ahead angle (PAA) compromises AO pre-compensation efficiency to an extent that depends on propagation conditions. The ability to properly assess the anisoplanatism impact in a wide variety of conditions is thus critical in designing the optical ground terminals. In this paper, we demonstrate the consistency of experimental coupled flux statistics with results coming from performance and end-to-end models, on an AO pre-compensated 13 km slant path in Tenerife. This validation is demonstrated in a wide variety of turbulence conditions, hence consolidating propagation channel models that are of critical importance for the reliability of future GEO feeder links. We then compare experimental results to theoretical on-sky performance, and discuss to what extent such slant path or horizontal path experiments can be representative of real GEO links.

The FEEDELIO experiment aims at demonstrating adaptive optics capability to mitigate the atmospheric turbulence channel disturbances in a GEO-Feeder relevant environment. In addition to the bi-directional data acquired in presence of anisoplanatism, this measurement campaign was an opportunity to assess the optical quality of the bench and to improve calibration procedures. To this end, we conducted a reciprocity experiment. We measured a channel reciprocity up to 95% and identified, through numerical simulations, that residual non common path aberrations were the primary source limiting the effective reciprocity.

To concurrently cope with the scarcity of RF frequency bands, the growing capacity demand and the required lower cost of the ground segment, Very High Throughput Satellites systems must rely on new technical solutions. Optical feeder links are considered as a promising alternative to surpass classical RF technology, offering assets inherent to optical technologies (large bandwidth, no frequency regulation, low beam divergence, components availability). Nevertheless the potential of this technology shall not conceal the remaining challenges to be overcome to make it relevant for operational missions : clouds, turbulence, power generation and high efficiency modulations. VERTIGO (Very High Throughput Satellite Ground Optical Link) is a 3-year H2020 project funded by the European commission and started mid-2019 focusing on the optical link itself regardless of site diversity aspect and aiming at demonstrating in a ground demonstration required technologies to implement very high capacity optical feeder links. In particular, VERTIGO is built on 3 pillars each addressing a key issue for the implementation of optical feerder links: 1) Throughput increase through the use of advanced schemes with high spectral and power efficiency compared to current modulations used in space, as well as RF-over-Fiber approach. 2) High optical power generation to close the demanding link budgets by developing on-board and ground means to raise the transmitted optical power, not only based on amplifier power increase, but also on incoherent/coherent power combining. 3) Opto-mechanical and digital techniques for the mitigation of atmospheric propagation impairments, to make full use of throughput and power increases. Several demonstrations in-flight or on-ground already demonstrated separately key aspects (atmospheric propagation and impairments mitigation techniques, modulation format, high power…), for the implementation of optical (feeder) links. These aspects are closely linked since the solutions to each of them are necessary but not sufficient to allow for high throughput transmissions. VERTIGO concept is to address each key issue with at least one solution and to combine them in an unprecedented manner. To reach these objectives, VERTIGO will lean on a highly skilled consortium composed of : CREONIC, ETH Zürich, Fraunhofer HHI, Gooch and Housego, Leo Space Photonics RD, ONERA, Thales Research and Technology, Thales Alenia Space in France and Switzerland. This paper will present the VERTIGO project and its status.

Mitigation of turbulence-induced disturbances is crucial for high data rate optical links in the atmosphere. Sensorless adaptive optics, based on the optimization of the coupling in a single mode fiber, is a promising solution as it circumvents the limitations of conventional wavefront sensing in strong perturbations. We propose the use of a spatial multiplexer to reduce the bandwidth of the temporal modulation required with such a technique. In this approach, after correction by a deformable mirror, the residual perturbations are analyzed thanks to the multiplexer. The concept and first results of laboratory tests are presented.