  Room temperature operation is mandatory for any optoelectronics technology
which aims to provide low-cost compact systems for widespread applications. In
recent years, an important technological effort in this direction has been made
in bolometric detection for thermal imaging$^1$, which has delivered relatively
high sensitivity and video rate performance ($\sim$ 60 Hz). However, room
temperature operation is still beyond reach for semiconductor photodetectors in
the 8-12 $\mu$m wavelength band$^2$, and all developments for applications such
as imaging, environmental remote sensing and laser-based free-space
communication$^{3-5}$ have therefore had to be realised at low temperatures.
For these devices, high sensitivity and high speed have never been compatible
with high temperature operation$^{6, 7}$. Here, we show that a 9 $\mu$m quantum
well infrared photodetector$^8$, implemented in a metamaterial made of
subwavelength metallic resonators$^{9-12}$, has strongly enhanced performances
up to room temperature. This occurs because the photonic collection area is
increased with respect to the electrical area for each resonator, thus
significantly reducing the dark current of the device$^{13}$. Furthermore, we
show that our photonic architecture overcomes intrinsic limitations of the
material, such as the drop of the electronic drift velocity with
temperature$^{14, 15}$, which constrains conventional geometries at cryogenic
operation$^6$. Finally, the reduced physical area of the device and its
increased responsivity allows us, for the first time, to take advantage of the
intrinsic high frequency response of the quantum detector$^7$ at room
temperature. By beating two quantum cascade lasers$^{16}$ we have measured the
heterodyne signal at high frequencies up to 4 GHz.
