Hello !
I've got a little question. Is it possible to communicate 2 arduinos with only one wire between them. I try to explain :
I want that an arduino send a "message" (impulsion ? Signal ?) in just one wire on a pin
So the second arduino can catch the message and transform it in a beep or a brighting LED for example.
I want to know if it's possible. If yes, which technology/code/function have I to use ?
No wireless solution please, just one wire.
Have a nice day ! 
Can't do it with one wire. Need signal and ground wires. That is 2 wires.
Ok, I'll byte...,..
Yes, it CAN be done with one cable.
Fiber Optic cable.
Electrically, no. There is no reference for the signal line. The receiving uno wouldn't know a high without knowing a reference point of the high (gnd of the 1st uno)
The OneWire library?
Just use one wire with 2 conductors 
aarg:
The OneWire library?
Has one power/data wire AND a ground wire.
It actually is possible but the effort required is massive for the desired outcome.
I made one of these once just to see if it worked, and it does.
Pretty well actually, but for practical purposes is next to useless, and I suspect not what the OP wants.
Fiber optic is the only option. Think of it as a really stretched out optocoupler. Would work just fine if your two Arduinos don't have line of sight.
Even though the obvious and simple IR LED is probably within your grasp, that'd be wireless which you nixed.
Thank you for your answers !
I tought it wasn't possible without a ground wire... Unfortunately... But I prefer ask before giving up. 
But I've found this : Turn your Arduino into an AM radio transmitter! - Exhibition / Gallery - Arduino Forum
So it's not possible to catch the AM signal with another arduino by wire ?
Wireless works because current isn't being sucked out of the system. A pulsating antenna doesn't shrug off electrons. The moment you hardwire it, the current needs a place to go via common ground.
It's really clear for me now. Thank you !
So it's not possible to catch the AM signal with another arduino by wire ?
But you said:-
tryptophane:
No wireless solution please, just one wire.
And that Turn your Arduino into an AM radio transmitter! - Exhibition / Gallery - Arduino Forum is a wireless solution is it not?
groundFungus:
Can't do it with one wire. Need signal and ground wires. That is 2 wires.
ACK.
One wire (GND)is connecting GND to GND of both Arduinos
The other wire (signal) is conncting an OUTPUT of one Arduino with an INPUT of another one.
Transmission can be done in pulse-pause-modulation transferring bit after bit after bit.