Jet-Loaded Cold Atomic Beam Source for Strontium Minho KwonAaron HolmanQuan Gan Chun-Wei Liu Matthew Molinelli Ian Stevenson and Sebastian Will Department of Physics Columbia University 538 West 120th Street New York New York 10027 USA

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Jet-Loaded Cold Atomic Beam Source for Strontium

Minho Kwon,∗Aaron Holman,∗Quan Gan, Chun-Wei Liu, Matthew Molinelli, Ian Stevenson, and Sebastian Will†

Department of Physics, Columbia University, 538 West 120th Street, New York, New York 10027, USA

(Dated: February 3, 2023)

We report on the design and characterization of a cold atom source for strontium (Sr) based on a two-

dimensional magneto-optical trap (MOT) that is directly loaded from the atom jet of a dispenser. We char-

acterize the atom ﬂux of the source by measuring the loading rate of a three-dimensional MOT. We ﬁnd loading

rates of up to 108atoms per second. The setup is compact, easy to construct, and has low power consumption. It

addresses the long standing challenge of reducing the complexity of cold beam sources for Sr, which is relevant

for optical atomic clocks and quantum simulation and computing devices based on ultracold Sr.

INTRODUCTION

In ultracold quantum science the impact of atoms with two

valence electrons, such as strontium (Sr) and ytterbium (Yb),

has dramatically increased over the past years [1–12]. With a

level structure that features singlet and triplet electronic states,

these atoms have a wide gamut of internal transitions [13, 14],

including transitions with broad linewidths (several MHz) that

are well-suited for highly effective laser cooling; with nar-

row linewidths (tens of kHz) that are used for laser cooling

to Doppler temperatures in the microkelvin range; and with

ultranarrow linewidths (less than Hz) that enable highly co-

herent quantum operations. Combined with the presence of

magic wavelengths [15–19], tune-out wavelengths [20], and

optically trappable Rydberg states [11, 21], which all are a re-

sult of the rich two-electron level structure, Sr and Yb have

emerged as important atoms for optical atomic clocks, quan-

tum simulators, and quantum computers.

Focusing on Sr, several groundbreaking advances in the

past few years have shown its exceptional potential for quan-

tum science and technology. Today’s most accurate atomic

clocks are using fermionic 87Sr trapped in optical lattices

[22, 23]. Building on these advances, the concept of Sr atomic

clocks is currently combined with optical tweezer trapping

technology, showing competitive clock performance [24–26].

Sr atomic clocks are a potential candidate for an upcoming re-

deﬁnition of the second [27, 28], replacing the deﬁnition from

1967 based on a microwave transition in cesium. In addition,

optical tweezer platforms utilizing 87Sr and 88Sr are showing

great promise for quantum computing [29, 30], including the

demonstration of highly coherent nuclear spin qubits [31] and

Bell state generation with extremely high ﬁdelity [21].

To realize the promise of Sr platforms on a broad scale and

allow for the construction of deployable Sr-based quantum de-

vices, robust and compact hardware for the preparation of ul-

tracold Sr is critical. In this context, Sr atomic sources face

particular technical challenges. Due to its high melting (769

◦C) and boiling point (1,384 ◦C), Sr tends to stick to view-

ports and the inner walls of room-temperature vacuum cham-

bers. As a result, sources based on a vapor cell, which are

highly functional for alkali atoms, such as Rb and Cs, can-

not be realized for Sr (similar to Yb, Er, Dy). Instead, Sr

sources often rely on an effusive oven combined with a Zee-

man slower. Such slowers have a cold atom ﬂux of up to 109

atoms per second, but are large (typically 1 m long) and use

electromagnets that are power-hungry and intricate to build

[14, 32, 33]. It has been shown that permanent magnets can

be used to replace electromagnets, while the overall dimen-

sions of the slower remain large [34]. A more compact solu-

tion that combines a Zeeman slower with transverse cooling is

available commercially [35], reaching a trappable cold atom

ﬂux of about 109atoms per second [36], but is technologically

complex, costly, and difﬁcult to service.

Two-dimensional (2D) magneto optical traps (MOTs), cre-

ating an atomic beam via transverse laser cooling in two di-

rections, are a popular alternative source concept [37–40]. For

alkali atoms, 2D MOTs have been shown to lead to high atom

ﬂux, while ensuring a small footprint, in particular if atoms

can be introduced into the system via dispensers instead of ef-

fusive ovens [41–43]. However, for atomic species with low

vapor pressure, such as Sr and Yb, sources that are solely

based on a 2D MOT are not widely used, yet. Recent work

has shown a Sr 2D MOT with a ﬂux of up to 108atoms per

second[44, 45], but the system requires an oven to be heated

to about 500 ◦C. Focusing on setups based on compact dis-

pensers, a 2D MOT for Yb with a ﬂux of 107atoms per sec-

ond has been realized in a highly customized setup [46]. For

Sr, dispenser-based 2D MOT designs have been demonstrated

with a ﬂux of 105atoms per second [47, 48].

In this letter, we demonstrate a dispenser-based 2D MOT

for Sr with a cold atom ﬂux of up to 108atoms per second.

