  We present a wide-field multi-pass implementation of label-free imaging flow
cytometry. Our technique is shown for high-speed flow imaging of ensembles of
human red blood cells with up to four passes, demonstrating x4 enhancement in
contrast and signal-to-noise. We show that our technique approaches close to
the quantum limit of measurement sensitivity, extending the range of optimal
imaging to samples in the weakly absorbing regime. This allows for near optimal
imaging sensitivity and throughput in a practical scenario of imaging a dynamic
sample under limited illumination intensity, surpassing the sensitivity
achieved with currently available quantum light sources.

Optics;Applied Physics;Quantum Physics