The CEA has two types of company buses, which can be used by anyone working in the IPhT:
Information for both the types of buses is available at: http://www-saclay.cea.fr/Phocea/Page/index.php?id=502
To receive an SMS if buses are cancelled or running late, go to www.cea-infobus.fr and create a personal account.
The region Ile de France has a journey planner: https://www.vianavigo.com/
The RER is a regional train that runs from Paris to Massy-Palaiseau, Le Guichet, Orsay and Gif. People often come to the lab using the RER and take a bus either at Massy-Palaiseau or Le Guichet. Note that the RER is generally considered to be rather unreliable and has frequent perturbations.
It is also possible to come to the lab by foot directly from the RER station La Hacquinière. It takes around half and hour and involves walking in a pretty steep forest which is not lit up at night: http://www.lsce.ipsl.fr/Images/Page/1/Gif-Orme.jpg
Three bus lines arrive at Orme des Merisiers, which is the closest bus stop to the lab:
There are night buses from Paris to Gif, Orsay and other places: http://www.noctilien.fr/
There are recently built cycle lanes around the lab. However do note that the lab is on a plateau and therefore coming by bike might involve a steep climb.