  Modern technology unintentionally provides resources that enable the trust of
everyday interactions to be undermined. Some authentication schemes address
this issue using devices that give unique outputs in response to a challenge.
These signatures are generated by hard-to-predict physical responses derived
from structural characteristics, which lend themselves to two different
architectures, known as unique objects (UNOs) and physically unclonable
functions (PUFs). The classical design of UNOs and PUFs limits their size and,
in some cases, their security. Here we show that quantum confinement lends
itself to the provision of unique identities at the nanoscale, by using
fluctuations in tunnelling measurements through quantum wells in resonant
tunnelling diodes (RTDs). This provides an uncomplicated measurement of
identity without conventional resource limitations whilst providing robust
security. The confined energy levels are highly sensitive to the specific
nanostructure within each RTD, resulting in a distinct tunnelling spectrum for
every device, as they contain a unique and unpredictable structure that is
presently impossible to clone. This new class of authentication device operates
with few resources in simple electronic structures above room temperature.
