COLLISIONS IN ULTRACOLD COLLISIONS IN ULTRACOLD METASTABLE HELIUM GASESMETASTABLE HELIUM GASES

G. B. Partridge, J.-C. Jaskula, M. Bonneau, D. Boiron, C. I. WestbrookLaboratoire Charles Fabry de l’Institut d’Optique, Palaiseau France

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OutlineOutline

Methods, apparatus, He*.

Motivation and Background--Optics, atomoptics, quantum optics, quantum atom optics… Optics, atomoptics, quantum optics, quantum atom optics…

Optical Trapping Optical Trapping andand Relative Number Relative Number

SqueezingSqueezing

Experiments:

4-wave mixing of matter 4-wave mixing of matter waveswaves

Spin MixturesSpin Mixtures

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Motivation, atom optics…Motivation, atom optics…

Optics : Photons, waves… wave particle duality.

Atomic physics atom optics :i.e – slits, interferrometers, etc

Bec coherent atom optics:Atom Laser, fringes, + nonlinear atom

optics (interactions): 4wm , solitons…

Quantum atom optics? -ex’s correlations, squeezing,

entanglement, teleportation…

Use counting, single particles, statistics…--Key is detection: metastable Helium

(He*).

T. Pfau (Stuttgart)

L. Deng et al. (NIST)

Strecker et al. (Rice)

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He* : What’s it hiding?He* : What’s it hiding?

The 23S1 state of He has a decay time ~ 8000 s !*

*single atom ~ spin polarized

This energy can kick off electrons & ionize atoms of surfaces that the atom meets.

The stored energy of the metastable state is 19.8 eV/atom.

Add in a potential, get an avalanche of electrons.

High gain amplifier = single atom sensitivity.

e-

+

(So what?)

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Trapping and Cooling He*Trapping and Cooling He*

Laser cooling helium?

Behaves a lot like an alkali-metal.

(Cycling Optical Transition, magnetically trappable )

I

I

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Single Atom DetectionSingle Atom Detection

Use a micro-channel plate (many e- avalanche detectors in parallel) to give position information.

Gather resulting electric pulses using crossed delay lines.

Use relative arrival times to reconstruct atoms’ positions

(time of flight) in 3D.

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A new tool for He*A new tool for He*

Statistical measurements: 1000’s of repetitions.

magnetic trap was not engineered for this…

(although we try anyway)Long term: favors Optical Trap

magnetic optical

TOF

Also, better geometry: aligns long axis of potential ( short TOF, short

correlation length) w/ high resolution direction, Z.

Gives freedom to try spin mixtures…

First step towards more complicated potentials for He*

(lattices, disorder etc.)

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BEC of He* in the optical BEC of He* in the optical traptrap

N0 = 105 r = 1.5 kHz, z = 8 Hz

Transfer from magnetic trap after some pre-cooling: N = 5 x 106, T = 15 K

Evaporate by reducing intensity of trap laser over ~ 4 sec.

G. B. Partridge, J.-C. Jaskula, M. Bonneau, D. Boiron, C. I. Westbrook, Phys. Rev. A 81, 053631 (2010).

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Quantum Optics: photon Quantum Optics: photon pairspairs

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Matter Wave FWM: atom Matter Wave FWM: atom pairspairs

S-wave interactions lead to spherical shell of scattered atoms at k=kS

spontaneous FWM

k0

k0kS

kS

k0 k0

Create an m = 0 condensate w/ raman pulse.

Split BEC into two momentum components with Bragg pulse: +/- k0

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The “intuitive” resultThe “intuitive” result

Scattered pairs are correlated…

0 Δt

P(Δt)

k0

k0

kS

kS

Like in photon pairs: “Enhanced

coincidence rate when phase

matching condition is met.”

