Network Clock Research Articles

Enable Precise Timing with Clock Ensemble in Packet Networks

The need for accurate and reliable time is increasingly important for network infrastructures with precise timing requirements. Traditional time distribution in networks is carried out through dedicated synchronization networks, whose networking mode may limit the applicability of time synchronization for emerging applications in terms of synchronization performance, cost-effectiveness, and reliability. This paper introduces a new concept of Network Clock Ensemble (NCE) and explores the feasibility of constructing an innovative mechanism for network time synchronization. Specifically, an NCE scheme is proposed based on a software-defined time synchronization network architecture to obtain time accuracy and stability by forming a synergistic effect between clocks in packet networks. The NCE model and method are used to reveal the precise timing mechanism. Simulation experiments and numerical analysis results show that our scheme can achieve significant performance improvements in terms of time accuracy and stability. Furthermore, with the improvement of the network time synchronization performance, the NCE results gradually approximate the Clock Ensemble (CE) results in the actual test, further verifying the effectiveness and feasibility of the proposed scheme.

Методика формирования комплексной системы синхронизации и доставки шкалы времени для крупной распределенной системы технологического назначения

Purpose: To develop scientific and technical solutions for constructing a comprehensive synchronization system and delivery of the time scale, which includes subsystems of network clock synchronization (NCS) and unified time (UT), as a unified timekeeping system addressing the challenges of frequency, time, and phase synchronization in large-scale distributed technological purpose systems. Methods: Research methods are based on the fundamental provisions of the theory of systems and the theory of decision-making. Results: A methodology for the formation of a comprehensive synchronization system and the delivery of a time scale is proposed, which is characterized by an integrated approach to building unified time and frequency systems in the communication networks of the technological segment, the independence of the types of generator equipment used and the means of delivery of NCS and UT signals, as well as meeting the requirements of potential consumers of frequency and time services. The proposed methodology for the formation of a comprehensive synchronization system and delivery of a time scale will allow to reasonably approach building systems for delivering unified time and frequency signals in complex heterogeneous communication networks. Practical significance: Solutions based on the presented methodology will allow the provision of an infrastructure independent of external communication operators to ensure the unity of network clock synchronization and unified time. This is necessary for the correct operation of telecommunication equipment of the technological communication network. The methodology will also meet the needs for providing a full range of synchronization and unified time signal services to existing and prospective consumers. Additionally, it will enable a gradual transition of the organization of communication networks from channel-swithching to packet-oriented technologies due to a flexible approach to the routes and transmission systems used for the distribution of synchronization signals and unified time.

Precisely Synced Clocks Pave the Way for New Science

Researchers working on Hawaiian mountain tops precisely synchronized two clocks with unprecedented efficiency, demonstrating the potential to create networks of such clocks for geodesy and more.

The human circadian system: physiology, pathophysiology and interactions with sleep and stress reactivity

The dramatic fluctuations in the energy demands of living organisms by the rhythmic succession of night and day on our planet has prompted a geophysical evolutionary need for a biological temporal organization necessary for maintenance of homeostasis and adaptation to environmental changes across phylogeny. The intrinsic circadian system (CS) represents a highly conserved and complex internal biological "clock", adjusted to the 24-hour rotation of the earth about itself. This system creates and maintains cellular and organismal rhythmicity and enables a nyctohemeral coordination of multi-level physiologic processes, ranging from gene expression to behaviour. The suprachiasmatic nucleus (SCN) of the hypothalamus is the primary pacemaker of the circadian system of the organism, while a ubiquitous peripheral oscillating network of cellular molecular clocks participates in a complex circadian hierarchy. A critical loss of this harmoniously timed circadian order at different organizational levels is defined as "chronodisruption", a condition that may alter the fundamental properties of basic homeostatic systems at molecular, cellular and organismal levels, and lead to a breakdown of biobehavioral adaptive mechanisms, resulting in maladaptive stress regulation and increased sensitivity and vulnerability to stress. Chronodisruption has been linked to neuroendocrine, immune, cardiometabolic and autonomic dysregulation, with blunted diurnal rhythms, specific sleep pattern pathologies and cognitive deficits, as well as with altered circadian gene expression. This condition may, thus, play a central role in the development of mental and somatic disease. Nevertheless, circadian and sleep disturbances are often clinically considered as "secondary" manifestations in most disorders, neglecting the potentially important pathophysiological role of CS. Understanding the pathophysiologic mechanisms of circadian dysregulation and their role in stress-related, systemic disease could provide new insights into disease mechanisms and could help advance chronobiological treatment possibilities and preventive strategies in populations at risk.

