  High-temperature superconductivity confined to nanometer-size interfaces has
been a long standing goal because of potential applications^{1,2} and the
opportunity to study quantum phenomena in reduced dimensions^{3,4}. However,
this is a challenging target: in conventional metals the high electron density
restricts interface effects such as carrier depletion/accumulation to a region
much narrower than the coherence length, the scale necessary for
superconductivity to occur. In contrast, in copper oxides the carrier density
is low while the critical temperature (T_c) is high and the coherence length
very short; so, this provides a breakthrough opportunity but at a price: the
interface must be atomically perfect. Here we report on superconductivity in
bilayers consisting of an insulator (La_2CuO_4) and a metal
(La_{1.55}Sr_{0.45}CuO_{4}), neither of which is superconducting in isolation.
However, in bilayers T_c is either ~15 K or ~30 K, depending on the layering
sequence. This highly robust phenomenon is confined within 2-3 nm from the
interface. If such a bilayer is exposed to ozone, T_c exceeds 50 K and this
enhanced superconductivity is also shown to originate from the interface layer
about 1-2 unit cell thick. Enhancement of T_c in bilayer systems was observed
previously^5 but the essential role of the interface was not recognized at the
time. Our results demonstrate that engineering artificial heterostructures
provides a novel, unconventional way to fabricate stable, quasi two-dimensional
high T_c phases and to significantly enhance superconducting properties in
known or new superconductors.