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Beyond Optics: smaller Beyond Optics: smaller spheresphere

|kS| < |k0|

k0

k0

kS

kS

Energy Cost to put atom into scattered mode (still overlapped w/

condensate).

k0 k0

kS

kS

(per atom)

Energy gain from removal of atom from

condensate mode <

““energy balance”energy balance”

V. Krachmalnicoff, J.-C. Jaskula, M. Bonneau, V. Leung, G. B. Partridge, D. Boiron, C. I. Westbrook, P. Deuar, P. Zin, M. Trippenbach, K. Kheruntsyan, Phys. Rev. Lett. 104,150402 (2010).

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Plus, the sphere’s not a Plus, the sphere’s not a spheresphere

After colliding, atoms still have to get out of the region of the condensates.

i.e. they roll down the mean field hill: V = 2g(r,t)

But the hill is collapsing out from under them.

Anisotropy of BEC’s leads to directional acceleration

Lesson Learned:Do Q.O. experiments using atoms, but be careful about

simple 1:1 intuition. There are differences, for better or worse…

Analogy? ponderomotive force in high harmonic generation

(Balcou et al PRA 1997)Phys. Rev. Lett. 104,150402 (2010).

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Intermediate Q.O.: Relative N Intermediate Q.O.: Relative N SqueezingSqueezing

Heidmann et al. PRL 59 2555 (1987)

BA

BA

II

IIR

A

B

Measurement of intensity noise between “twin” beams.

Reduction in noise, 30% below the shot noise limit!

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New Atom PairsNew Atom PairsRF + Bragg pulse.Optical Trap BEC

Back-to-Back Correlations: 3600 shotsCollision along long axis + better repeatability gives

improved S/N.Now what about squeezing?

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Matter Wave N SqueezingMatter Wave N Squeezing

1,

M

ji

jiij

NN

NNM

Divide scattered halo into sections, compare number difference in

geometrically opposing zones to that of non-opposing zones.

(for uncorrelated N, i.e. shot noise)

1

M

M 16 zones

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Details…Details…

Detail 1:

Raw data ~ -0.5 dB squeezing

Why isn’t it perfect?

(partly b/c its an experiment)

Specifically, the detector efficiency, , limits the measured variance.

Perfect correlations: M = (1- ) = 0.6 (“open area”) : -3 dB

= .13 (best estimate): -13 dB

Detail 2:Effect of of correlation length:

~Measurement bandwidth

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What’s next?What’s next?

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But! Trapped He* gases are prone to loss due to Ionization-enhanced

inelastic loss processes.

Spin Polarization in the mJ = 1 provides stabilization by ~5 orders

of magnitude.

What about other states and combinations of states?

State specific loss constants unconfirmed experimentally (only mJ = 1 is magnetically trappable)

With optical trap, we can think about using different spin states (mJ = +1,-1,0)

spin mixtures, spinor condensates …

RF transfers: spin mixtures

Alternate Future: spin mixturesAlternate Future: spin mixtures

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Loss Rates in a spin Loss Rates in a spin mixturemixture

Inelastic Loss Experiment 1: Put them all together and see what

survives…

“Small” loss rate: 01, 0-1, 11, -1-1

“Large” loss rate: 00, ±1

G. B. Partridge et al., Phys. Rev. A 81, 053631 (2010).

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Quantitative Loss RatesQuantitative Loss Rates

00 = 6.6(4) × 10 −10 cm3/s

±1 = 7.4(10) × 10 −10 cm3/s.

Not necessarily prohibitive! (for certain things…)

Inelastic Loss Experiment 2: Make careful measure of the dominant processes 00 ±1.

G. B. Partridge et al., Phys. Rev. A 81, 053631 (2010).

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SummarySummary

1. Quantum Atom Optics: Spontaneous FWM of deBroglie matter waves.

• Don’t forget they’re atoms.

2. Relative Number Squeezing for correlated atom pairs.

• Atomic version of a Quantum Optics Classic.

3. Spin Mixtures in of He* ? • Stay tuned…

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Thanks!

Questions?