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We present a JWST/MRS spectrum of the quasar J1120+0641 at z=7.0848, thefirst spectroscopic observation of a reionisation-era quasar in the rest-frameinfrared ($0.6<\lambda<3.4\mu$m). In the context of the mysterious fastassembly of the first supermassive black holes at z>7, our observations enablefor the first time the detection of hot torus dust, the H$\alpha$ emissionline, and the Paschen-series broad emission lines in a quasar at z>7. Hot torusdust is clearly detected as an upturn in the continuum emission at$\lambda_{\text{rest}}\simeq1.3\mu$m, leading to a black-body temperature of$T=1413.5^{+5.7}_{-7.4}$K. Compared to similarly-luminous quasars at 0<z<6, thehot dust in J1120+0641 is somewhat elevated in temperature (top 1%). Thetemperature is more typical among 6<z<6.5 quasars (top 25%), leading us topostulate a weak evolution in the hot dust temperature at z>6 ($2\sigma$significance). We measure the black hole mass of J1120+0641 based on theH$\alpha$ Balmer line, $M_{\text{BH}}=1.52\pm0.17\cdot 10^9 M_\odot$, which isin good agreement with the previous rest-UV MgII black hole mass measurement.The black hole mass based on the Paschen-series lines is also consistent,indicating no significant extinction in the rest-frame UV measurement. Thebroad H$\alpha$, Pa-$\alpha$ and Pa-$\beta$ emission lines are consistent withan origin in a common broad-line region (BLR) with densitylog$N_H/\text{cm}^{-3}\geq 12$, ionisation parameter $-7<$log$U<-4$, andextinction E(B-V)$\lesssim 0.1$mag. These BLR parameters are consistent withsimilarly-bright quasars at 0<z<4. Overall, we find that both J1120+0641's hotdust torus and hydrogen BLR properties show no significant peculiarity whencompared to luminous quasars down to z=0. The quasar accretion structures musthave therefore assembled very quickly, as they appear fully "mature" less than760 million years after the Big Bang.

First rest-frame infrared spectrum of a z>7 quasar: JWST/MRS observations of J1120+0641