Precision Measurement of the Hydrogen $1S\mathrm{\text{−}}2S$ Frequency via a 920-km Fiber Link

We have measured the frequency of the extremely narrow $1S\mathrm{\text{−}}2S$ two-photon transition in atomic hydrogen using a remote cesium fountain clock with the help of a 920 km stabilized optical fiber. With an improved detection method we obtain $f_{1S\mathrm{\text{−}}2S}=2466061413187018(11)\mathrm{Hz}$ with a relative uncertainty of $4.5\times {10}^{-15}$, confirming our previous measurement obtained with a local cesium clock [C. G. Parthey et al., Phys. Rev. Lett. 107, 203001 (2011)]. Combining these results with older measurements, we constrain the linear combinations of Lorentz boost symmetry violation parameters $c_{(TX)}=(3.1\pm 1.9)\times {10}^{-11}$ and $0.92c_{(TY)}+0.40c_{(TZ)}=(2.6\pm 5.3)\times {10}^{-11}$ in the standard model extension framework [D. Colladay, V. A. Kostelecký, Phys. Rev. D. 58, 116002 (1998)].

Figure 1

The experimental setup. The frequency of the external cavity diode laser (ECDL) for hydrogen spectroscopy is measured using a frequency comb which is referenced to an active hydrogen maser. The maser is calibrated against the cesium fountain clock CSF1 via a 920 km long actively stabilized fiber link. $f_{\mathrm{rep}}$ denotes repetition rate and $f_{\mathrm{CEO}}$ the CEO frequency.

Figure 2

Comparison of the data scattering of the previous photomultiplier detector (red circles) with the photoelectron detector (black squares). Every point corresponds to one measurement run which typically consists of 7-10 line scans taken during $\approx 1\mathrm{h}$. The blue line represents a weighted average.

Figure 3

Comparison of the hydrogen $1S\mathrm{\text{−}}2S$ centroid frequency derived from the $1S(F=1)\mathrm{\text{−}}2S(F=1)$ hyperfine component including the most recent November 2010 measurement [4, 6, 7]. Each black point represents a one-day average. The labeled red hollow points are the weighted mean values including systematic uncertainties. To measure the latter we also give ${\chi }^2$ per degree of freedom (d.o.f.) of the computed average.