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 m(j)(2), where m(j)(2) is the angular momentum of level vertical bar j > along the quantization axis, from large noisy shifts that are linear in m(j), such as those due to magnetic field noise. Using this method we measured the electric-quadrupole moment of the 4D(5/2) level in Sr-88(+) to be 2.973(-0.033)(+0.026)ea(0)(2). 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.