Session H6: Atomic Clocks

The Sr optical lattice clock at JILA: A new record in atomic clock performance

The exquisite control exhibited over quantum states of individual particles has revolutionized the field of precision measurement, as exemplified by highly accurate atomic clocks. Optical clocks have been the most accurate frequency standards for the better part of a decade, surpassing even the cesium microwave fountains upon which the SI second is based. Two classes of optical clocks have outperformed cesium: single-ion clocks and optical lattice clocks. Historically ion clocks have always been more accurate, and the precision of ion clocks and lattice clocks has been comparable. For years it has been unclear if lattice clocks can overcome key systematics and become more accurate than ion clocks. In this presentation I report the first lattice clock that has surpassed ion clocks in both precision and accuracy. These measurements represent a tenfold improvement in precision and a factor of 20 improvement in accuracy over the previous best lattice clock results. This work paves the way for a better realization of SI units, the development of more sophisticated quantum sensors, and precision tests of the fundamental laws of nature.   

Precision measurements of lattice-induced frequency shifts in the Yb optical lattice clock

Optical clocks based on ultra-cold, lattice trapped alkaline-earth-like atoms interrogated on the ultra-narrow $^1S_0\leftrightarrow^{3}\! P_0$ transition promise timing performance at unprecedented levels. Recently, our ytterbium optical lattice clock demonstrated record frequency instability of $1.6\times10^{-18}$ in fractional units. Evaluation of the clock uncertainty at the $10^{-18}$ level requires characterization of the atomic response to two main systematic frequency shifts: one due to ambient blackbody radiation and one due to the lattice itself. In this talk, we discuss an evaluation of the residual lattice Stark shifts. Ytterbium atoms are cooled and trapped in a cavity-enhanced standing wave of light (optical lattice). We operate the lattice near its magic wavelength, where the scalar Stark shift of each clock state is equal so that the natural transition frequency is preserved. The buildup cavity permits an enhancement of the lattice intensity ($I$) by a factor of more than twenty over levels limited by the total laser output power available. Exploiting the intense lattice electric field, we make precision measurements of the hyperpolarizability ($\propto I^2$) and M1 and E2 multipole ($\propto\sqrt{I}$) contributions to Stark shifts of the clock transition in Yb.   

Collective atomic effects in cavity QED with application to atomic clocks

Atoms placed in an optical cavity can experience cooperative effects where the atomic dipoles synchronize and collectively acquire a macroscopic phase. This process is reminiscent of the spontaneous symmetry breaking that often occurs in second-order phase transitions. The collective phase can be even more stable than the single-atom linewidth. We investigate Strontium atoms in an optical cavity, where the ultranarrow atomic transition considered is 1000 times narrower than the cavity linewidth. A strong probe laser is introduced along the cavity axis, and is scanned through the atomic resonance. For input intensities slightly above the optical bistablity region, an extremely sharp collective resonance feature occurs. Locking to this rapidly varying dispersion feature could be useful for a variety of precision measurement applications.   

Experimental investigation of collective atomic effects in cavity QED with application to atomic clocks

Atoms placed in an optical cavity can experience cooperative effects where the atomic dipoles synchronize and collectively acquire a macroscopic phase. This collective phase can significantly enhance the phase response of the system and open the possibility of using collective effects to improve the spectral purity of a clock laser. We demonstrate the first measurements of a collective phase behavior of $^{88}$Sr atoms trapped inside an optical cavity and we investigate the input probe power and atom number dependence of this collective behavior. We trap about $3\times10^{8}$ atoms in a standard Magneto Optical Trap (MOT) inside a low finesse ($F=85$) cavity and perform direct spectroscopy on the narrow optical $^{1}$S$_{0}\rightarrow^{3}$P$_{0}$ transition ($\Gamma=7.6$ kHz) at $689$ nm. The phase response is measured by performing cavity enhanced FM spectroscopy using the so-called NICE-OHMS technique. In addition, we will present our latest results for optimizing the collective phase response, demonstration of superradiance at an optical frequency and investigation of the time evolution of the phase synchronization of the atoms.   

A Robust Ramsey Interferometer for Atomic Timekeeping in Dynamic Environments

We present a laser-based approach to atomic timekeeping, in which atomic phase information is extracted using modified Raman pulses in a Ramsey sequence. We overcome systematic effects associated with differential AC Stark shifts by employing atom optics derived from Raman adiabatic rapid passage (ARP). ARP drives coherent transfer between two hyperfine ground states by sweeping the frequency difference of two optical fields and maintaining a large single-photon detuning. Compared to fixed-frequency Raman transitions, ARP atom optics afford a $\sim$100x reduction in sensitivity to differential AC Stark shifts in a Ramsey interferometer. We also demonstrate that ARP preserves fringe contrast in Ramsey interferometers for cloud displacements reaching the 1/e$^2$ intensity radius of the laser beam. ARP can thus be expected to improve the robustness of clock interferometers operating in dynamic environments. Copyright {\copyright} 2014 by The Charles Stark Draper Laboratory, Inc. All rights reserved.   

