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X-ray absorption spectroscopy (XAS) is a powerful and well establishedtechnique with sensitivity to elemental and chemical composition. Despite theseadvantages, its implementation has not kept pace with the development ofultrafast pulsed x-ray sources where XAS can capture femtosecond chemicalprocesses. X-ray Free Electron Lasers (XFELs) deliver femtosecond, narrowbandwidth ($\frac{\Delta E}{E} < 0.5\%$) pulses containing $\sim 10^{10}$photons. However, the energy contained in each pulse fluctuates thuscomplicating pulse by pulse efforts to quantify the number of photons.Improvements in counting the photons in each pulse have defined the state ofthe art for XAS sensitivity. Here we demonstrate a final step in theseimprovements through a balanced detection method that approaches the photoncounting shot noise limit. In doing so, we obtain high quality absorptionspectra from the insulator-metal transition in VO$_2$ and unlock a method toexplore dilute systems, subtle processes and nonlinear phenomena with ultrafastx-rays. The method is especially beneficial for x-ray light sources whereintegration and averaging are not viable options to improve sensitivity.

Balanced Detection in Femtosecond X-ray Absorption Spectroscopy to Reach the Ultimate Sensitivity Limit