Centro Español de Metrología (CEM) is developing a quantum frequency standard based on trapped calcium ions, marking its entry into the landscape of the second quantum revolution. Optical frequency standards offer unprecedented precision by referencing atomic transitions that are fundamentally stable and immune to environmental drift. However, the challenge of developing such a system from scratch is unaffordable for a medium-sized National Metrology Institute (NMI), which seems to limit the ability of an institute such as CEM to contribute to this field of research. To overcome this, CEM has adopted a hybrid strategy, combining commercially available components with custom integration to accelerate deployment. This paper defines and implements an architecture adapted to the constraints of a medium-size NMI, where the main contribution is the systematic design, selection, and interconnection of the subsystems required to realize this standard. The rationale behind the system design is presented, detailing the integration of key elements for ion trapping, laser stabilization, frequency measurement, and system control. Current progress, ongoing developments, and future research directions are outlined, establishing the foundation for spectroscopic measurements and uncertainty evaluation. The project represents a strategic step toward strengthening national capabilities in quantum metrology for a medium-sized NMI.
Centro Español de Metrología (CEM) is developing a quantum frequency standard based on trapped calcium ions, marking its entry into the landscape of the second quantum revolution. Optical frequency standards offer unprecedented precision by referencing atomic transitions that are fundamentally stable and immune to environmental drift. However, the challenge of developing such a system from scratch is unaffordable for a medium-sized National Metrology Institute (NMI), which seems to limit the ability of an institute such as CEM to contribute to this field of research. To overcome this, CEM has adopted a hybrid strategy, combining commercially available components with custom integration to accelerate deployment. This paper defines and implements an architecture adapted to the constraints of a medium-size NMI, where the main contribution is the systematic design, selection, and interconnection of the subsystems required to realize this standard. The rationale behind the system design is presented, detailing the integration of key elements for ion trapping, laser stabilization, frequency measurement, and system control. Current progress, ongoing developments, and future research directions are outlined, establishing the foundation for spectroscopic measurements and uncertainty evaluation. The project represents a strategic step toward strengthening national capabilities in quantum metrology for a medium-sized NMI.
Quantum technologies make it possible to obtain reproducible standards at any time and place without traceability chain. Centro Español de Metrología (CEM) has been using them for more than 20 years for standards such as voltage and resistance. Nowadays, the second quantum revolution has burst into the field of metrology in a radical way, offering tools and methods with unprecedented capabilities and transforming the classical view of this science, giving the opportunity to overcome limits that were considered unsurpassed. NMIs have a duty and obligation to take this potential and transform it into tangible benefits for science, industry and society. This goal, which is a major challenge for the whole community, is even greater for small and medium-sized NMIs. However, far from being a barrier, this should be an invitation to actively participate in this revolution. CEM has assumed a firm commitment to develop these quantum technologies. This has led to design and implement a comprehensive strategy to completely transform the Institute, from the most classical conception of metrology towards future quantum developments. As part of this strategy, several projects are being carried out with the aim of being an active actor in this global transformation, such as the following: An optical clock based on Calcium 40 ions is being implemented. This device will make use of an ion trap, a state-of-the-art technology which will enhance the capabilities of CEM offering higher levels of accuracy in our realisations. Furthermore, this project aims to establish, with ROA, national and international collaborations that will allow us to contribute to the advancement of this technology in the global scientific community. On another hand, optomechanical sensors have been proposed by the International Thermometry community which, by combining light-mater interaction with the vibrations of the device at the nanometric scale, allows the construction of primary quantum thermometer. CEM has been working in nano-photonic sensors based on photonic crystals in Si. It has been developed a free space-measuring set-up working at 1550 nm from 10 °C to 80 °C for the calibration of photonic integrated circuits (PICs). In addition, CEM is developing an experiment for the materialization of the pressure unit, the Pascal, by refractometry technique. This experiment has the aim of covering the pressure range of 1 Pa to 30 kPa with a target expanded uncertainty (k = 2) of 2 mPa + 10 ppm. This experiment is part of the European integrated metrological infrastructure for a SI-traceable quantum-pascal, and it is expected to be one pillar for primary materializations of other quantities. These developments are part of the ongoing transformation at CEM, with the aim of becoming a key player in the implementation of quantum technologies in metrology. Beyond our progress, the goal is to serve as a showcase and inspiration for all NMIs, regardless of their size or resources, to understand the importance of being part of this revolution. Our aim is to demonstrate that, with a clear strategy and a strong commitment, it is possible to be part of this historic change.
