| Abstract |
Spin-exchange collisions in alkali-metal vapors underlie several fundamental and
applied investigations such as nuclear structure studies and tests of fundamental
symmetries, ultrasensitive atomic magnetometers, magnetic resonance, and
biomagnetic imaging. The effect of spin-exchange collisions on the atomic system is very
rich and it can vary depending on the conditions: it can cause decoherence and produce
spin noise, or it can transfer spin-polarization and coherences without contributing to
spin relaxation. Recently, with the development of quantum measurements and
quantum technology in general, spin-exchange collisions have attracted a lot of interest
as a resource for quantum metrology and as a potential source of quantum noise.
We have performed a precision experiment regarding spin-noise correlations in
multispecies hot alkali vapors. In particular, we observe non-zero spin-noise correlations
and coherence transfer effects in equilibrium between overlapping 133Cs and 87Rb spin
ensembles, having different gyromagnetic ratios. Both observations stem from the
spontaneous and incessant spin-exchange collisions that tend to couple the two atomic
species through the coherent exchange of spin-polarization.
We investigate the behaviour of the spontaneous spin-noise correlations both in
a strong coupling regime at low magnetic fields where the spin-exchange rate is larger
than the difference of the Larmor frequencies and in a weak coupling regime, at higher
fields, where the fast Larmor precession tends to rapidly contract the formatted
correlations. On theoretical grounds, we develop a quantum-trajectory picture of
spin-exchange collisions, consistent with their long-standing ensemble description using
density matrices. We then use quantum trajectories to reveal the nature of spin-noise
correlations that spontaneously build up in multispecies atomic vapors, frequently
utilized in the most sensitive spin measurements.
Finally, we show that spin-exchange collisions in hot alkali vapors naturally
produce strong bipartite entanglement, which we explicitly quantify using the tools of
quantum information science. This entanglement is shown to have a lifetime at least as
long as the spin-exchange relaxation time, and to directly affect measurable spin noise
observables. We present a formal theoretical demonstration that a hot and dense
atomic vapor can support longlived bipartite and possibly higher-order entanglement.
Deeper understanding of spin-noise exchange in dual-species hot alkali vapors may
advance the operation of atomic devices like for example atomic magnetometers and
atomic gradiometers and it may also contribute to the development of new quantum
protocols and devices.
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Spin noise correlations in multispecies hot atomic vapors
