  To capture the high-field magnetization process of herbertsmithite
(ZnCu3(OH)6Cl2), Faraday rotation (FR) measurements were carried out on a
single crystal in magnetic fields of up to 190 T. The magnetization data
evaluated from the FR angle exhibited a saturation behavior above 150 T at low
temperatures, which was attributed to the 1/3 magnetization plateau. The
overall behavior of the magnetization process was reproduced by theoretical
models based on the nearest-neighbor Heisenberg model. This suggests that
herbertsmithite is a proximate kagome antiferromagnet hosting an ideal quantum
spin liquid in the ground state. A distinguishing feature is the superlinear
magnetization increase, which is in contrast to the Brillouin function-type
increase observed by conventional magnetization measurements and indicates a
reduced contribution from free spins located at the Zn sites to the FR signal.
