Final Report Summary - HYCODE (Atom chips on the submicron scale: Routes to hybrid cold atom-quantum electronics devices)
The main objective of the project HYCODE was to investigate ultra-close trapping distances (<1micron) between ultra-cold atoms and surface structures, which would open new gateways to improve the use of cold atoms in commercial applications as sensors or ultra-precise clocks. In order to reach this goal two approaches were originally proposed, first to use thinned-out SiN membranes (<10nm) to carry the structure of interest, secondly two dimensional electron gases (2DEG) within thin (<200nm) GaAs/(AlGa)As membranes to carry the trapping/manipulation currents. The first approaches aims for a reduction in Casimir-Polder (CP) interaction between the atoms and the surface, as the second will in addition reduce Johnson noise close to surface. Due to the low electron mobility of the GaAs/(AlGa)As membranes, the latter approach requires an evolved cryogenic environment to house an atom chip that is capable of providing traps for ultra-cold atoms close to a surface at roughly 4K. As this approach makes scaling and commercial use of ultra-close trapped atomic ensembles (for e.g. magnetic field or gravity sensors) difficult HYCODE focussed on an alternative approach by switching to free-standing graphene membranes.
The advantages of graphene lie in its high electron mobility, which allows the generation of sufficiently high currents to produce magnetic traps for ultra-cold atoms at room temperature, while still maintaining a low electron density to feature improved performance regarding Johnson-noise-induced spin flips. Calculations performed during this project have also shown that Casimir Polder forces are of the same magnitude as with 10nm SiN membranes.
The approach chosen to fabricate these freestanding graphene membranes was to place them on an isolated copper grid, commonly used in Transmission Electron Microscopy (TEM). For this task collaboration with the Andre group (University of Rutgers, USA) was founded. Further structuring of the membrane can be performed using focussed laser light to burn away individual sites of the grid so that a current path can be formed. This will allow forming individual wires, which will then generate the magnetic fields to form the atom traps (see Figure 1).
In parallel to the work on suitable membranes an ultra-high vacuum chamber capable of bearing all necessary parts of the experiment was set up (Figure 2a). Development of a high efficiency dual-colour Magneto Optical Trap (MOT) scheme allows for a higher initial atom number in the MOT which shortens the cycle time of the experiment pushing cold atom sensors in general towards industrial applicability (Figure 2b). In connection with a conventional 2D MOT this approach also reduces the necessary working pressures, which results in a lower contamination of the samples by background rubidium. This again adds value for commercial use in e.g. magnetic field microscopy on microscopic samples.
The advantages of graphene lie in its high electron mobility, which allows the generation of sufficiently high currents to produce magnetic traps for ultra-cold atoms at room temperature, while still maintaining a low electron density to feature improved performance regarding Johnson-noise-induced spin flips. Calculations performed during this project have also shown that Casimir Polder forces are of the same magnitude as with 10nm SiN membranes.
The approach chosen to fabricate these freestanding graphene membranes was to place them on an isolated copper grid, commonly used in Transmission Electron Microscopy (TEM). For this task collaboration with the Andre group (University of Rutgers, USA) was founded. Further structuring of the membrane can be performed using focussed laser light to burn away individual sites of the grid so that a current path can be formed. This will allow forming individual wires, which will then generate the magnetic fields to form the atom traps (see Figure 1).
In parallel to the work on suitable membranes an ultra-high vacuum chamber capable of bearing all necessary parts of the experiment was set up (Figure 2a). Development of a high efficiency dual-colour Magneto Optical Trap (MOT) scheme allows for a higher initial atom number in the MOT which shortens the cycle time of the experiment pushing cold atom sensors in general towards industrial applicability (Figure 2b). In connection with a conventional 2D MOT this approach also reduces the necessary working pressures, which results in a lower contamination of the samples by background rubidium. This again adds value for commercial use in e.g. magnetic field microscopy on microscopic samples.