© Gerald Gabrielse.
While the Heisenberg uncertainty principle predicts the scatter in the results for a single measurement, it does not limit the precision with which a physical property of a quantum mechanical object can be measured. One example is a dimensionless property of an electron called its g-factor. Quantum electrodynamics, the quantum mechanical theory of the electromagnetic field, predicts that this g-factor is almost, but not quite equal to 2. Gerald Gabrielse and his team at Harvard University used the apparatus shown here to confine an electron in a miniature cyclotron-like device. The actual device hangs suspended from a small refrigerator at the left of the apparatus. In operation, the refrigerator fits in a superconducting magnet in a low-temperature cryostat within the bulky housing. The result of the measurement was g/2 = 1.00115965218085, confirming the prediction of QED. The uncertainty in the measurement is 0.76 parts per trillion. (Unit: 5)