Optical clocks provide ultra-precise frequency references that are vital for international metrology as well as for tests of fundamental physics. To investigate the level of agreement between different clocks, we simultaneously measured the frequency ratios between ten optical clocks in six different countries, using fiber and satellite links. This is the largest coordinated comparison to date, from which we present a subset of 38 optical frequency ratios and an evaluation of the correlations between them. Four ratios were measured directly for the first time, while others had significantly lower uncertainties than previously achieved, supporting the advance towards a redefinition of the second and the use of optical standards for international time scales.

Low-power, compact, and narrow-linewidth light sources are crucial for applications such as wavelength- division multiplexing, coherent LIDAR, and high-resolution spectroscopy [1], for which high spectral purity, power efficiency, and small footprint are instrumental. In the 2 μm wavelength range, narrow-linewidth lasers are particularly valuable for gas sensing and remote sensing applications [2]. To this end, hybrid integration of GaSb- based semiconductor optical amplifiers (SOAs) with silicon photonic integrated circuits (PICs) have recently emerged as an attractive solution [3], [4]. Here we present the development of a hybrid distributed Bragg reflector (DBR) laser operating at 2 μm, achieving uncooled continuous wave (CW) operation with a narrow linewidth of 8 kHz at a 0.5 ms timescale and a maximum output power of 17.4 mW [5].

A polarization-switching pulse interleaver is shown to be effective in reducing timing noise due to polarization mode dispersion in time and frequency transfer based on mode-locked lasers and standard single-mode (SM) fibers. In closed-loop time transfer over a 30-km dispersion-compensated fiber link with 300 fs of differential group delay, polarization interleaving reduced the delay variations to <20 fs. The results indicate that the remaining drift is caused by polarization-dependent loss and by AM-to-PM noise conversion in the photodiodes, suggesting the need for a "double-balanced" phase detector in the receiver, i.e., a phase detector balanced in power and polarization. By mitigating the polarization dependence, this work demonstrates a simple approach that can potentially yield sub-femtosecond-level, long-term time transfer in long-haul fiber links utilizing standard single-mode fibers.

We report on establishing the mobile Finnish Seismic Instrument Pool (FINNSIP) that is owned and operated by Finnish academic and research institutions. The pool supports domestic and international collaborative seismic research. At the conclusion of the 2020 to 2024 build-up stage, the instrumentation includes 46 broadband seismometers and digitizers, 5 accelerometers, and 1216 and 71 Geospace and SmartSolo autonomous geophone units, respectively, making FINNSIP one of the largest and most coherent mobile seismic instrument pools in Europe in the public sector. We explain the utilization of the pool instruments and discuss the equipment, facilities, ownership and governance structure, fees, and the management and support system. Through Finland's membership in the Observatories and Research Facilities for European Seismology (ORFEUS) and the Finnish European Plate Observing System (EPOS) node, FINNSIP endorses and implements international data management standards and best practices as promoted in Europe. The importance of appropriate data and computing systems is highlighted by the ~90 TB volume of formatted data that has been collected in 25 large-N projects between October 2021 and December 2024. We summarize a checklist for building, operating, and managing this extensive seismic pool that can inform the planning and establishment of other research infrastructure.

Lasers with long coherence time and narrow linewidth are an essential tool for quantum sensors and clocks. Ultrastable cavities and laser systems are now commercially available with fractional frequency instabilities in the mid 10^-16 range. This document aims to provide technical guidance for researchers starting in the field of ultrastable lasers and to give an outlook toward the next generation of improved ultrastable lasers. These guidelines have arisen from the scope of the EMPIR project “Next generation ultrastable lasers” ( https://www.ptb.de/empir2021/nextlasers ) with contributions from the European project partners.

