(Disclaimer: I am not a physicist)

Probably they would just explain it with the normal Copenhagen interpretation?

https://en.wikipedia.org/wiki/Copenhagen_interpretation

In terms of quantum theory, the behaviour of a quantum computer is not fundamentally any different to the behaviour of any small scale particle or collection of particles.

Even buckyballs have been shown to self-interfere in a double-slit experiment, and that doesn't _prove_ multi-worlds over the Copenhagen interpretation.

https://web.archive.org/web/20120331115055/http://www.quantu...

There might be other philosophical reasons to believe in many-worlds, and I am partial to it myself, but experimental observation of this form would not to my understanding contribute to that argument.