An experiment will be pursued to search for the electric dipole moment(EDM) of the electron using cold Cs and Rb atoms. The atom processing and apparatus construction part of experiment will be completed and data collection will commence. Specifically, atoms will be loaded into a pair of parallel 1D far-off-resonant optical lattice traps in a magnetically shielded region of space, laser-cooled and optically pumped. Their EDMs will be measured by observing their coherent evolution in electric fields that are directed oppositely in the two traps. It is projected that the experiment will be sensitive to an EDM as small as 3x10^-30 e-cm, which is a 500-fold improvement over the current limit. This project is potentially transformative, because if an electric dipole moment were discovered this would have a profound affect on our understanding of the laws of physics at the most fundamental level. The work will also push the limits of sensitivity of measurements of the electron EDM using a novel new technique based on trapping atoms in a optical lattice.
A particle with a permanent EDM implies that both time-reversal invariance and parity invariance are violated. Both of these symmetries are in fact violated in the Standard Model of particle physics, which predicts very small, but non-zero EDMs for fundamental particles. Proposed extensions to the Standard Model tend to predict much larger EDMs, close to the current experimental upper limit. Continued non-observation of EDMs would rule out many posssible extensions to the Standard Model. Conversely, should an EDM be observed in the next several years, it would be the first experimental result of any kind that cannot be incorporated into the Standard Model. This experiment addresses a question of fundamental importance to elementary particle physics. Such questions are normally addressed with high energy experiments, but in this case, the precision tools of atomic physics can be applied to a much lower energy system, with a concomitant lower cost. In addition to these scientific broader impacts, this work has an important educational component as undergraduates and graduate students will be trained in the use of forefront research techniques.
This project has been to build an apparatus to measure the electron's electric dipole moment (eEDM). This physical constant is predicted to be perhaps unmeasurably small in the Standard Model (SM) of Physics (10^-38 e- cm), but extensions to the SM generically predict much larger values, larger than our target sensitivity of 3x10^-30 e-cm. Incredibly, the SM is consistent with every confirmed experimental observation, so If a larger eEDM were to be measured, it would be the first experimental observation of any type that is inconsistent with the SM. Still, many physicists believe that there must be additional physics that explains some seemingly coincidental very small dimensionless numbers in the SM, and that can perhaps play a role in incorporating gravity into the SM. A larger eEDM is considered to be a good candidate harbinger of this new physics. Our approach is to trap laser-cooled atoms in 1D optical lattices that thread three glass electric field plates. The measurement is quite sensitive to the eEDM, but it also has an unwanted sensitivity to magnetic fields, and most of the central design choices for the apparatus are made to avoid the effect of magnetic fields, while of course retaining eEDM sensitivity. With this in mind, we launch cold atoms 1 m above a magnetic trap into a region with four layers of mu-metal magnetic shields. The electric field plates are made of dielectric and transparent conducitng coated glass, which are suspended in a glass vacuum chamber. That is necessary because we use the atoms themselves to map out the magnetic field in space, so that they can be cancelled out. Everything within the shileds must be non-magnetic, and very little metal can be used, since thermal currents (Johnson noise) would otherwise come to dominate the magnetic field background. Also, the polarization of our optical trapping light must be extremely linear, or else the light creates a problematic fictitious magnetic field. Over the course of this grant we have devised a way to mount the electric field plates in vacuum and developed a technique to precisely measure their separation (to <10^-4 precision). We constructed and mounted the magnetic shields, and designed and built low noise current controllers to cancel residual fields. We designed and built a completely non-ferromagnetic photodiode array, and developed a low-noise pre-amplifier that could be located outside the shields with relatively little added noise (published in 2012). We built a system for stabilizing the high voltage to the electric field plates. With an eye toward making the trapping light as linearly polarized as possible, we developed a very low stress way to mount the vacuum windows so that they are minimially birefringent (published in 2011). We then worked on our atom processing and measurement, optimizing cooling in the lattices, optical pumping, and adiabatic rapid passage state transfer. We also designed a compact, very large field of view imaging system for use with our linear arrays of atoms. We implemented our measurement scheme for separately detecting the population in all seven of the Cs ground state's magnetic sublevels. This will be essential for the eEDM measurement. In this grant period we used that method to demonstrate the most perfectly linearly polarized light ever (10^-8) (published in 2013). All this apparatus buidling and experimental setup did not come off without a hitch, since in the middle of the grant period our electric field plates experienced electrical breakdown. This required a small (but obviously critically important) redesign of the coating. Although we contniued to work with the apparatus without an electric field for some time after this happened, new plates had to be manufactured and coated. In the last year we disassembled the whole ship in a bottle-like apparatus, replaced the field plates, and began the arduous process of reassembly, which is still in process.