Cognition and Consciousness Entwined.

We argue that cognition (information processing) and internal phenomenological sensations, including emotions, are intimately related and are not separable. We aver that phenomenological sensations are dynamical "modes" of firing behaviour that (i) exist over time and over large parts of the cortex's neuron-to-neuron network and (ii) are consequences of the network-of-networks architecture, coupling the individual neuronal dynamics and the necessary time delay incurred by neuron-to-neuron transmission: if you possess those system properties, then you will have the dynamical modes and, thus, the phenomenological sensations. These modes are consequences of incoming external stimuli and are competitive within the system, suppressing and locking-out one another. On the other hand, the presence of any such mode acts as a preconditioner for the immediate (dynamic) cognitive processing of information. Thus, internal phenomenological sensations, including emotions, reduce the immediate decision set (of feasible interpretations) and hence the cognitive load. For organisms with such a mental inner life, there would clearly be a large cognitive evolutionary advantage, resulting in the well-known "thinking fast, thinking slow" phenomena. We call this the entwinement hypothesis: how latent conscious phenomena arise from the dynamics of the cognitive processing load, and how these precondition the cognitive tasks immediately following. We discuss how internal dynamical modes, which are candidates for emotions down to single qualia, can be observed by reverse engineering large sets of simulations of system's stimulated responses, either using vast supercomputers (with full 10B neuronal network analyses) or else using laptops to do the same for appropriately generalised Kuramoto models (networks of k-dimensional clocks, each representing the 10,000 neurons within a single neural column). We explain why such simplifications are appropriate. We also discuss the consequent cognitive advantages for information-processing systems exhibiting internal sensations and the exciting implications for next-generation (non-binary) computation and for AI.

Cognitive and plastic recurrent neural network clock model for the judgment of time and its variations

The aim of this study in the field of computational neurosciences was to simulate and predict inter-individual variability in time judgements with different neuropsychological properties. We propose and test a Simple Recurrent Neural Network-based clock model that is able to account for inter-individual variability in time judgment by adding four new components into the clock system: the first relates to the plasticity of the neural system, the second to the attention allocated to time, the third to the memory of duration, and the fourth to the learning of duration by iteration. A simulation with this model explored its fit with participants’ time estimates in a temporal reproduction task undertaken by both children and adults, whose varied cognitive abilities were assessed with neuropsychological tests. The simulation successfully predicted 90% of temporal errors. Our Cognitive and Plastic RNN-Clock model (CP-RNN-Clock), that takes into account the interference arising from a clock system grounded in cognition, was thus validated.

Quantum memories for fundamental science in space

Investigating and verifying the connections between the foundations of quantum mechanics and general relativity will require extremely sensitive quantum experiments. To provide ultimate insight into this fascinating area of physics, the realization of dedicated experiments in space will sooner or later become a necessity. Quantum technologies, and among them quantum memories in particular, are providing novel approaches to reach conclusive experimental results due to their advanced state of development backed by decades of progress. Storing quantum states for prolonged time will make it possible to study Bell tests on astronomical baselines, to increase measurement precision for investigations of gravitational effects on quantum systems, or enable distributed networks of quantum sensors and clocks. We here promote the case of exploiting quantum memories for fundamental physics in space, and discuss both distinct experiments as well as potential quantum memory platforms and their performance.

