AION: Experimental Overview - Richard Hobson - experimental overview
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
AION: Experimental Overview Richard Hobson
Example: strontium atomic clock Lattice loading Readout State preparation Blue MOT Red MOT L Spectroscopy R t 26/09/2019 Optical lattice clocks at NPL 2
Strontium 87 813 nm Optical Lattice 461 nm – 1st stage cooling to 1 mK 689 nm – 2nd stage cooling to 1 μK (Sr1) 2.9 um – 2nd stage cooling to 5 μK (Sr2) 813 nm – optical lattice trap 698 nm – the clock transition 679 nm, 707 nm, 497 nm – repumping 26/09/2019 Optical lattice clocks at NPL 3
Blue MOT 200 ms Blue MOT Red MOT L Spectroscopy Readout 26/09/2019 Optical lattice clocks at NPL 4
Red MOT Blue MOT 300 ms Red MOT L Spectroscopy Readout 26/09/2019 Optical lattice clocks at NPL 5
Lattice Loading Blue MOT Red MOT 15 ms Lattice Spectroscopy Readout 26/09/2019 Optical lattice clocks at NPL 6
State Preparation Blue MOT Red MOT 15 ms State prep. Spectroscopy Readout 26/09/2019 Optical lattice clocks at NPL 7
Spectroscopy Blue MOT Red MOT L 100 ms Spectroscopy Readout 26/09/2019 Optical lattice clocks at NPL 8
Readout Blue MOT Red MOT Lattice Spectroscopy 15 ms Readout 26/09/2019 Optical lattice clocks at NPL 9
Result Lattice loading Readout State preparation Spectroscopy Blue MOT Red MOT L R 0 + t 26/09/2019 Optical lattice clocks at NPL 10
Outline • What are the main atomic physics challenges for GW detection? • High signal-to-noise ratio extreme LMT; high atoms/second • For LMT, homogenous Doppler shift need to be very cold vertically • For LMT, homogenous intensity + long flight time need to be very cold horizontally • Atom physics targets and benchmarks • AION targets (3 yr): (1) AION 10 demonstrator; (2) “Upgrade” R&D • Benchmarks: (1) current state of the art; (2) GW detection threshold • Building a long-term programme • Technology development (key features: reliability, documentation) • Strong collaboration needed – management structures, discussion forums, repos… 26/09/2019 Outline 11
Atomic physics challenges 3. Sequence of high- fidelity pi pulses 1. Big tube 2. Lots of atoms & squeezing Need to shrink quantum projection noise Δθ 26/09/2019 Atomic physics challenges 12
How cold? To get 40,000 ħk, we need to drive π pulses with > 99.999% fidelity* We need to avoid inhomogenous intensity and Doppler shifts Effect of Doppler shift Effect of intensity error *This assumes error in consecutive pulses is uncorrelated, probably reasonable since excitation laser phase will be noisy enough 26/09/2019 Atomic physics challenges 13
Vertical temperature Let’s assume a Rabi frequency of 8 kHz (This drives a Rabi π pulse in 62.5 us and needs 50 W/cm2 at 698 nm) Maximum permissible Doppler shift for 99.999% fidelity: 25 Hz Doppler shift is given by Δf = f0*v/c = f0*sqrt(kTvert/m)/c Maximum permissible vertical atom temperature: Tvert < 3 pK This is really cold! 26/09/2019 Atomic physics challenges 14
Horizontal temperature Start with state of the art phase space density [1]: n0 = 1.2e13 /cm^3; T = 44 nK cloud width for 1e6 atoms dx = 17 um; velocity spread dv = 0.002 m/s Ideal delta-kick preserves phase space density We have a fixed relationship between size dx and velocity spread dv: dxf=2*sqrt(dxi*dvi*flight time) Perfect delta-kick cooling will preserve phase space density. To minimise the final cloud size (and therefore the intensity inhomogeneity toward the end of the LMT sequence), then 26/09/2019 Atomic physics challenges 15
Horizontal temperature I didn’t have time to finish the horizontal calculation . But the take home message from Mike’s calculation was that we do need to be in (or close to) the degenerate regime. Even for T ~ 0.2 TF, the cloud width will be > 1mm for > 2e6 atoms Maximum permissible pulse area error is 2e-3 LMT beam width must be 1/sqrt(2e-3) = 20 times larger than cloud width ~ 20 mm waist for ~ 1e6 atoms at T/TF = 0.2 Lots of power needed at 698 nm! (E.g. 300 W to drive 62.5 us pi pulses) This gets worse with higher atom number… 26/09/2019 Atomic physics challenges 16
Can composite pulses help? Composite pulses suppress inhomogeneity in (1) intensity and (2) Doppler shifts Can use smaller LMT beam & higher Tvert Best options we found are plotted on right: (see https://arxiv.org/pdf/1406.2916.pdf) E.g. Knill allows Tvert to be 105 times larger! Still, the ultimate constraint on LMT beam is Rayleigh length for 698 nm light: w0 = 5 mm zR = 100 m For long baselines we need > 100 W at 698 nm to drive enough π pulses within the flight time Calculated by William Bowden 26/09/2019 Atomic physics challenges 17
