For the particle to leave a portal travelling downwards, it needs to enter travelling to the left.
Click to show hint
The combination of the left-arrow and the portals will get you to the first receiver.
Click to show hint
There's probably a reason the room is so long...
Click to show hint
Sending the particle the length of the room will you give you time to do something.
Click to show hint
You'll need to use the left arrow twice.
Click to show hint
Move the left arrow down to point at the receiver once the particle starts travelling along the length of the room. The portals should be set up to send the particle downwards towards the left-arrow's new position.