  We study coherent spin transport through helical edge states of topological
insulator tunnel-coupled to metallic leads.
  We demonstrate that unpolarized incoming electron beam acquires finite
polarization after transmission through such a setup provided that edges
contain at least one magnetic impurity. The finite polarization appears even in
the fully classical regime and is therefore robust to dephasing. There is also
a quantum magnetic field-tunable contribution to the polarization, which shows
sharp identical Aharonov-Bohm resonances as a function of magnetic flux - with
the period $hc/2e$ - and survives at relatively high temperature. We
demonstrate that this tunneling interferometer can be described in terms of
ensemble of flux-tunable qubits giving equal contributions to conductance and
spin polarization. The number of active qubits participating in the charge and
spin transport is given by the ratio of the temperature and the level spacing.
The interferometer can effectively operate at high temperature and can be used
for quantum calculations. In particular, the ensemble of qubits can be
described by a single Hadamard operator. The obtained results open wide avenue
for applications in the area of quantum computing.
