Measurement of the Two-Loop Lamb Shift in Lithiumlike $\mathrmU^89+$

Using the SuperEBIT electron beam ion trap, we have measured the 2s1/2-2p1/2 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 3.6×10-4. 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+.