Image credit: INRIM
Muochrony, a new subfield of muography, has been developed with the INRIM (Italian National Metrology Institute)-VMI (International Virtual Muography Institute) team in collaboration with MUOGRAPHIX. In this interview, Giancarlo Cerretto introduces and discusses the principles, experimental results and expectations for future developments of this new subfield.
Could you describe what the term muochrony means and how newly developed muochrony techniques (CTS and CTC) work jointly with other subfields of muography?
“Muochrony” is a neologism proposed through the collaboration between INRIM and Muographix/The University of Tokyo. It can be defined both as a noun and as an adjective:
muochrony (n) = (in italian muocronia (s.f.)) branch of knowledge studying the possibility of disciplining/synchronizing different oscillators by using muonic techniques. From muon (the elementary particle) and greek chronos (Χρόνος) “time”
muocronous/muochronized (adj) = (in italian muocrono/muocronizzato (agg.)) the adjective that indicates that different oscillators have been synchronized (or a slave is disciplined with respect to a master) by using muonic techniques
As is well known, due to their high penetration capabilities, muons generated within an Extensive Air Shower (EAS) can be utilized to create images of the interiors of large objects, such as volcanoes or pyramids, within a specialized field known as Muographic Imagery. Additionally, single muons from EAS can be considered “simultaneous” events for detectors within tens of nanoseconds, and the differences in arrival times between two or more detectors are pretty stable over time, affected by white Gaussian noise. This stability makes them particularly suitable for applications related to timing (T) within an emerging field of study called Muochrony, which draws an analogy to the more established field of Muographic Imagery and another emerging technology known as Muometry, focused on muonic Positioning and Navigation (PN). Muographic Imagery, Muometry (i.e., PN), and Muochrony (i.e., T) are the foundational pillars of the muonic scientific discipline referred to as Muography. Muochrony encompasses techniques such as CTS (Cosmic Time Synchronizer) and CTC (Cosmic Time Calibrator) for time synchronization and dissemination, as well as COSMOCAT (Cosmic Coding and Transfer) for cryptography. Since muonic PN detectors must be synchronized, Muography also serves as a key enabler of Muometry.
What is the FRATERNISE project? How was this team formed and what are your objectives?
The FRATERNISE project (Facility for High Temporal Accuracy for Fundamental Physics Experiments) was developed from INRIM’s previous experiences in the metrological characterization and calibration of timing devices used to investigate the superluminal properties of neutrinos in collaboration with the Borexino project (CNGS project, CERN neutrino to Gran Sasso), as well as in the detection of extensive and ultra-high-energy cosmic showers as part of the EEE (Extreme Energy Events) project.
Time is crucial in Fundamental Physics experiments, such as those in space and accelerator physics. It enables the evaluation of event time stamping and simultaneity, facilitating deductions about the characteristics of the observed phenomena. Furthermore, there is an increasing demand to correlate events from experiments conducted over vast distances or those measured by different experiments investigating similar phenomena in separate locations.
The primary objective of the FRATERNISE project was to establish a permanent facility at INRIM dedicated to the metrological characterization and calibration of timing devices for Fundamental Physics experiments and GNSS receivers used for the remote comparison of atomic clocks or time scales. The FRATERNISE facility is designed to support Fundamental Physics experiments that require enhancements to their timing systems, benefiting from direct synchronization with the Italian atomic reference time scale, UTC(IT).
In addition to the permanent facility, FRATERNISE is equipped with “traveling” units with tailored functional and metrological features to calibrate timing devices that cannot be physically transported to INRIM. Funded by Fondazione CRT (Cassa di Risparmio di Torino, an Italian foundation) and the Bank of Italy, the facility undergoes a gradual and continuous improvement, solidifying its objectives and evolving into a practical and functional system for the metrological characterization of timing devices used in Fundamental Physics experiments.
To support this goal, a survey was conducted to assess the timing requirements of the Particle Physics and Astrophysics community, engaging with various research groups to identify their primary needs and the driving factors behind their experiments.
Finally, it is worth noting that FRATERNISE is not just an acronym; it also reflects the hope that scientific collaborations can occur in an environment of cooperation and constructive engagement. This approach facilitates the integration of diverse expertise, methodologies, sensitivities, and interests, all aimed at advancing science and serving society collaboratively.
The core team of the FRATERNISE project includes Dr. Giancarlo Cerretto, Dr. Elena Cantoni, and Dr. Marco Sellone, scientists at the INRIM Time and Frequency Sector.
