Measurement of the two-loop lamb shift in lithiumlike U89+.

Using the SuperEBIT electron beam ion trap, we have measured the 2s(1/2)-2p(1/2) transitions in U88+ and U89+. The measured value of 280.645 +/- 0.015 eV for Li-like U89+ improves the available precision by nearly an order of magnitude and establishes a new benchmark for testing the total QED contribution to the transition energy within a fractional accuracy of 3.6 x 10(-4). We infer a value for the 2s two-loop Lamb shift in U89+ of -0.23 eV, from which we estimate a value of -1.27 eV for the 1s two-loop Lamb shift in U91+.