Free-space dissemination of time and frequency with 10-19 instability over 113 km.

Networks of optical clocks find applications in precise navigation1,2, in efforts to redefine the fundamental unit of the 'second'3-6 and ingravitational tests7. As the frequency instability for state-of-the-art optical clocks has reached the 10-19 level8,9, the vision of a global-scale optical network that achieves comparable performances requires the dissemination of time and frequency over a long-distance free-space link with a similar instability of 10-19. However, previous attempts at free-space dissemination of time and frequency at high precision did not extend beyond dozens of kilometres10,11. Here we report time-frequency dissemination withan offset of 6.3 × 10-20 ± 3.4 × 10-19 and an instability of less than 4 × 10-19 at 10,000 s througha free-space link of 113 km. Key technologies essential to this achievement include the deployment of high-power frequency combs, high-stability and high-efficiency optical transceiver systems and efficient linear optical sampling. We observe that the stability we have reached is retained for channel losses up to 89 dB. The technique we report can not only be directly used in ground-based applications, but could also lay the groundwork for future satellite time-frequency dissemination.

Absolute clock synchronization with a single time-correlated photon pair source over a 10 km optical fibre.

We demonstrate a point-to-point clock synchronization protocol based on bidirectionally propagating photons generated in a single spontaneous parametric down-conversion (SPDC) source. Tight timing correlations between photon pairs are used to determine the single and round-trip times measured by two separate clocks, providing sufficient information for distance-independent absolute synchronization secure against symmetric delay attacks. We show that the coincidence signature useful for determining the round-trip time of a synchronization channel, established using a 10 km telecommunications fiber, can be derived from photons reflected off the end face of the fiber without additional optics. Our technique allows the synchronization of multiple clocks with a single reference clock co-located with the source, without requiring additional pair sources, in a client-server configuration suitable for synchronizing a network of clocks.

Measuring the stability of fundamental constants with a network of clocks

The detection of variations of fundamental constants of the Standard Model would provide us with compelling evidence of new physics, and could lift the veil on the nature of dark matter and dark energy. In this work, we discuss how a network of atomic and molecular clocks can be used to look for such variations with unprecedented sensitivity over a wide range of time scales. This is precisely the goal of the recently launched QSNET project: A network of clocks for measuring the stability of fundamental constants. QSNET will include state-of-the-art atomic clocks, but will also develop next-generation molecular and highly charged ion clocks with enhanced sensitivity to variations of fundamental constants. We describe the technological and scientific aims of QSNET and evaluate its expected performance. We show that in the range of parameters probed by QSNET, either we will discover new physics, or we will impose new constraints on violations of fundamental symmetries and a range of theories beyond the Standard Model, including dark matter and dark energy models.

Cycle dynamics and synchronization in a coupled network of peripheral circadian clocks.

The intercellular interactions between peripheral circadian clocks, located in tissues and organs other than the suprachiasmatic nuclei of the hypothalamus, are still very poorly understood. We propose a theoretical and computational study of the coupling between two or more clocks, using a calibrated, reduced model of the circadian clock to describe some synchronization properties between peripheral cellular clocks. Based on a piecewise linearization of the dynamics of the mutual CLOCK:BMAL1/PER:CRY inactivation term, we suggest a segmentation of the circadian cycle into six stages, to help analyse different types of synchronization between two clocks, including single stage duration, total period and maximal amplitudes. Finally, our model reproduces some recent experimental results on the effects of different regimes of time-restricted feeding in liver circadian clocks of mice.

