Measurement of the Magnetic Interaction between Two Electrons
Since an electron is a tiny magnet, every two electrons should exert
torques on one another - as magnets do.
This fundamental interaction, however, was not observed at the atomic
scale, since there it is overwhelmed by the much stronger Coloumb
exchange forces. At distances greater than the atomic scale, the
energy, scaling as 1/r^3, is typically smaller than the effect of
magnetic field noise fluctuations.
By co-trapping two Sr^+ ions, we were able to measure the magnetic
spin-spin interaction between their valence electrons. The ions were
separated by 2-3 microns, resulting in a ~ milihertz interaction. Each
experiment was ~15 seconds long, during which spin-spin entanglement
was measured. By varying the distance between the ions, we were able
to recover the 1/r^3 dependence of the interaction.
The experiment was made possible by a set of
capabilities/understandings we developed in the past 3 years :
1) Measurement of a signal within a Decoherence Free Subspace.
2) Quantum Lock-in of weak signals.
3) Improved detection for long (>10 sec) experiments.
4) Single ion preparation and manipulation.
I will try to give an overview of these efforts as well as discuss the
fundamental aspects of e-e measurement.[“Measurement of the magnetic interaction between two bound electrons of two separate ions”http://www.nature.com/nature/journal/v510/n7505/full/nature13403.html][“Constraints on exotic dipole-dipole couplings between electrons at the micrometer scale”http://arxiv.org/abs/1501.07891]