Quantum reflection of ultracold atoms from semiconductor surfaces

Quantum reflection ofultracold atoms fromsemiconductor surfacesMark Fromhold, German Sinuco-León, Tom Judd,Robin Scott, Andrew Martin*, Peter Krüger,Bartek Kaczmareck*School of Physics, University of Melbourne

Outline• Coherent quantum control of atomic Bose-Einsteincondensates (BECs) by solid-state devices• Quantum reflection of a BEC from a planar Si surface– Experiments at MIT (Pasquini, Ketterle)– Analysis of the experiments– Interactions, vortex rings and cloud shape crucial• Interaction of BECs with two-dimensional electron gases– Re-writable alkali-atom lithography of quantum devices– Using 2DEGs to overcome present limitations of atom chips

Using solid surfacesto control BECs• Room-temperature surfaceswere, traditionally, the enemyof cold atoms• But, paradoxically, they arenow very useful for trappingand manipulating BECs

Using solid surfacesto control BECs• Atom chips (cold chips)– Current-carrying wires microfabricatedon a surface– More later...• Natural surfaces– Quantum reflection can shieldnK BECs from disruption by aa room-temperature surfaceonly a few microns awayR. Folman et al.,Phys. Rev. Lett. 84, 4749 (2000)Fortagh & ZimmermannRev. Mod. Phys. 79, 235 (2007)

Motivation for studyingquantum reflection• Manipulate BECs using only intrinsic surface potential• Make atom-optical elements, such as mirrors, lensesand cavities, without the need for external fields• Use the reflection process to probe both theatom-surface and atom-atom interactions

xQuantum reflection• Reverses the direction of motion where there isno classical turning point• It occurs when the potential varies rapidly with positione.g. a potential stepEnergy

xQuantum reflectionEnergy

The potential energy of an atom falls rapidly near a surfaceDue to mutual polarizationof the atom and surface……which creates anintrinsic atom-surfaceattraction:Casimir-Polder potential+-~3 mSolid+ -+ -+ -V CPxCxx

Effect of the Casimir-Polder potential on incident atomsIn a classical picture, noatoms would be reflectedSolid~3 mV CPxCx

Effect of the Casimir-Polder potential on incident atomsIn a quantum picture,reflection can occurSolid~3 mV CPxCx

Effect of the Casimir-Polder potential on incident atomsif the deBroglie wavelength spans rapidSolidpotential variation, which requires low v x1~3 mV CPxCxd~ dx

Observing quantum reflectionLow v x only realized in exceptional systems:1. Helium or hydrogen atoms incident on liquid heliumWhere low mass and weak atom-surface attraction allowquantum reflection to occur at energies ~ k B x 10 mK[Meyer et al., Cryogenics 3, 150 (1963)]2. Reflection of alkali atoms from a solid surfacerequires incident energy ~ k B x 10 nKFirst achieved for individual cold atoms grazingthe surface [F. Shimizu, PRL 86, 987 (2001)]

Quantum reflection for a BEC T. Pasquini et al.at normal incidence on a Si surface PRL 97, 093201 (2006)rSiliconwaferAtom density profile(red high)xBEC prepared in3D magnetic trapEquipotentials of3D magnetic trap x = 20 rad s -1

Quantum reflection for a BEC T. Pasquini et al.at normal incidence on a Si surface PRL 97, 093201 (2006)Equilibrium destroyedSiliconby shifting origin ofwaferthe harmonic trapDxBEC acceleratestowards Si surface & isincident with v x ~ ω x Δx

Experimental image of a BECTaken from http://cua.mit.edu/ketterle_group/• Containing 300,000 Na atoms at 10 nK60 μm

Quantum reflection from a Si surface: experimentPasquini et al. PRL 93, 223201 (2004)

Solved time-dependent Gross-Pitaevskii equation22i2VTRAP ( x,r ) C INTt 2mNonlinear repulsive potential increaseswith increasing atom densityFor a BEC with rotational symmetry about x-axisrxDensity profile in x-r plane

Reflection from a Si wall (Casimir-Polder potential)Potential profileHigh impact velocity: 2.1 mm/sLargedisplacementThe BEC reflects cleanly: no disruption occurs

Reflection from a Si wall (Casimir-Polder potential)Potential profileLow impact velocity: 1.2 mm/sSmalldisplacementThe BEC becomes disrupted

Frames from the movie for low incident speed 1.2 mm/st = 0 mst = 90 msDue to the inter-atomic interactions, the highdensity in the standing wave causes atoms to bepushed into “side-lobes”

Frames from the movie for low incident speed 1.2 mm/st = 122 mst = 90 mst = 0 msThe “side-lobes” are pushed back towards the axis ofcylindrical symmetry by the trap, producing a soliton

Frames from the movie for low incident speed 1.2 mm/st = 122 mst = 90 mst = 143 mst = 0 msThe soliton decays into two vortex ringsAt the end of the oscillation the atomcloud has a fragmented appearance

Frames from the movie for low incident speed 1.2 mm/st = 122 mst = 90 mst = 143 mst = 0 msFor lobes to form: lobe formation time < reflection timeRadial width Longitudinal width

Expect fragmentation whenImpact speed

Use this shape for studying BEC-surface interactionsThe aspect ratio of the atom cloud is crucialA pancake-shaped BEC reflects cleanly even for low v x = 1.2 mm/sbecause reflection is over before sidelobes have time to form

Diffraction of coherent matter waves by etched surface patterns:Zone-plate focusing of a BECMovie by Dominic Walliman

Polarization of adsorbed alkali atoms locally depletes 2DEGMovie by Dominic Walliman

Scan the zone plate to produce surface patterns that createquantum electronic devices e.g. a quantum dot ?Advantages over existing lithographic techniques:Movie by Dominic Walliman• Non-invasive• Erasable• Scalable

Predict that 2DEG conductors can overcome presentlimits on the functionality & miniaturization of atom chips• Can trap atoms few 100 nm from surface(as opposed to ≥ 5 µm for metal trapping wires)2DEGSinuco-León et al.Phys. Rev. A 83, 021401(R) (2011)BEC

Summary• Quantum reflection shields nK BECsfrom a room temperature surface• Inter-atomic interactions strongly affectthe reflection dynamics• To avoid fragmentation uselow-density pancake-shaped BECs• Interfacing BECs with quantumelectronic devices offer manypossibilities:– Re-writable, low-damage, lithography ?– Smooth, low-noise, near-surface trapsfor miniaturizing atom chipsFor details, see:PRL 100, 100402 (2008);New J. Phys. 12 063033 (2010); Phys. Rev. A 83, 021401(R) (2011); arXiv:1105.2486