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arXiv:1505.02241 · Physical Review Letters

Coherent Coupling of Alkali Atoms by Random Collisions

Or Katz, Or Peleg, Ofer Firstenberg

Published Sep 9, 2015• 36 citations
arXivHTMLPDF

Abstract

Random spin-exchange collisions in warm alkali vapor cause rapid decoherence and act to equilibrate the spin state of the atoms in the vapor. In contrast, here we demonstrate experimentally and theoretically a coherent coupling of one alkali species to another species, mediated by these random collisions. We show that the minor species (potassium) inherits the magnetic properties of the dominant species (rubidium), including its lifetime (T_{1}), coherence time (T_{2}), gyromagnetic ratio, and spin-exchange relaxation-free magnetic-field threshold. We further show that this coupling can be completely controlled by varying the strength of the magnetic field. Finally, we explain these phenomena analytically by mode mixing of the two species via spin-exchange collisions.

DOI
10.1103/physrevlett.115.113003

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Random spin-exchange collisions in warm alkali vapor cause rapid decoherence and act to equilibrate the spin state of the atoms in the vapor. In contrast, here we demonstrate experimentally and theoretically a coherent coupling of one alkali species to another species, mediated by these random collisions. We show that the minor species (potassium) inherits the magnetic properties of the dominant species (rubidium), including its lifetime (T_{1}), coherence time (T_{2}), gyromagnetic ratio, and spin-exchange relaxation-free magnetic-field threshold. We further show that this coupling can be completely controlled by varying the strength of the magnetic field. Finally, we explain these phenomena analytically by mode mixing of the two species via spin-exchange collisions.
Spin-exchange collisions in hot vapors are generally regarded as a decoherence mechanism. In contrast, we show that linear and nonlinear spin-exchange coupling can lead to the generation of atomic coherence in a Bell-Bloom magnetometer. In particular, we theoretically and experimentally demonstrate that nonlinear spin-exchange coupling, acting in an analogous way to a wave-mixing mechanism, can create additional modes of coherent excitation which inherit the magnetic properties of the natural Larmor coherence. The generated coherences further couple via linear spin-exchange interaction, leading to an increase of the natural coherence lifetime of the system. Notably, the measurements are performed in a low-density caesium vapor and for nonzero magnetic field, outside the standard conditions for collisional coherence transfer. The strategies discussed are important for the development of spin-exchange coupling into a resource for an improved measurement platform based on room-temperature alkali-metal vapors.
Spin-exchange collisions among alkali-metal atoms affect the coherence time and gyromagnetic ratio of atoms by redistributing the ground-state population. We theoretically study the interactions between magnetic fields and atoms in the presence of spin-exchange collisions. Using the modified Bloch equations derived from the density-matrix equation in a low spin-polarization limit, we obtain explicit formulas describing the role of spin-exchange collisions in the interactions between atoms and magnetic fields, especially the nonresonant radio-frequency fields, which are widely used to manipulate atoms. The results derived from these equations agree well with density-matrix simulations and previous experimental results, and provide a simple and precise way to quantitatively analyze the effect of spin-exchange collisions on atoms. Our paper is helpful to gain physical insight into the spin-exchange collisions and optimize the performance of atom-based precision measurement instruments.
It was recently discovered that when there are sufficiently rapid spin-exchange collisions in an alkali-metal vapor, the rf magnetic-resonance frequency of the ground-state atoms becomes a constant fractional value (e.g., 411\frac{4}{11}114​ for Cs) of the Larmor frequency, and the linewidth becomes inversely proportional to the spin-exchange rate. These properties are in complete contrast to those in the slow spin-exchange limit. We present a theoretical analysis of these properties, and we predict some additional new effects for rapid spin exchange. Four theories are presented; an intuitive-vector model, a perturbation treatment, numerical solutions of complex non-Hermitian matrices, and a statistical analysis of the effect of rapid transfers between two Zeeman multiplets. These theories all provide, to varying degrees, consistent quantitative answers to the problem of how spin exchange affects magnetic resonance in an alkali vapor.
The physics of spin exchange collisions has fueled a large number of discoveries in fundamental physics, chemistry and biology, and has led to several applications in medical imaging and nuclear magnetic resonance. We here report on the experimental observation and theoretical justification of a novel effect, the transfer of spin noise from one atomic species to another, through the mechanism of spin exchange. Essentially, we extend the foundational studies of spin exchange into the deeper layer of quantum fluctuations. The signature of spin noise exchange is an increase of the total spin noise power at low magnetic fields where the two-species spin noise resonances overlap.
We report an experimental and theoretical study of spin-noise correlations in a $^{87}\mathrm{Rb}\text{\ensuremath{-}}^{133}\mathrm{Cs}$ unpolarized alkali-metal vapor dominated by spin-exchange collisions. We observe strong unequal-time interspecies correlations and account for these with a first-principles theoretical model. Since the two atomic species have different spin precession frequencies, the dual-species vapor enables the use of an additional experimental handle, the applied magnetic field, for untangling various subtypes of spin correlations. In particular, the measured cross-correlation and autocorrelation spectra shed light on a number of spin-dynamic effects involving intra-atom, interatom, intraspecies, and interspecies correlations. Cross-correlation coefficients exceeding 6060%60 have been observed at low magnetic fields, where the two spin species couple strongly via spin-exchange collisions. The understanding of such spontaneously generated correlations can motivate the design of quantum-enhanced measurements with single or multispecies spin-polarized alkali-metal vapors used in quantum sensing applications.
Spin-exchange collisions often play a dominant role in the broadening of Zeeman resonances in an alkali-metal vapor. Contrary to intuitive expectations, at high alkali-metal densities this broadening can be completely eliminated by operating in a low magnetic field, allowing construction of ultrasensitive atomic magnetometers. We describe a detailed study of the Zeeman resonance frequencies and linewidths as a function of the magnetic field, alkali-metal density, and the degree of spin polarization of the atoms. Due to the nonlinear nature of the density matrix equations describing the spin-exchange collisions both the gyromagnetic ratio and the linewidth change as a function of the polarization. The results of experimental measurements are in excellent agreement with analytical and numerical solutions of the density matrix equations.