This paper presents the development of an optical frequency standard based on trapped ions within the MADQuantum-CM project, aimed at establishing a quantum communications network in the Community of Madrid. Centro Español de Metrología (CEM) has a key role as a node in this network, contributing to its synchronization through a high-precision frequency reference.The optical frequency standard developed at CEM utilizes the 4 S1/2↔ 3 D5/2transition in40Ca+ions at 729 nm. This system comprises an ion trapping setup using a Paul trap, laser cooling techniques, and an ultra-stable laser to achieve a linewidth in the order of 1 Hz. Additionally, a frequency comb is employed to ensure precise optical frequency measurements.By integrating this frequency standard into the quantum network, highly stable and accurate time references will be able to be distributed, which is essential for quantum communication protocols.
Emerging technologies demand innovations in metrology: • Agility – to keep pace with a rapidly changing technical landscape; • Ability – to develop novel, perhaps first-ever, measurement capabilities; • Comparability and interoperability – across vendors, in commercial timeframes; • Accelerated delivery – because formal standards may be obsolete by the time they’re published. The same quantum technologies that have revolutionized metrology, such as Josephson junctions that are used to create the world’s most accurate voltage standards and ion traps that are used in time and frequency standards, also underpin emerging new markets in quantum computing, networking and sensing – and these quantum-based products require entirely new approaches for measurement and characterization. The world’s metrology institutes have been at the forefront of the quantum revolution, being both early developers and adopters of quantum technologies for metrology, and are uniquely poised to spearhead the development of metrology for quantum – those measurements and related standards needed by industry to underpin quantum-based products. The metrology community is positioned to create trust in the emerging quantum economy due to its long history of objectivity, neutrality and international collaboration; anticipating industry’s needs to be ready with rigorous measurement capabilities as technologies evolve; and its having formed institutions such as the CIPM, BIPM and IMEKO to support collaborative efforts. The BIPM hosted a workshop in March 2024 which brought together 149 participants from 43 NMIs and designated institutes (DIs) from 39 economies, along with industry representatives, to discuss how best to leverage the expertise of the worlds national metrology institutes (NMIs) and their designated institutes (DIs) to accelerate the development and adoption of quantum technologies through measurements and standards. The workshop supported coordinated development and sharing of measurement “best practices” in support of future standardization, the mission of a new collaborative initiative referred to as NMI-Q. The NMI-Q organizing committee, comprised of representatives of the NMIs of Australia, Canada, Germany, Italy, Japan, the United Kingdom, and the United States, are working on a cooperation framework inspired by the successful VAMAS organization, which conducts coordinated pre-standardization work in material science. NMI-Q has initiated early work in quantum photonics, and expects to expand to a breadth of technical areas, such as measurement and characterization to support multiple quantum computing platforms, quantum networking models, and sensing modalities.
Guidelines for developing optical clocks with 10−18 fractional frequency uncertainty
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 1×10−18 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.
