Florian SchreckUniversity of Amsterdam

Towards ultracoldRbSr ground-state molecules

Many-body systems

www.thepistrophy.com

Quantum many-body physics

Strong quantum correlations render description difficult.Especially difficult:

Fermionic systems

?Frustrated systems Dynamics

𝑡𝑡

𝑂𝑂

Quantum simulation (and beyond)

Control everything

…

potential interactions internal state

difficult to studyquantum system

well-controlledultracold systemsimplified model

Alkaline-earth(-like) elementsYb, Sr, Ca

New ultracold systems

Magnetic atoms Cr, Dy, Er

Ground-state moleculesclosed-shell: KRb, NaK, RbCs,...

open-shell: RbYb, RbSr, YbCs

Rydberg atoms

More interesting interactions

Benefits

• long-range • anisotropic

More internal structure

Molecules

Richer structure than atomsrotational and vibrational states

Dipole-dipole interactions due to electric dipole moment

Example: quantum magnetismMagnetism requires spin dependent interactions.

Provided by super-exchange in ultracold ground-state atoms

𝑡𝑡𝑈𝑈

𝑡𝑡

spin dependent interaction

Requires extremely low motional temperature

In dipolar molecules simply provided by spin-state dependent dipole interaction

Requires only low spin temperature

Theory:Micheli et al., nature physics 2, 341 (2006)Gorshkov et al., PR A, 84, 033619 (2011)

Examples: dipolar gases in 2D

Interlayer Cooper pairing

G. Pupillo et al., PRL 104, 223002 (2010)

Monte-Carlo density distributions in dependence of interaction strength

Bosonic molecules Fermionic molecules

ReviewsM. A. Baranov, M. Dalmonte, G. Pupillo, and P. Zoller, Chem. Rev. 112, 5012 (2012)

M. L. Wall, K. R. A. Hazzard, and A. M. Rey, arXiv:1406.4758

Pikovski, A. et al., PRL 105, 215302 (2010)

Two paths to ultracold molecules

Atoms Molecules

hot

cold

cool

associate

cool

associate

Path 1

Path 2

Status

• KRb 0.5 Debye dipole moment Science 322, 231 (2008) Ye, Jin group• Cs2 0 Debye Faraday Disc., 284 (2009) Nägerl group• RbCs 1.2 Debye PRL 113, 205301 (2014) Ferlaino, Grimm, Nägerl gr.• NaK 2.7 Debye PRL 114, 205302 (2015) Zwierlein group

Observation of dipolar spin-exchange interactions with lattice-confined polar moleculesNature, 501, 521 (2013)

Many-body dynamics of dipolar molecules in an optical latticePRL 113, 195302 (2014)

Four systems, all based on alkali atoms:

Ye, Jin group

Challenge

Cooling to quantum degeneracy

First experiments studying magnetism:

Other alkali-alkali molecule experiments being set up by many groups.

Evaporative cooling

KRb + KRb K2 + Rb2 + energy

Challenge for evaporative cooling

Chemistry

© m

astil

oade

d.co

m

Counter measures

© m

astil

oade

d.co

m

Repulsive molecules in 1D lattice

might allow evaporative cooling to quantum gas

e.g. RbCs, NaK,…

Non-reactive species

Dangerous „sticky collisions“ ?Bohn group, PR A 87, 012709 (2013)

Repulsive van der Waals interactions

Gorshkov et al. PRL 101, 073201 (2008)

Bohn group, PR A 73, 022707 (2006)

Zoller group, PRL 98, 060404 (2007)

Bohn group, PR A 81, 060701(R) (2010)

RbSr ground-state molecules

unpaired electron

alkali + alkali atom

Our goal

Our group RbSrGörlitz group RbYbGupta, Takahashi groups LiYbCornish group CsYb

So faralkali + alkaline earth atom

only paired electrons

Repulsive van der Waals interactionsinstantaneous dipole – induced dipole interaction

Can be repulsive in excited state:

evaporative cooling of OH at mK temperatures, Ye group, Nature 492, 396 (2012)

E

383 G

N=0

N=1

magnetic field

E = 150 V/cm

RbSr energy, neglecting hyperfine structure

repulsive vdW interaction!