Figure 1: Design scheme of the INRIM FRATERNISE Facility. The left rack hosts the Time and Frequency devices for UTC(IT) and AHM time/frequency signals distribution, an Rb Clock, a Microstepper, Time Interval Counters (TIC), and permanent/traveling calibrated GNSS geodetic receivers for timing application. The rack on the right hosts the facility’s informatics system, timing GNSS receivers (not geodetic), and a second-level time and frequency signal distribution system to serve external systems like CTS. (Image Credit: INRIM)
Figure 2: The FRATERNISE facility as realized following the design scheme and project. On the right are details of some GNSS geodetic receivers for timing applications that are equipping the facility. (Image Credit: INRIM)
What contributions have MUOGRAPHIX made to FRATERNISE and what do you expect the next steps to be in this collaboration between INRIM and MUOGRAPHIX?
The University of Tokyo can be considered as the first external scientific client of the FRATERNISE Facility. The metrological characterization of the CTS technique—developed by the University of Tokyo—and the scientific research aimed at envisaging it as an innovative and alternative method for time synchronization and dissemination of reference time scales were made possible thanks to the FRATERNISE facility. Access to the time and frequency signals of the UTC(IT) Italian Standard Time, generated at the INRIM Time Laboratory, along with signals from other commercial atomic clocks, such as Active Hydrogen Masers (AHM) and Rubidium (Rb) clocks, was facilitated by the permanent facility. Additionally, the FRATERNISE’s permanent and calibrated GNSS geodetic receiver for timing applications will enable testing of the CTS’s capabilities to enhance GNSS-based time synchronization and dissemination techniques in areas not covered by satellite signals, such as indoors, underground, and underwater. The FRATERNISE traveling GNSS-based calibrated stations will also allow for the metrological characterization of the CTS when disseminating reference time scales at remote user sites.
Please describe the CTS system that has been installed at INRIM. How has this system been tested and what have been the experimental results thus far?
Following the promising results obtained at the University of Tokyo Laboratories, a CTS setup comprising four 30 cm x 30 cm scintillators and associated measurement equipment was shipped to INRIM. It was installed in the INRIM RadioNavigation Laboratory and integrated into the FRATERNISE facility in late October 2023. The CTS detectors were initially positioned within a 5-meter radius throughout the RadioNavigation Laboratory. This measurement system is intended for various tests and metrological characterization activities.
Initially, the Master sensor and three Slave sensors were connected to UTC(IT) via FRATERNISE to initiate a measurement campaign to assess the CTS system’s noise characteristics preliminarily. Following this campaign, the Master sensor remained connected to the UTC(IT) signal, which serves as a reference. In contrast, the Slave sensors were linked to Active Hydrogen Maser (AHM) signals provided by FRATERNISE. This configuration allowed for the evaluation of the CTS’s capabilities in estimating the relative frequency offset and drift of the AHM to UTC(IT) compared to other state-of-the-art time metrology measurement systems operated at the INRIM Time and RadioNavigation Laboratories.
Based on these results, initial tests were conducted to discipline the AHM to UTC(IT) and to remotely disseminate a UTC(IT) replica (denoted as UTC(IT)_CTS), leveraging the short-term characteristics of the AHM alongside the medium-to-long-term stability of UTC(IT). Although preliminary, the results are promising, indicating that the CTS can generate a UTC(IT) replica AHM based remotely within a 5-meter range, achieving precision within two nanoseconds of UTC(IT). Building on these findings, further studies have commenced, involving the relocation of one of the three SCMs to 30 meters horizontally and – then – underground, utilizing the time and frequency signals from a FRATERNISE commercial Rubidium clock as the reference for the SCM in addition to UTC(IT) and the AHM.
Figure 3: CTS setup at INRIM RadioNavigation Laboratory. On the left is a view of the rack hosting the CTS power supply system, Master and Slave TDCs, and ancillary measurement systems. The details of the 30 cm x 30 cm plastic scintillators, PMT (Photo Multiplier Tubes), and HV (High Voltage) units are on the right. (Image Credit: INRIM)
Why is it essential to develop more precise time synchronization solutions? How are muochrony techniques different than other time synchronization techniques and what are their unique advantages?
Time synchronization techniques are essential for time metrology and are increasingly recognized in fundamental science and various industrial sectors. In time metrology, these techniques are employed for calculating TAI (International Atomic Time) and UTC (Coordinated Universal Time) international atomic time scales, as well as for the remote comparison of national realizations of UTC (i.e., UTC(k)). Timing is also crucial in fundamental physics experiments, where it enables the timestamping of events and the analysis of observed phenomena based on their coincidences.