Atoms: targets and benchmarks Target: AION 10 Target AION “Upgrade” R&D Benchmark: Current tech Benchmark: GW Desiderata Priorities: simple & reliable Priorities: high SNR, shot rate These numbers Nice to have: high SNR, shot rate Must be feasible in 3.5 years should scare you! Blue MOT 5e7 atoms @ 1 second > 5e8 atoms @ 1 second Sources 5e9-1e10 atoms/s 88Sr: Schreck, Singapore, Shanghai, MPQ: picture to right Red MOT 1e6 atoms @ 2 uk > 1e7 atoms @ 2 uK Up to 1e7 atoms @ 1.5 uK in 87Sr red MOT: Schreck, Ye, MPQ Dipole trap 5e5 atoms @ 2 uK Deliverable: 2e5 atoms @ < 100 nK Up to 2e5 atoms @ 50 nK in 87Sr: Schreck, Stretch: 1e8 atoms/sec @ < 100 nK Ye Transport 2e5 atoms @ 5 uK in tube Deliverable: 1e6 atoms @ < 1uK Tuneable lens; Mechanical translation ~ 1e8 atoms/s @ Stretch: > 1e7 atoms and DKC to < 1nK stage; Bessel beam 1e6 atoms/sec in tube; contrast = 1* ~ 10 pK Launching Copy MAGIS ? Try in transport R&D system? Tim Kovachy Thesis pg 50 Delta-kick Copy MAGIS ? Try in transport R&D system? 50 pK horizontal (Kasevich using Rb; note use of initial magnetic DKC stage) LMT 1 ħk @ 10 m baseline Adaptive optics? Composite pulses? 100 ħk (Kasevich with Rb [1,2]) 4e4 ħk 1 (Poli group with Sr [1,2]) 100 ħk @ 10 m baseline* Squeezing None Deliverable: QND on > 1e5 atoms 20 dB (Kasevich Rb), 13 dB (Vuletic Yb), 20 dB Stretch: Useful squeezing > 1e6 atoms 17.7 dB (Thompson Rb), Thompson Sr weak detection, Another good paper Detection Florescence onto CCD ? Any activities here? < 1e-5 resolution 26/09/2019 *These seem too difficult Targets and benchmarks Red = taken from IRB proposal 18
Atoms: What techniques to employ? Technique 1 Challenges 1 Technique 2 Challenges 2 Blue MOT Load into magnetic trap Easy route to large atom number, but Really big blue MOT Rescatter 1e10 atoms occupies ~ 1cm3 slow (~ 10 s) and inefficient transfer MOT becomes unstable. Also you get especially for 87Sr inelastic collisions need high flux source Red MOT SWAP cooling Easy to implement, already use at NPL. 688 nm transparency Might help to get towards recoil limit ~ 180 Improves transfer and speeds up cooling nK before evaporation in dipole trap but doesn’t help final temperature much. beam dark SPOT Dipole trap 1064 nm crossed dipole Can copy trap dimensions from Schreck, Ye groups (~ 60 x 60 x 18 um), but this trap only supports ~ e5 degenerate atoms Transport Moving dipole trap Beam needs to support against gravity Launch and recapture Could combine with delta-kick cooling. over a 50 cm horizontal distance Bessel Launch lattice needs excellent alignment, (lattice assisted?) beam, tunable lens, or mechanical stage via lattice wavefront and phase stability Launching Blue detuned 689 nm Beam needs excellent chirped lattice Delta-kick Combine with ballistic Implement at apogee on stationary atoms Use clock transition This is usually wasteful – atoms in the wrong can apply vertical delta kick as well as velocity class get thrown away. However, we transport horizontal for velocity selection could move atoms into the same velocity class using a series of selection pulses and state-selective lattice launches… LMT Squeezing Cavity (non-destructive > 0 dB Rydberg? Only theoretical. Inelastic loss and decoherence would need investigating measurement; twisting) 26/09/2019 Targets and benchmarks 19 If we squeeze before launching anyway, then
Technology development • Modules needed (note our ambitions for unprecedentedly reliable and reproducible tech) • Lasers • Laser stabilization system • Individual lasers and distribution optics (auto-aligning?) • Special research focus: High power LMT laser at 698 nm • Electronics • Experimental control (sequence generator, data acquisition, machine learning, laser (re-)locks) • Coil drivers, AOM drivers, RF sources, fancy DDS • Vacuum system • Sidearm chamber with Sr source and delivery optics • Very big tube! AION-100… • Documentation and collaboration • Git repositories (completely open? Shared with MAGIS?)? Discussion forums? Meetings? 26/09/2019 Building a long-term programme 20
