Is the Emperor Quartet the German National Anthem? No

No. The Emperor Quartet (Kaiserquartett) is String Quartet No. 62 in C major, Op. 76 No. 3 by Joseph Haydn (1797). Its second movement uses the melody "Gott erhalte Franz den Kaiser" ("God Save Emperor Francis"), composed for Emperor Francis II of the Holy Roman Empire.

Germany's National Anthem is the "Deutschlandlied" ("Song of Germany"), also by Haydn (same melody, 1797), with lyrics by August Heinrich Hoffmann von Fallersleben (1841). Only the third stanza ("Einigkeit und Recht und Freiheit") is official since 1952.

The Emperor Quartet is a chamber music masterpiece; the Deutschlandlied is the national anthem. Same composer, same tune origin, different purpose.