Fast loaded dual species magneto-optical trap of cold Sodium and Potassium atoms with light-assisted inter-species interaction Sagar Sutradhar1Anirban Misra1Gourab Pal1Sayari Majumder1Sanjukta Roy1and Saptarishi Chaudhuri1

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Fast loaded dual species magneto-optical trap of cold Sodium and

Potassium atoms with light-assisted inter-species interaction

Sagar Sutradhar,1Anirban Misra,1Gourab Pal,1Sayari Majumder,1Sanjukta Roy,1and Saptarishi Chaudhuri1

Raman Research Institute, C. V. Raman Avenue, Sadashivanagar, Bangalore 560080, India

(*Electronic mail: srishic@rri.res.in)

(Dated: 23 August 2023)

We present the design, implementation and detailed experimental characterization and comparison with numerical sim-

ulations of two-dimensional Magneto-optical traps (MOT) of bosonic 23Na and 39K atoms for loading the cold atomic

mixture in a dual-species 3DMOT with a large number of atoms. We report our various measurements pertaining to the

characterisation of the two 2D+MOTs via the capture rate in the 3DMOT and also present the optimised parameters

for the best performance of the system of the cold atomic mixture. In the optimised condition, we capture more than

3×1010 39K atoms and 5.8×108 23Na atoms in the 3DMOT simultaneously from the individual 2D+MOTs with the

capture rate of 5 ×1010 atoms/sec and 3.5×108atoms/sec for 39K and 23Na, respectively. We also demonstrate im-

provements of more than a factor of 5 in the capture rate into the 3DMOT from the cold atomic sources when a relatively

high-power ultra-violet light is used to cause light-induced atomic desorption in the 2D+MOT glass cells. A detailed

study of the light assisted interspecies cold collisions between the co-trapped atoms is presented and interspecies loss

coefﬁcients have been extracted to be, βNaK ∼2×10−12 cm3/sec. The cold atomic mixture would be useful for further

experiments on Quantum simulation with ultra-cold quantum mixtures in optical potentials.

I. INTRODUCTION

Ultra-cold quantum gases in optical potentials offer a

versatile platform for Quantum Simulation1–5, precision

measurements6and Quantum Technologies7due to the high

degree of controllability of such systems such as inter-atomic

interaction, dimensionality, spin states and external potentials.

This makes ultra-cold atomic ensembles an ideal ‘quantum

toolbox’ leading to unprecedented progress in this research

ﬁeld.

Quantum gas mixtures with dual atomic species has at-

tracted considerable interest since they offer a wealth of novel

possibilities. Quantum degenerate mixtures realized by using

single atomic species in different Zeeman sub-levels8–10, mul-

tiple isotopes of same species or different atomic species11–26

can be used to investigate novel quantum phases hitherto un-

explored in single atomic species. For example, the physics

of impurities coupled to degenerate gas27–29 is of fundamen-

tal importance in condensed matter systems. Novel exotic

quantum phases such as quantum droplets in a spin mixture

of Bose gases have recently been proposed30 and observed

in homo- and hetero-nuclear quantum mixtures31–33. Quan-

tum mixtures can also be used to create hetero-nuclear sta-

ble polar molecules34–36 which is useful to study controlled

ultra-cold chemistry37 as well as long-range anisotropic dipo-

lar interactions38–41.

A Quantum degenerate mixture of sodium and potassium is

an attractive combination for a hetero-nuclear quantum mix-

ture experiment. Both the Bose-Bose mixture (23Na-39K,

23Na-41K) and Bose-Fermi mixture (23Na-40K) can be ob-

tained opening up a myriad of possibilities for exploring the

many-body physics arising due to the interplay between inter-

species and intra-species interaction with quantum statistics

playing a signiﬁcant role. Another important advantage of

the combination of sodium and potassium for the hetero-

nuclear quantum mixture is that the Na-K ground-state po-

lar molecules42,43 are chemically stable as compared to other

combinations of inter-species hetero-nuclear molecules with a

large dipole moment of ∼2.72 Debye paving the way to ex-

plore long-range dipolar interaction for quantum simulation44.

In this article, we describe our experimental setup to real-

ize an ultra-cold atomic mixture of 23Na and 39K atoms in a

dual-species magneto-optical trap (3DMOT) loaded from cold

atomic beams produced via two independent, compact and ef-

ﬁcient two-dimensional magneto-optical traps (2D+MOTs) of

23Na and 39K. We also present the detailed characterisation of

the performance of the cold atom sources of both 23Na and

39K atoms to obtain the optimised experimental parameters

for the best possible performance of the cold atomic beam

sources.

The various sections in this article are organised as follows:

In section II, we provide a detailed description of the experi-

mental system including the ultra-high vacuum assembly and

laser systems. In section III, we focus on the characterisa-

tion and performance of the cold atomic beam sources. In

section IV we give a detailed theoretical description of the nu-

merical simulations performed in order to compare with the

experimental results of the atomic sources. We have provided

a complete system performance study in section V. Finally,

we discuss about the interspecies light-assisted collisions be-

tween hetero-nuclear cold atoms in section VI.

