Atomic quadrupole moment measurement using dynamic decoupling

We present a method that uses dynamic decoupling of a multilevel quantum probe to distinguish small frequency shifts that depend on mj2, where mj2 is the angular momentum of level |j| along the quantization axis, from large noisy shifts that are linear in mj, such as those due to magnetic field noise. Using this method we measured the electric-quadrupole moment of the 4D5/2 level in Sr+88 to be 2.973-0.033+0.026ea02. Our measurement improves the uncertainty of this value by an order of magnitude and thus helps mitigate an important systematic uncertainty in Sr+88 based optical atomic clocks and verifies complicated many-body quantum calculations.