A method to align a Sagnac loop for an entangled photon source comprising the steps of providing: I) first arrangement of optical components in a Sagnac loop configuration, wherein the optical components comprise of - a first light source emitting a polarized photon beam , and - the Sagnac loop , formed by a first polarizing beam splitter , and two mirrors arranged in a first and second output path of the first polarizing beam splitter , forming a clockwise and counter-clockwise path for the photon beam behind the first polarizing beam splitter , wherein the polarized photon beam of the first light source enters the first polarizing beam splitter on a third or fourth output path, II) one or more optimization steps performed by a tip/tilt movement of the two mirrors and an observation of the clockwise and counter-clockwise beams in an output path of a second and/or third polarizing beam splitter , in order to optimize the spatial overlap of the clockwise and counter-clockwise photon beams in the Sagnac loop , characterized in that the second and/or third polarizing beam splitter is arranged in the third and/or fourth output path of the first polarizing beam splitter , and the observation in the output path of the second and/or third polarizing beam splitter is performed by - a measurement of the optical power of the clockwise and counter-clockwise photon beams with a power-meter , and/or - a detection of the interference pattern of the clockwise and counter-clockwise photon beams with a camera or an optical component with a surface to monitor the interference. I) first arrangement of optical components in a Sagnac loop configuration, wherein the optical components comprise of - a first light source emitting a polarized photon beam , and - the Sagnac loop , formed by a first polarizing beam splitter , and two mirrors arranged in a first and second output path of the first polarizing beam splitter , forming a clockwise and counter-clockwise path for the photon beam behind the first polarizing beam splitter , wherein the polarized photon beam of the first light source enters the first polarizing beam splitter on a third or fourth output path, - a first light source emitting a polarized photon beam , and - the Sagnac loop , formed by a first polarizing beam splitter , and two mirrors arranged in a first and second output path of the first polarizing beam splitter , forming a clockwise and counter-clockwise path for the photon beam behind the first polarizing beam splitter , wherein the polarized photon beam of the first light source enters the first polarizing beam splitter on a third or fourth output path, II) one or more optimization steps performed by a tip/tilt movement of the two mirrors and an observation of the clockwise and counter-clockwise beams in an output path of a second and/or third polarizing beam splitter , in order to optimize the spatial overlap of the clockwise and counter-clockwise photon beams in the Sagnac loop , the second and/or third polarizing beam splitter is arranged in the third and/or fourth output path of the first polarizing beam splitter , and the observation in the output path of the second and/or third polarizing beam splitter is performed by - a measurement of the optical power of the clockwise and counter-clockwise photon beams with a power-meter , and/or - a detection of the interference pattern of the clockwise and counter-clockwise photon beams with a camera or an optical component with a surface to monitor the interference. - a measurement of the optical power of the clockwise and counter-clockwise photon beams with a power-meter , and/or - a detection of the interference pattern of the clockwise and counter-clockwise photon beams with a camera or an optical component with a surface to monitor the interference. <EOS>