We report in this Letter the outstanding frequency stability performance of an autonomous cryogenic sapphire oscillator (CSO) presenting a flicker frequency noise floor below 2 × 10 16 near 1000 s of integration time and a long-term Allan deviation limited by a random walk process of 1 × 10 18 τ. The frequency stability qualification at this level called for the implementation of sophisticated instrumentation associated with ultra-stable frequency references. This result is technologically sound as it demonstrates the potentiality of the CSO technology. From the physical point of view, it sets an upper limit to the ultimate noise floor of the cryogenic microwave resonator that is competitive to that of the ultra-stable optical Fabry–Pérot cavities.

1.
P.
Wolf
,
S.
Bize
,
A.
Clairon
et al, “
Test of Lorentz invariance using a microwave resonator
,”
Phys. Rev. Lett.
90
,
060402
(
2003
).
2.
C.
Eisele
,
A. Y.
Nevsky
, and
S.
Schiller
, “
Laboratory test of the isotropy of light propagation at the 10 17 level
,”
Phys. Rev. Lett.
103
(
9
),
090401
(
2009
).
3.
M.
Takamoto
,
I.
Ushijima
,
N.
Ohmae
et al, “
Test of general relativity by a pair of transportable optical lattice clocks
,”
Nat. Photonics
14
(
7
),
411
415
(
2020
).
4.
W. M.
Campbell
,
B. T.
McAllister
,
M.
Goryachev
et al, “
Searching for scalar dark matter via coupling to fundamental constants with photonic, atomic, and mechanical oscillators
,”
Phys. Rev. Lett.
126
(
7
),
071301
(
2021
).
5.
M.
Rioja
,
R.
Dodson
,
Y.
Asaki
et al, “
The impact of frequency standards on coherence in VLBI at the highest frequencies
,”
Astronomical J.
144
(
4
),
121
(
2012
).
6.
B.
Alachkar
,
A.
Wilkinson
, and
K.
Grainge
, “
Frequency reference stability and coherence loss in radio astronomy interferometers application to the SKA
,”
J. Astronomical Instrum.
07
(
01
),
1850001
(
2018
).
7.
V.
Dolgovskiy
,
S.
Schilt
,
N.
Bucalovic
et al, “
Ultra-stable microwave generation with a diode-pumped solid-state laser in the 1.5-μm range
,”
Appl. Phys. B
116
(
3
),
593
601
(
2014
).
8.
J. W.
Zobel
,
M.
Giunta
,
A. J.
Goers
et al, “
Comparison of optical frequency comb and sapphire loaded cavity microwave oscillators
,”
IEEE Photonics Technol. Lett.
31
(
16
),
1323
1326
(
2019
).
9.
G.
Santarelli
,
C.
Audoin
,
A.
Makdissi
et al, “
Frequency stability degradation of an oscillator slaved to a periodically interrogated atomic resonator
,”
IEEE Trans. Ultrason., Ferroelectr., Freq. Control
45
(
4
),
887
894
(
1998
).
10.
M.
Abgrall
,
J.
Guéna
,
M.
Lours
et al, “
High-stability comparison of atomic fountains using two different cryogenic oscillators
,”
IEEE Trans. Ultrason., Ferroelectr., Freq. Control
63
(
8
),
1198
1203
(
2016
).
11.
J. M.
Robinson
,
E.
Oelker
,
W. R.
Milner
et al, “
Crystalline optical cavity at 4 K with thermal-noise-limited instability and ultralow drift
,”
Optica
6
(
2
),
240
243
(
2019
).
12.
W. H.
Horton
and
G. E.
Hague
, “
Dynamic measurement of amplitude-frequency effect of VHF resonators
,”
IEEE Trans. Ultrason., Ferroelectr., Freq. Control
53
(
1
),
159
166
(
2006
).
13.
S.
Chang
,
A. G.
Mann
,
A. N.
Luiten
et al, “
Measurements of radiation pressure effect in cryogenic sapphire dielectric resonators
,”
Phys. Rev. Lett.
79
,
2141
2144
(
1997
).
14.
V.
Giordano
,
S.
Grop
,
P.-Y.