We demonstrate a cryogenic electro-optic sampling (EOS) setup that allows for the measurement of microwave signals at arbitrary positions on a cryogenic chip-scale device. We use a Josephson Arbitrary Waveform Synthesizer (JAWS) to generate quantum-accurate voltage signals and measure them with the EOS setup, allowing for the calibration of its response, yielding traceability of the microwave measurements to a quantum standard. We use the EOS setup to determine the time-domain response of ultrafast cryogenic photodiodes and the electrical reflection coefficient, i.e., the S11 scattering parameter, in a superconducting transmission line. Finally, we introduce an optical femtosecond pulse source which can be used to study the fidelity of superconducting transmission lines and terminations, as well as reflections from elements like Josephson junction arrays imbedded in them.

A narrow linewidth hybrid integrated distributed Bragg reflector (DBR) laser platform operating at 2 μm wavelength region is demonstrated. The laser architecture comprises AlGaInAsSb/GaSb type-I quantum well reflective semiconductor optical amplifiers butt-coupled to a Si3N4 photonic integrated circuit (PIC), incorporating a narrow-band DBR. The DBR is realized with a long spiral-shaped waveguide structure with periodic circular posts placed adjacent to the waveguide. At room temperature operating conditions, the laser exhibits a maximum continuous wave output power of more than 17 mW for emission near 2 μm. Linewidth properties are analyzed with a heterodyne measurement technique, involving the mixing of the laser signal with a frequency comb phase-locked to an ultra-stable laser. The hybrid laser exhibits a narrow linewidth of ∼8 kHz in 1 ms timescale and ∼50 kHz in 10 ms timescale.

Ultra-stable laser systems are needed for precision measurements, e.g. with optical clocks, where the performance directly depends on the lasers' fractional frequency instability. This instability is fundamentally limited by the thermal noise in the systems' ultra-stable resonators. To reach the thermal noise floor, the technical noise of the laser system must be reduced below the thermal noise limit. This includes, noise resulting from laser power fluctuations, from residual amplitude modulation in the PDH servo or from seismic noise acting on the cavity.

Signal repeaters for fiber-optic communication can be realized with back-to-back connected transceivers. This configuration can provide high gain ( ≈ 30 dB) at low cost, and the needed semiconductor lasers and modulators can be realized for practically any relevant wavelength. Unfortunately, for time and frequency (TF) transfer the uncorrelated wavelength drifts in the transceiver lasers can compromise transfer stability, and device replacement due to failure may result in large time offsets that have to be measured via global navigation satellite services (GNSS). This work demonstrates that good results can nevertheless be obtained with standard telecom dense wavelength-division multiplexing (DWDM) transceivers over long time periods (years). More importantly, a simple wavelength-symmetric repeater is proposed that can be used to cancel the detrimental effects of wavelength drifts and which lessens the need for link recalibrations after transceiver replacements. In a proof-of-concept test setup, timing drift due to wavelength drift of a repeater laser is reduced by approximately two orders of magnitude.

Ultra-stable laser systems are needed for precision measurements, e.g. with optical clocks, where the performance directly depends on the lasers' fractional frequency instability. This instability is fundamentally limited by the thermal noise in the systems' ultra-stable resonators. To reach the thermal noise floor, the technical noise of the laser system must be reduced below the thermal noise limit. This includes, noise resulting from laser power fluctuations, from residual amplitude modulation in the PDH servo or from seismic noise acting on the cavity.

We develop technologies for future cryogenics that enable scaleable electrical quantum technology. In particular, our goal is to replace electrical radio-frequency cables by optical fibers that allow fast data transfer between room temperature and cryogenics without the excessive heat conduction of electrical wiring.

The aim of this paper is to reduce the noise level in the time signals of the Global Navigation Satellite Systems (GNSS). This is done by finding patterns in the Common Generic GNSS Timing Transfer Standard (CGGTTS) data, as the pseudorange residuals in this data appear to include patterns that repeat every day. The reduced noise level allows for easier detection of possible anomalies in the time signals of individual GNSS satellites and hence increases the resilience of the GNSS time measurement. The observed patterns are explainable by multipath, repeating every time the satellite is at a certain position in its groundtrack.

A simple scheme for all-optical time interpolation using spectral interferometry is put forward that is, in principle, capable of single-shot measurements. In this method, the arrival time of optical timing pulses is encoded into the spectrum of a time-stretched supercontinuum via cross phase modulation. The proof-of-concept test setup points toward femtosecond-level absolute timing capabilities with only minor additions to modern optical clockwork.

