Permissions for Reuse

Antirelaxation surface coatings reduce the effects of spin relaxation at the walls of alkali vapor cells, allowing for significantly enhanced atomic spin polarization lifetimes. Coated cells have been used for atomic magnetometry,^1,2,3,4,5,6 atomic clocks,^7,8 magneto-optical traps,^9,10,11 and slow light¹² and quantum memory experiments.¹³ To date, the most effective, widely used coating is paraffin, which allows an alkali atom to collide up to 10 000 times with the cell wall before depolarizing.^14,15,16 However, paraffin typically melts at temperatures of about 60–80 °C and so cannot be used for applications such as high alkali vapor density spin-exchange relaxation-free (SERF) magnetometers,^17,18 which must operate at higher temperatures. Various silane molecules have also been demonstrated^9,19,20,21 to reduce surface relaxation rates; in particular, octadecyltrichlorosilane (OTS) has been shown to allow up to 2000 bounces on the surface before depolarization and to operate at higher temperatures than paraffin.⁶

When an alkali atom collides with the wall of a cell, it interacts with the surface for a brief period of time, during which it is subject to large local electric and magnetic fields, and its spin coherence with the other atoms in the cell may be lost. Wall collisions typically occur more frequently than other relaxation mechanisms (i.e., spin-exchange and spin-destruction collisions) and can completely dominate the lifetime of the atomic polarization. Surface coatings prevent direct interaction between the atoms and cell walls, and coatings with low polarizibility, such as paraffin, feature significantly smaller local fields than the bare surface, allowing the atoms to bounce off a coated surface many times while maintaining their polarization.¹⁴ Surface relaxation is also suppressed in vapor cells filled with a buffer gas that slows diffusion of atoms to the walls, but coated cells without buffer gas have several advantages. Although coated cells may still require a small amount of quenching gas to prevent radiation trapping,²² without a high-pressure buffer gas atoms are still free to move between different parts of the cell. Pump and probe laser beams, therefore, do not need to fill the entire cell and can be made small, since the atoms are likely to pass through each beam at least once during a polarization lifetime. In addition, depolarization due to magnetic field gradients is suppressed because the atoms effectively average the field over the entire cell volume.²³ Also, in the absence of pressure broadening due to buffer gas, the optical linewidths of atomic transitions are narrower, allowing for the use of less-powerful lasers and resulting in larger optical rotation signals.²⁴

Recent advances in alkali-metal magnetometry have achieved extremely high sensitivity by operating at high alkali atom density (~10¹³–10¹⁴  cm⁻³), requiring temperatures outside the effective operating range of paraffin (T>100 °C for cesium, T>150 °C for potassium). These include the SERF magnetometer^17,18 with demonstrated sensitivity of 0.75  fT/Hz (Ref. 25) for near-dc magnetic fields and a tunable radio-frequency magnetometer^26,27 with demonstrated sensitivity of 0.24  fT/Hz (Ref. 28) for fields in the range of several kilohertz to several megahertz. Both types of magnetometers have fundamental sensitivity below 0.01  fT/Hz. Previous demonstrations of these high-density magnetometers have used uncoated cells filled with buffer gas because of the unavailability of high-quality surface coatings that can operate at these temperatures; therefore, the development of robust wall coatings that allow many bounces at high temperatures would benefit these applications.

In order to evaluate and develop effective coatings, a method is needed which allows comparison of multiple coatings under identical experimental conditions. This paper describes the development and use of an easily reusable alkali vapor cell for the purpose of testing multiple coatings for effectiveness and suitability for high-density atomic magnetometry. Previous experiments investigating the effectiveness of coatings^{9,19,20,21,29} have used individual coated cells, whereas the use of a cell that can accommodate easily removable test surfaces allows for efficient testing of many different types of coatings under the same conditions. In the design described here, individual glass or silicon slides may be coated and then inserted into the reusable cell for testing. Flat slides can be coated more easily and uniformly than closed-geometry cells, and this geometry allows for additional surface characterization tests, such as infrared spectroscopy, atomic force microscopy, and x-ray photoelectron spectroscopy, to be conducted both before and after exposure to the alkali vapor.