Spin depolarization of a quantum particle in nonuniform magnetic environments

Yuto Ashida

Yu. N. Pokotilovski

Mechanisms of the depolarization of ultracold neutrons in traps reflecting from the trap wall are considered. One is due to neutron spin-flip elastic or quasielastic incoherent scattering from protons of surface hydrogen contaminations. According to the second one, significant depolarization may take place because of a sudden change in the neutron trajectory on reflection from the wall when the neutron moves even at large adiabaticity parameters in nonuniform magnetic field.

Masato R. Nakamura, Kazuo Kitahara

The spin depolarization of a quantum particle in random magnetic field is treated in terms of a path integral in the spin states as well as the translational motion. The path integral of the spin operator, represented in terms of Fermion operators, is performed in the Fermion coherent state representation. An expression for the spin depolarization is obtained for an arbitrary magnetic field distribution. A particular case, in which the magnetic field is oriented in the z -direction and the magnitude is Gaussianly distributed, is considered and the spin decays exponentially in time.

V. V. Dodonov, Olga V. Man’ko

Masato R. Nakamura

Yue Chang, Shuangai Wan, Shichao Dong +1 more

Field-inhomogeneity-induced relaxation of atomic spins confined in vapor cells with depolarizing walls is studied. In contrast to nuclear spins, such as noble-gas spins, which experience minimal polarization loss at cell walls, atomic spins in uncoated cells undergo randomization at the boundaries. This distinct boundary condition results in a varied dependence of the relaxation rate on the field gradient. By solving the Bloch-Torrey equation under fully depolarizing boundary conditions, we illustrate that the relaxation rate induced by field inhomogeneity is more pronounced for spins with a smaller original relaxation rate (in the absence of the inhomogeneous field). We establish an upper limit for the relaxation rate through calculations in the perturbation regime. Moreover, we connect it to the spin-exchange-relaxation-free magnetometers, demonstrating that its linewidth is most sensitive to inhomogeneous fields along the magnetometer's sensitive axis. Our theoretical result agrees with the experimental data for cells subjected to small pump power. A deviation in high input-power scenarios arises from pump field attenuation, resulting in a nonuniformly distributed light shift that behaves like an inhomogeneous magnetic field.