I have been messing around with auth0.com for a while on bubble.
Most recently with Sendpilot to allow us to attach multiple social media accounts with oauth, and also to do the infamous oauth1 Twitter posting.
One of the things that auth0 offers out of the box and free … is passwordless authentication via SMS or Email.
You set it up just like any API Connector, and configure the auth0 back end as you need.
Really very quick.
Here is a scrappy little demo using email.
Note, this uses the Bubble user process as standard, so no tokens or codes are shared or stored.