II. EXPERIMENTAL SETUP

In this experimental setup, a large number of 23Na and 39K

atoms are simultaneously captured in a dual-species 3DMOT

from two independent sources of the cold atomic beams.

There are stringent requirements on the design of the appa-

ratus such as good optical access for trapping laser beams as

well as detection, ultra-high vacuum to ensure longer trap life-

time of the atoms and high magnetic ﬁeld gradient for mag-

arXiv:2210.14084v2 [physics.atom-ph] 22 Aug 2023

FIG. 1. (Color online) A schematic of the vacuum assembly. The two-species MOT is loaded from two independent 2D+MOTs as sources

of cold 23Na and 39K atoms. The dual-species 3DMOT is produced in a spherical octagonal chamber. The UHV side is pumped by three

large-capacity ion pumps whereas the two independent source regions are pumped with two 20 l/s ion pumps. Coils made of hollow-cored

water-cooled copper tubes placed outside 3DMOT chamber are used to generate the quadrupole magnetic ﬁeld for trapping of atoms. A single-

arm magnetic transport allows transferring the cloud to the ‘science cell’ with large optical access.

netic trapping. Our experimental setup is designed and built

up to fulﬁl these requirements and enable further experiments

on the quantum degenerate mixture in both magnetic and op-

tical potentials.

The conﬂicting requirements of having a large number of

atoms for experiments on degenerate quantum gases as well

as a long lifetime of the atomic cloud has led to the design of

multi-chamber vacuum systems for such experiments where

the MOT is loaded from a cold atomic beam source instead of

the background vapour. Examples of such cold atomic beam

sources are: Zeeman slower45, Low velocity intense source46,

2DMOT47, 2D+MOT48,49, and pyramidal MOTs50. Amongst

such possibilities, 2D+MOT offers the most compact design

with the most efﬁcient performance. For 23Na and 39K atoms,

Zeeman slowers45,51,52 and 2DMOTs52–55, have been realized.

In the case of hetero-nuclear atomic species mixture, to the

best of our knowledge, our experiment is the ﬁrst demonstra-

tion where both the atomic species are simultaneously derived

from compact 2D+MOT conﬁgurations.

A. Vacuum assembly

A schematic view of our vacuum system is shown in

Fig.1. A spherical octagon-shaped chamber for 3DMOT,

made with non-magnetic stainless steel (Kimball physics-

MCF600-SphOct-F2C8) placed at the centre of the vacuum

manifold is attached with two independent 2D+MOT glass

cells (Precision Glassblowing, Colorado, USA). For both

23Na and 39K atoms, the vacuum chamber of the 2D+MOT

consists of a cuboidal glass cell (dimensions 85 mm ×40

mm ×40 mm), whose longitudinal axis is aligned horizon-

tally and placed along the axis of a differential pumping tube

connecting the 2D+MOT glass cell and the 3DMOT chamber.

The atomic beam is prepared along the longitudinal axis of

the glass cell. The differential pumping tube was made from

a single block of oxygen-free highly conductive (OFHC) cop-

per. One end of the tube is a 45◦-angled polished mirror with a

round surface of diameter 18 mm and placed inside the glass

cell. The other end of the tube has a disk shape of diameter

≈48 mm and a thickness of 10 mm. This disk acts as a gas-

ket between the two CF40 ﬂanges of the 2D+MOT and the

3DMOT chamber. The 45◦surface of the copper tube allows

the alignment of the longitudinal cooling laser beams as de-

scribed later in this article.

The differential tube has a hole which originates at the cen-

ter of the 45◦surface and runs along the axis of the tube and

ends up at the UHV side of the 3DMOT chamber. The differ-

ential pumping hole starts with a diameter of 2 mm and then

widens up in two steps over a total distance of 270 mm. The

hole reaches a diameter of 8 mm (6 mm) after the ﬁrst 20 mm

length and subsequently widens up to 14 mm (12 mm) after

the next 120 mm length for 23Na (39K) tubes.

FIG. 2. (Color online) Schematic diagram of the 2D+MOT. Two

transverse cooling beams are retro-reﬂected using two helicity-

preserving right-angled prisms. In addition, a pair of longitudinal

cooling beams (pushing and retarding beams) are aligned along the

line of zero magnetic ﬁeld created due to the conﬁguration of the four

race-track-shaped coils. The copper tube with a differential pumping

hole connecting the 2D+MOT and the 3DMOT sides is cut at an

angle of 45oand mirror-polished to facilitate the passage of the re-

tarding beam. An additional pushing beam is used to direct the cold

atomic beam to the 3DMOT chamber through the differential pump-

ing hole.

The differential pumping tube has a conductance of 0.043

l/s (0.038 l/s) for the 23Na (39K) side. The two 2D+MOT glass

cells are individually pumped using two 20 l/s Ion pumps. The

3D-MOT chamber is pumped by a 75 l/s Ion pump, and the

generated pressure ratio between the two chambers is 1200

(1400) for 23Na (39K) side.

