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quencies or tones spanning the
optical regime. The repetition
rate can be referenced to the
microwave-based International
System of Units (SI) definition
of the second. Because of a di-
rect relation between the rep-
etition rate and the comb tone
spacing, the absolute frequency
of each tone can be determined.
Measuring the optical beat be-
tween a single comb tone and
an unknown optical frequency
provides a method for directly
linking optical frequencies to
the microwave reference. The
inverse should also be true, as
the stability and accuracy pro-
vided by an optical frequency
standard can be used as a ref-
erence for the OFC. This means
locking the comb in step with
the optical oscillations. The
comb is then expected to pro-
duce an equally stable output
in the microwave regime (see
the figure), and the measure-
ment of this seemed the obvious next step
( 5 ). The devil was in the details, and it took
nearly 20 years to come full circle to obtain
exceptional accuracy and long-term stabil-
ity in microwave generation.
Stable and accurate microwave signals
are used in a variety of arenas including
communication, navigation, radar, radio
astronomy, and fundamental physics re-
search. Currently, the most accurate and
stable frequency comparison and dissemi-
nation is achieved with ultrastable lasers
transmitted by optical fiber networks. These
systems are being used to lay the ground-
work for a redefinition of the SI second by
enabling direct optical-to-optical compari-
sons of frequency standards separated by
large geographical distances. Furthermore,
frequency transfer networks are used in
probes of fundamental physics ( 6 ) and de-
tection of submarine earthquakes by means
of deep-sea fiber optic cables ( 7 ), among
other applications.
Maintaining the stability and accuracy of
microwaves over long distances, whether
for synchronization, communication, or
navigation, requires overcoming some ob-
stacles in transmission and signal processing.
Atmospheric and delay compensation has im-
proved—for example, with two-way satellite
time and frequency transfer for satellite- and
ground-based systems. Space-based applica-
tions using ultrastable microwaves avoid the
issues of atmospheric disturbance altogether.
By using the ultralong baselines only acces-
sible to space-based systems, radio telescope
arrays in space synchronized by ultrastable

microwaves have the potential to greatly in-

crease the resolution of the signals ( 8 ).

Unlike for satellite-based transmission,

Doppler radar systems can immediately

take advantage of stability and accuracy

improvements in microwave reference sig-

nals. These radar systems simply compare

an outgoing signal and a received echo, us-

ing a single common clock signal and path.

Upgraded microwave stability directly in-

creases the attainable accuracy, unblurring

the images of distant and moving targets

( 9 ). This improved accuracy will help in

efforts to upgrade navigation and tracking

systems across multiple sectors.

The directed development of new tech-

nologies as a result of scientific need runs

through several recent innovations impor-

tant for improved accuracy and stability.

An essential component for the optical-to-

electrical conversion through OFC is the

optical detector that turns the regularly

spaced output pulses of light into a signal

of ultrastable microwaves. Extensive and

focused research and development was re-

quired to produce high-speed photodiodes

with high linearity even when processing

high peak intensities in the pulse train.

Exceptionally stable microwave genera-

tion and measurement required exploiting

the growing field of software-defined radio

( 10 ), which combines multiple systems of

microwave hardware devices into one re-

configurable software-driven device. This

approach is gaining widespread use for fre-

quency production and manipulation. This

trend highlights a move from analog to

digital frequency synthesis and

for integration of signal gen-

eration and processing. Finally,

innovations in the OFC technol-

ogy itself were key to improved

microwave generation and can

now be found in commercially

available devices.

Miniaturization and integra-

tion of several technologies

will be necessary to enable

widespread stable microwave

and optical frequency produc-

tion and dissemination. The

next step likely involves the

continued development of por-

table atomic clocks and opti-

cal frequency “microcombs”

(microOFCs) ( 11 ). Far-ranging

operation could involve optical

clock–level time and frequency

provided by fully integrated

systems of miniature optical

clocks, microOFCs, and noise-

reducing detectors and mi-

crowave electronics. Because

optical clocks have achieved un-

precedented levels of accuracy ( 12 ) and sta-

bility ( 13 ), linking the frequencies provided

by these optical standards with distantly lo-

cated devices would allow direct calibration

of microwave clocks to the future optical SI

second. This combination would markedly

improve synchronization capabilities be-

tween multiple locations and devices, and

over large distances.

Like the revolution that OFC technology

produced in the field of optical frequency

metrology, Nakamura et al.’s ability to pro-

duce microwaves with the stability and ac-

curacy afforded by an optical clock signal is

a paradigm shift in the field of microwave

metrology. The impact will extend to ap-

plications in fundamental physics, com-

munication, navigation, and microwave

engineering. Growing access to this new

frontier of ultrastable microwave sources

will only push these sectors and others to

new innovative heights. j

REFERENCES AND NOTES

1. A. D. Ludlow, M. M. Boyd, J. Ye, E. Peik, P. O. Schmidt, Rev.
   Mod. Phys. 87 , 637 (2015).


2. C. Clivati et al., Sci. Rep. 7 , 40992 (2017).


3. T. Nakamura et al., Science 368 , 889 (2020).


4. T. Fortier, E. Baumann, Commun. Phys. 2 , 153 (2019).


5. S. A. Diddams et al., Science 293 , 825 (2001).


6. P. Delva et al., Phys. Rev. Lett. 118 , 221102 (2017).


7. G. Marra et al., Science 361 , 486 (2018).


8. L. I. Gurvits, Adv. Space Res. 65 , 868 (2020).


9. P. Ghelfi et al., Nature 507 , 341 (2014).


10. J. A. Sherman, R. Jördens, Rev. Sci. Instrum. 87 , 054711
    (2016).


11. P. Del’Haye et al., Nat. Photonics 10 , 516 (2016).


12. S. M. Brewer et al., Phys. Rev. Lett. 123 , 033201 (2019).


13. E. Oelker et al., Nat. Photonics 13 , 714 (2019).

10.1126/science.abb8629

INSIGHTS | PERSPECTIVES

GRAPHIC. V. ALTOUNIAN/

SCIENCE

Optical-to-microwave conversion

Optical frequency comb tones

Optical frequency comb

Lock comb

to optical

frequency

standard

Comb

repetition

rate frep

Optical fast “tick”

Microwave slow “tick”

Actual gear ratio

~26,000:1

Atomic reference

Atomic optical frequency standard

To atomic

system

To optical

frequency comb

Stabilized

laser

Time

Amplitude

Photodiode

Optical frequency comb output

Ultrastable

microwave

1/frep output

Making microwaves

An optical frequency standard uses

a signal generated from an atomic

excitation to steer a stabilized

laser output. Combining this with

the output of an optical frequency

comb produces an interference

beat. Stabilizing the beat frequency

forces the repetition rate of the

comb to lock in step with the

frequency reference, only with a

microwave output.

826 22 MAY 2020 • VOL 368 ISSUE 6493

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