  Accurate 3D imaging is essential for machines to map and interact with the
physical world. While numerous 3D imaging technologies exist, each addressing
niche applications with varying degrees of success, none have achieved the
breadth of applicability and impact that digital image sensors have achieved in
the 2D imaging world. A large-scale two-dimensional array of coherent detector
pixels operating as a light detection and ranging (LiDAR) system could serve as
a universal 3D imaging platform. Such a system would offer high depth accuracy
and immunity to interference from sunlight, as well as the ability to directly
measure the velocity of moving objects. However, due to difficulties in
providing electrical and photonic connections to every pixel, previous systems
have been restricted to fewer than 20 pixels. Here, we demonstrate the first
large-scale coherent detector array consisting of 512 ($32 \times 16$) pixels,
and its operation in a 3D imaging system. Leveraging recent advances in the
monolithic integration of photonic and electronic circuits, a dense array of
optical heterodyne detectors is combined with an integrated electronic readout
architecture, enabling straightforward scaling to arbitrarily large arrays.
Meanwhile, two-axis solid-state beam steering eliminates any tradeoff between
field of view and range. Operating at the quantum noise limit, our system
achieves an accuracy of $3.1~\mathrm{mm}$ at a distance of 75 metres using only
$4~\mathrm{mW}$ of light, an order of magnitude more accurate than existing
solid-state systems at such ranges. Future reductions of pixel size using
state-of-the-art components could yield resolutions in excess of 20 megapixels
for arrays the size of a consumer camera sensor. This result paves the way for
the development and proliferation of low cost, compact, and high performance 3D
imaging cameras.
