Measurement of Optical Rubidium Clock Frequency Spanning 65 Days

Abstract

Optical clocks are emerging as next-generation timekeeping devices with technological and scientific use cases. Simplified atomic sources such as vapor cells may offer a straightforward path to field use, but suffer from long-term frequency drifts and environmental sensitivities. Here, we measure a laboratory optical clock based on warm rubidium atoms and find low levels of drift on the month-long timescale. We observe and quantify helium contamination inside the glass vapor cell by gradually removing the helium via a vacuum apparatus. We quantify a drift rate of 4×10-15/day, a 10 day Allan deviation less than 5×10-15, and an absolute frequency of the Rb-87 two-photon clock transition of 385,284,566,371,190(1970) Hz. These results support the premise that optical vapor cell clocks will be able to meet future technology needs in navigation and communications as sensors of time and frequency.

Keywords: atomic clock; helium permeation; two-photon spectroscopy.

Figure 1

Diagram of the two-photon clock. Laser light at 1556 nm is amplified, frequency doubled, and delivered by optical fiber into a vacuum chamber via a vacuum feedthrough. From there, the light is launched into free space and encounters a filter, pickoff (for monitoring power), and polarizer before entering the vapor cells. A cat’s eye retroreflector returns the beam anti-parallel to create counter-propagating beams. A photomultiplier tube (PMT) monitors for 420 nm fluorescence from the Rb atoms. An optical frequency comb (OFC) divides the laser frequency optical frequency down to an RF signal. ISO: isolator. SPL: splitter. WDM: wavelength division multiplexer. BT: beam terminator. BC: beam coupler. PD: photodiode. SHG: second harmonic generation.

Figure 2

Daily average frequency data over time (dots), along with an exponential decay fitting function (red line) that also includes a small linear drift. The data and fit are manually offset so that they gradually approach y = 0, which corresponds to a helium-free condition. The observed time constant $T_{\mathrm{cell}}$ is 38(10) days. The right y axis displays the inferred remaining partial pressure of helium in mTorr.

Figure 3

Total Allan Deviation determination of optical clock stability after removing the effect of helium by fitting and subtracting an exponential decay function with linear drift (black markers). Additionally shown in gray markers is the Total Allan deviation of the same data prior to removing the drift. At one day (86,400 s) the fractional frequency deviation is approximately $5\times {10}^{-15}$. The dashed line corresponds to $5\times {10}^{-13}/\tau$, with $\tau$ the averaging time in seconds.

Figure 4

Time deviation determination of optical clock stability after removing the effect of helium by fitting and subtracting an exponential decay function (black circles). Additionally shown in gray markers is the time deviation for the same data prior to removal of the drift. At one day (86,400 s), the time deviation is approximately 200 ps.