Summerfield has made four appearances in all competitions for the Pilgrims so far this season, but moves to the London club to gain more regular first-team experience.
The 21-year-old joins in time to go straight into Williams' squad to face Millwall this weekend.
Williams told Orient's official website: "Luke is a very good footballer. He's got experience in the Championship and played 20-odd games for them at that level last year.
"I went down to watch him play earlier in the week and he's hungry - he wants to get out there and play.
"We're delighted that he couldn't wait to come down and join us and we're delighted to have him."