Determine the best route via your preferred app. Our Institute is referenced as IPhT Saclay on google maps.
You can reach IPhT directly by walking from the train stations of La Hacquinière or Gif-sur-Yvette on RER B. This requires both a valid access badge to Saclay and a good knowledge of the neighborhood. But the gate beside the swivel is generally open during the business hours.
It is not a valid option on your first visit. The walk includes a steep forest path uphill, which can be very muddy, thus take good shoes. It takes about 30 minutes for an experienced hiker to reach IPhT from La Hacquinière (see map).