Parity violation in atomic ytterbium: Experimental sensitivity and systematics

We present a detailed description of the observation of parity violation in the ${}^1{S}_0$-${}^3{D}_1$ 408-nm forbidden transition of ytterbium, a brief report of which appeared earlier. Linearly polarized 408-nm light interacts with Yb atoms in crossed $E$ and $B$ fields. The probability of the 408-nm transition contains a parity-violating term, proportional to $(ℰ·\mathbf{B})[(\mathbf{E}\times ℰ)·\mathbf{B}]$, arising from interference between the parity-violating amplitude and the Stark amplitude due to the $E$ field ($ℰ$ is the electric field of the light). The transition probability is detected by measuring the population of the ${}^3{P}_0$ state, to which $65\%$ of the atoms excited to the ${}^3{D}_1$ state spontaneously decay. The population of the ${}^3{P}_0$ state is determined by resonantly exciting the atoms with 649-nm light to the $6s7s\hspace{0.5em}{}^3{S}_1$ state and collecting the fluorescence resulting from its decay. Systematic corrections due to $E$-field and $B$-field imperfections are determined in auxiliary experiments. The statistical uncertainty is dominated by parasitic frequency excursions of the 408-nm excitation light due to the imperfect stabilization of the optical reference with respect to the atomic resonance. The present uncertainties are $9\%$ statistical and $8\%$ systematic. Methods of improving the accuracy for future experiments are discussed.

Figure 1

Low-lying energy eigenstates of Yb and transitions relevant to the APV experiment.

Figure 2

Orientation of fields for PV-Stark interference experiment and schematic of the present APV apparatus. Not shown is the vacuum chamber containing all the depicted elements, except the photomultiplier (PMT) and the photodiode (PD). PBC is the power buildup cavity. Light is applied collinearly with x.

Figure 3

Discrimination of the PV effect by $E$-field modulation under static magnetic field. The Zeeman pattern is shown for the polarization angle $\theta =\pi /4$.

Figure 4

Schematic of the power buildup cavity.

Figure 5

Schematic of the optical setup. Light at 408 nm is produced by frequency doubling the output of a Ti:sapphire laser (Coherent $899$) using the WaveTrain cw ring-resonator doubler. The laser is locked to the PBC using the FM-sideband technique. The PBC is locked to a confocal Fabry-Pérot étalon. This scannable étalon provides the master frequency. The 649-nm excitation light is derived from a single-frequency diode laser (New Focus Vortex). The diode laser is locked to a frequency-stabilized He-Ne laser using another scanning Fabry-Pérot étalon.

Figure 6

Schematic of the $E$-field electrodes assembly, and a result of the $E$-field modeling showing an $X$-$Z$ slice of the amplitude of the $E$-field $z$ component $e_z$ in a midplane ($Y=0$) of the assembly normalized by the total $E$-field amplitude $E$. The voltage is applied to electrode 1, and electrode 2 and the correction electrodes are grounded.

Figure 7

Profile of the $B$-field-split 408-nm spectral line of ${}^{174}\mathrm{Yb}$ recorded at first and second harmonic of the modulation. Also shown is a simulated PV contribution in the first-harmonic signal. $\mathrm{Ẽ}=5$ kV$/$cm; dc offset = 40 $\mathrm{V}/$cm; $\theta =\pi /4$; the effective integration time is 200 ms per point.

Figure 8

PV interference parameter $\zeta /\beta$. Mean value: $39(4){}_{\mathrm{stat}.}(3){}_{\mathrm{syst}.}\hspace{0.5em}\text{mV}/\text{cm}$, $\left|\zeta \right|=8.7\pm 1.4\times {10}^{-10}e\hspace{0.5em}{a}_0$.

Figure 9

Impact of the frequency excursions of the Fabry-Pérot étalon on the noise level in the harmonics ratio. A change in the noise level when the optical system was tuned from the wing of the atomic resonance to its peak is shown. In the inserts above the excitation light spectral position is shown schematically with respect to the atomic resonance. Arrows denote the fluctuations.

Figure 10

Application of the cavity-ring-down method for the determination of the finesse of PBC.