  A quantum emitter interacting with photons in a single optical mode
constitutes a one-dimensional atom. A coherent and efficiently coupled
one-dimensional atom provides a large nonlinearity, enabling photonic quantum
gates. Achieving a high coupling efficiency ($\beta$-factor) and low dephasing
is challenging. Here, we use a semiconductor quantum dot in an open microcavity
as an implementation of a one-dimensional atom. With a weak laser input, we
achieve an extinction of $99.2\%$ in transmission and a concomitant bunching in
the photon statistics of $g^{(2)}(0) = 587$, showcasing the reflection of the
single-photon component and the transmission of the multi-photon components of
the coherent input. The tunable nature of the microcavity allows $\beta$ to be
adjusted and gives control over the photon statistics -- from strong bunching
to anti-bunching -- as well as over the phase of the transmitted photons. We
obtain excellent agreement between experiment and theory by going beyond the
single-mode Jaynes-Cummings model. Our results pave the way towards the
creation of exotic photonic states and two-photon phase gates.
