Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Optical coherence between atomic species at the second scale: improved
  clock comparisons via differential spectroscopy   Comparisons of high-accuracy optical atomic clocks \cite{Ludlow2015} are
essential for precision tests of fundamental physics \cite{Safronova2018},
relativistic geodesy \cite{McGrew2018, Grotti2018, Delva2019}, and the
anticipated redefinition of the SI second \cite{Riehle2018}. The scientific
reach of these applications is restricted by the statistical precision of
interspecies comparison measurements. The instability of individual clocks is
limited by the finite coherence time of the optical local oscillator (OLO),
which bounds the maximum atomic interrogation time. In this letter, we
experimentally demonstrate differential spectroscopy \cite{Hume2016}, a
comparison protocol that enables interrogating beyond the OLO coherence time.
By phase-coherently linking a zero-dead-time (ZDT) \cite{Schioppo2017} Yb
optical lattice clock with an Al$^+$ single-ion clock via an optical frequency
comb and performing synchronised Ramsey spectroscopy, we show an improvement in
comparison instability relative to our previous result
\cite{network2020frequency} of nearly an order of magnitude. To our knowledge,
this result represents the most stable interspecies clock comparison to date.

17
The categories of this paper are: q-fin
26
The categories of this paper are: bayes-an
5
The categories of this paper are: gr-qc
31
The categories of this paper are: ao-sci
28
The categories of this paper are: alg-geom
18
The categories of this paper are: econ
8
The categories of this paper are: hep-ex
7
The categories of this paper are: hep-th
9
The categories of this paper are: nlin
2
The categories of this paper are: math
4
The categories of this paper are: cond-mat
20
The categories of this paper are: acc-phys
21
The categories of this paper are: adap-org
25
The categories of this paper are: patt-sol
35
The categories of this paper are: mtrl-th
11
The categories of this paper are: hep-lat