Phase-stabilized distribution of radio frequency (RF) signal is an essential requirement for many modern technological applications and advanced scientific experiments including clock synchronization or synchronization of distributed sensors. Coaxial cables are commonly used for the distribution of RF signal, but the phase of the RF signal is very much sensitive to the variation of ambient conditions, especially temperature variation. In this letter, we introduced a technique and verified with 10-MHz signal for real-time measurement of phase-drift due to varying ambient condition along with phase-stabilized RF signal transmission through the coaxial cables by active phase compensation.

The present article describes the design and development of a universal device for precise and accurate distribution of time or frequency signal, i.e., one pulse per second (PPS) or 10 MHz and 5 MHz sinusoidal signal generated from an atomic clock or ensemble of atomic clocks. The frequency distribution unit provides four identical outputs of excellent stability along with suppression of all high harmonics by more than 50 dB with respect to the fundamental frequency. For time (PPS) distribution, all four outputs also provide very much similar signal as the input with excellent stability. The rising time of the output pulses is very much similar to the input pulse and stays within 3 ns.

An optical fibre link has been established for precise time and frequency transfer utilizing White Rabbit (WR) network. Temperature sensitivity of WR network components on time offset error has been studied in detail. WR node shows significant sensitivity to the ambient temperature variation when it is locked to an external reference frequency. The change in time offset between two nodes reaches up to one nanosecond for a temperature variation of 25 °C. The temperature variation of the optical fibres does not show any noticeable change in time offset during two-way communication as the precision time protocol (PTP) compensates the delay arises for any local temperature variation. However, time offset variation shows significant sensitivity to temperature variation of the fibre when the PTP is not functional.

Precise transfer of time and frequency signals over long distances as well as clock synchronization to an ultra‐stable reference are very crucial for many of the technological applications as well as for advanced scientific research. These reinforces a wide range of applications such as navigation, power grid management, mobile communication, and so on. In order to compare the performances of two highly stable and accurate atomic clocks, it is desirable that the link between the two clocks, that is, the transmission link has higher level of stability than those clocks. This article describes establishment of an ultra‐stable optical fiber link employing White Rabbit network for transfer of time and frequency signals and also for comparing performance of atomic clocks. Utilizing this link, time signals have been transferred within an uncertainty of ±130 ps at ambient temperature (30°C–40°C) and the instability of the link in terms of modified Allan‐deviation reaches to ∼10 −16 within one day of integration time.

Nowadays, most of the standards of measurement are based on fundamental constants, and among all, the SI units of time and frequency are realized with the highest precision. The SI unit of time interval, i.e. second, is realized on the basis of a hyperfine transition of ground state of 133Cesium atom in the microwave region. Atomic clocks operating at the optical frequencies have potential of providing better accuracy and higher stability than the microwave atomic clocks, and it is expected that SI second will be redefined on the basis of an optical transition. In this article, we focus on different atomic frequency standards operating in the optical domain of the spectrum by interrogating neutral atoms in optical lattice or a single ion within a radiofrequency ion trap. Recent worldwide developments along with activities at CSIR-National Physical Laboratory (CSIR-NPL) towards building optical atomic clock or optical frequency standard have also been presented.

Precise time and frequency transfer as well as time synchronization among clocks have immense applications in different advanced technologies along with strategic applications. White Rabbit time synchronization protocol is one of the advanced technique and being utilized for precise time and frequency transfer in a large network. The present work describes establishment of a precise and stable optical fibre link for accurate time and frequency transfer utilizing White Rabbit Network. The link has been utilized for transferring time from an atomic clock in order to compare its performance with respect to another reference clock. The time deviation between the two clocks have been recorded over a long period of time and finally the fractional frequency offset between the two clocks has been derived.

AbstractWe report an easy to construct imaging system that can resolve particles separated by $$\ge $$ ≥ 0.68 $$\upmu $$ μ m with minimum aberrations. Its first photon collecting lens is placed at a distance of 31.6 mm giving wide optical access. The microscope has a Numerical Aperture (NA) of 0.33, which is able to collect signal over 0.36 sr. The diffraction limited objective and magnifier recollects 77% photons into the central disc of the image with a transverse spherical aberration of 0.05 mm and magnification upto 238. The system has a depth of field of 142 $$\upmu $$ μ m and a field of view of 56 $$\upmu $$ μ m which images a large ensemble of atoms. The imaging system gives a diffraction limited performance over visible to near-infrared wavelengths on optimization of the working distance and the distance between the objective and magnifier.

The present article describes an experimental study on precise and accurate time transfer through an optical fibre link for in house dissemination of time from an ultra-stable reference clock. The link consists of couple of single mode optical fibres and a White Rabbit (WR) link, using commercially available White Rabbit equipment along with necessary devices and software. We demonstrate time transfer accuracy remains well within 50 ps with respect to a primary clock over a very long period of time and the achieved link stability is about 10 -16 at 2000 sec integration time.

