"The Hop", recorded by Radio Citizen, is a song featured in the Person of Interest episode “2πR”. It is played in the club where Carter goes to find a guy whose cheek swab she would later use to replace the one the FBI took from Reese.
"The Hop" is one of the few songs that is source music (sometimes called diegetic music). Unlike the episode's score, source music is heard by the characters in an episode and originates within the story itself.
Lyrics for the song can be found here.