  Sensors are indispensable tools of modern life that are ubiquitously used in
diverse settings ranging from smartphones and autonomous vehicles to the
healthcare industry and space technology. By interfacing multiple sensors that
collectively interact with the signal to be measured, one can go beyond the
signal-to-noise ratios (SNR) than those attainable by the individual
constituting elements. Such distributed sensing techniques have also been
implemented in the quantum regime, where a linear increase in the SNR has been
achieved via using entangled states. Along similar lines, coupled non-
Hermitian systems have provided yet additional degrees of freedom to obtain
better sensors via higher-order exceptional points. Quite recently, a new class
of non-Hermitian systems, known as non-Hermitian topological sensors (NTOS) has
been theoretically proposed. Remarkably, the synergistic interplay between
non-Hermiticity and topology is expected to bestow such sensors with an
enhanced sensitivity that grows exponentially with the size of the sensor
network. Here, we experimentally demonstrate NTOS using a network of photonic
time-multiplexed resonators in the synthetic dimension represented by optical
pulses. By judiciously programming the delay lines in such a network, we
realize the archetypical Hatano-Nelson model for our non-Hermitian topological
sensing scheme. Our experimentally measured sensitivities for different lattice
sizes confirm the characteristic exponential enhancement of NTOS. We show that
this peculiar response arises due to the combined synergy between
non-Hermiticity and topology, something that is absent in Hermitian topological
lattices. Our demonstration of NTOS paves the way for realizing sensors with
unprecedented sensitivities.

Optics