C. State-of-the-art of LIGO

Why twin detectors

  The LIGO “observatory” is made up of two identical and widely separated interferometers located in two uninhabited places: LIGO Hanford in shrub-steppe region of southeastern Washington State, and LIGO Livingston, 3002 kilometers away in a forest east of Baton Rouge,Louisiana.
"LIGO was designed with two detectors so far apart for good reason. LIGO’s detectors are so sensitive that they can 'feel' the tiniest vibrations on the Earth from sources very nearby to sources hundreds or thousands of miles away. Things like earthquakes, acoustic noise (e.g., trucks driving on nearby roads, farmers plowing fields, things that people can hear and feel), and even internal laser fluctuations can cause disturbances that can mask or mimic a gravitational wave signal in each interferometer. If the instruments were located close together, they would detect the same vibrations at the same times--both from Earth-sources and from gravitaitonal waves and it would be nearly impossible to distinguish a vibration from a gravitational wave from the local noise."\cite{7wxnuuc}

Operation 

  When a gravitational wave passes through the interferometer, the spacetime in the local area is altered. Depending on the source of the wave and its polarization, this results in an effective change in length of one or both of the cavities. The effective length change between the beams will cause the light currently in the cavity to become very slightly out of phase (antiphase) with the incoming light. The cavity will therefore periodically get very slightly out of coherence and the beams, which are tuned to destructively interfere at the detector, will have a very slight periodically varying detuning. This results in a measurable signal.\cite{thorne2016} 

The enhanced and advanced LIGO project

  The primary LIGO started to detect gravatational waves in 2002, while it hadn't detected any gravitational waves after the fifth run of the detector ended in 2007. After 2007, there comes the first upgrade for  LIGO,  called the enhanced LIGO.The improvements include:
  LIGO went offline in 2010 and started its second upgrade, called the advanced LIGO. After installing the new detectors, its sensitivity increased 3 times comparing to the initial LIGO. \cite{kimberly2015}This upgrade is a huge success, as it brought the first detection of gravatational waves which won the Nobel Prize for Kip Thorne and Rainer Weiss.