Signal-to-noise-ratio and maximum-signal-to-noise-ratio detection statistics in template-bank searches for exotic physics transients with networks of quantum sensors

Quantum sensor networks such as the existing networks of atomic clocks and magnetometers offer intriguing capabilities in searches for transient signals such as dark matter or fields sourced by powerful astrophysical events. The common matched-filter technique relies on the signal-to-noise-ratio (SNR) detection statistic to probe such transients. For macroscopic dark matter objects, the network would register a sweeping signal as the object propagates through at galactic velocities. A potential event is registered when the SNR from a specific template exceeds a threshold set by a desired false positive rate. Generically, to span the continuous parameter space for the network-exotic-physics encounter, one has to deal with multiple templates. In such template-bank searches, the natural generalization of the SNR statistic is the maximum-signal-to-noise-ratio (SNR-max) statistic, defined as the maximum of the absolute values of SNRs determined from individual template matching. While individual SNR realizations are Gaussian distributed, SNR-max probability distribution is non-Gaussian. Moreover, as the individual template-bank SNRs are computed using the same network data streams, SNRs become correlated between the templates. Cross-template correlations have a sizable effect on the SNR-max probability and cumulative distributions, and on the threshold SNR-max values. Computing threshold SNR-max values for large template banks is computationally prohibitive and we develop analytic approaches to computing properties of the SNR-max statistic. This is done for cases when the template bank is nearly orthogonal (small cross-template correlations) and for banks with cross-template correlation coefficient distribution ``squeezed'' about the most probable cross-template correlation value. Since the cross-template correlation coefficients quantify the similarity of templates, increasing correlations tend to decrease SNR-max thresholds for specific values of false positive rates. Increasing the number of templates in the bank increases the SNR-max thresholds. Our derivations are carried out for networks that may exhibit colored noise and cross-node correlations. Specific applications are illustrated with a dark matter search with atomic clocks onboard satellites of a Global Positioning System and with a ``toy'' planar network with cyclic rotational symmetry.

Ultrastable Free-Space Laser Links for a Global Network of Optical Atomic Clocks.

A global network of optical atomic clocks will enable unprecedented measurement precision in fields including tests of fundamental physics, dark matter searches, geodesy, and navigation. Free-space laser links through the turbulent atmosphere are needed to fully exploit this global network, by enabling comparisons to airborne and spaceborne clocks. We demonstrate frequency transfer over a 2.4km atmospheric link with turbulence comparable to that of a ground-to-space link, achieving a fractional frequency stability of 6.1×10^{-21} in 300s of integration time. We also show that clock comparison between ground and low Earth orbit will be limited by the stability of the clocks themselves after only a few seconds of integration. This significantly advances the technologies needed to realize a global timescale network of optical atomic clocks.

Hardware in the Loop Testing of Main and Backup Protection Scheme for Systems with High Penetration of Inverter-Based Resources

Several challenges arise when designing an adequate protection scheme for transmission systems with inverter-based resources. The communication network plays a vital role in help- ing the protective relays to overcome the complexities of the archi- tecture and provide a suitable protection technique. This paper presents a co-simulation platform linking the simulated model of the modified IEEE 14 bus transmission system simulated on the RTDS simulator and the commercial SEL-421 protective relays to protect the system. The protection scheme utilizes voltage signals from the buses to identify the faulted section and determine the fault type. Voltages and currents are communicated to the relays by sending SV messages from RTDS to the relays using the GTNETx2 interface, synchronized by the GTSYNC interface card and SEL-2488 Satellite-Synchronized Network Clock. Several cases were simulated to investigate the protection system under breaker and relay failures to provide a suitable backup solution and allow the healthy feeders to serve the system during abnormal conditions. The real-time results show the capability of the sug- gested protection scheme.

