cold atoms for magnetic microscopy Amruta Gadge (University of - - PowerPoint PPT Presentation

Sub-micron trapping of cold atoms for magnetic microscopy

Amruta Gadge (University of Nottingham) Quantum Technology Conference VI, Warsaw 25th June 2015

Tim James, Christian Koller, Jessica O Maclean, Chris Mellor, Mark Fromhold, Fedja Orucevic, Peter Kruger Samples: Rutgers, NPL, Chalmers, IBM

Quantum sensors

• Exploiting quantum effects for precision

measurements

• Cold atoms are ideal candidates for sensing

applications.

Outline

Ultra-close trapping of atoms

• Motivation
• Atom-surface interactions
• Possibility of sub-micron trapping
• Ideal samples

The experiment

• Experimental set up
• Dual colour MOT
• Magnetic transport of atoms

Chip based magnetic field sensor

• Trapped cold atoms are extremely

sensitive to magnetic field landscape. They are ideal as magnetic field sensors.

• Local magnetic fields can be deduced

from the density distribution of the trapped atoms.

Wildermuth et al. NATURE|Vol 435|26 May 2005

Submicron trapping of the atoms

• Limitation on trapping separations~ 10-100µm.
• Atom cloud positioned very close to the surface, will be a

sensor with high sensitivity, high resolution and with large field of view What are the limitations?

• Distance dependent atom- surface interactions become

dominant.

• Casimir force, Johnson noise, stray potentials from

adsorbates, wire corrugations, technical noise

• Modification of trapping potential due to surface forces

leading to losses

• Reduction of life time

Thermal magnetic near field noise

• Thermal of motion of electrons in a conductor causes current

fluctuations- Johnson Nyquist Noise

• Current fluctuations lead to magnetic field fluctuations
• These fluctuations couple to atomic spin inducing transition to anti

trapped state.

) , , ( 16 ) (     



h d g s d T k S

B B



Minimizing thermal noise

• . For thin wire of finite width
• For d<<h,

(Bulk behavior)

• For d>>h,

d d h d h 1 ) (  

2

) ( d h d h d h   For d=1μm, h= 100nm Ten times increase in lifetime!

1 . 0   

d w w d h h g 2 .    )) , ( ( d h  

d w w d h h d T KB

B s

2 . 640 9

2 2

        

Casimir Polder interaction

• Attractive interaction between neutral atom and the surface.
• Strongly distance dependent

U z C z m U

j j z total

         ) ( ) ( 2

2 2

Nathan Welsh (2015)

Effects of surface interaction

• Modification of trapping potential
• Reduction of trap depth
• Modification of trap frequency
• Shift in trap position

) ( 1 1 32 3

2 4 r r r

c C           

Interaction depends on material properties This interaction is weaker between atom and a thin dielectric surface.

Stray potential due to adsorbates

• Atoms adsorbed on the surface get polarized and

generate electric field perpendicular to the surface.

• Combined field due to adsorbed atoms can be

significant.

• Potential generated can be given as,

2

) ( 2 r E Va   

Is submicron trapping possible?

Yes if the surface is carefully chosen! Ideal sample

• Very thin thin membranes (silicon nitride

membranes)

• Dielectric Silicon nitride membranes
• Possibility of in-situ cleaning

Silicon Nitride membranes

• Thin silicon nitride membranes of thickness 10 and 100nm

supported on a silicon frame.

• Each sample has 9 membrane windows.
• Different patterns are made on each of these

membrane windows.

• Patterning is done by milling away material using focused

ion beam technique.

Samples from Norcada Inc, Canada FIB patterning: Dr Leo Gross, IBM, Zurich

Transport PCB Ion beam E-beam Sample holder

Some patterns on 100nm membranes

Silicon Nitride

10nm membrane 100nm membrane

Our experiment

Initial trap

• Magneto optical trap for Rubidium 87 atoms
• Macroscopic magnetic trap

Atomic manipulation

• Transport chip

(fabricated using printed circuit board technology)

• Atom chip

Experiment

Location of various samples

Our experiment

Transport chip Reflection QWP

Atom chip

Samples

5 beam MOT

Beam1 Beam2 Beam3 Beam4 Beam5 Imaging

Two colour scheme

|g> |e> 𝜕𝑚 𝜕1 𝜕2 𝜕𝑓𝑕 − 𝜀 𝜕𝑓𝑕 𝛽 𝛽

• Increased atom number in MOT by using slightly detuned two frequencies

Single color MOT

Cao Qiang et. al. Chin. Phys. B Vol. 21, No. 4 (2012)

Chip based atom traps

• By turning these wires either in

U or Z shape, confinement in third direction can be obtained.

Z-currnet: Ioffe-Pritchard trap Single wire: Side guide R Folman, P Kruger , J Schmiedmayer , J Denschlag, C Henkel(2002)

Transport of atoms

I1 I1 I2 I2 Transport wires

• Four wire guide for tight confinement of atoms in two directions.
• Set of orthogonal wires for weak confinement in third direction.
• By modulating currents in these transport wires, trap position can

be shifted from one wire to next.

Long et.al Eur. Phys. J. D 35, 125–133 (2005)

Transport PCB

Our design

• 10 layer printed circuit board
• Top layer contains transport wires, vias and contact pads for atom chip.
• Bottom layers contain quadrupole wires.

I1 I2 I3 I4 I5 I6

Atom chip

• Two layer atom chip to manipulate and precisely

position atoms very close to the surface.

• Grid of wires of widths 30µm and 100µm on top and

bottom layers respectively.

Summary

Transport chip Reflection QWP

Atom chip

Samples

Summary

• At separations below a micron various surface forces

impede separation of trapped atoms

• Ultra-close atom trapping is possible with use of thin

membranes.

• We are in process of developing a system to trap

atoms very close surfaces.

• Magnetic field microscopy with much better resolution

can be performed with such a system.

Thank you!