  We introduce a sliding window decoder based on belief propagation (BP) with
guided decimation for the purposes of decoding quantum low-density parity-check
codes in the presence of circuit-level noise. Windowed decoding keeps the
decoding complexity reasonable when, as is typically the case, repeated rounds
of syndrome extraction are required to decode. Within each window, we employ
several rounds of BP with decimation of the variable node that we expect to be
the most likely to flip in each round, Furthermore, we employ ensemble decoding
to keep both decimation options (guesses) open in a small number of chosen
rounds. We term the resulting decoder BP with guided decimation guessing (GDG).
Applied to bivariate bicycle codes, GDG achieves a similar logical error rate
as BP with an additional OSD post-processing stage (BP+OSD) and
combination-sweep of order 10. For a window size of three syndrome cycles, a
multi-threaded CPU implementation of GDG achieves a worst-case decoding latency
of 3ms per window for the [[144,12,12]] code.
