Development of Atomic Frequency Standards at CSIR-NPL, India

Amitava Sen Gupta, Poonam Arora, Subhadeep De, Aishik Acharya, Neha Batra, Suchi Yadav, Subhasis

Panja and Ashish Agarwal

Time & Frequency Division, CSIR-NPL, Dr. K. S. Krishnan Marg, New Delhi – 110012

Abstract

Time & Frequency division at CSIR-NPL is responsible for the highest level of time and frequency measurements in India, maintenance of IST (Indian Standard Time), its dissemination and keeping it traceable to the International Bureau of Weights and Measures (BIPM) using ultra-precise satellite links. The main focus of the division is on the state-of-the-art R&D activities on atomic frequency standards namely Cesium atomic fountains, single trapped Ytterbium ion clock and Rubidium atomic clocks for space applications. In this paper, recent efforts and results on atomic frequency standards development are discussed

1. Introduction

National Physical Laboratory India (NPLI) is the Time Keeper of the Country and is responsible for highest level of time and frequency measurements in India at par with the international standards. Apart from timescale (UTC (NPLI)) generation and dissemination, major focus is on the R & D on time standards (namely Cs fountain clocks, optical clock based on single trapped Ytterbium ion and Rubidium clocks for space). Since having a stable and reliable timescale is crucial for operation and evaluation of primary frequency standards, the current architecture and operation of timescale at NPLI is discussed first. In the next sections, progress and results on development of Cs atomic fountain clocks, single trapped Ytterbium ion clock and Rubidium atomic clocks is described, repectively. The timescale UTC (NPLI) is maintained with the help of a five Cesium (Cs) atomic clocks and a Hydrogen Maser (shown in Fig. 1) with stringent environment control and clean uninterrupted power. All 5 Cs clocks and the H-Maser are contributing to International Atomic Time (TAI) through precise, regulated, automated inter comparison of clocks, precise satellite links and automated daily upload of data to BIPM. Dissemination of Time and frequency to users is being done using two techniques: Teleclock service and Network Time Protocol (NTP) service. Time transfer using ultra stable time links is being done by NPLI to contribute to TAI and maintain the National Time Scale. Regular time and frequency transfer sessions are being conducted using two way satellite time and frequency transfer system with 7 timing laboratories participating in Eu-Asia network using Russian satellite AM-2 at different frequencies & time slots which includes the measurement of clock difference of the two labs at a time. The uncertainty of time transfer link has been significantly reduced by introducing dual frequency GPS receivers, which are capable of phase tracking a GPS carrier and processing the data by the precise point positioning technique. With the time scale and time links in place, R & D on time and frequency standards is the most important activity.

2. Cesium Atomic Fountain Clocks

The primary standard of time and frequency is a Cs fountain clock. Most of the developed countries have developed such clocks which are already operating as primary standards [1-6]. T & F division at CSIR-NPL started efforts to realize India’s first fountain clock only few years back. The first Cesium atomic fountain (India-CsF1) frequency standard [7], as shown in

Fig. 1, is now completely assembled and op India-CsF1 has a (0, 0, 1) geometry of tgeometry, four out of the six cooling beamsare first loaded and cooled in MOT followemolasses (frequency detuning of vertical beabout 107 Cs atoms, cool them to about 7 µand launch them up by moving molasses msignals are detected in the detection zone. Aviz. Ramsey fringes (Fig. 2a), C-field mapcontinuously operated and its frequency is btotal uncertainty is close to 2.5 x 10-15. The are presently working to make more accurat

Fig. 2 (a): Ramsey Fringes corresponding tb

In 2011, NPLI has started a project to desigto carefully investigate the systematic er1016. Optical pumping will be used to incredistributed cavity phase shifts and the Mimprovements, it is expected that the 2nd fou

3. S

The variance of the frequency measuremen

(a)

perational.

the magneto-optical trap (MOT) for cooling and launcs are in horizontal plane and other are going up and downed by further cooling in optical molasses (OM). They areeams) and cooled further with polarization gradient coolµK by both magneto-optical trap (MOT) and polarizationmethod. The atoms are tossed up to 75 cm above the tr

At present, the fountain is fully assembled and we have gopping, frequency locking and stability analysis (Fig. 2bbeing evaluated for all kinds of statistical and systematic umajor contribution to the total uncertainty comes from thte estimations of collision coefficient.

