Stability of the Proton-to-Electron Mass Ratio

We report a limit on the fractional temporal variation of the proton-to-electron mass ratio as $\frac{1}{(m_P/m_e)}\frac{\partial }{\partial t}(m_P/m_e)=(-3.8\pm 5.6)\times {10}^{-14}{\mathrm{yr}}^{-1}$, obtained by comparing the frequency of a rovibrational transition in ${\mathrm{SF}}_6$ with the fundamental hyperfine transition in Cs. The ${\mathrm{SF}}_6$ transition was accessed using a ${\mathrm{CO}}_2$ laser to interrogate spatial 2-photon Ramsey fringes. The atomic transition was accessed using a primary standard controlled with a Cs fountain. This result is direct and model-free.

Figure 1

Experimental setup.

Figure 2

Fringes at 200 Hz, obtained using a 1 m interzone separation. Experimental conditions: pure ${\mathrm{SF}}_6$ beam, input pressure $5\times {10}^5\mathrm{Pa}$, 12 mW inside $U$ cavity FM modulation at 115 Hz index 0.43, $75\mu \mathrm{W}$ inside the detection cavity, time constant for detection 0.1 s. Average of 5 up-down sweeps, 200 points, averaging 1 s per point. Signal-to-noise ratio 30.

Figure 3

Absolute frequency of the central fringe displayed as a function of time. The $y$ axis is offset by 28 412 764 347 000 Hz. The least-squares best fit line has a slope of $1.88\times {10}^{-14}{\mathrm{yr}}^{-1}$.