Low-noise environment for probing fundamental symmetries

TL;DR

This work presents a meticulously engineered low-noise environment for electron EDM measurements using a beam of ultracold YbF molecules. It combines ceramic, TiN-coated Al$_2$O$_3$ field plates inside a glass vacuum tube with a four-layer mu-metal shield and an array of atomic magnetometers to suppress magnetic Johnson noise and external magnetic fluctuations, aiming for EDM sensitivity near $d_e\sim10^{-31}~e\,\mathrm{cm}$. Analytical and numerical analyses quantify Johnson-noise contributions from conductors and shields, while detailed measurements validate shielding performance and magnetic-field stability, including $B_E$ correlations with the electric-field reversal. The results establish a practical, broadly applicable framework for high-precision symmetry tests in molecules, atoms, and neutrons, and point to concrete improvements to reach ultimate sensitivity limits.

Abstract

We present the design and characterization of a low-noise environment for measuring the electron's electric dipole moment (EDM) with a beam of molecules. To minimize magnetic Johnson noise from metals, the design features ceramic electric field plates housed in a glass vacuum chamber. To suppress external magnetic noise the apparatus is enclosed within a cylindrical four-layer mu-metal shield with a shielding factor exceeding $10^6$ in one radial direction and $10^5$ in the other. Finite element modelling shows that the difference between these shielding factors is due to imperfect joints between sections of mu-metal. Using atomic magnetometers to monitor the magnetic field inside the shield, we measure noise below 40 fT/$\sqrt{\rm Hz}$ at 1 Hz and above, rising to 500 fT/$\sqrt{\rm Hz}$ at 0.1 Hz. Analytical and numerical studies show that residual magnetic Johnson noise contributes approximately 13 fT/$\sqrt{\rm Hz}$. The background magnetic field averaged along the beamline is maintained below 3 pT, with typical gradients of a few nT/m. An electric field of 20 kV/cm is applied without discharges and with leakage currents below 1 nA. Each magnetometer measures the magnetic field correlated with the direction of the applied electric field with a precision of 0.11 fT in 104 hours of data. These results demonstrate that the apparatus is suitable for measuring the electron EDM with precision at the $10^{-31}$ e cm level. The design principles and characterization techniques presented here are broadly applicable to precision measurements probing fundamental symmetries in molecules, atoms, and neutrons.

Figures (15)

• Figure 1: Overview of the interaction region, showing the electric field plates, vacuum chambers, magnetometers and magnetic shields. Gravity is along $-x$. Molecules travel along $y$. Electric field is along $z$.
• Figure 2: Construction of the electric field plates showing how two modules are connected together and how the plates are supported at the two ends.
• Figure 3: Leakage currents measured at each plate (labelled W and E) as a function of the applied voltage. The measurement is made for two polarities, labelled 1 when W is positive and E is negative, and 2 for the opposite case. The currents flowing to W and E are shown in red and blue, using triangles for polarity 1, and circles for polarity 2.
• Figure 4: A scan of the work function of a test sample of TiN-coated alumina. This contour plot shows a mean work function of 4753m with an RMS variation across the sample of 54m.
• Figure 5: Design of the four-layer magnetic shield. A half section is shown.