their position relative to some massive object is
The current best limits are primarily based on
systems optimized for general performance as
clocks, but certain systems have properties that
enhance their sensitivity to variations in constants. This can take place when contributions
to the energies from two physical mechanisms
accidentally cancel such that two energy levels
are far closer than normal. Tiny changes in a
fundamental constant can then have disproportionately large effects in the energy-level splitting. Examples include Dy atoms [where current
experiments are competitive with standard clocks
(46 )], molecules [where vibrational energies can
cancel electronic energy differences (47–49)],
highly charged atomic ions (50), and the 229Th
nucleus [where strong-force and electromagnetic
effects cancel to make a pair of levels close enough
to excite with a laser (51, 52)]. These systems have
the promise to enable orders-of-magnitude improved sensitivity in variations to µ and a. Finally, clocks will likely continue to improve at a
rapid pace, potentially greatly improving searches
for time variation of constants.
Whether the sort of ultralight quantum fields
that could give rise to varying constants actually
exist is anybody’s guess. What seems to be certain
is that the steady improvements in clocks and
related high-precision measurements of quantized energy levels are leading to a new series of
unprecedented tests about the nature of the fabric of space itself. Because the expansion of the
universe we observe is faster than ever before,
now is a good time, in this way, to look for dark
We have highlighted a few types of tabletop-scale
experiments that are exploring the frontiers of
fundamental physics. The recent progress in these
experiments has been enabled by remarkable
advances in techniques from the fields of atomic,
molecular, optical, and condensed-matter physics.
This includes methods such as laser-based manipulation of molecular states (used for EDM
searches), cooling and trapping of atoms (used
for clocks), the advent of new materials, and
development of detectors and devices that operate at the quantum limits (both used for axion
searches). These fields continue to thrive and will
likely produce even better tools in the near future. For example, techniques now under development, such as quantum entanglement–based
methods to surpass classical limits on statistical
sensitivity (53), are likely to yield big improvements in sensitivity for both EDM searches and
clock-based searches for variation of fundamental constants. With other new methods, such as
laser cooling and trapping of molecules (54), it is
conceivable, within a decade, to reach a sensitivity to EDMs that would probe for particles with
masses 1000 times heavier than those accessible to the LHC. Likewise, quantum squeezing
techniques could be used to substantially improve the sensitivity of axion searches. This widening range of new experimental approaches to
studying fundamental physics may hold the keys
to unlocking some of the deepest puzzles about
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D.D. acknowledges support by NSF grant PHY1404146, the
John Templeton Foundation grant 58468, and the Heising-Simons
Foundation grant 2016-035. J.M.D. acknowledges support by
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994 8 SEPTEMBER 2017 • VOL 357 ISSUE 6355 sciencemag.org SCIENCE
Fig. 4. Variation of fundamental constants. Overlapping constraints from a variety of clock-based
measurements constrain slow drifts in a and m in the current era [reprinted with permission
from (56); copyright (2014) by the American Physical Society]. Bands are 1s uncertainty regions.
Comparison with cosmological constraints is highly model-dependent. However, very roughly,
the astrophysical data sets limits on fractional changes of <0.1 part per million (ppm) for m (57)
and <2 ppm for a (42) over roughly 6 billion years. Thus, clock work is highly complementary
to astro-observational measurements.