We report the results of measurements of the total hemispherical emissivity coefficients of the most popular vacuum compatible materials used in designs of optical atomic clocks. The experimental vacuum test chamber was designed to precisely quantify the heat transfer between the material's samples and a calibrated plate that serves as a thermometer. The temperatures of vacuum chamber's internal surfaces were measured and their heat balance equations were solved to designate their emissivities. The simplicity of the experimental set-up allows for analytical calculations of all necessary parameters without use of numerical finite elements methods (FEM). The reported results cover the temperature range between 0.0 and 25.0 degrees C, that is crucial in optical clocks set-ups operating at room temperatures. The linear model of the emissivity change with the temperature is applied to the measured data to make them applicable in further calculations or FEM simulations.

We would like to report on a design of new type of optical atomic clock - a continuous active clock with cold strontium atoms, as well as on progress with lattice clocks in KL FAMO. The active clock design will use blue detuned optical lattice to provide the Lamb-Dicke regime in the optical cavity, where conditions for superradiance emission on the clock transition will be meet. Studies of the blue detuned magic wavelength for 88Sr will be presented together with possibility to create a state dependence, blue detuned optical lattice.

There has been tremendous progress in the performance of optical frequency standards since the first proposals to carry out precision spectroscopy on trapped, single ions in the 1970s. The estimated fractional frequency uncertainty of today's leading optical standards is currently in the $10^{-18}$ range, approximately two orders of magnitude better than that of the best caesium primary frequency standards. This exceptional accuracy and stability is resulting in a growing number of research groups developing optical clocks. While good review papers covering the topic already exist, more practical guidelines are needed as a complement. The purpose of this document is therefore to provide technical guidance for researchers starting in the field of optical clocks. The target audience includes national metrology institutes (NMIs) wanting to set up optical clocks (or subsystems thereof) and PhD students and postdocs entering the field. Another potential audience is academic groups with experience in atomic physics and atom or ion trapping, but with less experience of time and frequency metrology and optical clock requirements. These guidelines have arisen from the scope of the EMPIR project "Optical clocks with $1 \times 10^{-18}$ uncertainty" (OC18). Therefore, the examples are from European laboratories even though similar work is carried out all over the world. The goal of OC18 was to push the development of optical clocks by improving each of the necessary subsystems: ultrastable lasers, neutral-atom and single-ion traps, and interrogation techniques. This document shares the knowledge acquired by the OC18 project consortium and gives practical guidance on each of these aspects.

We report on a photoassociation approach to determine the scattering lengths for 1 So- 3 Po and 3 Po- 3 Po cold collisions. Positions of molecular states, connected with asymptotes of 1 So+ 3 S1 and 3 Po+ 3 S1 states could be determined using a tunable 679 nm laser and atoms trapped in an optical lattice. This approach allows an indirect calculation (without direct probing of 1 S0- 3 P0 molecular state) of the effect of cold collisions on an optical atomic clock operation. We also propose to realize a molecular clock with bosonic ytterbium molecules.

We present an approach to reducing blackbody radiation shift uncertainty in optical lattice clocks based on precise temperature measurements and thermal simulations of heat transfer in vacuum chambers. Two approaches are discussed in this article - the first one is based only on the numerical simulation and the second one contains also analytical solutions of heat transfer problem. The preliminary results of this approach have been discussed.

We describe optical atomic clocks readouts' analysis and provide a recipe for analysing data from transcontinental network made of already existing optical atomic clocks to search for dark-matter signatures. We show how to correlate the data and we discuss methods of computing cross-correlation of more than two readouts. Furthermore, we show how to analyse the data from a network of many clocks to exceed previously reported limits on oscillating massive scalar fields couplings to standard matter.

We report on the first earth-scale quantum sensor network based on optical atomic clocks aimed at dark matter (DM) detection. Exploiting differences in the susceptibilities to the fine-structure constant of essential parts of an optical atomic clock, i.e. the cold atoms and the optical reference cavity, we can perform sensitive searches for dark matter signatures without the need of real-time comparisons of the clocks. We report a two orders of magnitude improvement in constraints on transient variations of the fine-structure constant, which considerably improves the detection limit for the standard model (SM) - DM coupling. We use Yb and Sr optical atomic clocks at four laboratories on three continents to search for both topological defect (TD) and massive scalar field candidates. No signal consistent with a dark-matter coupling is identified, leading to significantly improved constraints on the DM-SM couplings.

We report on the first Earth-scale quantum sensor network based on optical atomic clocks aimed at dark matter (DM) detection. Exploiting differences in the susceptibilities to the fine-structure constant of essential parts of an optical atomic clock, i.e., the cold atoms and the optical reference cavity, we can perform sensitive searches for DM signatures without the need for real-time comparisons of the clocks. We report a two orders of magnitude improvement in constraints on transient variations of the fine-structure constant, which considerably improves the detection limit for the standard model (SM)-DM coupling. We use Yb and Sr optical atomic clocks at four laboratories on three continents to search for both topological defect and massive scalar field candidates. No signal consistent with a DM coupling is identified, leading to considerably improved constraints on the DM-SM couplings.

We report preliminary results of dark mater searches within the worldwide network made of our laboratories. We demonstrate that data routinely collected by our currently operating optical atomic clocks without any further developments of the experimental set-ups may be used to run a global program aimed on searches of dark matter.

