  Controlling the energy of unauthorized light signals in a quantum
cryptosystem is an essential criterion for implementation security. Here, we
propose a passive optical power limiter device based on thermo-optical
defocusing effects providing a reliable power limiting threshold which can be
readily adjusted to suit various quantum applications. In addition, the device
is robust against a wide variety of signal variations (e.g. wavelength, pulse
width), which is important for implementation security. Moreover, we
experimentally show that the proposed device does not compromise quantum
communication signals, in that it has only a very minimal impact (if not,
negligible impact) on the intensity, phase, or polarization degrees of freedom
of the photon, thus making it suitable for general communication purposes. To
show its practical utility for quantum cryptography, we demonstrate and discuss
three potential applications: (1) measurement-device-independent quantum key
distribution with enhanced security against a general class of Trojan-horse
attacks, (2) using the power limiter as a countermeasure against bright
illumination attacks, and (3) the application of power limiters to potentially
enhance the implementation security of plug-and-play quantum key distribution.
