If you vary the inlet and outlet geometry of a ducted fan you can have your fan running at subsonic, while the plane moves supersonic.

You could also heat the airstream after the compressor.

> electric motors which will certainly always be sub-sonic

Why is that?

Electric motors have a top speed which is governed by the forces generated by induction. At higher speeds they spend more and more torque countering those forces. Contrast this with turbines, where a higher inlet speed translates into a higher output speed - the amount of energy a turbine uses to keep itself spinning is constant rather than proportional to airspeed. This is why electric motors would have issues flying at high speeds.

Can you not simply use overdrive gearing to keep motor speed down?

... or spin the motor with another motor. It could be motors all the way down . . .

What may still be simpler than the fusion reactor some posts up.

For a plane, make that a neutron-less kind of fusion.

After fusion becomes commercially viable, and the entire world concerts, we'll still be about as far as neutron-less fusion as we are from fusion right now.