We present a proof–of–principle experiment demonstrating the use of atomic optical clocks as a frequency reference in Doppler–limited molecular spectroscopy. We report the determination of an unperturbed line position with a relative uncertainty of 2 × 10-11.

The quality of Very Long Baseline Interferometry (VLBI) radio observations predominantly relies on precise and ultra-stable time and frequency (T&F) standards, usually hydrogen masers (HM), maintained locally at each VLBI station. Here, we present an operational solution in which the VLBI observations are routinely carried out without use of a local HM, but using remote synchronization via a stabilized, long-distance fibre-optic link. The T&F reference signals, traceable to international atomic timescale (TAI), are delivered to the VLBI station from a dedicated timekeeping laboratory. Moreover, we describe a proof-of-concept experiment where the VLBI station is synchronized to a remote strontium optical lattice clock during the observation.

We present current status of work on blackbody radiation impact on ultra-narrow optical resonances. The emissivities of the most popular materials, which are used for construction of vacuum chambers, have been measured in the temperatures close to the room temperature. We have developed a new vacuum system designed exclusively for the emissivity measurements. We have measured the emissivity of samples made from different materials and with different finishing. This data has been also used to perform numerical simulation of temperature distribution of an optical clock vacuum set-up.

We measured absolute frequency of the unperturbed P7 P7 O-2 B-band transition v(0) =434783.5084857(82)GHz and the collisional self-shift coefficient delta =-9.381(62) x 10(-21) GHz/(molecule/cm(3)). With Doppler-limited spectroscopy we achieved the relative standard uncertainty of 2 x 10(-11) on line position, typical for Doppler-free techniques. Shapes of the spectral line were measured with a Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectrometer referenced to an Sr-88 optical atomic clock via an optical frequency comb. (C) 2017 Elsevier Ltd. All rights reserved.

Spectral line shapes can be described by the transport-relaxation equation (TRE). When ab initio collisional operator is incorporated, the TRE needs to be solved numericaly. We report a pure numerical problems encountered during tests of the iterative approach to solving the TRE. As a reference we have used Voigt profile, which can be easily calculated analytically with error function, as well as numerically by solving TRE with simple collisional operator. Our studies lead us to the conclusions about impact of numerical precion and matrix operators dimensions on the accuracy of the calculations.

In direct frequency comb spectroscopy with a VIPA spectrometer, the resolution of the spectrometer is usually insufficient to resolve the comb modes. Thus, one can either filter the modes, reducing the density of spectral elements or cope with the inability to uniquely identify spectral elements with individual comb modes by calibrating the spectrometer itself. Here, we present a way to make use of the inherent frequency accuracy of a stabilized frequency comb to calibrate the spectrometer. We also present a comparison between two commonly used schemes to stabilize the coupling between a frequency comb and cavity resonances: the Pound-Drever-Hall locking scheme and the swept coupling scheme.

We present a cavity-enhanced direct optical frequency comb spectroscopy system with a virtually imaged phased array (VIPA) spectrometer and either a dither or a Pound-Drever-Hall (PDH) locking scheme used for stable transmission of the comb through the cavity. A self-referenced scheme for frequency axis calibration is shown along with an analysis of its accuracy. A careful comparison between both locking schemes is performed based on near-IR measurements of the carbon monoxide ν=3←0 band P branch transitions in a gas sample with known composition. The noise-equivalent absorptions (NEA) for the PDH and dither schemes are 9.9×10(-10) cm(-1) and 5.3×10(-9) cm(-1), respectively.

We report a system of two independent strontium optical lattice standards with 88 Sr probed with a single shared ultra-narrow laser. We achieved frequency stability (frequency between two standards) of 7 × 10 -17 . The absolute frequency of the clock transition can be measured by the use of an optical frequency comb referenced to the UTC(AOS) and UTC(PL) via the 330-km stabilized fibre optic link of the OPTIME network. The 15.5-km stabilized fibre optic link between National Laboratory for Atomic, Molecular, and Optical Physics (KL FAMO) and Toruń Centre for Astronomy made it possible to use the optical clocks as a frequency reference for the 32-metre precise parabolic antenna of the radio telescope in the Toruń Centre for Astronomy participating in the VLBI networks. We report the world's first astronomical VLBI measurements referenced to an optical atomic clock.

We perform cavity-enhanced direct frequency comb spectroscopy with a self-referenced VIPA spectrometer. We compare PDH and dither cavity stabilization in near-IR measurements of carbon monoxide and measure absorption sensitivities of 9.9×10-10 and 5.3×10-9 cm-1 respectively.

This paper presents a detailed accuracy budget of two independent strontium optical lattice clocks at the National Laboratory FAMO (KL FAMO) probed with a single shared ultra-narrow laser. The combined instability of the two frequency standards was 7 x 10(-17) after 10(5)s of averaging.

We report a system of two independent strontium optical lattice standards probed with a single shared ultra-narrow laser. This allows verification of relative stability of both optical standards. The absolute frequency of the clocks can be roughly verified by the use of an optical frequency comb with the GPS-disciplined Rb frequency standard or, more accurately, by a long distance stabilized fiber optic link with the UTC(AOS) and UTC(PL) via the OPTIME network.