Since 1967, the Second has been defined by the CGPM as 9,192,631,770 periods of oscillation corresponding to a transition between two hyperfine levels in a Cesium atom with zero external fields. Key to this definition was the fact that as experimentalists found new and more precise ways to measure this frequency, the definition of the second would become more and more accurate with time. However, in the last 30 years, new technologies based on tunable lasers addressing optical transitions in atoms, ions, and molecules have offered an entirely new approach to defining the Second with significantly higher precision and accuracy. Here, I will show that by trapping thousands of atoms inside a specially engineered optical lattice one can create an extremely accurate frequency standard with 2 orders of magnitude improvement over current Cs standards. Furthermore, I will explain that standards based on this technology are fundamentally more stable than the primary standard by 3 orders of magnitude. Leapfrogging the currently held accuracy records of ion clocks, this work documents the first optical lattice clock to best all other atomic clock implementations, a mere 8 years after the first proof of principle experiments. In this thesis I will describe how we have overcome a number of important systematic uncertainties to achieve these results. This revolution in accuracy and precision opens the door for new experiments utilizing the clock as a probe of quantum many-body physics. To this end, a new apparatus has been designed that combines the unprecedented precision of clock experiments with the amazing progress attained in quantum gas experiments.
Atomic clocks constitute a fundamental tool for time and frequency metrology and their application is widespread in many technological fields. In particular, the International System of units (SI) defines the second on a microwave transition of Caesium atoms. The realization is made by clocks reaching uncertainties of few parts in 10^−16, making the second to be the quantity realized with the smallest uncertainty in the SI. However, a new generation of atomic clocks, called optical clocks, have already demonstrated to surpass Caesium standards both in accuracy and stability. The research performed during my PhD activity has been focused on the development and characterization of Ytterbium (Yb) optical lattice clocks. These systems operate with a large number of ultra-cold neutral atoms having a clock transition in the visible region of the electromagnetic spectrum. The atomic sample is trapped in an periodical optical potential called optical lattice that gives the advantage to interrogate many quantum absorbers for an extended time, with small perturbations, allowing to achieve an unprecedented stability and accuracy. The main experimental work has been carried out in the laboratories of the Istituto Nazionale di Ricerca Metrologica (INRIM) in Turin, where several atomic clocks are present including the Italian primary frequency standard, the Caesium fountain ITCsF2, and where an Yb optical lattice clock is currently being developed. This thesis describes the functioning of the system along with the full characterization of systematic effects, the complete uncertainty budget and its first absolute frequency measurement against the primary frequency standard. The clock exhibited an accuracy of 1.6 × 10^−16 and the comparison with the Cs fountain resulted in a frequency of f171Yb = 518 295 836 590 863.59(31) Hz, limited by the fountain uncertainty. This measurement is in agreement with the ytterbium frequency recommended as a secondary representation of the second in the SI and constitutes the first measurement of a Yb clock in Europe and the second one in the world against a primary frequency standard. Several upgrades have been applied after the absolute measurement. In particular, the design and realization a system capable to frequency stabilize several lasers on a single optical cavity is illustrated. This cavity has been implemented to lock the lasers used to cool and trap the atomic sample at 399, 556 and 759nm using the offset sideband locking technique, a modified version of the Pound–Drever–Hall method that gives an extended frequency tunability. The system proved to be an easy-to-use and reliable tool for the experimental activity showing a linewidth below 300 Hz at 556nm, which is the wavelength that requires the most stringent performance, and a long term drift below 20 kHz per day at 759nm. That is suitable for operating the lattice laser with a light shift uncertainty below 1 × 10−18. During my PhD I have been guest researcher at the National Institute of Standards and Technology (NIST) of Boulder, Colorado, for nine months in 2016. In these laboratories two Yb optical lattice clocks are operative. I worked on the instability measurement of a composite system exploiting the two clocks to suppress the Dick effect, called zero-dead-time (ZDT) clock, which demonstrated a fractional instability of 3 × 10−17 at 1s. The two clocks can also be operated to extend the interrogation time obtaining a spectroscopic feature after 4 s of 120(20)mHz corresponding to a quality factor Q > 4 × 10^15. I also worked on the characterization of several systematic shifts that allowed to complete the uncertainty budget of the clocks at 1.6 × 10^−18. In particular, I contributed to the characterization of lattice light shifts considering the effect of atomic sample temperature and the identification of a metrological regime called operational magic frequency where frequency shifts are insensitive to changes in trap depth.