Bourgeois
et al, “
Influence of the electron spin resonance saturation on the power sensitivity of cryogenic sapphire resonators
,”
J. Appl. Phys.
116
(
5
),
054901
(
2014
).
15.
F.
Vernotte
,
C. E.
Calosso
, and
E.
Rubiola
, “
Three-cornered hat versus Allan covariance
,” in
Proceedings of 2016 IEEE International Frequency Control Symposium (IFCS)
(
IEEE
,
2016
), pp.
1
6
.
16.
C.
Fluhr
,
S.
Grop
,
B.
Dubois
et al, “
Characterization of the individual short-term frequency stability of cryogenic sapphire oscillators at the 10 16 level
,”
IEEE Trans. Ultrason., Ferroelectr. Freq. Control
63
(
6
),
915
921
(
2016
).
17.
C. E.
Calosso
,
F.
Vernotte
,
V.
Giordano
et al, “
Frequency stability measurement of cryogenic sapphire oscillators with a multichannel tracking DDS and the two-sample covariance
,”
IEEE Trans. Ultrason., Ferroelectr., Freq. Control
66
(
3
),
616
623
(
2019
).
18.
E.
Oelker
,
R.
Hutson
,
C.
Kennedy
et al, “
Demonstration of 4.8 × 10 17 stability at 1 s for two independent optical clocks
,”
Nat. Photonics
13
(
10
),
714
719
(
2019
).
19.
V.
Giordano
,
S.
Grop
,
B.
Dubois
et al, “
New generation of cryogenic sapphire microwave oscillator for space, metrology and scientific applications
,”
Rev. Sci. Instrum.
83
,
085113
(
2012
).
20.
V.
Giordano
,
S.
Grop
,
C.
Fluhr
et al, “
The autonomous cryocooled sapphire oscillator: A reference for frequency stability and phase noise measurements
,”
J. Phys.: Conf. Ser.
723
(
1
),
012030
(
2016
).
21.
See http://www.uliss-st.fr/ for information about the CSO ULISS-2G.
22.
C.
Fluhr
,
B.
Dubois
,
S.
Grop
et al, “
A low power cryocooled autonomous ultra-stable oscillator
,”
Cryogenics
80
,
164
173
(
2016
).
23.
C.
Fluhr
,
B.
Dubois
,
G.
Le Tetu
et al, “
ULISS-2G ultra stable cryocooled microwave sapphire oscillator: A mature and reproducible technology
,” in
2022 3rd URSI Atlantic and Asia Pacific Radio Science Meeting (at-AP-RASC)
(
IEEE
,
2022
), pp.
1
3
.
24.
C.
Fluhr
,
B.
Dubois
,
G.
Le Tetu
et al, “
Reliability and reproducibility of the cryogenic sapphire oscillator technology
,”
IEEE Trans. Instrum. Meas.
72
,
1
(
2023
).
25.
C. E.
Calosso
, “
Tracking DDS in time and frequency metrology
,” in
2013 Joint European Frequency and Time Forum & International Frequency Control Symposium (EFTF/IFC)
(
IEEE
,
2013
), pp.
747
749
.
26.
See https://www.femto-engineering.fr/en/equipement/oscillator-instability-measurement-platform/ for information about the metrological platform Oscillator-IMP.
27.
S.
Chang
and
A. G.
Mann
, “
Mechanical stress caused frequency drift in cryogenic sapphire resonators,” in Proceedings of the
2001 IEEE International Frequency Control Symposium and PDA Exhibition
,
Seattle, WA, USA
(
IEEE
,
2001
), pp.
710
714
.
28.
M. E.
Tobar
,
E. N.
Ivanov
,
C. R.
Locke
et al, “
Long-term operation and performance of cryogenic sapphire oscillators
,”
IEEE Trans. Ultrason., Ferroelectr., Freq. Control
53
(
12
),
2386
2393
(
2006
).
29.
S.
Grop
,
W.
Schäfer
,
P.-Y.
Bourgeois
et al, “
Unprecedented long-term frequency stability with a microwave resonator oscillator
,”
IEEE Trans. Ultrason., Ferroelectr., Freq. Control
58
(
8
),
1694
1697
(
2011
).
You do not currently have access to this content.