“Beyond 5G” and precision timekeeping are allies
Prepared for: NICT Beyond 5G open discussion (11 March 2022 JST)Dr. Jeff A. Sherman (jeff.sherman@nist.gov
NIST // Time and Frequency Divisio
Boulder, CO, USA)n(image attribution: unknown, https://i.imgur.com/v4xbEz7.jpg)

All radio communication requires transmission and modulation of pure tones“Carrier frequency”
Data
Basic modulation types:
Amplitude

Frequency

Phase

Adapted from Huawei, “WDM Basics” (https://info.support.huawei.com/ta/wdmwiki/index.html#/en/basic)

Beyond 5G will likely mean:
1. Higher carrier frequencies, more bandwidth

Figure: Wikimedia Commons, EM spectrum (https://commons.wikimedia.org/wiki/File:EM_spectrum.svg)

1 THz = 1000 GHz

1 GHz

Beyond 5G will likely mean:
1. Higher carrier frequencies, more bandwidth“… there are more high frequencies than low frequencies …”
Isaac Asimov(Understanding Physics, p. 130, 1993)

Beyond 5G will likely mean:
2. Coherent techniques to make better use of spectrum (e.g. beamforming, massive MIMO)

Left: REMCOM “Design and simulation of 28 GHz beamforming system and antenna array for 5G network base stations”
https://www.remcom.com/examples/2019/3/19/design-and-simulation-of-28-ghz-beamforming-system-and-antenna-array-for-5g-network-base-stations

Problem: few stable, practical oscillators 100 GHz - 1 THz
Idea 1: multiply 1 GHz (microwave) sources by n ≈ 100
Idea 2: divide 100 THz (optical or infrared) sources by n ≈ 100

Figure: Wikimedia Commons, EM spectrum (https://commons.wikimedia.org/wiki/File:EM_spectrum.svg)

Problem: few stable, practical oscillators 100 GHz - 1 THz
Idea 1: multiply 1 GHz (microwave) sources by n ≈ 100
Idea 2: divide 100 THz (optical or infrared) sources by n ≈ 100

Frequency division improves phase noise power spectral density by n2 ≈ 10,000!
Figure: Wikimedia Commons, EM spectrum (https://commons.wikimedia.org/wiki/File:EM_spectrum.svg)

Beyond 5G may bene t from coherent division of stable infrared/optical oscillators

÷fiFigure: Wikimedia Commons, EM spectrum (https://commons.wikimedia.org/wiki/File:EM_spectrum.svg)

mm-waves

Stable oscillator? That’s a clock!

~31 Dec 2020
23:59:59.012

Oscillator

Counter

ControlFrequency reference

Timekeeping also bene ts from higher frequencies, reduced e ects of phase noise.

fffi~ 1 fs

Optical clocks forecast last century

Weekly instability of Earth’s length-of-day

Figure: DW Allan, N Ashby, C Hodge, HP application note 1289: Science of Timekeeping (1997)

Microwave

Rough comparison of oscillator technologies

OpticalFrequency range:

10 kHz — 10 GHz

100 THz — 500 THz

Materials:

Quartz, ceramic resonators, MEMS

Si, fused silica (low-expansion), sapphire

Mode:

Active; electro-mechanicalPassive; laser stabilized to length

Stability @ 1 s average

10-11

10-16

Cost (best available)~ $1k USD

~ $1M USD

Likely fully developed

Early in development; cryogenics,
vibration sensitivity, large physical size,
cost, “unique” components, etc.

Complications

Left: http://lowpowerradio.blogspot.com/2017/02/Instant-AM-radio-station-hacking-1-mhz-crystal-oscillator.html
Right: PTB & JILA, photo reproduced at https://www.sciencedaily.com/releases/2017/06/170629101709.htm

Other optical oscillator schemes directly use atoms/molecules

ffTh Schuldt, K Döringsho , E Kovalchuk, et al., Appl. Optics 56(4), 1101 (2017)

Fractional frequency instability

Accurate, bias-free, optical-to-microwave conversion (errors at 10-18)[1]: T Nakamura, JD Rodriguez, H Leopardi, et al., “Coherent optical clock down-conversion for
microwave frequencies with 10-18 stability”, Science 368, 889-892 (2020).

Wide distribution model for stable/accurate frequency

GNSS/GPS

Users

Primary atomic clocks& frequency references

National standards laboratories

5G/6G networks(…mm-scale terrestrial PNT beacons?)

Evolution of the de nition of frequency?
“The second is … 9 192 631 770 periods of the radiation corresponding to the transition
between two hyper ne levels of the ground state of the cesium-133 atom... at rest and at atemperature of 0 K.”
Volume 106, Number 1, January–February 2001

Journal of Research of the National Institute of Standards and Technology2014) 174

T P Heavner et al~

Total deviation (TOTDEV) of NIST-F2 operating atensity where the stability is ≈1.7 × 10−13 τ −1/2 . The
number is, approximately, 2 × 104 . The measurementng a commercial hydrogen maser as a reference.

