Atomic Clocks and Primary Frequency Sources
WSTS 2021 TUTORIAL SESSION March 24, 2021
Marc A. Weiss, Ph.D. Marc Weiss Consulting LLC [email protected] Atomic Frequency Standards: Produce Frequency Locked to an Atomic Transition
(F, mF) (4,4) Definition of the (4,3) second is the (4,2) definition of (4,1) frequency accuracy: 9.2 (4,0) (4,-1) agreement with the (4,-2) Cs clock transition (4,-3) (4,-4) 9.192,631,770 GHz E = h (3,-3) (3,-2) Absorption of photon at (3,-1) 9,192,631,770Hz results in transition from state (3,0) 0 (3,0) to state (4,0) (3,1) transition from state (3,0) (3,2) Cs. Energy Levels (Frequency,LevelsEnergy Cs. GHz) to state (4,0) results in (3,3) radiation of photon at
Magnetic Field HO 9,192,631,770Hz Basic Passive Atomic Clock 1. Obtain atoms to measure
2. Depopulate one hyperfine level
3. Radiate the state-selected sample with frequency
4. Measure how many atoms change state
5. Correct to maximize measured atoms in changed state Block Diagram of Atomic Clock Passive Standard Atomic Response Atomic Resonator
Frequency = u (e.g. 9,192,631,770Hz )
Frequency Frequency Synthesis Output Control Amp. Frequency eg 5 MHz
Local (Qz) Divider Output Oscillator (Counter) Time Ticks Types of Commercial Atomic Clocks
• Cesium thermal beam standard – Best long-term frequency stability • Rubidium cell standard – Small size, low cost • Hydrogen maser – Best stability at 1 to 10 days (short-term stability) – Expensive several $100K • Chip Scale Atomic Clock (CSAC) – Very small size, low power • Note that new clocks are under development! Chip Scale Atomic Clock (CSAC)
• Cs or Rb miniature cell standard – not a Cs beam tube! • Coherent Population Trapping (CPT) • Very small size and low power consumption, but better performance than a quartz oscillator
106 MEMS TCXO
4 OCXO-miniature s) 10 MCXO OCXO
2 10 CSAC Rb
100
•Better Cs beam 10-2 H-maser Timing@Error1 ( day 10-4 10-6 10-4 10-2 100 102 104 Power Consumption (W) Oscillator Comparison
Intrinsic Stability Stability Aging (/day) Technology initial to Applications Accuracy (1s) (floor) ultimate
Wristwatch, computer, Cheap Quartz, 10-6 ~10-11 ~10-11 10-7 to 10-8 cell phone, household TCXO clock/appliance,…
Network sync, test Hi-quality 10-8 ~10-12 ~10-12 10-9 to 10-11 equipment, radar, Quartz, OCXO comms, nav,…
Wireless comms Rb Oscillator ~10-9 ~10-11 ~10-13 10-11 to 10-13 infrastructure, lab equipment, GPS, …
Timekeeping, Navigation, Cesium Beam ~10-13 ~10-11 ~10-14 nil GPS, Science, Wireline comms infrastructure,…
Hydrogen Timekeeping, Radio ~10-11 ~10-13 ~10-15 10-15 to 10-16 Maser astronomy, Science,… Oscillator Comparison (continued)
Technology Size Weight Power World Market Cost
Cheap Quartz, 1 cm3 10 g 10 mW 109s/year $1s TCXO
Hi-quality 50 cm3 500 g 10 W 10Ks/year $100s Quartz, OCXO
Rb Oscillator 200 cm3 500 g 10 W 10Ks/year $1000s
Cesium Beam 30,000 cm3 20 kg 50 W 100s/year $10Ks
Hydrogen 1 m3 200 kg 100 W 10s/year $100Ks Maser Holding a Microsecond after Loss of Sync (circa 2019)
Temperature Oven Chip Scale Rb Cs Compensated Controlled Atomic Oscillator Beam-Tube Crystal Crystal Clock (5E-12/mo. Oscillator Oscillator Oscillator (CSAC) aging) (TCXO) (OCXO) Range of 10 minutes – 1 – 24 hours 3-15 hours 8 hours – 3 10-300 days times to hold 1 hour days a microsecond Cost Range $5-20 $50-250 $1.5K-3K $500-1500 $20K - $50K Conclusions: Atomic Standards
• Rubidium, cesium, and hydrogen atomic frequency standards share a common theme: the stabilization of an electronic (quartz) oscillator with respect to an atomic resonance.
