Strontium Ion Optical Clocks at NPL
Geoffrey Barwood1, Patrick Gill1, Guilong Huang1, Yao Huang2 and Hugh Klein1.
1National Physical Laboratory (NPL), Teddington, Middlesex, TW11 0LW, United Kingdom. 2Wuhan Institute of Physics and Mathematics (WIPM), Chinese Academy of Sciences, Wuhan 430071, China.
References[1] H.S. Margolis, “Optical Frequency Standards and Clocks”, Contemporary Physics 51 (1)
37-58 (2010).[2] G.P. Barwood, P. Gill, G. Huang and H.A. Klein, “Observation of a sub-10 Hz linewidth
88Sr+ 2S1/2 - 2D5/2 clock transition at 674 nm”, IEEE TIM 56 226-229 (2007).[3] Patrick Gill, Helen Margolis, Anne Curtis, Hugh Klein, Stephen Webster and Peter Whibberley,
“Optical Atomic Clocks for Space”, Tech. Supporting Doc. ESTEC/Contract 21641/08/NL/PA (Nov. 2008): http://www.npl.co.uk/upload/pdf/atomic_clocks_space.pdf
[4] S.A. Webster et al, Phys Rev A75 011801(R) (2007)
AcknowledgementThis work was funded by the UK Department for Business, Innovation and Skills (BIS), as part of the National Measurement System (NMS) “Pathfinder” programme. Discussion with collaboration partners and colleagues at WIPM, ESA, JPL and NPL is gratefully acknowledged.
Partial term scheme of 88Sr+ ionOptical frequency standards offer a step improvement of one to two orders of magnitude in the future ability to realise the SI second [1]. The NPL strontium ion optical clock project [2] is targeted on creating systems with better performance than microwave clocks, for both space [3] and terrestrial environments. Recent activities have centred on compact transportable system development and long-term unattended operation with quantum-limited performance. One ring and two end-cap 88Sr+ trap systems based on the 2S1/2 - 2D5/2 674 nm optical-clock quadrupole transition are under development.
Introduction Overall schematic of the trap and laser systems Wednesday, 25 May 2011
844 nmDFB
Cooling radiationat 422 nm
PPKTP doublingcrystal
1092 nm DFB laser
674 nm probelaser system
"Clock" laser at 674 nm
Polarisationmodulator
AOM2
AOM1
Sr trap+PMT
1033 nmDFB
Primary servo:
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ν1ν
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Secondary servo:Average the quantum jump imbalance (QJI) over a few minutes and then add a frequency:
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674 nm probe laser frequency
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The slope is adjusted to make the average QJI zero
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674 nm probe laser frequency
ν1ν
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Locking to the strontium transitionWednesday, 25 May 2011
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• By comparing the two Sr+ end-cap traps, we have demonstrated single-ion stability of 7 parts in 1016
at 1000 seconds
Stability and reproducibility
• Implement optical pumping to increase quantum jump rate• Complete two-trap comparison and systematics evaluation• Frequency measurement relative to NPL Cs fountain using
fs combs• Compact trap development for long-term uninterrupted
operation; demonstrate capability to operate in space
Outlook
• Lock monitoring and recovery for 674 nm and 844 nm
• 1092 nm and 1033 nm diode laser drift control via wavemeter
• Up to 60 hours of continuous two-trap comparison data taken
Software for data taking and error recoveryW
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• In five hours, with a 200 ms probe pulse, calculations indicate one should be able to achieve relative frequency stabilities of parts in 1017
Systematics projected to be:Doppler shifts (secular & micromotion) ~10-17
Zeeman shifts (residual linear & quadratic) <10-18
Quadrupole shift (nulled) ~2 x10-17
Stark (trapping fields, secular motion etc) ~3 x10-17
Blackbody Stark shift (±1 K) ~10-17
Gravitational shift (± 0.1 m) ~10-17
Compact Paul ring trap (r0 = 1.5 mm) Trap operates with small 2 l/s pump
RingCompensation electrode
End cap
Cooling laser system
844 nm laser1092 nm laser
844 nm etalon
PPKTP doubling crystal: 422 nm
output)
Sr+ ion end-cap traps
V1
V cos tAC Ω
V ~ 200 V~ 17 MHz
V , V ~ few volts
AC
1 2
Ω
0.56 mm
Probe laser system
Isolator
674 nmlaser
ULE high-finesse etalon
λ /4plate
PBS
APD
FMlock
Fast feedback(via current)
Vibration isolation platform
Fibrelink
Outputto trap(via fibre)
To fslaser
To laserPZT
Slow feedback
Phasemodulator
AOM @~70 MHz
• Insensitive to horizontal and vertical acceleration
• Mounted on an AVI
• Cavity spacer linear thermal expansion zero at ~20°C
Cut-out cavity in vacuum chamber [4]
88Sr+ cold ion 674 nm Zeeman component of ~9Hz width (transform limited with 100 ms pulse)
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Frequency (Hz)
Allan deviation between two 674 nm clock lasers
The 674 nm clock lasers, locked to ULE cavities, reach stabilities of 2 to 3 parts in 1015 between 1 and 500 s, with the linear and quadratic drifts removed.
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Averaging time (s) Beat frequency between two 674 nm clock lasers with 3 sec averaging time Assuming white noise and that lasers are similar; individual laser widths 1.5Hz. Rises to 6 Hz over 30 sec; ~ 4 Hz per laser.
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∆ mj =-2 -2 -1 -1 0 0 +1 +1 +2 +2
Zeeman structure of the 674 nm 88Sr+ optical clock transition
• 70 mm cube
• Two-layer mu-metal shield, volume of the delivery optics and magnetically shielded vacuum chamber less than a tenth of the end-cap trap optical clock systems
• 12 mm diameter photo-multiplier was installed and two layers of magnetic shielding
Wednesday, 25 May 2011
Frequency (Hz)
2 Hz beat width