Figure 2. A sectional view of the NIST-F2 physics package. The

laser-cooled Cs source chamber operates in the (1,1,1) geometry.
Directly above the source region and within the same vacuum
chamber is the two-level detection zone. A short bellows with aresonance is not sufficiently close to the atomic thermal break (not shown here) connects the lower half of theDB Sullivan, JC Bergquist, JJ Bollinger, et al., J.system
Res. toNIST
106, 47–63(2011)cavity structure. The cavity
the cryogenicmicrowave
TP Heavner, EA Donley, Filippo Levi, et al., Metrologia
174–182(2014)
structure 51,resides
on a coldplate that is in thermal contact with thefifi

ign concepts for NIST-F2 were first presented inFig. 4. NIST-F1 Ramsey pattern. The upper portion of the figure shows the entire ground state(F = 3 ↔ F = 4) pattern, while the lower portion of the figure shows an expanded view of the central
fringe. The full linewidth at half maximum is about 1 Hz.

Two leading technologies for absolute optical frequency references (errors at 10-18)
Single (rf-) trapped ion✅ possibly lower systematics✅ path to low BBR uncertainty❌ residual motional e ects

Optical-lattice trapped atoms✅ higher S/N✅ path to low BBR uncertainty❌ uncanceled Stark shifts

ffIllustration: Phil Saunders. From E Cartlidge, “Toward the optical second”, Opt. & Phot. (OSA) 2019(https://www.osa-opn.org/home/articles/volume_30/february_2019/features/toward_the_optical_second/)Absolute atomic optical frequency reference
Example: Yb “optical lattice” at NISTz
H11.
0±z
H.71

2851

36809563859sha1_base64="JT4mjRlkKi3VuglpQODd/5nPGDc=">AAACC3icdZDLSsNAFIYn9VbrLerSzdAiuLEkbantruDGZQV7gSaGyXTSDp0kw8xEDKF7N76KGxeKuPUF3Pk2Ti+Civ4w8PGfczhzfp8zKpVlfRi5ldW19Y38ZmFre2d3z9w/6Mo4EZh0cMxi0feRJIxGpKOoYqTPBUGhz0jPn5zP6r0bIiSNoyuVcuKGaBTRgGKktOWZRUfSUYi8FDqIcxHfwgp0FA2JhLZ1nZ3a9alnlqxyc6Y6XEC1qcGq2Va1Cu2yNVcJLNX2zHdnGOMkJJHCDEk5sC2u3AwJRTEj04KTSMIRnqARGWiMkN7mZvNbpvBYO0MYxEK/SMG5+30iQ6GUaejrzhCpsfxdm5l/1QaJChpuRiOeKBLhxaIgYVDFcBYMHFJBsGKpBoQF1X+FeIwEwkrHV9AhfF0K/4dupWxrvqyVWo1lHHlwBIrgBNjgDLTABWiDDsDgDjyAJ/Bs3BuPxovxumjNGcuZQ/BDxtsnm+aaGQ==</latexit><latexit

WF McGrew, X Zhang, H Leopardi, et al., Optica 6(4) 448 (2019); plot edited to remove annotation
Photo: Nate Phillips, NIST

Recent examples from Japan (1)NMIJ/AIST

NMIJ

NICT

NICT: link between optical clocks (Japan/Italy)[1] T Kobayashi, D Akamatsu, K Hosaka, Y Hisai, M Wada, H Inaba, T Suzuyama, F-L Hong, M Tasuda, “Demonstration of the nearly continuous operation of an 171Yb optical lattice clock for half a year”, Metrologia 57, 065021 (2020)[2] M Pizzocaro, M Sekido, K Takefuji, et al., “Intercontinental comparison of optical atomic clocks through VLBI”, Nature Physics 17, p. 223–227 (2021)

Recent examples from Japan (2)M Takamoto, I Ushijima, N Ohmae, T Yahagi, K Kokado, H Shinkai, H Katori, “Test of general relativity by a pair of transportable optical lattice clocks,” Nat. Photonics 14, pp. 411–415 (2020).

Summary
Beyond 5G likely requires:• Higher carrier frequencies• Higher bandwidths• Highly coherent transmitters (e.g. beamforming)

All these needs served by coherent-division of optical oscillators

A parallel, fascinating technology track in precision optical timekeeping exists:• Highly stable optical oscillators• Coherent optical-to-microwave division• Highly accurate optical frequency references• Motivation for rede nition of the SI unit of time
• Supports fundamental science and novel sensingfi(image attribution: unknown, https://i.imgur.com/v4xbEz7.jpg)