In industrial applications, precise timing is vital for synchronizing systems in mobile communications, digital radio/television broadcasting, smart grids and for accurately timestamping financial transactions in agreement with international regulations.
Time synchronization is typically achieved through local oscillators (either high-quality quartz or atomic), internet-based protocols (e.g. NTP. PTP), optical fibers over defined protocols (e.g., White Rabbit), and satellite techniques, each offering different performance levels, functional characteristics, limitations, target markets, and user bases.
Currently, satellite synchronization techniques are the most widely adopted, utilizing positioning systems (e.g., GPS and Galileo) or geostationary systems (commonly used for radio and television applications and internet services). These methods allow for the remote synchronization of atomic clocks and time scales with uncertainties on the order of nanoseconds as part of an ongoing international effort to enhance accuracy, precision, latency, and reliability. Additionally, local oscillators can be disciplined using GPS (or GNSS in general) to generate reference time and frequency signals, combining the short-term stability of the local oscillator with the medium- to long-term reliability of the GPS atomic reference time scale. These systems are known as GPS Disciplined Oscillators (GPS DOs).
However, the inherent characteristics of radiofrequency (RF) signals make these systems potentially vulnerable to jamming or spoofing and unsuitable for indoor or underwater applications. As a result, there is an increasing need for backup or alternative time synchronization techniques to improve timing robustness and resilience in critical infrastructures.
Recent theoretical studies, simulations, and initial metrological results suggest that Cosmic Ray Extensive Air Shower (EAS) muons can be utilized in the CTS wireless clock network resynchronization scheme, which operates on a hectometer-to-kilometer scale with a precision of 10–100 ns. The low interaction cross-section of EAS muons enables cost-effective, long-term wireless synchronization of atomic clocks and dissemination of reference time scales within the EAS shower disc area, providing enhanced resistance to jamming and spoofing. This approach is suitable for indoor, underground, and underwater applications, although it necessitates a denser detector network as the water equivalent depth increases in underground environments.
Figure 4: First outcomes on the possibility of disciplining atomic clocks using CTS. The plot shows the time difference between UTC(IT) realized at the INRIM Time Laboratory and its remote reconstruction by disciplining an Active Hydrogen Maser through muon detection. For the period considered, the UTC(IT)-UTC(IT)_CTS has been kept within 1-2 ns. The red areas indicate moments when the AHM system was not locked to UTC(IT) employing CTS measurements. (Image Credit: G. Cerretto, M. Sellone, E. Cantoni, C. E. Calosso, I. Gnesi, and H.K.M. Tanaka. “Muochrony: timing with muons. First experimental results at INRIM on the synchronization of atomic clocks and dissemination of reference timescales”. ESA 9th International Colloquium on Scientific and Fundamental Aspects of GNSS, Wrocław, Poland, 25-27 September, 2024 )
Figure 5: CTS rationale and possible fields of application, ranging from secure time synchronization for financial districts, underground tunnels, underground mining operations, and underwater environments to extending GNSS capabilities in areas not reached by satellite RF signals. (Image Credit: MUOGRAPHIX)
What is the principal role of the INRIM VMI (International Virtual Muography Institute) team?
As previously indicated, CTS has emerged as the first significant practical application of FRATERNISE following its completion. The formal collaboration between INRIM and the University of Tokyo has been established through a Letter of Intent (LOI) signed by the President of INRIM and the Director of Muographix/The University of Tokyo. As a result, the core team of FRATERNISE is now affiliated with the VMI (International Virtual Muography Institute) and has adopted the designation of INRIM VMI Team.
This dual identity of FRATERNISE and VMI adds substantial value to the group, allowing it to continue managing the FRATERNISE facility in alignment with its original objectives while also serving as the inaugural operational research nucleus of INRIM, focused on time metrology and, in the long term, metrology in general, to support applications utilizing muons. Initially, the focus is on Muochrony; however, the team will also be available to the international Muography community to address any metrological needs. In this context, the INRIM VMI Team is dedicated to strengthening its connections with the Muography community through ongoing scientific research activities related to the CTS technique, as well as on other innovative approaches that utilize muons, like CTC, COSMOCAT, muPS, etc. INRIM VMI Team is also willing to foster collaboration between the two communities by organizing outreach events, such as seminars and workshops.
