
High-Stability Single-Ion Clock with 5.5×10
− 19
Systematic Uncertainty
Mason C. Marshall ,
1,*
Daniel A. Rodriguez Castillo ,
1,2
Willa J. Arthur-Dworschack ,
1,2
Alexander Aeppli ,
2,3
Kyungtae Kim ,
2,3
Dahyeon Lee ,
2,3
William Warfield ,
2,3
Joost Hinrichs ,
1,4
Nicholas V. Nardelli ,
1
Tara M. Fortier,
1
Jun Ye ,
2,3
David R. Leibrandt ,
1,2,5
and David B. Hume
1,2,†
1
Time and Frequency Division, National Institute of Standards and Technology, Boulder, Colorado, USA
2
Department of Physics, University of Colorado, Boulder, Colorado, USA
3
JILA, National Institute of Standards and Technology and the University of Colorado, Boulder, Colorado, USA
4
Institute of Quantum Optics, Leibniz University Hannover, Hannover, Germany
5
Department of Physics and Astronomy, University of California, Los Angeles, California, USA
(Received 25 April 2025; accepted 17 June 2025; published 14 July 2025)
We report a single-ion optical atomic clock with a fractional frequency uncertainty of 5.5 × 10
−19
and
fractional frequency stability of 3.5 × 10
−16
=
ffiffiffiffiffiffiffi
τ=s
p
, based on quantum logic spectroscopy of a single
27
Al
þ
ion. A cotrapped
25
Mg
þ
ion provides sympathetic cooling and quantum logic readout of the
27
Al
þ1
S
0
↔
3
P
0
clock transition. A Rabi probe duration of 1 s, enabled by laser stability transfer from a remote
cryogenic silicon cavity across a 3.6 km fiber link, results in a threefold reduction in instability compared to
previous
27
Al
þ
clocks. Systematic uncertainties are lower due to an improved ion trap electrical design,
which reduces excess micromotion, and a new vacuum system, which reduces collisional shifts. We also
perform a direction-sensitive measurement of the ac magnet ic field due to the rf ion trap, eliminating
systematic uncertainty due to field orientation.
DOI: 10.1103/hb3c-dk28
Introduction—Optical atomic clocks based on spectros-
copy of dipole-forbidden electronic transitions in isolated,
trapped atoms are among the most precise instruments
developed, capable of measuring time more precisely than
the cesium clocks that currently define the second [1].
Accordingly, optical clock frequency ratios are some of the
most accurate measurements [2,3] and are used as probes
for new physics [4], including time variation of funda-
mental constants [5,6]; violations of local position invari-
ance [7]; constraints on dark matter [2,8,9]; and general
relativity at small scales [10,11]. Optical clocks based on
single trapped ions [12,13] and neutral atoms in optical
lattices [14] have reached fractional frequency uncertainties
below 10
−18
; further advances open new possibilities for
these investigations. Additionally, as the scientific com-
munity moves toward the redefinition of the second,
advances in the state of the art for clock accuracy and
stability are critical [15].
The exquisite degree of control and access to environ-
mentally insensitive transitions offered by trapped atomic
ions have made them a leading technology for measure-
ment accuracy. In particular, the
1
S
0
↔
3
P
0
transition in
singly ionized aluminum offers a high transition frequency,
long excited-state lifetime, and one of the lowest known
sensitivities to blackbody radiation [16–19]. In this Letter,
we report the accuracy and stability evaluation of the
current-generation
27
Al
þ
quantum logic clock at the
National Institute of Standards and Technology (NIST).
This clock realizes the lowest fractional frequency uncer-
tainty of any clock to date, at Δν=ν ¼ 5.5 × 10
−19
. Its
fractional instability of 3.5 × 10
−16
=
ffiffiffiffiffiffiffi
τ=s
p
represents a
threefold reduction in instability compared to the previous
NIST quantum logic clock [12]. Critical to these achieve-
ments are a more stable clock laser, an improved Paul trap
electrical design with reduced excess micromotion, and a
150× improvement in background gas pressure from a new
ultrahigh vacuum system.
Clock operation and stability—The operation of the
clock is similar to that described in [12,20]. The clock
cycle begins with preparation of the
27
Al
þ
clock ion into
one of the j
1
S
0
;m
F
¼5=2i states via optical pumping on
the
1
S
0
↔
3
P
1
transition. Sympathetic cooling on the
25
Mg
þ
logic ion then brings the ion pair to the Doppler
temperature limit. Finally, we probe the
27
Al
þ
clock
transition using Rabi spectroscopy followed by quantum
logic readout [21,22].
We probe both the m
F
¼þ5=2 and m
F
¼ −5=2
1
S
0
↔
3
P
0
transitions and generate a “virtual” first-order mag-
netic-field-insensitive transition from their mean frequency
[23]. Additionally, we alternate probing from opposite
directions, with the two probe beams counterpropagating
through the same single-mode optical fibers; the average of
opposite directions is insensitive to possible first-order
*
Contact author: mason.marshall@nist.gov
†
Contact author: david.hume@nist.gov
PHYSICAL REVIEW LETTERS 135, 033201 (2025)
Editors' Suggestion
0031-9007=25=135(3)=033201(6) 033201-1 © 2025 American Physical Society