We show that seafloor-cables, when combined with ultra-stable interferometry, can act as arrays of environmental sensors for earthquakes, ocean currents and other ocean signals with potentially game -changing applications in Earth monitoring.

Optical fiber-based sensing technology can drastically improve Earth observations by enabling the use of existing submarine communication cables as seafloor sensors. Previous interferometric and polarizationbased techniques demonstrated environmental sensing over cable lengths up to 10,500 kilometers. However, measurements were limited to the integrated changes over the entire length of the cable. We demonstrate the detection of earthquakes and ocean signals on individual spans between repeaters of a 5860-kilometer-long transatlantic cable rather than the whole cable. By applying this technique to the existing undersea communication cables, which have a repeater-to-repeater span length of 45 to 90 kilometers, the largely unmonitored ocean floor could be instrumented with thousands of permanent real-time environmental sensors without changes to the underwater infrastructure.

NPL is leading a programme called the National Timing Centre (NTC) to develop a robust, geographically distributed time scale infrastructure across the UK. The aims of the programme are to improve security and resilience in national time and frequency distribution, whilst supporting UK innovation and skills. Recent government studies have highlighted the dependence of services such as telecommunications, broadcasting, energy and finance on Global Navigation Satellite Systems (GNSS) for time and frequency reference signals. As GNSS signals are vulnerable to disruption, the availability of accurate time and frequency signals from a robust national timing infrastructure will, in future, complement GNSS and mitigate the impact of any disruption to GNSS signals to these critical services. At present, the UK’s national time scale UTC(NPL) is maintained by NPL in Teddington, London. Time dissemination services are provided over the whole country via the millisecond-level accuracy MSF radio time signal and an NTP internet time service. The NPLTime® service provides higher accuracy to a number of users in the London area and in several European locations. The new National Timing Centre programme will significantly improve the resilience and capability of this UK time scale infrastructure. In future, a distributed UTC(NPL) time scale infrastructure will operate from several locations across the UK, including a new NPL facility in Teddington, interconnected by two or more time and frequency transfer methods. The time scale design is based on signal measurement, frequency steering and distribution from hydrogen masers with significant built-in hardware redundancy, which will be replicated at each site. At NPL, development of the new time scale facility is underway with the arrival of new hydrogen masers and construction of an additional caesium fountain. Work is also in progress to develop a suite of software to automate, monitor and control the time scale. To enable geographically widespread availability of signals from this new time scale, several distribution nodes will provide points of access across the UK. At these nodes, a range of distribution technologies including fibre, communication satellites, terrestrial broadcasts and GNSS will deliver resilient, UTC-traceable time and frequency signals to critical national infrastructure, businesses and academia. As a result, the NTC programme will not only support essential services, it will enable technological growth and innovation in industry through the development of new time and frequency products and applications. Additionally, the NTC programme will enhance the UK’s research and development capabilities in time and frequency generation and dissemination. The NTC programme will also respond to the specialist skills shortage in the field of time and frequency. An assessment of the current and future workforce requirements in this field has been carried out to create a Training & Education Blueprint. Through collaboration with industry and academia, new training opportunities will be created, suitable for learners at a range of levels.

Distributed cavity phase (DCP) frequency shifts are a leading systematic effect in atomic fountain frequency standards. They originate from the phase variations of the field in the microwave cavity combined with different positions of the atoms in the cavity on the ascent and descent. Here we demonstrate techniques to precisely determine the position of the cloud of atoms in the microwave cavity, using either the approximately linear variation of the transverse components of the microwave field or the quadratic variation of the longitudinal microwave field amplitude in the cavity. We also show that shifting the initial position of the atoms gives a significantly higher sensitivity to DCP variations than the often-used tilting of fountains. A demonstrated centring precision of order 50 mu m will enable DCP frequency shift uncertainties to be reduced to less than 10(-17)and thereby contribute insignificantly to the accuracy budget of a standard. These techniques to vertically align a fountain are straightforward to automate for routine operation and require a negligible fraction of the standard's averaging time.

Maintaining a robust and largely autonomous timescale is a task not only for national measurement institutes, but also for timing labs at large scientific facilities, military and security agencies or telecom and financial service providers. The most stable local timescales consist of a flywheel clock (usually a hydrogen maser) that is frequently steered using corrections provided by an atomic fountain frequency standard whose local oscillator is weakly locked to that clock.

The paper firstly argues from conservation principles that, when dealing with physics aside from elementary particle interactions, the number of naturally independent quantities, and hence the minimum number of base quantities within a unit system, is five. These can be, for example, mass, charge, length, time, and angle. It also highlights the benefits of expressing the laws of physics using equations that are invariant when the size of the chosen unit for any of these base quantities is changed. Following the pioneering work in this area by Buckingham, these are termed 'complete' equations, in contrast with equations that require a specific unit to be used. Using complete equations is shown to remove much ambiguity and confusion, especially where angles are involved. As an example, some quantities relating to atomic frequencies are clarified. Also, the reduced Planck constant ħ, as commonly used, is shown to represent two distinct quantities, one an action (energy x time), and the other an angular momentum (action/angle). There would be benefits in giving these two quantities different symbols. Lastly, the freedom to choose how base units are defined is shown to allow, in principle, measurements of changes over time to dimensional fundamental constants like c.

