arXiv:1905.03202v2 [physics.atom-ph] 3 Dec 2019 Study of loss dynamics of strontium in a magneto-optical trap Chetan Vishwakarma, 1 Kushal Patel, 1 Jay Mangaonkar, 1 Jamie L. MacLennan, 2 Korak Biswas, 1 and Umakant D. Rapol 1, ∗ 1 Department of Physics, Indian Institute of Science Education and Research, Pune 411008, Maharashtra, India 2 Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA Collisions with background atoms are known to induce a significant shift in the frequency of state- of-the-art optical atomic clocks and contribute to state decoherence in cold atom experiments. The effects of these collisions can be quantified by measuring their cross sections. We experimentally measured the collision cross section between 88 Sr-N2 in a Magneto-Optical Trap (MOT). The measurement was carried out by monitoring the atom number loss rate as a function of background pressure of N2 and the cross section thus obtained was 8.1(4)×10 -18 m 2 . The measured collision cross section has been utilized for the determination of C6 coefficient of the ground state ( 1 S0) of 88 Sr atom, which can be useful to estimate the relative frequency shift in the clock transition. We also estimate the loss rate induced by the combined effect of the decay of atoms in the long-lived 3 P 0 state and temperature-induced atomic losses from the capture volume of the MOT. We find that the contribution due to the latter is dominant in comparison to the other atomic loss channels and must be included in the studies that rely on the total loss rate measurement. I. INTRODUCTION The invention of optical frequency comb [1] in con- junction with laser cooling techniques has boosted ef- forts towards using optical transitions as a universal time standard. Among various architectures [2], neutral atom based platforms have become popular in realizing the next generation optical clocks which hold promise to revolutionize global timekeeping, precision sensing and probing the stability of fundamental constants [2–4]. The clock accuracy depends on accurately quantifying var- ious systematic shifts in the clock transition frequency; further progress will require reducing their respective un- certainties. Frequency shifts due to Background Gas Collisions (BGCs) are currently one of the largest sources of un- certainty in many of the best atomic clocks of various types [5–13]. This has motivated a number of theoreti- cal [14–18] and experimental [7, 19] efforts to study these shifts. In particular, in Ref. [14] a model is developed relating the shifts to dispersion coefficients (and thus to collisional cross sections), facilitating the estimation of shifts without directly probing the clock transitions. One of the leading candidates in neutral-atom based clocks is the Sr optical lattice clock, with a current per- formance of 2.0-2.1×10 −18 uncertainty in the two most- accurate ones [5, 6]. In these, BGC-induced shifts are the third largest uncertainty, contributing 4-6×10 −19 to the uncertainty budget. Recent work [19] had enabled a reduction in this contribution in the former clock [5] by a measurement of the frequency shift due to collisions with H 2 , with ongoing studies to investigate the effect of other species present in the background gas. A theoreti- cal estimate for the shift due to Sr-H 2 collisions was also recently made [15, 20]. * Electronic mail: umakant.rapol@iiserpune.ac.in While previous studies on BGC-induced shifts have mainly focused on H 2 because it is typically the domi- nant species present in the vacuum chamber, it is antici- pated that as the clock uncertainties continue to improve, it may become necessary to account for the contribution of other background gas species such as N 2 . Dispersion coefficients have been estimated for collisions of N 2 with alkali, noble, and various molecular gases and collisional cross sections have been reported for Rb-N 2 [21, 22], Ne*-N 2 [23, 24], Na-N 2 [25] and Ar-N 2 [26], but so far Sr-N 2 collisional properties have not been investigated. The traditional way to determine the collision cross section is to employ crossed beam technique [27, 28] where, two collimated atomic/molecular beams are gen- erated and made to intersect in a well-defined interac- tion region. Uncertainties in the number of target atoms and the volume of the intersection region are two major sources of uncertainty in the measurement. During an experiment, these are the two major sources of errors. Since the invention of the technique of laser cooling, an alternative way to measure the cross section is the atomic loss rate from the MOT, magnetic trap (MT), or the Op- tical Dipole Trap (ODT). Collision cross sections deter- mined in this manner have been shown to be more accu- rate with respect to the beam-based method [23]. There have been extensive experimental studies performed in the same spirit using Rubidium [21, 29, 30], Ytterbium [31], Cesium [32], Neon [23, 33] etc. In this article, we study the dynamics of a Sr MOT, by observing the loading and the loss rate under various con- ditions and repumping schemes. Using this data, we also report the first experimental determination of 88 Sr-N 2 collision cross section of the 1 S 0 ground state by measur- ing the loss rate of atoms from 88 Sr MOT operating with the first stage cooling transition. The collision cross sec- tion is determined by injecting nitrogen (N 2 ) inside the vacuum chamber in a controlled manner and studying the atomic loss rate at different background pressures. This measurement is helpful in evaluation of systematic shift