Figure 6: The INRIM VMI Team. From left, to right: Dr. Giancarlo Cerretto, Dr. Marco Sellone and Dr. Elena Cantoni. (Image Credit: INRIM)
What have been the most technically challenging aspects that the INRIM-VMI team have encountered in the process of developing muochrony techniques?
The primary challenge that emerged at the outset of the project involved integrating two distinct scientific disciplines—Time Metrology and Particle Physics—around a common research theme. This integration presented some difficulties due to differing areas of expertise, scientific terminologies, and working methodologies. However, what initially seemed a limitation soon evolved into a significant strength of the collaboration among INRIM, VMI, and MUOGRAPHIX/The University of Tokyo. The differences that once posed challenges were transformed into added value, thanks to the efforts of Prof. Hiroyuki Tanaka’s team and the INRIM VMI team, who identified a shared and common goal. This goal was pursued with passion, dedication, mutual support, and a commitment to growth, all aimed at making an innovative and meaningful contribution to science and society.
From a technological perspective, although Muography and Time Metrology are two distinct and seemingly unrelated scientific approaches, they have surprisingly merged fruitfully, giving rise to a new discipline known as Muochrony. This field combines the ability to detect muons with the capability to discipline an oscillator using classical Time Metrology techniques. No significant challenges were encountered in setting up the measurement apparatus and conducting the experiments, demonstrating the strong compatibility of these two techniques in creating something novel and unexpectedly innovative.
How might muochrony be applied to global financial systems? What are some other applications for these techniques?
As previously mentioned, the Cosmic Time Synchronization (CTS) system, which employs cosmic muons instead of RF signals, represents a promising candidate for secure synchronization and reference time dissemination in critical applications, such as financial systems. This system can be envisaged to work in a standalone mode or as a backup to other established methods, typically based on GNSS or, although at a higher cost, optical fiber.
For the specific application of disseminating a reference time scale, CTS is likely to be integrated with other long-haul dissemination techniques to transmit a remote UTC-traceable time scale to the CTS system when local time references are unavailable. To maintain the system’s robustness, the long-range dissemination technique that provides CTS with the reference time scale must be characterized by enhanced resilience against jamming and spoofing.
Conversely, in scenarios where user applications are less critical and more functional, combining CTS and GPS (GNSS) could provide an effective solution to extend GNSS coverage in areas where signals are unavailable, such as indoors, underground, and underwater. To accommodate diverse user requirements, the development of customized master and slave detectors will be essential. In this context, prototype versions of a compact muonic Disciplined Oscillator (u-DO), analogous to the more common GPS-DO, are currently being designed and developed by INRIM, featuring tailored specifications regarding detection, electronics, and disciplining algorithms.
What do you anticipate for the future of muochrony techniques in the next decade?
Muocrony is a new discipline that has begun to demonstrate promising potential. However, several factors must be carefully considered before any technique or set of techniques can achieve widespread acceptance and consolidation. In particular, from a metrological standpoint, thorough characterization of CTS is crucial for assessing the technique’s ultimate attributes regarding precision, accuracy, and robustness. Specifically, establishing the system’s uncertainty budget and developing specific calibration and operational procedures – alongside the definition of proper disciplining algorithms – are essential for classifying CTS as a traceable time dissemination system. This undertaking, especially for metrological applications, necessitates time and dedication.
Encouragingly, initial results from the collaborative efforts between the INRIM VMI Team and Prof. Hiroyuki Tanaka’s Team suggest that Muochronic techniques may have a role in the future landscape of international time synchronization and dissemination methods. Specifically, CTS could be employed in standalone/alternate mode in critical contexts or combined with GNSS-based timing techniques to enhance coverage in areas lacking satellite signal availability, such as indoor, underground, or underwater environments.
In this regard, a preliminary expression of interest has been noted by the European Space Agency (ESA) following the presentation at its “9th Colloquium on Scientific and Fundamental Aspects of GNSS” of a collaborative work by INRIM, VMI, and Muographix/The University of Tokyo, which outlined the current state of research on the CTS technique and Muochrony and received the recognition as the best presentation of the “Time and Frequency Transfer, Timing Aspects, Frequency Standards and Clock Technologies” session. Furthermore, through the Joint Research Centre (JRC), the European Union has shown preliminary interest in considering the potential to discipline atomic clocks using muons as an emerging technique within its Complementary/Continuous Positioning, Navigation, and Timing (C-PNT) framework.
Figure 7: The INRIM VMI Team. From left, to right: Dr. Giancarlo Cerretto, Dr. Elena Cantoni and Dr. Marco Sellone. (Image Credit: INRIM)