Hi all and Derek in particular,
I'm really disappointed that 20%C is the best we can expect out of laser pumped light sails (they stop being solar sails when you start pumping them with lasers).
Magsails are promising. See: https://en.wikipedia.org/wiki/Magnetic_sail
. Particle beams have a much higher energy density than lasers. I've read of theoretical top speeds of 50% C to 65% C. Of cause the faster you go the harder it is to slow down at the other end if you don't have a particle beam generator network at your destination. But going to Alpha Centauri doesn't really present that problems. The solar wind from the main star is intense, and you have three major gravity wells to play around with to do velocity reducing fly bys.
The question really is why would anyone want to visit AC. The solar system is hideously hostile, radiation wise. Its a big freaking star burning through its fuel real fast. Almost every other star in the <10 LY is friendly. Visiting AC is only worthwhile as a brief visit by a extremely rad-hardened probe. Us frail organics dare not take the trip.