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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674 nm probe laser frequency

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tsff Δ+

><+ QJIgss s

Secondary servo:Average the quantum jump imbalance (QJI) over a few minutes and then add a frequency:

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The slope is adjusted to make the average QJI zero

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Locking to the strontium transitionWednesday, 25 May 2011

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674 nm probe laser frequency

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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