Repulsive van der Waals interactions

0

1

2

3

4

5

6

7

8

9

10

600 800 1000 1200 1400 1600 1800 2000

Ener

gy (µ

K)

preliminary result by John Bohn, JILA, neglecting hyperfine interaction

E = 150 V/cm B = 383 G

RbSr interaction potential

r (Bohr radii)

expectedtemperatureof molecular gas

Evaporative cooling possible?

Repulsive van der Waals interactions

Experimental realizationRb-Sr mixtures87Rb

N = 1.3 x 105

84Sr

N = 2.3 x 105

Molecule association

Standard technique

magneto association Sr is non-magnetic magneto association not trivial

Alternative technique

STIRAP association let‘s demonstrate it by creating Sr2

Hutson group, PRL 105, 153201 (2010)

SchemeBEC

Mott insulator

ramp on optical lattice

associate molecules

BEC

Molecule association by STIRAP

1S0 + 1S0

1S0 + 3P1

external trap(lattice well)

|e>

Ω1Ω2

|m>

|a>

Γ

Molecule association by STIRAP

1S0 + 1S0

1S0 + 3P1

external trap(lattice well)

|e>

Ω2

|m>

|a>

Γ

Molecule association by STIRAP

1S0 + 1S0

1S0 + 3P1

external trap(lattice well)

|e>

Ω1

|m>

|a>

Γ

Molecule association by STIRAP

1S0 + 1S0

1S0 + 3P1

external trap(lattice well)

|e>

Ω1Ω2

|m>

|a>

|Ψ> = Ω1|m> + Ω2|a>

Γ

0 20 40 60 80 1000,0

0,5

1,0

L2 L1

Inte

nsity

Time (µs)

Molecule association by STIRAP

1S0 + 1S0

1S0 + 3P1

external trap(lattice well)

|e>

Ω1Ω2

|m>

|a>

Γ

STIRAP transition

singlet states triplet states

461 nm30 MHz

689 nm7.4 kHz

1S0

1P1

3PJ

698 nmmHz

210671 nm

mHz

STIRAP transition

singlet states triplet states

689 nm7.4 kHz

1S0

1P1

3PJ210

L1L2

Result

See also 88Sr2 work by Tanya Zelevinsky, PRL 109, 115303 (2012).

30% conversion efficiency

60µs lifetime

L1L2

Result

See also 88Sr2 work by Tanya Zelevinsky, PRL 109, 115303 (2012).

30% conversion efficiency

60µs lifetime

What are the limitations?

Can we improve performance?

Short lifetime of molecules

532 nm (Verdi) used in Innsbruck

1064 nm

A. Stein et al., Eur. Phys. J. D 64, 227 (2011)

Sr2 molecular potentials

Lifetime in 1064nm lattice

110(25) s lifetime

STIRAP efficiency: 55%

Lightshift on binding energy

1S0 + 1S0

1S0 + 3P1

|e>

L1

|m>

|a>

228 MHz

compensation beam

75 MHz

Compensating binding energy shift

1S0 + 1S0

1S0 + 3P1

|e>

L1

|m>

|a>

228 MHz

compensation beam

75 MHz

compensation beam intensity / L1 intensity (%)

STIRAP with compensation

~50% single pass efficiency

80% single pass efficiency

Inte

nsity

(abr

)

L2 L1 C

STIRAP with compensation

80% single pass STIRAP efficiency

efficiency increase bycompensation beam

Why initially lower efficiency?

~50% single pass efficiency

80% single pass efficiency

Inte

nsity

(abr

)

L1 L2 C

STIRAP parameter dependenceatom number after STIRAP round-trip

|e>

|m>|a>

What limits STIRAP efficiency?

2) dark state lifetime of 3.5 ms

1) off-resonant scattering of photons on atomic line

Next goal

RbSr molecules

Sr2 molecules

84Sr87Rb

Rb-Sr mixture87Rb

N = 1.3 x 105

84Sr

N = 2.3 x 105

RbSr molecular potentialsZuchowski et al., PR A 90, 012507 (2014)

RbSr molecular potentialsZuchowski et al., PR A 90, 012507 (2014)

Photoassociation spectroscopy

88Sr-87Rb 84Sr-87Rb

Photoassociation spectroscopy

88Sr-87Rb 84Sr-87Rb

ObservationRb hyperfine splitting decreases by3 MHz for 2GHz binding energy of F=2 state

Observations• Transition linewidth increases

from 20kHz to 400kHz withincreasing binding energy

• 84Sr87Rb Rabi frequencies ~10x lower than Sr2 or 88Sr87Rb Rabi frequencies at given intensity

Molecular potential fit resultsTo fit data, ground-state molecular potential needs to be 10% lower than predicted.

