Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Femtosecond Synchronization of Optical Clocks Off of a Flying Quadcopter   Future optical clock networks will require free-space optical time-frequency
transfer between flying clocks. However, simple one-way or standard two-way
time transfer between flying clocks will completely break down because of the
time-of-flight variations and Doppler shifts associated with the strongly
time-varying optical link distances. Here, we demonstrate an advanced,
comb-based optical two-way time-frequency transfer that can successfully
synchronize the optical timescales at two sites connected via a time-varying
turbulent air path. The link between the two sites is established using either
a quadcopter-mounted retroreflector or a swept delay line at speeds up to 24
m/s. Despite 50-ps breakdown in time-of-flight reciprocity, the sites'
timescales are synchronized to < 1 fs in time deviation. The corresponding
sites' frequencies agree to $\sim$ $10^{-18}$ despite $10^{-7}$ Doppler shifts.
This work demonstrates comb-based O-TWTFT can enable free-space optical
networks between airborne or satellite-borne optical clocks for precision
navigation, timing and probes of fundamental science.

28
The categories of this paper are: alg-geom
9
The categories of this paper are: nlin
1
The categories of this paper are: cs
21
The categories of this paper are: adap-org
37
The categories of this paper are: supr-con
34
The categories of this paper are: chem-ph
33
The categories of this paper are: plasm-ph
30
The categories of this paper are: q-alg
18
The categories of this paper are: econ
35
The categories of this paper are: mtrl-th
4
The categories of this paper are: cond-mat
15
The categories of this paper are: nucl-ex
20
The categories of this paper are: acc-phys
23
The categories of this paper are: solv-int
7
The categories of this paper are: hep-th
31
The categories of this paper are: ao-sci