  Ever since its discovery, the electron spin has only been measured or
manipulated through the application of an electromagnetic force acting on the
associated magnetic moment. In this work, we propose a spin Aharonov-Bohm
effect in which the electron spin is controlled by a magnetic flux while no
electromagnetic field is acting on the electron. Such a nonlocal spin
manipulation is realized in an Aharonov-Bohm ring made from the recently
discovered quantum spin Hall insulator, by taking advantage of the defining
property of the quantum spin Hall edge states: the one-to-one correspondence
between spin polarization and direction of propagation. The proposed setup can
be used to realize a new spintronics device, the topological spin transistor,
in which the spin rotation is completely controlled by a magnetic flux of
hc/2e, independently of the details of the sample.