An ultra-stable light source and optical detection set-up for high-precision measurements of 114 Cd absorption lines is presented. The experimental setup for ultraviolet light at 326.2 nm is described with an aim toward primary thermometry.

The non-reciprocal and dynamic nature of polarization mode dispersion (PMD) in optical fibers can be a problem for accurate time and frequency transfer. Here, a simple, passive solution is put forward that is based on transmitting optical pulses with alternating orthogonal polarization. The fast and deterministic polarization modulation means that the PMD noise is pushed far away from the frequencies of interest. Furthermore, upon reflection from a Faraday mirror at the receiver, the pulses have a well-defined polarization when they return to the transmitter, which facilitates stable optical phase detection and fibre phase compensation. In an open-loop test setup that uses a mode-locked laser and a simple pulse interleaver, the polarization mode dispersion is shown to be reduced by more than two orders of magnitude.

Critical infrastructures are becoming increasingly dependent on accurate and continuous position, navigation, and timing (PNT) services provided by Global Navigation Satellite Systems (GNSS). PNT services are critical for, e.g., stock market, electricity transmission, banking and security information systems, building industry, logistics and transport (maritime and road transport as well as aviation), wireless communications, and rescue services. These critical services will not be available or they will need to rely on backup services if GNSS signals are unavailable in the area. This makes these services vulnerable when it comes to disruption in GNSS signals as a result of natural or intentional interference, or occurrence of unexpected GNSS constellation level problems. This calls for continuous monitoring of the GNSS signal quality so that any anomalies can be detected, isolated, and reported to authorities and a seamless shift to back-up solutions can be made. This study aims at improving the security of supply of the services that rely on GNSS-enabled PNT by the use of emerging Machine Learning methods (such as Deep Learning) for improved situational awareness in GNSS throughout Finland. The study is based on a GNSS-Finland monitoring platform, which uses the permanent GNSS reference network in Finland (FinnRef) to detect and localize the disruptions in the GNSS signals. Using the big data available from GNSS-Finland, Deep Learning (DL) methods will be developed to investigate possible trends in signal quality, and to detect or predict signal anomalies. This will provide an assessment of the continuity and forecast of critical failures in positioning and timing information and thus improve the resilience of critical PNTdependent services and operations in Finland. For the improved resilience of timing services, we also aim to explore solutions for cost-effective, fibre-optic time transfer to a large number of geographical locations as well as develop software-defined-radio-based technologies for monitoring low-frequency timing signals and other signals of opportunity. As a future effort, case studies in critical locations are planned in collaboration with end users, both for monitoring the GNSS signal quality and to explore the potential of using back-up timing services.

HCl measurements in a cleanroom environment are carried out using a cavity ring-down analyser in order to get an estimate of the HCl concentrations that can be expected to be present in semiconductor manufacturing. HCl is a reactive gas and the materials used for gas sampling and transportation have a significant effect on the measurement response time; hence, data on HCl adsorption to different materials is also provided. Finally, a light source operating at 1742 nm that can be used for HCl detection in a multi-gas analyser based on multipass photo-acoustic spectroscopy is presented. Preliminary results indicate that sub-ppb detection levels are achievable using this approach even for HCl.

An optoelectronic module for generating high frequency bipolar current pulses at 4 K was developed. Bipolar signals are generated by connecting a pair of photodiodes in series to coplanar waveguides on silicon. The transmission line pulse propagation was simulated in COMSOL. A planar optical fiber connector was fabricated, where the end fiber faces where angle polished to reflect the beam perpendicular to the fiber. An assembled prototype was fabricated by flip-chip bonding photodiodes to the silicon photodiode carrier. The optical connector was aligned and bonded using epoxy. The prototype was tested using a mode-locked laser producing pulses with sub-20 ps pulses. Bipolar output pulses were generated at 4 K and measured using an oscilloscope. The measured output pulses were affected by ringing and were broader than the reference signal. Comparison with simulations indicate that the ringing was caused by reflections at wirebonds.