Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Determining $G$ with Laser Spectroscopy to 38 ppb

Noah Bray-Ali

Abstract

A precision measurement is proposed to determine, in a couple hours of integration time, the axion Compton frequency using a modest power (3 mW) tunable external-cavity diode laser at 2458 nm as input to drive a free-space table-top Mach-Zehnder interferometer whose sensing arm passes the expanded beam-waist ($3~{\rm mm}$) light beam through a $1~{\rm T}$ strong, $40~{\rm cm}$ long dipole magnetic field created by a custom-built permanent-magnet assembly with a large but achievable ($6~{\rm mm}$) gap between poles. As the laser frequency is slowly modulated at 1 kHz through a 65 MHz wide window that is well within the 30 GHz fine-tuning range of the laser, a small but readily observable modulation appears in the dark-port optical power of the dark-fringe phase-locked interferometer due to photons converting into axions within the light beam as it passes through the magnetic field. Measuring the axion Compton frequency, $ν_A\approx{\rm 122~THz}$, where the dark-port power modulation peaks, to within the line-width of the laser, $Δν_A=1~{\rm MHz}$, then determines $G$ to 38 ppb, a roughly 600-fold improvement, through a relation between $ν_A$ and $G$, involving $h$, $c$, and nucleon masses.

Determining $G$ with Laser Spectroscopy to 38 ppb

Abstract

A precision measurement is proposed to determine, in a couple hours of integration time, the axion Compton frequency using a modest power (3 mW) tunable external-cavity diode laser at 2458 nm as input to drive a free-space table-top Mach-Zehnder interferometer whose sensing arm passes the expanded beam-waist () light beam through a strong, long dipole magnetic field created by a custom-built permanent-magnet assembly with a large but achievable () gap between poles. As the laser frequency is slowly modulated at 1 kHz through a 65 MHz wide window that is well within the 30 GHz fine-tuning range of the laser, a small but readily observable modulation appears in the dark-port optical power of the dark-fringe phase-locked interferometer due to photons converting into axions within the light beam as it passes through the magnetic field. Measuring the axion Compton frequency, , where the dark-port power modulation peaks, to within the line-width of the laser, , then determines to 38 ppb, a roughly 600-fold improvement, through a relation between and , involving , , and nucleon masses.

Paper Structure

This paper contains 9 equations, 1 figure, 1 table.

Figures (1)

  • Figure 1: Determing $G$ with Laser Spectroscopy to 38 ppb— Schematic of proposed Year One (FY 2027) precision measurement showing a frequency-modulated and phase-locked free-space Mach-Zehnder inteferometer for sensing modulation of the transmitted intensity of near-infrared light passing through a static magnetic field. The tunable 3 mW external cavity diode laser input at 2458 nm with 1 MHz line-width is frequency-modulated at $1~{\rm kHz}$ through a $\pm100~{\rm MHz}$ window around the predicted axion Compton frequency, $\nu_A=121~944~463~(65)~{\rm MHz}$laser. The laser beam-waist is broadened to $w=3~{\rm mm}$ as it passes through the dipole magnetic field with strength, $B_{\rm DC}=1~{\rm T}$, and length, $\ell=40~{\rm cm}$, within a custom-built permanent magnet assembly in the "sensing arm" of the interferometer. A resonant electro-optical modulator (EOM 1) at 4 MHz provides the phase-lock reference and a broadband electro-optical modulator (EOM 2) in the "control arm" compensates phase drifts with a PID controller. Other components include mirrors (M), beam splitters (BS), a half-wave plate (HWP), photodiodes (PD), and a low-pass filter (LPF).