At redshifts 2 to 5, the thermal state of the intergalactic medium (IGM) is a relic of the epoch of hydrogen reionization and reflects the impact of heating from Helium reionization. Gas in the IGM traces dark matter fluctuations on Mpc scales, but on smaller scales of hundreds of kpc, fluctuations are suppressed as the reheated gas is pressure supported against gravity, according to the classical Jeans argument. This Jeans filtering scale thus quantifies the small-scale structure of the IGM and provides a record of cosmic reionization and thermal evolution. Although the IGM temperature loses memory of the epoch of hydrogen reionization by the observed redshifts, the filtering scale can retain it, thus probing high-z reionization physics. Recently we have shown that it is possible to directly measure the Jeans filtering scale by characterizing the coherence of correlated Ly-alpha forest absorption in close quasar pairs. I will describe a statistical method based on quasar pair spectra which is highly sensitive to the filtering scale, and present the first measurement of the filtering scale from real quasar pair data based on this method. Our preliminary results suggest that the filtering scale is smaller than expected from the standard model of the IGM and current measurements of IGM temperatures.