When was Tantric Buddhism systematically introduced into China? And by whom? And what class of tantra? Which lineage(s)?
Do I really need to be your encyclopedia, or are you testing me?
At the earliest it was during the reign period of Kaiyuan 開元 (713-741) during the Tang Dynasty (618-907) emperor Xuanzong 玄宗(712-756). The three eminent masters Śubhakarasiṃha 善無畏 (637-735), Vajrabodhi (671-741) 金剛智 and Amoghavajra (705-774) 不空 arrived in China. Prior to a great amount of esoteric texts being translated in this period, there were already many scattered esoteric sūtras and dhāraṇīs in Chinese. What became Shingon in Japan includes everything up to yoga-tantra.
Further, during the Song Dynasty (960-1279) monks were dispatched to India to collect sūtras. Dānapāla 施護 (d.1017), Dharmarakṣa 法護 (?-1058), Dharmadeva 法天 (d. 1001) and Tian Xi Zai 天息災 (d. 1000) translated a great amount of Vajrayāna texts, though their quality were sometimes poor and moreover not widely read. The Hevajra Tantra
is an example of annutara-yoga-tantra being translated into Chinese.
Who are the "some Japanese scholars", and according to them, when is the "middle period", and when is the "later period"? Also, which earlier developments were preserved?
This is just the general convention. If you read Japanese and the relevant academic literature, you would understand that this is a general convention. Early period esoteric Buddhism is understood as assorted dhāraṇī and sorcery practices before the need for abhiṣeka. Middle period would include all practices requiring abhiṣeka plus contemporary texts and traditions up to annutara-yoga-tantra. The later period is generally understood as annutara-yoga-tantra and all subsequent developments. Again, this is just a general outline, and one that I've encountered in my readings of Japanese academic literature.
In East Asia, specifically Japan, the early and middle periods are in particular preserved fairly well, at least in Chinese translation.
It is further unclear how extensive Buddhism actually was within the Yarlung state considering the history.
History of Tibet
states the following in the introduction:
The History of Tibet Volume I The Early Period: to c. AD 850 The Yarlung Dynasty, edited by Alex McKay (London, UK: RoutledgeCurzon, 2003), 29.
- Thus, while we may acknowledge genuine piety among the community of believers and the attraction of the coherent world view offered by the Buddhist message, we must conclude that Buddhism provided a unifying socio-cultural model that was promoted by Yarlung dynasty kings for political purposes.
Now, granted, the Tang state was likewise interested in the political utility of Buddhism, though Buddhism was already quite extensive and well-developed in China already as compared to Tibet at the time, meaning many dedicated adherents and clergy were already trained and well-learned in China. Moreover, the Chinese economy could and probably did support a larger more systematic introduction of tantric Buddhism when compared to the Yarlung state.
This is why I believe the earlier development of tantra was better preserved in the Sinosphere. Annutara-yoga-tantra of course was best preserved in Tibet, but by that time the Chinese had lost interest in the subject and the state translation bureau was performing rather poorly.