Zeeman-Sisyphus Deceleration for Heavy Molecules with Perturbed Excited-State Structure Hiromitsu Sawaoka1 2Alexander Frenett1 2Abdullah Nasir1 2Tasuku

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Zeeman-Sisyphus Deceleration for Heavy Molecules with Perturbed Excited-State

Structure

Hiromitsu Sawaoka∗,1, 2 Alexander Frenett∗,1, 2 Abdullah Nasir,1, 2 Tasuku

Ono,1, 2 Benjamin L. Augenbraun,1, 2 Timothy C. Steimle,3and John M. Doyle1, 2

1Department of Physics, Harvard University, Cambridge, MA 02138, USA

2Harvard-MIT Center for Ultracold Atoms, Cambridge, MA 02138, USA

3School of Molecular Sciences, Arizona State University, Tempe, AZ 85287, USA

(Dated: October 18, 2022)

We demonstrate and characterize Zeeman-Sisyphus (ZS) deceleration of a beam of ytterbium mono-

hydroxide (YbOH). Our method uses a combination of large magnetic ﬁelds (∼2.5 T) and optical

spin-ﬂip transitions to decelerate molecules while scattering only ∼10 photons per molecule. We

study the challenges associated with the presence of internal molecular perturbations among the

excited electronic states and discuss the methods used to overcome these challenges, including a

modiﬁed ZS decelerator using microwave and optical transitions.

I. INTRODUCTION

The rich internal structures of molecules can be utilized to precisely probe for physics beyond the Standard Model

(BSM) [1,2]. Current and planned experiments search for fundamental symmetry violations [3–7], time-variation of

fundamental constants, and dark matter [8–12]. In many cases molecules with heavy atomic constituents are used

because heavy nuclei cause relativistic enhancements that provide higher intrinsic sensitivity to fundamental symmetry

violating eﬀects originating from BSM particles (e.g. electric dipole moments (EDM) of elementary particles). Recent

proposals have also targeted polyatomic molecules, which generically possess nearly degenerate rotational-vibrational

states. Such structure leads to high molecular polarization at low electric ﬁelds, parity doublets (useful for EDM

experiments), as well as high sensitivty to time variation of fundamental constants. [12–14]. These features, which

are helpful for precision measurements, may also lead to perturbations that make it technically challenging to achieve

the necessary level of quantum state control.

Taking full advantage of molecules for precision measurements requires cooling them to ultracold (.100 µK)

temperatures. Low temperatures suppress broadening mechanisms and allows for trapping and concomitant long

interaction times [15,16]. One approach to creating ultracold molecules is laser cooling, where molecules are ﬁrst

cooled cryogenically (creating a cold beam), then radiatively slowed, and then loaded into a magneto-optical trap

(MOT). Additional methods can then be used for cooling and loading into traps, for example an optical dipole trap

(ODT). These steps have led to the successful cooling and loading into an ODT of diatomic (SrF, CaF, YO) and

triatomic (CaOH) molecules [17–20], and the magnetic trapping of CaF [21]. Radiative slowing is widely recognized

as one of the most diﬃcult steps in this process because, under typical conditions, ∼104photons per molecule are

required, a constraint that leads to technical challenges due to leakage into dark states. Transverse pluming of the

molecular beam due to many photon momentum kicks also leads to loss of molecular ﬂux.

Many polyatomic molecules, including those proposed for next-generation BSM measurements [13,14,22–26], can be

easily photon cycled hundreds of times—but cycling 104photons is challenging due to these molecules’ complex internal

structures. Because radiative slowing consumes the most photons in the laser cooling sequence, ﬁnding alternative

(non-radiative) methods for the slowing step would open the methods of MOT loading and optical trapping to a

broader set of molecules.

One alternative to radiative slowing is a deceleration method using magnetic ﬁelds and single optical pumping

in a ”Zeeman-Sisyphus” (ZS) conﬁguration [27,28]. This method builds on previously demonstrated magnetic trap

loading techniques [29], extended to multiple stages. The ﬁrst realization of the ZS decelerator leveraged the large

(∼1–3 K) energy shifts experienced by paramagnetic molecules in Tesla-level magnetic ﬁelds, as demonstrated on the

polyatomic species CaOH. Nearly 10% of a molecular beam was decelerated to velocities suﬃciently low for direct

MOT loading [30]. The average slowed molecule in that work scattered fewer than 10 photons per molecule, meaning

the “photon budget” for deceleration to MOT capture velocities was reduced by a factor of ∼103. Open questions

about the method remained, however, such as whether the deceleration method will be equally applicable to heavy

molecules, particularly when their excited electronic states are perturbed and/or possess large magnetic g-factors.

