  The quantum spin Hall (QSH) state is a genuinely new state of matter
characterized by a non-trivial topology of its band structure. Its key feature
is conducting edge channels whose spin polarization has potential for
spintronic and quantum information applications. The QSH state was predicted
and experimentally demonstrated to exist in HgTe quantum wells. The existence
of the edge channels has been inferred from the fact that local and non-local
conductance values in sufficiently small devices are close to the quantized
values expected for ideal edge channels and from signatures of the spin
polarization. The robustness of the edge channels in larger devices and the
interplay between the edge channels and a conducting bulk are relatively
unexplored experimentally, and are difficult to assess via transport
measurements. Here we image the current in large Hallbars made from HgTe
quantum wells by probing the magnetic field generated by the current using a
scanning superconducting quantum interference device (SQUID). We observe that
the current flows along the edge of the device in the QSH regime, and
furthermore that an identifiable edge channel exists even in the presence of
disorder and considerable bulk conduction as the device is gated or its
temperature is raised. Our results represent a versatile method for the
characterization of new quantum spin Hall materials systems, and confirm both
the existence and the robustness of the predicted edge channels.
