  In nanostructures, the light-matter interaction can be engineered to be
chiral. In the fully quantum regime, a chiral one-dimensional atom, a photon
propagating in one direction interacts with the atom; a photon propagating in
the other direction does not. Chiral quantum optics has applications in
creating nanoscopic single-photon routers, circulators, phase-shifters and
two-photon gates. Furthermore, the directional photon-exchange between many
emitters in a chiral system may enable the creation of highly exotic quantum
states. Here, we present a new way of implementing chiral quantum optics $-$ we
use a low-noise quantum dot in an open microcavity. Specifically, we
demonstrate the non-reciprocal absorption of single photons, a single-photon
diode. The non-reciprocity, the ratio of the transmission in the
forward-direction to the transmission in the reverse direction, is as high as
10.7 dB, and is optimised $\textit{in situ}$ by tuning the photon-emitter
coupling to the optimal operating condition ($\beta = 0.5$). Proof that the
non-reciprocity arises from a single quantum emitter lies in the nonlinearity
with increasing input laser power, and in the photon statistics $-$
ultralow-power laser light propagating in the diode's reverse direction results
in a highly bunched output ($g^{(2)}(0) = 101$), showing that the single-photon
component is largely removed. The results pave the way to a single-photon phase
shifter, and, by exploiting a quantum dot spin, to two-photon gates and quantum
non-demolition single-photon detectors.
