How to unlock: You must complete the Fire Support quest, agree to help Danse and then enter the Cambridge Police Station (M5,1).
Following Paladin Danse
The quest will start automatically after you finish the Fire Support quest and agree to help Paladin Danse and the Brotherhood of Steel. In first order you must walk into the Cambridge Police Station. Before the conversation, walk through the whole room and collect all valuable items. Once you're ready, talk with Danse. Now you must follow the soldier and help him in combat. At the end of each phase of the journey you must talk with him again.
Finding a way to open the door
Once you reach the ArcJet Systems building, after some exploring you will find a room with closed door. In order to open it, walk to the terminal. You will be able to change the password. Once you do it, walk to the computer in front of the door that you want to open. Select the proper option and leave the system. The door will open. Watch out, immediately after that a battle with few Synths will start. Keep following Danse.
Restoring the power
After a while you will walk down to the underground chamber. In order to use the elevator, you must restore the power. Go to the engine room and hack into the computer (novice level). Once you restore the power, Danse will be attacked by a large group of Synths. Leave the engine room and press the button in the room with the glass. Remain in the room until the countdown ends. Once all enemies are destroyed, go check what happened to Danse.
Reclaiming the deep range transmitter
Use the elevator after the conversation. Don't worry if your companion won't enter it, after a while he will be automatically teleported to you. Go upstairs. Defeat another group of opponents and take the Deep Range transmitter from the body of one of them. Enter another elevator. Leave the building and talk to Danse. He will give you a good laser rifle. He will also ask you if you want to join the Brotherhood of Steel. If you agree, the Semper Invicta quest will start.
Quest rewards: experience, legendary rifle - Righteous Authority.