In this paper, we use YbOH to study the application of ZS deceleration to heavier molecules with more complex

structure. We ﬁrst review the ZS deceleration scheme, and then present a spectroscopic characterization of the YbOH

*These authors contributed equally to this work.

arXiv:2210.10859v1 [physics.atom-ph] 19 Oct 2022

FIG. 1. Overview of the ZS deceleration scheme. (a) Simpliﬁed representation of the magnetic ﬁeld tuning for an ideal

2Π−2Σ+electronic transition. (b) Level diagram indicating the background-free detection scheme. (c) Schematic rendering

of the decelerator magnets and detection region.

level structure in magnetic ﬁelds up to ∼2.5 T. Next, we describe how this structure aﬀects the eﬃciency of the

ZS deceleration. The features discussed are expected to be generic to many heavy molecules of current interest.

Finally, we demonstrate near-optimal optical pumping eﬃciency and deceleration of YbOH using an abbreviated ZS

decelerator, accumulating molecules near 20 m/s. Our results show that ZS deceleration can be extended to heavy

polyatomic molecules, but care must be paid to the complex level structure in planning the experiment.

II. EXPERIMENTAL SETUP

The ZS decelerator (see Fig. 1) has been described in detail previously [30]. In brief, molecules in a WFS state

are incident on a region of increasing magnetic ﬁeld magnitude and decelerate as they proceed into higher magnetic

ﬁelds. Near the magnetic ﬁeld maximum, the molecules are optically pumped through an electronically excited state

to a SFS state and continue to decelerate as they exit the high-ﬁeld region. The process can be repeated to remove

more energy. In the experiments described here, we use a two-stage decelerator.

In this paper, we denote speciﬁc energy levels in the 2Σ+ground state using |˜

X2Σ+, v00

1, N00 , M00

si, where v00

1is the

quanta in the ﬁrst vibration mode (other vibration modes have 0 quanta for the states we are interested in), N00 is

the rotational quantum number in Hund’s case (b), and M00

sis either strong-ﬁeld-seeking (SFS) or weak-ﬁeld-seeking

(WFS) at nonzero magnetic ﬁelds. The parity of the state is determined by p00 = (−1)N00 . If not speciﬁed, v00

1= 0

and N00 = 1 is assumed. In the excited 2Π1/2state we use the notation |˜

A2Π1/2, v0

1, J0, p0i, where v0

1is the quanta in

the ﬁrst vibration mode, J0is the total electronic angular momentum in Hund’s case (a), and p0is the parity. If not

speciﬁed, v0

1= 0, J0= 1/2 and p0= + is assumed.

YbOH molecules are produced by laser ablation of a target pressed from a stoichiometric mixture of Yb and Yb(OH)3

powders. The YbOH molecules are produced inside a buﬀer gas cell held at ∼2 K and ﬁlled with He buﬀer gas at

a typical atom density of 1015 cm−3. The molecules thermalize to the cell temperature and are hydrodynamically

extracted from the cell through a 7 mm diameter aperture [31]. The molecules then enter a second gas collisional cell

that is 20 mm long [31,32] held at 0.9 K by a pumped 3He pot. A 2 mm gap between the ﬁrst and second cells is

used to tune the buﬀer gas density in the second stage, optimizing the molecular beam’s forward velocity and ﬂux.

Typical peak forward velocities after the second cell are between 30 and 50 m/s (equivalent kinetic energy ∼15 K).

The molecules propagate 55 cm to the decelerator, which consists of two superconducting Helmholtz magnets.

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Zeeman-SisyphusDecelerationforHeavyMoleculeswithPerturbedExcited-StateStructureHiromitsuSawaoka,1,2AlexanderFrenett,1,2AbdullahNasir,1,2TasukuOno,1,2BenjaminL.Augenbraun,1,2TimothyC.Steimle,3andJohnM.Doyle1,21DepartmentofPhysics,HarvardUniversity,Cambridge,MA02138,USA2Harvard-MITCenterforUltracold...

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Zeeman-Sisyphus Deceleration for Heavy Molecules with Perturbed Excited-State Structure Hiromitsu Sawaoka1 2Alexander Frenett1 2Abdullah Nasir1 2Tasuku

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