Collective State Raman Atomic Clock Using Trapped Atoms

Atomic clock has set the standard as the most accurate clock in the world. So far, the approach to making the atomic clock has been limited to utilizing individual atomic states. We have developed the framework for a collective atomic clock--in an ensemble of cold atoms using the method of separated Raman-Ramsey fields--by conceiving a method to detect the collective states, analyzing the signal to noise ratio, and finding the bounds for efficiency of our detector. The width of the Raman-Ramsey fringe in such a clock is narrower than that of a conventional Raman-Ramsey fringe by a factor of root-N, where N is the number of atoms in the ensemble. When the collection efficiency of the detection process is taken into account, such a clock can have a frequency stability that is expected to be better than that of a conventional Raman-Ramsey clock. The ultra-narrow fringe may also offer many other potential advantages, such as suppression of errors due to fluctuations in the bias field used for lifting Zeeman sublevel degeneracy, and the long-term bias drift. We will present the theoretical model, and describe the status of our experimental efforts towards demonstrating such a clock.   

High-Precison, Accurate, and Robust Optical Frequency Source Using a Non-Narrow Linewidth Laser

We stabilize the center frequency of an off-the-shelf diode laser to the saturated-absorption resonances of the ${}^{87}$Rb D${}_2$ hyperfine transitions. The laser makes no use of optical feedback from an external cavity for line narrowing or frequency tuning. Using two such sources, we characterize the output frequency stability, frequency accuracy, line width, and coherence time of the source. Despite the short-term megahertz-level line width of the source, our results demonstrate that the stabilized laser provides a frequency stability of $\sim$1 kHz at an integration time of 10 s, and a frequency accuracy of $\sim$150 kHz over a tuning range of 230 MHz and at 100 ms integration time. Our laser source is relatively simple, compact, portable, and is expected to be far less susceptible to environmental influences such as vibration and temperature variation than a stabilized laser system which relies on an external cavity. Therefore, our stabilized laser source has potential applications for atom-based sensors for field use. Results will also be presented demonstrating use of the source for measurement of hyperfine frequency intervals, and measurement of frequency-accurate saturated-absorption spectra in ${}^{87}$Rb.   

Quantum Synchronization of Two Ensembles of Atoms

We present a system that exhibits quantum synchronization as a modern analogue of the Huygens experiment which is implemented using state-of-the-art neutral atom lattice clocks of the highest precision. In particular, we study the correlated phase dynamics of two mesoscopic ensembles of atoms through their collective coupling to an optical cavity. We find a dynamical quantum phase transition induced by pump noise and cavity output-coupling. The spectral properties of the superradiant light emitted from the cavity show that at a critical pump rate the system undergoes a transition from the independent behavior of two disparate oscillators to the phase-locking that is the signature of quantum synchronization. Besides being of fundamental importance in nonequilibrium quantum many-body physics, this work could have broad implications for many practical applications of ultrastable lasers and precision measurements. This work was supported by the DARPA QuASAR program, the NSF, and NIST.   

Hunting for topological dark matter with atomic clocks

The cosmological applications of atomic clocks so far have been limited to searches of the uniform-in-time drift of fundamental constants. In this paper, we point out that a transient in time change of fundamental constants can be induced by dark matter objects that have large spatial extent, and are built from light non-Standard Model fields. The stability of this type of dark matter can be dictated by the topological reasons. We point out that correlated networks of atomic clocks, some of them already in existence, can be used as a powerful tool to search for the topological defect dark matter, thus providing another important fundamental physics application to the ever-improving accuracy of atomic clocks. During the encounter with a topological defect, as it sweeps through the network, initially synchronized clocks will become desynchronized. Time discrepancies between spatially-separated clocks are expected to exhibit a distinct signature, encoding defect's space structure and its interaction strength with the Standard Model fields. Details can be found in arXiv:1311.1244   

Initial results from a green astro-comb for exoplanet searches at HARPS-N

Astro-combs, a combination of a laser frequency comb, a coherent wavelength shifting mechanism (such as a doubling crystal and photonic crystal fiber), and a mode-filtering Fabry-Perot cavity, are promising tools that enable searches for Earth-like extra-solar planets (exoplanets) and the direct observation of the accelerating expansion of the universe. In this talk, We will present recent results from our ``green astro-comb'' that has been operating at the HARPS-N spectrograph in the 3.6 m Telescopio Nazionale Galileo (TNG) in the Canary Islands for the past year. The green astro-comb consists of $\sim6000$ lines equally spaced by $\sim20$ GHz in the 500 nm - 600 nm optical band and is derived from a 1 GHz Ti:Sapphire comb laser, a custom tapered photonic crystal fiber that spectrally shifts the comb lines to the visible, and two mode-filtering Fabry-Perot cavities that increase the line spacing to match the $R=100000$ HARPS-N spectrograph. Results from initial investigations with the green astro-comb characterizing the performance of the HARPS-N spectrograph will be presented.