Fig. 1: First Cs fountain India-CsF1

to a toss height of 65.4 cm above the trap center; (b) Allan deviabetween the fountain and Hydrogen maser.

gn and build a second Cs fountain with special design rrors in order to enhance the accuracy of our frequency sease the S/N ratio in this fountain. The cavity design has

MOT geometry of (1,1,1) has been chosen to reduce untain clock will be an order of magnitude more accurate

ingle Trapped Ytterbium Ion Clock

nt, Allan deviation, is inversely proportional to the energy

(b)

ching operations. In this , respectively. The atoms e launched using moving ling. It is possible to trap n gradient cooling (PGC) rap center and the return ot the preliminary results

b). The fountain is being uncertainties. The current e cold collision shift. We

ation of the frequnecy offset

features that enable us standard to a few parts in been improved to reduce light shifts. With these than India-CsF1.

y carried by each photons

and proportional to the linewidth of the interrogated transition. We are developing the first optical frequency standard in India with a single trapped Ytterbium ion (171Yb+) which will provide 100-1000 times better accuracy than the microwave clocks. A single ion will be trapped and laser cooled to about mK for precision frequency measurement of the ultra-narrow transition at the wavelength 467 nm. We present our ion trap design of end-cap geometry and its potential profile which has fractional anharmonicities 0.017 and 0.092 along axial and radial directions respectively.

Fig. 3: Lowest lying energy levels of 171Yb+ relevant to frequency standards (dashed lines) and laser cooling (solid lines). We will interrogate the |2S1/2; F=0, mF=0> - |2D3/2; F=1, mF=0> quadrupole transition at 435.5 nm.

Ytterbium atoms coming from a resistively heated oven will be photoionized using a pair of laser beams at wavelengths 399 nm and 369.5 nm. The trapped ions will be cooled to ~mK using a laser which drives the 0;1; 2/1

22/1

2 =−= FPFS cooling

transition at wavelength 369.5 nm (Fig. 3). A closed transition for an efficient laser cooling requires depopulating the 2/3

2D

and 2/7

2 F hyperfine states. Pair of additional lasers at 935 nm and 760 nm are therefore required for repumping atoms from

these states. Cold ions are required for confinement within sub wavelength spatial extension (Lamb Dicke regime). Hence the ions will be free from first order Doppler shift, which is particularly necessary for precision measurements. The fluorescence from the trapped ions at the 369.5 nm will be detected using a photomultiplier tube. Three orthogonal pairs of combined laser beams for detection (same as cooling) and probing are required to overlap within a volume of radius ~10 μm for measuring micromotions of the ion along three directions.

Fig. 4: Drawing of our (a) end-cap trap mounted on a standard conflat (CF-35) feed-through, (b) enlarged view of the electrodes where dimensions are given in mm and (c) potential along the z-direction (green). We fit harmonic (red) and anharmonic (yellow) functions to find out their strengths.

The Paul trap requires combination of a static, U, and oscillating, V cosΩ t, electric fields, Vt = U + V cosΩ t, to alter the local minima of the potential Φ(x,y,z,t) along radial and axial directions. The ion remains trapped in a 3D potential which alternates at a frequencyΩ, much faster than the motion of the ion. A harmonic potential is desired since the restoring force increases linearly in all directions from the centre of the trap, which can be achieved by an appropriate combination of U, V and Ω. We have chosen the end-cap geometry [1] due to its most optimal optical access which will be required for five lasers and the Yb-atomic beam, all overlapping at the centre of the trap. Also in an end-cap trap, a very steep confining potential can be produced for trapping few or a single ion. The end cap trap consists of a pair of electrodes facing each other. Figure 4a shows CAD drawing of our trap mounted on a ultra-high vacuum (UHV) feedthrough. Each of the electrodes is a combination of two co-axial electrodes – the inner one is made out of tantalum rod and the outer one is made out of tantalum tube and their edges are machined at particular angles (Fig. 4b). The diameter of the electrodes, their separation and also the subtended angle of the electrode tip determines shape of the potential (Fig. 4c). In practice Φ(x,y,z,t) has always anharmonic contribution due to imperfect alignment of the electrodes or tolerance in machining. Any nonzero anharmonic potential introduces systematic uncertainties to the measurement, which gives the most significant contribution to the accuracy of the precision measurement of the frequency in any atomic clock. The perturbation from a pure quadrupole field can be estimated from the higher order contributions and the dominating contribution arises from the octupole term. Design of the ion trap and UHV chamber is ready for fabrication. An rf resonator for efficient delivery of high frequency voltage to the electrodes is ready to use. We are engaged in designing the optical setup associated to photoionization, laser cooling of the ions, frequency stabilization of all the lasers and setting up very high resolution imaging optics for detection of the small signal coming from a single ion.

4. Rubidium Atomic Clocks

Another R & D activity is developing Rubidium clocks for India’s space applications. NPLI is contributing to India's strategic space programme by developing navigational space clocks. It has developed and transferred the critical technology of Rubidium atomic clock to ISRO. Initial model has been developed at NPL and is undergoing further developments at Satellite Applications Center before being integrated in the payload of the Indian Regional Navigation Satellite system. Further critical process for development of glass technology of Rubidium bulbs and cells is under development at NPL for making the indigenization of space clocks complete. For this process, a rare, highly enriched, isotope of Rubidium has been extracted in the lab. This has been achieved by a chemical reaction under ultra-high vacuum at 700o C. A modified ultra-high vacuum system is being developed for making the quantum devices required for making a stable atomic clock. \

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