An optical clock or frequency standard based on the interrogation of an atomic transition in the optical domain, operates at ~10 14 Hz is capable of providing orders of magnitude better accuracy than the present primary standard of frequency or time operating in the microwave range. Optical transitions of an ions, confined within a radio frequency ion trap and laser cooled to ~mK temperature, provides better long term stability since they are free from Coulomb and intra-atomic interactions. Optical frequency standards have been realized with several ions, e.g., 199 Hg + , 171 Yb + , 115 In + , 88 Sr + , 40 Ca + , 27 Al + and among those 171 Y + has multiple ultra-narrow optical transitions suitable for serving as frequency standards [1-2].

Nearly collimated atomic beam is of interest for a variety of experiments. This article reports a simple way of modifying the atomic beam distribution using a dark wall oven and describes detailed study of outcoming atoms' spatial distribution. A simple design is obtained by employing the fact that inhomogeneous thermal distribution along a capillary results due to its partial resistive heating. Based on this phenomenon, we have designed a dark wall oven consisting of a reservoir, collimator, and cold absorber at the exit end of atoms, where all three are fabricated out of a single stainless steel capillary. The nearly collimated spatial distribution of the atoms resulting due to the absorber eliminating the atoms diverging above a certain angle is modeled and experimentally verified. A divergence as minimum as 1.2(1)° corresponding to a half angle θ1/2 = 0.9(1)° is measured at an oven temperature of 250 °C that produces an atomic flux of about 8 × 109 atoms s-1. Total flux as estimated using our measured spatial distribution of atoms matches well with the numerically simulated values of it for the dark wall oven.

Time transfer using television (TV) signal transmitted over microwave link was first demonstrated by Tolman et. al [1] and utilizing that technique they could set the clocks to microsecond accuracy. Similar level of accuracy was also achieved with another timing system via television networks employed Line-10 as a passive time synchronization technique [2]. TV signals through geostationary satellites are also been used for precise time transfer between remote clocks [3]. With the advent of technologies, all analog TV are being replaced with digital ones and developing an easy deployable and accurate time transfer technique utilizing digital TV signal will be extremely useful for many of the applications like estimating clock offset or drift among servers and clocks. In this article we described a very simple and inexpensive method of time transfer based on the digital TV signal from the geostationary satellites. The basic principle of this kind of time transfer is that it identifies a specific part of the digital TV signal as the time marker and every time the marker signals arrive at any particular station, its time of arrival being recorded with respect to the local clock. Consider two such clocks located at two different places (Fig. 1) A and B and recording time of arrival of the marker signal from a commonly viewed geostationary satellite. If τ A and τ B considered to be the travelling time or delay of the marker signal from the satellite to the position A and B respectively and T A and T B to be the local time of arrival of that marker signal. The time offset (∇T AB ) between the two local clocks can be estimated as.

We report an all-in-one waveform generator, lock-in amplifier and Proportional-Integral-Differential (PID) controller, embedded in a single Field Programmable Gate Array (FPGA). The PID controller is advanced with a novel automatic relocking mechanism, which is capable of self-finding and relocking at the desired setpoint upon unlocking of the PID due to disturbances. The instrument is designed in such a way that these devices can either be used individually or simultaneously. Digital implementation via software processing of the associated modules is used and the firmware is embedded in an FPGA IC, which makes it compact and reconfigurable. The instrument consists of a hardware for establishing external linkage and a Python based computer-controlled interface to control it from a remote PC as well as for acquiring and plotting relevant data. A steep roll-off of the filters (6 dB/octave to 24 dB/octave), low noise density (30 nV/√Hz @ 100 kHz) in the lock-in amplifier and a PID bandwidth of 100 kHz makes it useful for wide range of applications. Versatility of the instrument is demonstrated in three different experiments, (i) Auger electron spectroscopy, (ii) low coherence optical tomography and (iii) laser frequency stabilization followed by self identification of the setpoint upon unlocking and automatic relocking to that.

CSIR-National Physical Laboratory (CSIR-NPL), New Delhi is the custodian of primary time scale generating Indian Standard Time (IST). In the present paper we report the major reduction of the uncertainty of Primary Time Scale from 20 ns to 7.2 ns and finally to 2.8 ns. We also discuss current time dissemination methods being employed and the tasks undertaken for upgradation and strengthening of the primary time scale to serve the pan India in a better way at par with the international level.