Additionally, our experimental system includes a magnetic

transport tube and a glass cell (‘science cell’) of dimension

85 mm ×30 mm ×30 mm pumped by two more Ion pumps

with 40 l/s and 75 l/s pumping speeds. We also occasion-

ally use a Titanium Sublimation pump to maintain the base

pressure below 10−11 mbar near the ‘science cell’. The base

pressure near the 3DMOT chamber is measured using an ion-

isation gauge to be ∼7×10−11 mbar which is also consistent

with our observed cold atom trap lifetime of ∼48 s. On the

other hand, both the 2D+MOT glass cells are maintained at a

base pressure below 10−9mbar.

We have used a natural abundance source (ingot) of sodium

( Sigma Aldrich(262714-5G)). The ingot is placed inside a

CF16 full nipple and attached to the glass cell through a CF16

angle gate valve (MDC vacuum). Heatings tapes are wrapped

around the full nipple and the gate valve in such a way that, we

could maintain a temperature gradient from the oven towards

the glass cell, which ensures the sodium drifts into the cell and

remains there. The purpose of the gate valve is two-fold, ﬁrst,

it determines the amount of ﬂow of sodium vapour into the

glass cell and, second, during replenishment of the source it

would allow us to isolate the oven from the rest of the vacuum

system.

We have also used a natural abundance source (ingot) of

potassium from Sigma Aldrich (244856-5G) as the source for

loading atoms in the 39K 2D+MOT. The design of the potas-

sium oven is similar to the sodium one. Here we have kept

natural abundance potassium and enriched 40K (10% enrich-

ment, from Precision Glassblowing, USA), inside two differ-

ent CF-16 full nipples, followed by respective CF-16 angle

gate valves. These two ovens are connected and integrated

with the 2D+MOT glass cell.

B. Laser systems

The cooling and repumping beams for the laser cooling of

sodium atoms were derived from a frequency-doubled Diode

laser system (Toptica TA-SHG pro) which typically gives a

total output power of 1100 mW at 589 nm (23Na D2 transi-

tion). The laser beam from the TA-SHG pro is divided into

several beams. A low-power beam (typically 5 mW) is fed

into an AOM (AA optics, centre frequency 110 MHz) double-

pass assembly and subsequently directed into the saturation

absorption spectroscopy (SAS) setup. The spectroscopy for

sodium is realized using a vapour cell of length 75 cm from

Triad technologies (TT-NA-75-V-P), which is heated to 150◦C

to create a sufﬁciently high vapour pressure for absorption.

The cooling beams for the 2D+MOT as well as the 3DMOT

are generated using two independent AOM (Isomet 110

MHz) double-pass setups and tuned appropriately red-detuned

from the 32S1/2|F=2⟩ → 32P

3/2|F′=3⟩transition. The

repumping beams are tuned in resonance with the transi-

tion 32S1/2|F=1⟩ → 32P

3/2|F′=2⟩, by passing the cool-

ing beams through two independent Electro-optic modulators

(EOM) (QuBig-EO-Na1.7M3). The EOMs are powered by

two independent drivers (QuBig-E3.93KC), and each side-

band has typically 20% of the power of the carrier (cool-

ing) frequency. The co-propagating cooling and repump-

ing beams are injected into their respective polarization-

maintaining (PM) ﬁbers and transferred to the experimen-

tal optical table for the realization of the 2D+MOT and the

3DMOT.

For potassium atoms, we use two independent External

Cavity Diode Lasers (ECDL) from Toptica Photonics for de-

riving the cooling (DL pro) and repumping (DL 100) laser

beams. Each of these laser outputs is ampliﬁed using two

independent tapered ampliﬁers (Toptica BoosTA pro) with a

maximum output power reaching 2 W. The output of each

of the Potassium lasers is divided into two beams, the one

with low power ≈5mW is fed into the SAS setup. The spec-

troscopy is realized with a glass vapour cell of length 5 cm, in

which a K-sample with natural abundance is heated to 50◦C.

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Fastloadeddualspeciesmagneto-opticaltrapofcoldSodiumandPotassiumatomswithlight-assistedinter-speciesinteractionSagarSutradhar,1AnirbanMisra,1GourabPal,1SayariMajumder,1SanjuktaRoy,1andSaptarishiChaudhuri1RamanResearchInstitute,C.V.RamanAvenue,Sadashivanagar,Bangalore560080,India(*Electronicmail:sris...

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Fast loaded dual species magneto-optical trap of cold Sodium and Potassium atoms with light-assisted inter-species interaction Sagar Sutradhar1Anirban Misra1Gourab Pal1Sayari Majumder1Sanjukta Roy1and Saptarishi Chaudhuri1.pdf

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Fast loaded dual species magneto-optical trap of cold Sodium and Potassium atoms with light-assisted inter-species interaction Sagar Sutradhar1Anirban Misra1Gourab Pal1Sayari Majumder1Sanjukta Roy1and Saptarishi Chaudhuri1

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