Conclusion • The atomic physics challenges are daunting • Complex sequence of cooling, transport and LMT • Very ambitious long-term needs for temperature, atom number, and squeezing • AION targets and benchmarks • AION-10 needs to be simple, reliable, and quick to set up • AION upgrade R&D needs to catch up with state-of-the-art, and overtake where possible • Need to lay out roadmap for meeting GW detection benchmarks • To make this work we need collaborative development of reliable, repeatable technology • Also need a very powerful 698 nm laser… 26/09/2019 Building a long-term programme 21
End of AION slides… 26/09/2019 Building a long-term programme 22
Accuracy: Error budget Lots of Pt thermistors… Limit: thermal gradients across chamber ~ 0.3 K Extrapolated using high/low lattice depth Measured with Sr Rydbergs (with help from Matt Jones) This is < 0.3 nm/s (!) thanks to lattice trap 26/09/2019 Optical lattice clocks at NPL 23
Rydberg DC Stark EIT signal 5s75d 1D2 413 nm (Tuneable) 5s5p 1P1 No E field applied 461 nm symmetric (Fixed) 5s2 1S0 E field applied skewed Residual shift to Sr clock: −1.6+0.4 −1.6 × 10 −20 Phys. Rev. A 96, 023419 26/09/2019 Optical lattice clocks at NPL 24
Instability: Three causes ∝N (1) Detection noise (2) Quantum projection noise (3) Local oscillator noise + dead time “Dick effect” 26/09/2019 Optical lattice clocks at NPL 25
Instability: NPL Sr 26/09/2019 Optical lattice clocks at NPL 26
Comparing clocks & the fibre network Clocks in the network: NPL Sr lattice, Yb+ ion SYRTE Sr lattice, Hg lattice PTB Sr lattice, Yb+ ion Tests of fundamental physics • Lorentz invariance - PRL 118, 221102 (2017) • Dark matter - Science Advances 4, 12 (2018) • Do fundamental constants change? PRL 113, 210801 (2014) 26/09/2019 Optical lattice clocks at NPL 27
Sr2 • Pyramid MOT • Metastable MOT • Cavity-enhanced lattice trap • Cavity non-destructive detection • Reduced detection noise? • Reduced Dick effect? • Spin squeezing reduced QPN? Optical lattice clocks at NPL 26/09/2019 28
Sr2: In-vacuum design • Two in-vacuum cavities (L = 37.5 mm) + a hexagonal pyramid MOT 26/09/2019 Optical lattice clocks at NPL 29
Sr2: Pyramid MOT 26/09/2019 Optical lattice clocks at NPL 30
Sr2: Metastable MOT Up to 1e5 spin-polarised atoms at 5 uK in lattice Clock works 26/09/2019 Optical lattice clocks at NPL 31
Sr2: Cavity-enhanced 1D lattice Power enhancement factor = 2000 Negligible heating rates, long trap lifetime Good PDH lock Bad PDH lock 26/09/2019 Optical lattice clocks at NPL 32
Sr2: Cavity-enhanced 2D lattice Overlapped cavities 2D lattice (But clock runs fine in 1D) 26/09/2019 Optical lattice clocks at NPL 33
Sr2: Non-destructive detection ∝N ∝N Fluorescence imaging Cavity non-destructive detection Easy to set up SNR enhanced by x Atoms are heated out of the lattice Atoms remain in the lattice and can be probed again x Only a small fraction of the Enables spin-squeezing via weak measurement information is collected We already have the atoms in the cavity! 26/09/2019 Optical lattice clocks at NPL 34
Sr2: Non-destructive detection setup δωn ≈ 2π x 55 Hz/atom Cavity finesse 13000 @ 461 nm 26/09/2019 Optical lattice clocks at NPL 35
Sr2: Non-destructive detection signal 1. Switch on 461 probe 2. Probe settles start averaging 3. Atom escape in a few milliseconds Orange: Initial settling effect due to probe Stark shifts Probe creates lattice potential & Sysiphus cooling Solve by adding extra Stark compensation sidebands 26/09/2019 Optical lattice clocks at NPL 36
Sr2: Non-destructive detection signal • Signal to noise good enough to resolve 4 atoms • But still ~ 6 times higher than shot noise limit Solutions: • Filter cavity for TA amplified spontaneous emission • Lower noise photodetector • More optical power 26/09/2019 Optical lattice clocks at NPL 37
Sr2: Spin squeezing? Calculated minimum N for spin squeezing (shot noise limit using our cavity): 35 Should be possible - just need lower technical noise So… does anyone have a transimpedance amplifier with < 3 pA/rt(Hz) at 100 MHz? 26/09/2019 Optical lattice clocks at NPL 38
Outlook • Improved non-destructive detection • Recycle atoms in clock cycle • Spin squeezing? • Stable/accurate ratios of Sr1/Sr2 (see right) • New improved cavity even better stability • Comparisons against Sr, Hg, Yb+ clocks over fibre link Still not QPN limited… we need to work on it 26/09/2019 Optical lattice clocks at NPL 39
Thanks for your attention Richard Hobson Alvise William Ian Hill Marco Vianello Bowden Schioppo 26/09/2019 Optical lattice clocks at NPL 40
Sr2: Assembly 26/09/2019 Optical lattice clocks at NPL 41
You can also read
