Bulletin of the American Physical Society
2013 Joint Meeting of the APS Division of Atomic, Molecular & Optical Physics and the CAP Division of Atomic, Molecular & Optical Physics, Canada
Volume 58, Number 6
Monday–Friday, June 3–7, 2013; Quebec City, Canada
Session T7: Focus Session: Advances in Atomic Clocks |
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Chair: Ken O'Hara, Pennsylvania State University Room: 303 |
Friday, June 7, 2013 8:00AM - 8:30AM |
T7.00001: $^{88}$Sr$^{+}$ 445-THz Single-Ion Reference at the 10$^{-17}$ Level Invited Speaker: Alan Madej We report experiments and precision measurements on a trapped and laser cooled single ion of $^{88}$Sr$^{+}$ which when probed on the narrow 5$s$~$^{2}$S$_{1/2}$ -- 4$d$~$^{2}$D$_{5/2}$ transition at 445-THz (674 nm) provides a reference yielding an evaluated fractional inaccuracy of 2.3~x~10$^{-17}$ and which significantly outperforms the current realization of the SI second. The extremely low systematic shifts obtained are a result of our ability to evaluate, control and in some instances cancel some of the main perturbations that the trapped ion experiences. The fractional uncertainty on the micromotion induced shifts of the trapped ion has been evaluated to better than 1~x~10$^{-18}$. This is achieved by minimizing any spurious displacement of the ion from trap center using DC trim electrodes and operating the system at a ``magic'' trap frequency where there is anti-correlation between the micromotion induced second order Doppler and Stark shifts resulting in near complete cancellation of this form of perturbation. The electrical quadrupole shift seen in many trapped ion systems is reduced to the 10$^{-19}$ level by averaging the measured shifts of several pairs of Zeeman components. As in many optical frequency references, the dominant source of uncertainty arises from the blackbody radiation shift. We have been able to reduce the uncertainties associated by this shift using a recent theoretical evaluation of the differential scalar polarizability of the reference transition together with experimental measurements of the trap heating behavior and modeling of the blackbody field at the ion location. The present measurements are performed with resolution of spectral features down to the 4 Hz level (1~part in 10$^{14}$) together with continuous measurement periods exceeding a few days allowing the possibility for the device to be used as an optical atomic time standard. As part of the effort to link this ultra accurate standard with current time/frequency standards, an absolute frequency measurement of the reference was performed over a two-month period relative to a H maser referenced to the SI second via GPS time transfer. A centre frequency of the transition of 444~779~044~095~485.5(9)~Hz was obtained. Comparison of the reference frequency between two different ion trap systems is currently underway which may further improve our knowledge on key shift parameters. Results will be reported at the meeting. [Preview Abstract] |
Friday, June 7, 2013 8:30AM - 9:00AM |
T7.00002: Optical lattice clocks near the QPN limit: a tenfold improvement in optical clock stability Invited Speaker: Travis Nicholson Two classes of optical atomic clocks have surpassed microwave frequency standards: single-ion clocks and optical lattice clocks. Single-ion clocks hold the record for the lowest systematic uncertainty [1]; however, many-atom lattice clocks have the potential to outperform single-ion clocks because the standard quantum limit to atomic clock instability (known as quantum projection noise or QPN) scales as $1/\sqrt{N_{atoms}}$ [2]. For realistic atom numbers and coherence times, QPN-limited lattice clocks could average down to a given stability hundreds of times faster than the best ion clocks. Up to now lattice clocks with $\sim 1000$ atoms have not shown improvement over the stability of single-ion clocks. Lattice clock stability has been limited by laser noise (via the optical Dick effect). To address this problem, we constructed a new clock laser with a thermal noise floor of $1 \times 10^{-16}$---an order of magnitude improvement over our previous clock laser. With this laser, we compare two lattice clocks, reaching instability of $1 \times 10^{-17}$ in 2000 s for a single clock. This instability is within a factor of 2 of the theoretical QPN limit for $\sim 1000$ atoms, representing the lowest reported instability for an independent clock [3]. The high stability of many-particle clocks can come at the price of larger systematic uncertainty due to a frequency shift from atomic interactions. To minimize this shift, we use a cavity-enhanced lattice [4] for our second clock. The high circulating power inside the cavity allows for a large trap volume, yielding a density at 2000 atoms that is 27 times smaller (than in our first clock) and permitting us to trap as many as $5 \times 10^4$ atoms. For 2000 atoms in our lattice, we measure a value for this shift (which is linear in density) of $-3.11 \times 10^{-17}$ with an uncertainty of $8.2 \times 10^{-19}$ [3].\\[4pt] [1] Chou, et al., PRL 104, 070802 (2010)\\[0pt] [2] Ludlow, et al., Science 319 1805 (2008)\\[0pt] [3] Nicholson, et al., PRL 109, 230801 (2012)\\[0pt] [4] Westergaard, et al., PRL 106, 210801 (2011) [Preview Abstract] |
