The period from 206 B.C. to 220 A.D. is known as the Han Dynasty. Under the Han rulers, science and technology made remarkable strides; paper, the compass, and the seismograph were invented; and steel was manufactured. The empire expanded into southern China, northern Vietnam and parts of Korea, and forged trade routes through Central Asia to India and Persia. Confucianism was reinterpreted and adopted as the official state ideology, and a national university was established for the training of Confucian officials. The political unity of the Qin was preserved, but sanctioned by Confucianism so that Chinese of today still look back on this epoch with pride and call themselves "men of Han." Diversity developed within the culture: native and foreign, Confucian and Taoist, courtly and popular.