Flux qubit based detector of microwave photons

A theory of detection of microwave photons with a flux qubit based detector is presented. We consider a semiclassical approximation with the electromagnetic field being in a coherent state. The flux qubit is considered as a multilevel quantum system (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>qu</a:mi><a:mi>d</a:mi><a:mi>it</a:mi></a:math>). By solving the Lindblad equation, we describe the time evolution of occupations of the qudit's levels for readout and reset stages of detection. When considering the reset stage, the time evolution is described by multiple avoided-level crossings, thus providing a multilevel Landau-Zener-Stückelberg-Majorana (LZSM) problem. In addition to numerical calculations, we present an approximate solution for the description of the reset stage dynamics based on the adiabatic-impulse approximation and rate equation approach. Our theory may be useful for the theoretical description of driven-dissipative dynamics of <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mi>qu</b:mi><b:mi>d</b:mi><b:mi>its</b:mi></b:math>, including applications such as single-photon detection. Published by the American Physical Society 2024