VOLUME 78, NUMBER 19 PHYSICAL REVIEW LETTERS 12 MAY 1997
Buffer-Gas Loading and Magnetic Trapping of Atomic Europium
Jinha Kim, Bretislav Friedrich, Daniel P. Katz, David Patterson,
Jonathan D. Weinstein, Robert DeCarvalho, and John M. Doyle
Department of Physics, Harvard University, Cambridge, Massachusetts 02138
(Received 13 January 1997)
Atomic europium has been magnetically trapped using buffer-gas loading. Laser ablated Eu
8
S
72
atoms are thermalized to 800 mK in a
4
He buffer gas (to 250 mK in a
3
He buffer gas). Anti-Helmholtz
superconducting coils produce a quadrupole magnetic field to trap the M
J
72 state of Eu. Detection
is via absorption spectroscopy at 462.7 nm. Up to 1 3 10
12
Eu atoms are loaded at a central density
of 5 3 10
12
cm
23
. Atoms can be held for longer than 100 s. [S0031-9007(97)03151-7]
PACS numbers: 32.80.Pj, 06.30.Ft, 32.30.Jc, 39.30. + w
Magnetic trapping of atoms has opened up important
areas of research including the creation and study of
weakly interacting atomic Bose condensates [1], high
resolution spectroscopy of atomic hydrogen [2], and the
realization of an “atom laser” [3,4]. However, despite the
fact that many ground state atoms are paramagnetic (about
70% of the periodic table) [5], only the alkali atoms and
atomic hydrogen have been magnetically trapped. This
paucity of trapped species is mainly due to the lack of
a general technique to load atoms into magnetic traps.
Recent efforts in atom trapping have been mostly limited to
atomic species whose structure allows for optical cooling; a
notable exception is atomic hydrogen which can be cooled
by thermalizing with liquid helium films [6,7].
A general technique for loading atomic and molecu-
lar species into a magnetic trap was recently proposed
by Doyle et al. [5]. It is based on the use of a cold he-
lium buffer gas to thermalize the species to energies below
the depth of the trap. After thermalization, the helium is
(cryo-)pumped away, leaving a thermally isolated, trapped
sample. Because buffer-gas loading relies on elastic col-
lisions it is essentially independent of the structure of the
trapped species and should therefore find wide application
in filling atomic and molecular traps. This feature of the
technique is especially important for the case of molecules
where the complex energy level structure precludes a
simple method for radiative cooling. Trapping is an im-
portant step towards ultrahigh resolution spectroscopy of
molecules and may find application in improved searches
for an elementary particle electric dipole moment [8].
We report magnetic trapping of atomic europium using
this buffer-gas loading technique. Eu atoms (vaporized by
laser ablation) are thermalized by a He buffer gas and are
loaded into a static magnetic trap. Our choice of ground
state Eu atoms
8
S
72
, g 1.993 as a test species was
led in part by considerations of experimental convenience:
The large paramagnetism of Eu makes it easy to trap and
the high oscillator strength of its visible transitions makes
it easy to detect.
Briefly, our trapping procedure begins with a cryogenic
cell filled with either
4
He or
3
He gas. The cell is inside a
spherical quadrupole magnetic trapping field with a depth
of up to 2.8 T produced by superconducting coils in an
anti-Helmholtz configuration. The cell, and therefore the
He gas, is maintained at 800 mK for
4
He (250 mK for
3
He). Europium atoms are vaporized by laser ablation
and thermalized by colliding with the He. Those atoms in
low-field seeking states whose kinetic energies are below
the trap depth are trapped. After loading, the buffer gas is
cryopumped away. The atoms are detected via absorption
spectroscopy in the y
8
P
72
2 a
8
S
72
band at 462.7 nm.
The cross-section of the cryogenic apparatus is de-
picted in Fig. 1. The apparatus consists of four parts: the
superconducting magnet, the cryogenic cell, the He can,
and the dilution refrigerator. The magnet (5.1 cm diame-
ter clear bore) consists of two NbTi superconducting
solenoids encased in a titanium cask. The two coils are
oriented in the anti-Helmholtz configuration and there-
fore repel each other. The repulsive force (of up to
2.5 3 10
5
N) is taken up by the cask. The individual
solenoids are both 2.8 cm thick with an inner and outer
diameter of 5.3 and 13.0 cm, respectively. Their centers
are separated by 3.3 cm. The entire magnet assembly is
immersed in liquid helium.
The cell is positioned at the center of the magnet. It
resides in vacuum and is separated from the magnet (and
the liquid helium) by a stainless steel vacuum can, a tube
of 5 cm diameter and 0.8 mm wall thickness. The cell
(innerouter diameter of 4.27 cm4.50 cm and length of
6.8 cm) is made of OFE copper with a 4.4 cm diameter
fused silica window sealing the bottom. On the inner top
surface there is a 1 cm diameter mirror. A solid lump
of Eu (the source of the atoms) is positioned near the
mirror. The top of the cell is thermally anchored to the
mixing chamber of the dilution refrigerator via a copper
rod of 1.3 cm diameter. The temperature of the cell can
be varied from 100 to 800 mK using a resistive heater.
The purpose of the He can is to allow the helium
buffer gas to be cryopumped out of the cell. The He
can is connected to the cell through a 3.8 cm diameter
thin-walled stainless steel tube. The tube allows gas to
flow freely between the cell and the can. The He can is
connected to the mixing chamber with a 2 cm diameter
copper rod.
0031-9007 97 78(19) 3665(4)$10.00 © 1997 The American Physical Society 3665