Optical clocks, present and
future fundamental physics tests
Pierre Lemonde
LNE-SYRTE
Fractional accuracy of atomic clocks
Systematic effects-accuracy
•
•
•
•
•
Zeeman effect:
– Independent on the clock transition frequency
Potential gain 104
Spectral purity, leakage,...:
– Independent on the clock transition frequency
Potential gain 104
Cold collisions:
– Independent on the clock transition frequency
Potential gain 104
Neighbouring transitions:
– Independent on the clock transition frequency
Potential gain 104
Blackbody radiation shift: differential in fountains
– Cs: 1.7 10-14, Sr, Yb ~ 5 10-15, Hg : 2.4 10-16, Al+ 8 10-18
Potential gain 102
@ Optical frequencies all these effects seem controllable at 10-18 or better !
•
Doppler effect:
– Proportional to the clock frequency for free atoms, a trap is required
Interest of optical clocks
Ultimate gain on the frequency stability : 104
Q~4 1014, N~106, Tc ~ 1s
Ultimate gain on the frequency accuracy > 102
<10-18
-A « good » clock transition
Key ingredients
-Ability to control external degrees of freedom.
-Ultra-stable lasers
Single ion clocks an neutral atom lattice clocks are two possible ways forward
Quantum references: ions or atoms
Multipolar couplings: E2, E3
2P
2P
1/2
2D
3/2
1/2
2D
5/2
369 nm
422 nm
d=3 Hz
674 nm
467 nm
d=0.4Hz
2S
2F
7/2
436 nm
2S
1/2
d=10-9 Hz
1/2
Yb+(PTB, NPL)
Sr+ (NPL,NRC)
Other ions: Hg+ (NIST), Ca+(Innsbruck, Osaka, PIIM)
Intercombination transitions
1P
1P
1
3P
461 nm
698 nm
1
3P
0
167 nm
267 nm
d=1 mHz
1S
0
Sr (Tokyo, JILA, SYRTE,…),
Yb (NIST, INRIM, Tokyo,…)
Hg (SYRTE, Tokyo), In+
d=8 mHz
1S
0
Al+ (NIST)
0
Quantum logic clock
One logic ion for cooling and detection
One clock ion for spectroscopy
External degrees of freedom are coupled via Coulomb interaction
Al+ clocks
C. Chou et al. Science 329, 1630 (2010)
C. Chou et al. PRL 104 070802 (2010)
Al+ clock accuracy budget
Ion clock with sub 10-17 accuracy
C. Chou et al. PRL 104 070802 (2010)
Neutral atom clocks
Trapping neutral atoms
Confinement : standing wave
Trapping : dipole force
(intense laser)
1
0.5
0
Optical lattice
clocks
0
-2.5
-5
-7.5
-10
Trap shifts
-0.5
-0.25
0
0.25
0.5
l/2
D> 10-10
reaching 10-18, effect must be controlled to within 10-8
Problems linked to trapping
Trap depth : light shift of clock states
3 parameters : polarisation, frequency, intensity
Trap depth required to cancel motional effects to within 10-18 : at
least 10 Er (i.e. 36 kHz, or 10-11 in fractional units for Sr)
Both states are shifted. The differential shift should be considered
P. Lemonde, P. Wolf, Phys. Rev. A 72 033409 (2005)
Solution to the trapping problem
Polarisation : use J=0  J=0 transition, which is a forbidden by
selection rules
Intensity : one uses the frequency dependence to cancel the
intensity dependence
Such a configuration exists for alkaline earths 1S0  3P0
3P
0
3S
1
Sr
679 nm
1S
1P
1
0
lm : "longueur d'onde magique"
M. Takamoto et al, Nature 453, 231 (2005)
461 nm
1S
3D
1
3P
0
698 nm
0
2.56 µm
Experimental setup
Ultra-narrow resonance
Lattice clock comparison
Trap effects
E2-M1 Effects
E1 interaction
Traps atoms at the electric field maxima
M1 and E2 interactions
Creates a potential with a different spatial
dependence
E2-M1 Effects
E1 interaction
Traps atoms at the electric field maxima
M1 and E2 interactions
Creates a potential with a different spatial
dependence
This leads to a clock shift
E2-M1 effects
Measurements
The shift is measured by changing n and the
trap depth U0=100-500 Er
•The effect is not resolved, not a problem
•Upper bound 10-17 for U0=800 Er
Trap shifts
•Hyperpolarisability
d<1 µHz/Er2
•Tensor and vector shift. Fully caracterized and under control <10-17
•All known trap effects are well understood and not problematic <10-17
P.G. Westergaard et al., arxiv 1102.1797
87Sr
lattice clock accuracy budget
A. Ludlow et al. Science, 319, 1805 (2008)
• Frequency
difference between Sr clocks at SYRTE <10-16
• 10-17 feasible at room temperature. BBR, a quite hard limit. Next step: cryogenic, Hg ?
