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The precision measurement outlined in Section 2.1 takes place in the finely controlled envi- ronment of the interaction region. Figures 2.1.1 and 3.0.1 show schematics of the relevant apparatus, and full descriptions and diagrams can be found in references [11, 144,181,189]. We will briefly summarize the most critical components.

Vacuum system: The interaction region is an aluminum, o-ring-sealed vacuum chamber slightly over half a meter in length. It was welded by Atlas, Inc. [189]. Everything in and on it is made of non-magnetic materials in order to avoid creating stray fields that might interact with the molecules and cause spurious precession phases.

Magnetic field coils: Surrounding the vacuum chamber is a set of coils wound on a cylinder in an approximate cosine-θconfiguration. These coils apply a uniformB-field alongzˆ. There are also several sets of shim coils for smoothing out the field and intentionally applying magnetic gradients. The experiment is typically run with applied magnetic fields of 0– 40mG.

Magnetic shields: The vacuum chamber and field coils are surrounded by five nested cylin- ders of1/16” thick mu-metal alloy, a high-magnetic-permeability material that magnetically shields the volume it encloses. Our set of shields provides a factor of ∼ 105 _{suppression of}

Earth’s (∼500 mG) magnetic field and stray DC or slowly varying fields in the lab environ- ment. The residual field on the beamline in the interaction region is ∼20µG. Our shields were fabricated by Amuneal Manufacturing Corp [189].

Electric field plates: A uniform electric field along zˆis provided by a pair of 43 cm long, 23cm tall, parallel Borofloat glass plates from Custom Scientific coated on their inner surfaces with a 200 nm thick layer of the transparent conductor indium tin oxide (ITO) [189]. The

inner surfaces of the plates are separated by 25 mm and are aligned with respect to each other using an interferometer built by Ivan Kozyryev and a kinematic mounting structure developed by Amar Vutha and Emil Kirilov [11, 181]. They are secured to the mounting structure by gold-coated copper guard rings that run all the way around the edges of the plates. The electrical contacts to the voltage supply leads are made underneath the guard rings. The plates are positioned in the chamber with their faces perpendicular tozˆ; one plate is to the west of the molecule beam, and the other is to the east. The final molecule beam collimator is secured to the upstream end of the field plate mounting structure. Typical applied fields are between about 40and 140 V/cm.

Laser systems: The 943 nm optical pumping laser and the 1090 nm state preparation and readout lasers discussed in Section 2.1 are critical to the measurement scheme and must be carefully shaped and stabilized before being sent into the interaction region. All three lasers are vertically stretched to be taller than the≈1cm molecule beam on they-axis, while along the beamline (x) axis, they are only ≈1 mm or so wide. The few hundred mW of 943 nm light required to saturate the optical pumping transition is produced by a diode-laser-seeded Toptica tapered amplifier. The few W of 1090 nm light required to saturate the H → C

transition is provided by a pair of diode-laser-seeded fiber amplifiers from Nufern. The seed lasers are locked to a transfer Fabry-Perot cavity which is in turn referenced to an Nd:YAG laser locked to a molecular iodine clock transition via a vapor cell. The locking system was developed by Yulia Gurevich [91], and the iodine reference was built by Dan Farkas [74]. In order to measure the asymmetry (see Eq. (2.11)) with shot-noise-limited statistics [118], the readout laser polarization is rapidly switched between xˆand yˆwith a frequency of 100kHz using AOMs [11]. Additional subtleties of the laser system are covered in references [100, 144, 181].

Light collection and detection optics: The apparatus for collecting the laser-induced fluo- rescence that constitutes our experimental signal consists of an array of eight lens doublets, four on each side of the field plates, that are aimed at the detection region. Each of the lenses

focuses fluorescence onto the end of a fiber bundle, and the four fiber bundles on either side of the field plates are married together into a single bundle, which is coupled with optical gel into a light pipe. The two light pipes pass out of the vacuum chamber via an o-ring feedthrough. A Hamamatsu R8900U-20 PMT, fitted with appropriate bandpass filters to transmit 690 nm light while rejecting the other wavelengths used in the experiment and the background light from computer monitors, etc. in the room, is placed at the end of each of the two light pipes. Details on the design and characterization of the light collection system are found in the theses of Nick Huzler [105] and Ben Spaun [181], and details on the PMT setups are found in the thesis of Paul Hess [100].

Experiment control, data acquisition, and logging: Needless to say, much of the hardware described here must ultimately be controlled, have its status monitored, and have its outputs read and logged by computers. While this is not, strictly speaking, part of the interaction region, it certainly belongs in any discussion of the core workings of the ACME experiment. We used a LabView 2009 based software system with NI DAQs to control the ACME ex- periment, acquire data, and log various control states and auxiliary parameters. Much of the code was written by Paul Hess [100], and various important aspects of the software and instrumentation are described in [91, 144, 181].