The ﬂux is measured via the loading rate of a 3D MOT, which

constitutes a conservative lower bound for the cold atom ﬂux

produced by the 2D MOT. The setup is compact, maintenance

free, consumes minimal electrical power due to the use of per-

manent magnets, and does not require a mechanical shutter to

stop the atomic ﬂux out of the source. While the performance

is about an order of magnitude below Zeeman slowers, for

most uses, such as optical lattice clocks and optical tweezer

arrays (requiring 100 to 10,000 atoms), the performance is

sufﬁcient. Straightforward modiﬁcations, discussed at the end

of the letter, should allow reaching a ﬂux similar to Sr Zeeman

slowers. Our setup is particularly suited for applications with

low SWaP (size, weight, and power) requirements.

arXiv:2210.14186v2 [physics.atom-ph] 2 Feb 2023

(a) (b) (c)

Permanent

magnets

Dispensers

λ/4 waveplates

2D MOT

Differential

pumping tube

Shields

Mirrors

Bias

electromagnets

Electrical

connections

Shields

Push beam

Dispensers

Differential

pumping tube

x1 cm

1 cm

FIG. 1. Overview of the technical implementation of the Sr 2D MOT. Schematic of the setup showing an axial view (a) and side view (b). (c)

Image of the dispensers and the mounting structure. The cylinder in the middle is the end of a mounting tube that holds the dispenser assembly.

SETUP

We start with a technical overview of the setup. Figure 1

shows the Sr 2D MOT design, illustrating the compact vac-

uum system and dispenser assembly.

Vacuum system

The vacuum system is mostly constructed from commer-

cially available ultra-high vacuum (UHV) components. The

2D-MOT chamber is a six-way cross made of non-magnetic

stainless steel (316SS). The vacuum is maintained by an ion

pump with a pumping speed of 20 l/s. The 2D-MOT cham-

ber is connected to the science chamber of the main apparatus

through an exit port. The exit port is comprised of a tube that

is 90 mm long with an inner diameter of 2 mm and serves as a

differential pumping tube, allowing for a pressure differential

of about 104between the 2D MOT and the science chamber.

The bore is vertically offset 3 mm above the center of the six-

way cross to account for the gravitational drop of the atomic

beam on the way to the science chamber. Transverse cooling

light enters from four uncoated Kodial glass viewports on the

sides. The axial ﬂange opposite to the exit port is designed to

accept mounting structures for the dispensers and the electric

connections, and has a through hole for the push beam.

The design allows for a relatively short distance between

the 2D MOT and the science chamber. The 2D MOT is formed

about 1 cm away from the opening of the differential pumping

tube and exits the differential pumping tube after about 10

cm of travel. The total travel distance between the 2D MOT

and the center of the 3D MOT is 43 cm. This is substantially

shorter than the 75 cm in an earlier realization of a Sr 2D MOT

[47]. Closer proximity increases the usable atomic ﬂux as the

atomic beam fans out less due to transverse motion on the way

to the science chamber. This issue is more pronounced than in

alkali 2D MOTs, as the transverse temperature of Sr remains

relatively high (about 1 mK). In our setup, the solid angle of

the atomic beam that can be captured is 126 mrad. We discuss

below how this can be further increased.

During operation of the 2D MOT, the pressure in the 2D-

MOT chamber remains as low as 1×10−9torr due to the low

vapor pressure of Sr.

Dispenser assembly

Sr atoms are introduced into our system by generating a hot

atomic jet that emerges from a resistively heated dispenser,

containing bulk atomic Sr. The dispenser assembly is custom-

designed. An important design criterion is to bring the output

opening of the dispensers as close as possible to the 2D MOT

trapping region (see Fig. 1). This allows for direct capture of

atoms from the dispenser jet, minimizing the amount of atoms

that ﬂy-by uncaptured and stick to the chamber walls.

To this end, we utilize two U-shaped dispensers produced

by a commercial vendor (AlfaVakuo) (see Fig. 1 (c)). They

are comprised of a steel tube, ﬁlled with bulk Sr with natural

abundance; the opening is a 5 mm long slit that before activa-

tion is sealed with indium. For the data reported here, we use

dispensers with 2 mm diameter and a ﬁlling of 40 mg of Sr.

Larger capacity dispensers with a ﬁlling of more than 200 mg

of Sr can be accommodated with a similar design. The dis-

tance between the output opening and the 2D-MOT trapping

region is about 1.5 cm. For electrical connection, the ﬂat legs

of the dispensers are connected to BeCu in-line barrel con-

nectors that are isolated from the vacuum ﬂange with ceramic

spacers (FTACERB068, Kurt J. Lesker).

In order to block the hot atom jet from coating the view

ports, we have placed L-shaped shields around the dispensers

made from stainless steel (SAE 304) sheet metal (see Fig. 1

(c)). The shields have a cut-out that restricts the solid angle

of the fanned-out hot atom ﬂux; the cut-out is narrow enough

to protect the viewports from Sr coating and large enough to

fully expose the trapping region.

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Jet-LoadedColdAtomicBeamSourceforStrontiumMinhoKwon,AaronHolman,QuanGan,Chun-WeiLiu,MatthewMolinelli,IanStevenson,andSebastianWillDepartmentofPhysics,ColumbiaUniversity,538West120thStreet,NewYork,NewYork10027,USA(Dated:February3,2023)Wereportonthedesignandcharacterizationofacoldatomsourceforstron...

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Jet-Loaded Cold Atomic Beam Source for Strontium Minho KwonAaron HolmanQuan Gan Chun-Wei Liu Matthew Molinelli Ian Stevenson and Sebastian Will Department of Physics Columbia University 538 West 120th Street New York New York 10027 USA

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