A strong demand of a separate time zone by northeast populace has been a matter of great debate for a very long period. However, no implementable solution to this genuine problem has yet been proposed. The CSIR-National Physical Laboratory, CSIR-NPL (the National Measurement Institute, NMI, of India and custodian of Indian Standard Time, 1ST) proposes an implementable solution that puts the country in two time zones: (1) IST-I (UTC + 5 : 30 h, represented by longitude passing through 82 degrees 33E) covering the regions falling between longitude 68 degrees 7 E and 89 degrees 52 E and (ii) IST-II (UTC + 6 : 30 h, represented by longitude passing through 97 degrees 30 E) encompassing the regions between 89 degrees 52 E and 97 degrees 25 E. The proposed demarcation line between IST-I and IST-II, falling at longitude 89 degrees 52 E, is derived from analyses of synchronizing the circadian clocks to normal office hours (9 : 00 a.m. to 5 : 30 p.m.). This demarcation line passes through the border of West Bengal and Assam and has a narrow spatial extension, which makes it easier to implement from the railways point of view. Once approved, the implementation would require establishment of a laboratory for 'Primary Time Ensemble - II' generating IST-II in any of the north-eastern states, which would be equivalent to the existing 'Primary Time Ensemble-I' at CSIR-NPL, New Delhi.

We have developed a digitally operated addressing and control module (DACM) for addressing and controlling of equipment from a remote computer using a communication protocol developed in-house. This is useful for automation of an experiment that uses multiple equipment in a pre-decided synchronized manner. We also report design of a multipurpose high voltage direct current (DC) source that provides output of 0–100 V with an average stability of 1.90 (36) mV and has minimum step size of 3 mV. Operation of the DACM is examined by selecting the desired equipment, which in this case is the dc source, and remotely controlling its output from a computer. We also show that this can generate voltage with different waveforms within a 0–10 Hz frequency bandwidth. Such computer controlled ultra-stable high voltage sources tuneable to any arbitrary waveforms at low frequencies have many applications such as, driving a piezo for smooth scanning of laser frequencies, tuning length of a Fabry–Perot cavity, biasing of the electrodes in an ion trap and so on.

The resonant frequency of a helical resonator, that efficiently delivers high voltage rf to an electrodynamic ion-trap, depends on capacitive and inductive loads. Estimation of these loads for the parts that are attached to the resonator are thus necessary for designing a resonator operating at a desired frequency. On the other hand, the trapped ion experiences a voltage induced by the capacitance resulting from the electrodes. This results to excess micromotion to the ion. Here, we estimate the capacitive and resistive loads arising from different parts of ourtrap.

Capacitive, inductive and resistive loads of an ion-trap system, which can be modelled as LCR circuits, are important to know for building a high accuracy experiment. Accurate estimation of these loads is necessary for delivering the desired radio frequency (RF) signal to an ion trap via an RF resonator. Of particular relevance to the trapped ion optical atomic clock, determination of these loads lead to accurate evaluation of the Black-Body Radiation (BBR) shift resulting from the inaccurate machining of the ion-trap itself. We have identified different sources of these loads and estimated their values using analytical and finite element analysis methods, which are found to be well in agreement with the experimentally measured values. For our trap geometry, we obtained values of the effective inductive, capacitive and resistive loads as: 3.1 μH, 3.71 (1) μH, 3.68 (6) μH; 50.4 pF, 51.4 (7) pF, 40.7 (2) pF; and 1.373 Ω, 1.273 (3) Ω, 1.183 (9) Ω by using analytical, numerical and experimental methods, respectively. The BBR shift induced by the excess capacitive load arising due to machining inaccuracy in the RF carrying parts has been accurately estimated, which results to a fractional frequency shift of 6.6 × 10−17 for an RF of 1 kV at 2π × 15 MHz and with ±10 μm machining inaccuracy. This needs to be incorporated into the total systematic uncertainty budget of a frequency standard as it is about one order of magnitude higher than the present precision of the trapped ion optical clocks.

We present precise values of the dipole polarizabilities (α) of the ground [4f6s] S1/2 and metastable [4f 5d] D3/2 states of Yb , that are important in reducing systematics in the clock frequency of the [4f6s] S1/2 → [4f 5d] D3/2 transition. The static values of α for the ground and [4f5d] D3/2 states are estimated to be 9.8(1)×10 JmV and 17.6(5)×10 JmV, respectively, while the tensor contribution to the [4f5d] D3/2 state as −12.3(3)× 10 −40 JmV compared to the experimental value −13.6(2.2)× 10 JmV. This corresponds to the differential scalar polarizability value of the above transition as −7.8(5)× 10 JmV in contrast to the available experimental value −6.9(1.4)× 10 JmV. This results in the black-body radiation (BBR) shift of the clock transition as −0.44(3) Hz at the room temperature, which is large as compared to the previously estimated values. Using the dynamic α values, we report the tune-out and magic wavelengths that could be of interest to subdue systematics due to the Stark shifts and for constructing lattice optical clock using Yb.

Supplying high voltage radio frequency (RF) is a critical part of ion trapping system due to impedance mismatching between RF source and the ion trap. A helical resonator has been constructed in order to deliver narrow bandwidth and high voltage RF to the ion trap for stable confinement of ions. The performances of the helical resonator have been studied for different capacitive load of the ion trap. Both the resonant frequency and quality factor of the resonator show strong dependence on external capacitive loads.