Performance Testing and Assessment of Protection Scheme Using Real-Time Hardware-in-the-Loop and IEC 61850 Standard

The main challenge of the microgrid is to design a suitable protection scheme due to the complexity of the architecture of the microgrid. The importance of the proposed protection technique is threefold. First, it presents a co-simulation platform to integrate between a simulated model on power system computer aided design (PSCAD)/real time digital simulator computer aided design (RSCAD) software's and physical devices schweitzer engineering laboratories (SEL) 421-7 relays to protect the microgrid that includes different resources connected based on inverter interface. Second, it presents a comprehensive hardware/software setup to test the protective relays in a closed loop system and shows how to configure the protective relay's International Electrotechnical Commission 61850 communications. Third, IEEE 1588 standard is used to provide sub nanoseconds latency between the simulated model that emulated on real time digital simulator (RTDS) and the external devices. Also, the measurement signals are synchronized between RTDS and the external devices using giga-transceiver synchronization card (GTSYNC) interface card and SEL-2488 satellite-synchronized network clock. The results showed that the co-simulation infrastructure introduces a highly dependable design, analysis, and testing environment for cyber and physical data flow in the system. Besides that, the voltages at ac/dc sides and frequency at fault condition were maintained due to the energy storage device contributions at different modes of operation.

Potential and scientific requirements of optical clock networks for validating satellite-derived time-variable gravity data

SUMMARY The Gravity Recovery and Climate Experiment (GRACE) and GRACE-FO missions have provided an unprecedented quantification of large-scale changes in the water cycle. However, it is still an open problem of how these missions’ data can be referenced to a ground truth. Meanwhile, stationary optical clocks show fractional instabilities below 10−18 when averaged over an hour, and continue to be improved in terms of stability and accuracy, uptime and transportability. The frequency of a clock is affected by the gravitational redshift, and thus depends on the local geopotential; a relative frequency change of 10−18 corresponds to a geoid height change of about 1 cm. Here we suggest that this effect could be exploited for sensing large-scale temporal geopotential changes via a network of clocks distributed at the Earth’s surface. In fact, several projects have already proposed to create an ensemble of optical clocks connected across Europe via optical fibre links. Our hypothesis is that a clock network with collocated GNSS receivers spread over Europe—for which the physical infrastructure is already partly in place—would enable us to determine temporal variations of the Earth’s gravity field at timescales of days and beyond, and thus provide a new means for validating satellite missions such as GRACE-FO or a future gravity mission. Here, we show through simulations how glacial, hydrological and atmospheric variations over Europe could be observed with clock comparisons in a future network that follows current design concepts in the metrology community. We assume different scenarios for clock and GNSS uncertainties and find that even under conservative assumptions—a clock error of 10−18 and vertical height control error of 1.4 mm for daily measurements—hydrological signals at the annual timescale and atmospheric signals down to the weekly timescale could be observed.

Circadian rhythms: influence on physiology, pharmacology, and therapeutic interventions.

Circadian rhythms are ubiquitous phenomena that recur daily in a self-sustaining, entrainable, and oscillatory manner, and orchestrate a wide range of molecular, physiological, and behavioral processes. Circadian clocks are comprised of a hierarchical network of central and peripheral clocks that generate, sustain, and synchronize the circadian rhythms. The functioning of the peripheral clock is regulated by signals from autonomic innervation (from the central clock), endocrine networks, feeding, and other external cues. The critical role played by circadian rhythms in maintaining both systemic and tissue-level homeostasis is well established, and disruption of the rhythm has direct consequence for human health, disorders, and diseases. Circadian oscillations in both pharmacokinetics and pharmacodynamic processes are known to affect efficacy and toxicity of several therapeutic agents. A variety of modeling approaches ranging from empirical to more complex systems modeling approaches have been applied to characterize circadian biology and its influence on drug actions, optimize time of dosing, and identify opportunities for pharmacological modulation of the clock mechanisms and their downstream effects. In this review, we summarize current understanding of circadian rhythms and its influence on physiology, pharmacology, and therapeutic interventions, and discuss the role of chronopharmacometrics in gaining new insights into circadian rhythms and its applications in chronopharmacology.

Frequency ratio measurements at 18-digit accuracy using an optical clock network.

