  Programmable photonic circuits performing universal linear-optical
transformations underpin vital functions in photonic quantum information
processing, quantum-enhanced sensor networks, machine learning and many other
intriguing applications. Recent advances in photonic integrated circuits
facilitate monolithic integration of externally controlled Mach-Zehnder
interferometers which can implement arbitrary unitary transformation on a large
number of input/output modes. In this work, we demonstrate a 4x4 programmable
linear photonic circuit on lithium niobate on insulator platform employing
fast, power-efficient and low-loss electro-optical phase shifters, showing
enormous advantages in terms of configuration rate and power consumption. Our
device is capable of fast switching with 500 ps rise time and 1.7 ns fall time,
and possesses a total on-chip power dissipation of only 0.015 mW when operated
at 1 MHz modulation, and an insertion loss of 0.15 dB for each modulator and an
on-chip extinction ratio of -34 dB for both cross and bar routes.
