** Shares of quantum computing firms tumble in extended trade after Nvidia CEO Jensen Huang said computers based on the emerging technology are as much as 30 years away

** "So if you kind of said 15 years for very useful quantum computers, that would probably be on the early side. If you said, you know, 30 is probably on the late side, but if you picked 20, I think a whole bunch of us would believe it" — Huang

** Rigetti Computing falls 10.3% while IonQ Inc

is down 8.5%, Quantum Computing Inc down 6.8% and D-Wave Quantum down 7.4%

** Quantum-computing ETF Defiance Quantum ETF also falls 2.4%

(Reporting by Juby Babu in Mexico City)

((Juby.Babu@thomsonreuters.com))