Synchronisation of a remote clock to a time reference can be challenging. Within the timing community these challenges have been addressed, and robust time transfer and calibration techniques have been developed offering differing levels of synchronization accuracy to the international time reference UTC, Coordinated Universal Time. These techniques can be applied to timing equipment at ground-based Cal/Val (Calibration/Validation) sites in Western Crete and elsewhere to achieve FRM (fiducial reference measurements) for altimetry, satisfying their requirement for SI (International System of units) traceability. Continuous monitoring of the remote sites is required to maintain traceability to the reference time, and a holdover clock may also be needed. This paper discusses how UTC or TAI (International Atomic Time) could be used as a time reference for timestamped measurements taken at Cal/Val sites, improving measurement uncertainty and linking fiducial reference measurements for satellite altimetry back to the SI unit of time: the second.

Galileo System Time (GST) is the cornerstone for the operations and performance of Galileo, the European GNSS (Global Navigation Satellite System). GST must be stable, traceable to UTC, and always available. GST is currently generated from an ensemble of physical on-ground clocks steered to UTC. This approach limits certain types of performance. For example, if a clock breaks down, a downtime is caused to the operations or a procedure needs to put in place to switch to a redundant clock. Moreover, for very long time periods (comparable to the design operational lifetime of GNSS, i.e., in the order of decades) the performance of the physical clock will degrade. A time scale built with pulsar measurements, i.e., measurements from celestial objects emitting radiation in pulses, would typically be less stable than one built using atomic or optical clocks in the short term, but could be competitive in the very long term (several decades, a period over which individual atomic clocks will cease to work). An additional justification for a pulsar time scale is that it would be independent of the clock technology for the generation of the oscillation mechanism (neutron star rotational period as opposed to atomic transitions in rubidium, caesium or hydrogen atoms). The objective of the PulChron project, an abbreviation built with the words "Pulsar" and "Chronos" ( ), which is the ancient Greek term for "Time", is to demonstrate the effectiveness of a pulsar time scale for the generation and monitoring of system timing in Positioning, Navigation and Timing (PNT) in general, and of GST in particular. The PulChron project has been developed in the frame of NAVISP (Navigation Innovation and Support Programme), an ESA programme aiming at fostering innovation in the PNT field while supporting industry and ESA member states interests. In this context, it was considered interesting to demonstrate the implementation of a "real time clock" and a "paper time scale" based on pulsar measurements for PNT monitoring.

We demonstrate a convenient experimental method, that facilitates faster and more accurate evaluation of the distributed cavity phase, a leading systematic effect in atomic fountain clocks. We expand on earlier work by Nemitz et al., which showed that displacement of the atoms from the microwave cavity axis can lead to an asymmetry in resonances observed for hyperfine transitions with Δ mF = 1. By applying an additional small transverse magnetic field, with controlled magnitude and direction, we are able to extract from the measured asymmetry the atom cloud's crossing position to within a fraction of a millimeter.

Non-adiabatic decay rates for a radio-frequency dressed magnetic trap are calculated using Fermi's Golden Rule: that is, we examine the probability for a single atom to make transitions out of the dressed trap and into a continuum in the adiabatic limit, where perturbation theory can be applied. This approach can be compared to the semi-classical Landau-Zener theory of a resonant dressed atom trap, and it is found that, when carefully implemented, the Landau-Zener theory overestimates the rate of non-adiabatic spin flip transitions in the adiabatic limit. This indicates that care is needed when determining requirements on trap Rabi frequency and magnetic field gradient in practical atom traps.

Recent progress in optics, atomic physics and material science has paved the way to study quantum effects in ultracold atomic alkali gases confined to non-trivial geometries. Multiply connected traps for cold atoms can be prepared by combining inhomogeneous distributions of DC and radio-frequency electromagnetic fields with optical fields that require complex systems for frequency control and stabilization. Here we propose a flexible and robust scheme that creates closed quasi-one-dimensional guides for ultracold atoms through the ‘dressing’ of hyperfine sublevels of the atomic ground state, where the dressing field is spatially modulated by inductive effects over a micro-engineered conducting loop. Remarkably, for commonly used atomic species (for example, 7 Li and 87 Rb), the guide operation relies entirely on controlling static and low-frequency fields in the regimes of radio-frequency and microwave frequencies. This novel trapping scheme can be implemented with current technology for micro-fabrication and electronic control.

We propose a ring trap based on spatially modulated coupling of the hyperfine manifolds of the atomic ground state, whose degeneracy is lifted by a bias field BDC. This results in a ring trap operating at microwave frequencies rather than radio-frequency. A sketched of our proposal is shown in a figure in this paper, where typical values of relevant parameters are included. We perform calculations for 87Rb and study conditions under which this trap can be realized, including considerations about the life-time of trapped states.