Atomic clocks are vital in a wide array of technologies and experiments, including tests of fundamental physics1. Clocks operating at optical frequencies have now demonstrated fractional stability and reproducibility at the 10-18 level, two orders of magnitude beyond their microwave predecessors2. Frequency ratio measurements between optical clocks are the basis for many of the applications that take advantage of this remarkable precision. However, the highest reported accuracy for frequency ratio measurements has remained largely unchanged for more than a decade3-5. Here we operate a network of optical clocks based on 27Al+(ref. 6), 87Sr (ref. 7) and 171Yb (ref. 8), and measure their frequency ratios with fractional uncertainties at or below 8×10-18. Exploiting this precision, we derive improved constraints on the potential coupling of ultralight bosonic dark matter to standard model fields9,10. Our optical clock network utilizes not just optical fibre11, but also a 1.5-kilometre free-space link12,13. This advance in frequency ratio measurements lays the groundwork for future networks of mobile, airborne and remote optical clocks that will be used to test physical laws1, perform relativistic geodesy14 and substantially improve international timekeeping15.

ACES/PHARAO: high-performance space-to-ground and ground-to-ground clock comparison for fundamental physics

The Atomic Clock Ensemble in Space is a fundamental physics mission of the European Space Agency to be launched in August 2021. It relies on a high-performance clock onboard the International Space Station (ISS), a network of high-performance clocks on ground, a dedicated 2-way microwave link (MWL) enabling space-to-ground and ground-to-ground clock comparisons, as well as an optical link (ELT). PHARAO/SHM (Projet d’Horloge Atomique par Refroidissement d’Atomes en Orbite/Space Hydrogen Maser), the clock onboard the ISS, has a relative frequency accuracy at the \({10}^{-16}\) level, a relative frequency stability (Allan deviation) equal to \({10}^{-13}/\sqrt{\tau }\) (\(\tau\) being the integration time in seconds) and a time deviation of 12 picoseconds after one day of integration. The MWL is designed to reach a time deviation below 7 ps after one day of integration. While space-to-ground clock comparisons will enable precise tests of the gravitational redshift, tests of deviations from General Relativity at the \({10}^{-6}\) level, and tests of local Lorentz invariance at the \({10}^{-10}\) level, ground-to-ground clock comparisons will enable a search of the time variation of fundamental constants with uncertainty at the \({10}^{-17}\) level after one year. In this contribution, we review the mission setup with a particular emphasis on the MWL, discuss the simulation and data analysis software developed to investigate mission performance, focusing on its primary scientific objective: the test of the gravitational redshift.

Contributions of white and brown adipose tissues to the circadian regulation of energy metabolism.

The term energy metabolism comprises the entirety of chemical processes associated with uptake, conversion, storage, and breakdown of nutrients. All these must be tightly regulated in time and space to ensure metabolic homeostasis in an environment characterized by cycles such as the succession of day and night. Most organisms evolved endogenous circadian clocks to achieve this goal. In mammals, a ubiquitous network of cellular clocks is coordinated by a pacemaker residing in the hypothalamic suprachiasmatic nucleus. Adipocytes harbor their own circadian clocks, and large aspects of adipose physiology are regulated in a circadian manner through transcriptional regulation of clock-controlled genes. White adipose tissue (WAT) stores energy in the form of triglycerides at times of high energy levels that then serve as fuel in times of need. It also functions as an endocrine organ, releasing factors in a circadian manner to regulate food intake and energy turnover in other tissues. Brown adipose tissue (BAT) produces heat through nonshivering thermogenesis, a process also controlled by the circadian clock. We here review how WAT and BAT contribute to the circadian regulation of energy metabolism. We describe how adipose rhythms are regulated by the interplay of systemic signals and local clocks and summarize how adipose-originating circadian factors feed-back on metabolic homeostasis. The role of adipose tissue in the circadian control of metabolism becomes increasingly clear as circadian disruption leads to alterations in adipose tissue regulation, promoting obesity and its sequelae. Stabilizing adipose tissue rhythms, in turn, may help to combat disrupted energy homeostasis and obesity.