Towards a Hg lattice clock
• First
lattice bound spectroscopy of Hg atoms
• First
experimental determination of Hg magic wavelength 362.53 (21) nm
L. Yi et al., Phys. Rev. Lett. 106, 073005 (2011)
Optical clocks worldwide
• Ion clocks
– NIST (Al+, Hg+), PTB-QUEST (Yb+, Al+), NPL (Yb+, Sr+),
Innsbruck (Ca+)…
• Neutral atom clocks
– Tokyo (Sr, Hg), JILA (Sr), SYRTE (Sr, Hg), NIST (Yb), PTB
(Sr),…
• Space projects
– SOC project (ESA – HHUD, PTB, SYRTE, U-Firenze)
– SOC2 (EU-FP7)
– Optical clock as an option for STE-QUEST mission
Performing fundamental physics tests implies comparing these clocks
Clock comparisons
• « Round-trip » method for noise compensation
Ultra-stable
1.542 µm laser
Noise
correction
2FP
Fiber
Accumulated
Phase noise
LAB 1
FP
LAB 2
Round-trip noise detection
Link instability
measurement
• Demonstrated at the 10-19 level over hundreds of km over telecom network
• Global comparisons = satellite based systems
•ACES-MWL 2014-2017 down to a few 10-17, L. Cacciapuoti (next talk)
•Mini-DOLL coherent optical link, K. Djerroud et al. Opt. Lett. 35, 1479 (2009)
Fundamental tests on ground
• Stability of fundamental constants
 a/a expected improvement by 2 orders of magnitude 10-18/yr
 m/m limited by microwave clocks. Possible improvements if
nuclear transitions are used.
• Dependence of a to local gravitational potential
– Expected improvement by 2 orders of magnitude 10-8 d(GM/rc2)
• Massive redondancy due to the large number of atomic
species/transitions
Optical clocks in space
• Earth orbit
– Highly elliptical orbit. x100 improvement on ACES goals
– Optional optical clock for STE-QUEST mission (pre-selected as
M mission in CV2).
S. Schiller et al. Exp. Astron. (2009) 23, 573
• Solar system probe
– Outer solar system (SAGAS-like). Further improvement by 2
orders of magnitude on gravitational red-shift and coupling of a
to gravity. Probe long range gravity.
– Inner solar system. Probe GR in high field.
P. Wolf et al. Exp. Astron. (2009) 23, 651
Transportable Strontium Source (LENS/U.Firenze)-SOC project
main requirements:
1. compact design
2. reliability
3. low power consumption
optical breadboard 120 cm x 90 cm
main planning choices:
1. compact breadboard
for frequency production
2. all lights fiber delivered
3. custom flange holding MOT coils
and oven with 2D cooling
Schioppo et al, Proc. EFTF (2010)
Conclusions
 Optival clocks with ions and neutrals now clearly outperform
microwave standards. Present accuracy and long term stability 10-17 .
Where is the limit ?
 Long distance comparisons techniques are progressing rapidly.
Different types of clocks, using different atoms and different kind of
transitions allow extremely complete tests of fundamental physics:
stability of fundamental constants, probing gravity and couplings to
other interactions. Redondancy is important in case violations are seen.
 Space projects.
 Further improvements ? Higher frequencies (UV-X) ? Nuclear
transitions ? Molecular transitions ?