Scattering lengths obtained from fit (preliminary)

84Sr 86Sr 87Sr 88Sr85Rb 300a0 90a0 50a0 -10a0

87Rb 90a0 -15a0 -400a0 275a0

Simplest mixtures to work with

84Sr-87Rb

87Sr-87Rb

Bose - Bose

Fermi - Bose

Sr intraspecies 122a0 800a0 96a0 -2a0

1-color PA: thermal cloud vs. BEC

-20 -15 -10 -5 0 5 100

5

10

15

Rb a

tom

num

ber (

104 )

Detuning (MHz)

thermal clouds

BECs

Spatial overlap

interspecies scattering length intraspecies scattering lengths87Rb - 84Sr: 90 a0

87Rb: 105 a0 , 84Sr: 124 a0

no phase separationexpected

Phase-separation if

<

;

position

dens

ityde

nsity

position

low interspeciesscattering length

large interspeciesscattering length

Spatial overlapWe use 1064 nm dipole trap: 3x deeper for Rb than for Sr

• gravitational sagging• weaker trap for Sr

Vertical position (µm)

Den

sity

(101

4cm

-3)

87Rb

84Sr

Vertical position (µm)

Pote

ntia

l (µK

)

1064nm dipole trap

Rb trap

Sr trap

Bi-chromatic dipole trapBalance trap by 532nm dipole trap(attractive for Sr, repulsive for Rb) 1064nm

dipole trap

532nmdipole trap

Vertical position (µm)

Den

sity

(101

4cm

-3)

87Rb

84Sr

Vertical position (µm)

Pote

ntia

l (µK

)

-20 -15 -10 -5 0 5 100

5

10

15

Rb a

tom

num

ber (

104 )

Detuning (MHz)

PA with trap compensation

thermal clouds

BECs, different potentials

-20 -15 -10 -5 0 5 100

5

10

15

Rb a

tom

num

ber (

104 )

Detuning (MHz)

BECs, similar potentials

87Rb-84Sr Mott insulator

PA of double Mott insulator

20 000 siteswith atoms of each species

-4 -3 -2 -1 0 1 2 3 4 50

1

2

3

4

5

6

7

Rb a

tom

num

ber (

104 )

Detuning (MHz)

STIRAP attempts with 87Rb-84Sr

Rb 2S1/2 + Sr 1S0

Rb 2S1/2 + Sr 3P1|e>

Ω1Ω2

|m>

Γ

|a>

All attempts failed because of weak free-bound transition

Why is this transition so weak?

Free-bound transition strength

88Sr - 87Rb

84Sr - 87Rb 7x10-4

4x10-3

87Sr - 87Rb 3x10-2

174

273

115

Excited statebinding energy

(MHz)

Franck-Condonfactor

40 times higherRabi frequency!

Try with 87Sr - 87Rb !

Predictions from ab-initio model fitted to data

2S1/2 + 1S0

2S1/2 + 3P1

Towards the RbSr ground state

few 100 MHz binding energy~50 a0 radius

30 THz binding energy9 a0 radius

~500 a0 radius

Spectroscopy of "hot" RbSr

HeN

Rb pickup cellSr pickup cell

pulsedlaser

Pickup process

Spectroscopy of "hot" RbSrJ. Chem. Phys.141(23):234309 (2014)

12000 12100 12200 12300 12400 12500

E [cm-1]

0

1

2

3absorption[arb. units] simulation

experiment

Current projectsRbSr molecules

Perpetual atom laser

Li-Sr quantum gas microscope

K quantum gases

Veni & Vici

The team

AlexBayerle(PhD)

FlorianSchreck

(PI)

Chun-ChiaChen(PhD)

AlessioCiamei(PhD)

ShayneBennetts

(PhD)

GeorgiosSiviloglou

(Marie Curiefellow)

OleksiyOnichshenko

(PhD)

NamrataDutta-Mazumdar

(master)

Funding

Veni & Vici

VincentBarbé(PhD)

SergeyPyatchenkov

(PhD)BenjaminPasquiou(Veni PI)

DenisKurlov(PhD)