Manipulating the spin degrees of freedom of atomic vapors has in recent years led to exciting advances in atomic physics and quantum metrology. In particular, understanding spin relaxation phenomena in collision-dominated atomic vapors, like cesium vapors used in atomic clocks, has lead to novel ultrasensitive magnetometers, having a number of applications, from low-field NMR to biomagnetic imaging. One of the fundamental limitations of this kind of precision measurements is spontaneous spin noise. This is a quantum fluctuation of the atomic vapor’s collective spin driven by atomic collisions, and sets the noise level and hence the precision of all relevant spin measurements.
We focused on high resolution measurements of spin noise at low magnetic fields, showing how one can obtain information about spin relaxation from the spontaneous spin noise resonance width. Most recently, we have demonstrated a new phenomenon related to spin-exchange collisions, namely that in a multi-species atomic vapor and low magnetic fields, spin-exchange collisions induce positive inter-species spin-noise correlations. This is an extension in the realm of quantum fluctuations of the well known property of spin exchange, the inter-species transfer of a deterministic spin polarization. The new effect, termed spin-noise exchange, was revealed by a low-field increase of the total spin-noise power.
Instead of using the indirect signature of increased spin-noise power, we wish to directly demonstrate the spin-noise correlations that spontaneously emerge in multi-species vapors and further study their ramifications for measurement precision in spin-exchange dominated atomic vapors. Studying this effect at even lower magnetic fields where the magnetic resonance line width is not anymore dominated by spin-exchange is a natural next step. We would also like to connect these studies with the general goal of quantum metrology of achieving sub-shot-noise spin fluctuations towards enhancing the precision of spin measurements such as in atomic magnetometers.