• Although the use of atoms brings with it new quantum mechanical problems, the resulting long-term stability is unmatched by traditional classical oscillators. Thanks for your attention!
Extra slides follow Clock Stability
Clock (in)stability is given by: f 1 1 f Q(S / N ) ( / )(S / N )
Atomic Line Q Signal to Noise
Clock stability can be improved by:
Increase Ramsey (observation) times (decrease Δ=1/TRamsey) Improve the S/N (more atoms!) Increase the frequency of the clock transition (e.g. optical) Classical Cesium Standard with Magnetic State Selection Note there is a newer design with Optical Pumping and Detection of states
•DC •C-FIELD •MAGNETIC SHIELD •POWER SUPPLY
•HOT WIRE •IONIZER •“C-FIELD” •B-MAGNET •GETTER
•Cs-BEAM
•A-MAGNET •CAVITY •GETTER •ION •COLLECTOR
•DETECTOR •PUMP •VACUUM •SIGNAL •ENVELOPE •FREQUENCY •INPUT •OVEN •PUMP •9,192,631,770 Hz •HEATER •POWER •DETECTOR •POWER •SUPPLY •POWER •SUPPLY •SUPPLY • Atoms come from an oven in a beam • • A magnet is used to deflect the atoms in different • hyper-fine states • Atoms pass through a Ramsey cavity in a magnetic field to be exposed to microwaves at frequency = 9.193 GHz • A second magnet selects atoms which have made the transition • The number of detected atoms is used to tune the frequency Rubidium Standard
• Two major differences from a cesium standard 1. Cell standard (doesn’t use up rubidium) 2. Optically pumped (no state selection magnets)
• Used where low cost and small size are important Rubidium Standard
•Magnetic shield •Magnetic shield •“C-Field” •Absorption •cell
• 85Rb •87Rb • Rb-87 •+ buffer •Light •+ buffer •lamp • gas • gas
•Photo •Detector •Filter •cell • Cell •output •Cavity
•Frequency •input •6.834,685 GHz •rf •Power supplies •C-field •lamp •for lamp, filter •power •exciter •and absorption •supply •cell thermostats
•Adapted from figure by John Vig Optical Microwave Double Resonance Simplified Rb energy level diagram nm 780 • 6.835 6.835 GHz 3.035 GHz • • •87 Rb •85 Rb •B •A • Optical pumping is used to deplete one hyper-fine level
• Light tuned to the transition frequency from “A” to the • unstable excited state puts all of the atoms in the • hyper-fine state “B” • Microwaves at = 6.835 GHz stimulate the transition from “B” to “A” • The absorption of light is measured • The frequency is tuned to minimize the light coming through the 87 Rb cell Frequency Stability of a Rubidium Standard
1E-11
1E-12
1E-13 Overlapping Allan Deviation Allan Overlapping 1E-14 Commercial Rubidium Oscillator GPS Rubidium (PerkinElmer RAFS) High Performance Cesium
100 101 102 103 104 105 106 107 Averaging Time
•Courtesy of Robert Lutwak, Symmetricom Hydrogen Maser (Active Standard)
•Teflon coated •storage bulb •Microwave •cavity •Microwav e •output
•State •selector
•Hydrogen •atoms
•Adapted from a figure by John Vig Hydrogen Maser (Active Standard) Frequency Drift of a Commercial Cesium Standard and a Hydrogen Maser
-1x10-13
-13
y -2x10 c
n Maser #5 e u
q -13
e -3x10 r F l a
n -13
o -4x10 i t c a . r Cesium #4 F -5x10-13
-6x10-13 50500 51000 51500 52000 52500 53000 MJD Frequency Stability of a Cesium Standard (No frequency drift removed)
avhp182.grf 1x10-11
1x10-12 n o i t -13 a 1x10 i v e D
n -14
a 1x10 l l A 1x10-15
1x10-16 1x10-1 1x100 1x101 1x102 1x103 1x104 1x105 1x106 1x107 1x10 8 (sec.) Frequency Stability of a Hydrogen Maser (Frequency drift removed – 1x10-16/day typical)
avst0005.grf 1x10-11
1x10-12 500Hz BW
n 1 Hz BW o i t -13 a 1x10 i v e D
n -14
a 1x10 l l A 1x10-15
Low 2nd order drift 1x10-16 1x10-1 1x100 1x101 1x102 1x103 1x104 1x105 1x106 1x107 1x10 8 (sec.)