Friday, June 7, 2013 9:00AM - 9:12AM |
T7.00003: Laser spectral analysis using $^{87}$Sr atoms with a quantum projection limited noise floor Michael Bishof, Xibo Zhang, Michael J. Martin, Jun Ye Ultra-stable lasers are essential tools in a variety of precision measurement experiments and their stability often dictates the performance of the experiments they serve. For example, a Sr clock recently demonstrated record clock stability by using a laser with $10^{-16}$ fractional stability[1]. Despite the importance of laser performance, evaluating noise spectra of state-of-the-art lasers remains challenging. Often, multiple lasers of similar performance are built for the sole purpose of evaluating laser noise [2]. We demonstrate a technique to measure the noise spectrum of a single ultra-stable laser using optical lattice-trapped $^{87}$Sr atoms as a quantum projection noise-limited reference. Using a simple theoretical framework, we deduce the laser spectrum from measured fluctuations in atomic excitation. Measurements using a variety of probe sequences are consistent with resonant features observed in an optical beat with a less stable laser. Furthermore, we use features from this beat to actively reduce resonant noise in our ultra-stable laser. Finally, we show how knowledge of our laser's spectrum informs the optimal conditions for clock operation.\\[4pt] [1] T. L. Nicholson et al., PRL 109, 23081 (2012).\\[0pt] [2] T. Kessler et al., Nature Photon. 6, 687-692 (2012). [Preview Abstract] |
Friday, June 7, 2013 9:12AM - 9:24AM |
T7.00004: Ytterbium in quantum gases and atomic clocks: van der Waals interactions and blackbody shifts Sergey Porsev, Marianna Safronova, Charles Clark We evaluated the $C_6$ coefficients of Yb-Yb, Yb-alkali, and Yb-group II van der Waals interactions with 2\% uncertainty. The only existing experimental result for such quantities is for the Yb-Yb dimer. Our value, $C_6=1929(39)$ a.u., is in excellent agreement with the recent experimental determination of 1932(35) a.u. We have also developed a new approach for the calculation of the dynamic correction to the blackbody radiation shift. We have calculated this quantity for the Yb $6s^{2}~ ^1\!$S$_0 - 6s6p ~^3\!$P$_0^o$ clock transition with 3.5\% uncertainty. This reduces the fractional uncertainty due to the blackbody radiation shift in the Yb optical clock at 300 K to the $10^{-18}$ level. [Preview Abstract] |
Friday, June 7, 2013 9:24AM - 9:36AM |
T7.00005: Precise realization of the thermal radiation environment for an optical lattice clock Kyle Beloy, Jeff A. Sherman, Nathaniel B. Phillips, Nathan Hinkley, Chris W. Oates, Andrew D. Ludlow The Stark shift due to thermal radiation contributes one of the largest known perturbations to the clock transition frequency of optical lattice clocks. Consequently, the uncertainty stemming from this shift has played a dominant role in the total uncertainty of these standards. Following recent works focused on atomic response factors (e.g., the differential polarizability), uncertainty in this perturbation is now limited by imprecise knowledge of the environment itself. Here we present progress towards precise realization of the thermal radiation environment in a Yb optical lattice clock by trapping the atoms in a highly uniform radiation shield at a well-known temperature. We characterize the non-ideal aspects of this approach, including less than unit emissivity, contamination of the blackbody environment from the ambient environment, and thermal non-uniformities. [Preview Abstract] |
Friday, June 7, 2013 9:36AM - 9:48AM |
T7.00006: Precision measurement of the Stark effect on a Yb lattice clock Nathan Hinkley, Jeff A. Sherman, Kyle Beloy, Nathaniel B. Phillips, Richard W. Fox, Chris W. Oates, Andrew D. Ludlow Ultracold alkaline-earth-like atoms, confined within an optical lattice and exploiting the ultra-narrow $^{1}\!S_{0}$ to $^{3}\!P_{0}$ atomic transition, are utilized as high-accuracy frequency standards and precision timekeepers. The blackbody Stark effect and residual lattice ac-Stark shifts not canceled at the magic wavelength (where scalar Stark shifts between clock states $^{1}\!S_{0}$ and $^{3}\!P_{0}$ are balanced) both remain as the principle contributions to the frequency uncertainty. We describe precision measurements that carefully characterize these effects, paving the way towards optical lattice clock systems with $10^{-17}$ level uncertainty. First, we determine the dynamic effect of blackbody radiation (BBR) on the atomic clock states, constraining the BBR shift uncertainty from an ideal blackbody environment to $1.1\times10^{-18}$. Next, we discuss precision measurements of the lattice-induced Stark shifts from the E1 polarizability, hyperpolarizability, and multipolar terms. Finally, we demonstrate the proficiency of lattice clock systems for precision frequency measurements by directly comparing two such Yb standards, and achieve $10^{-17}$ frequency stability in $<\!1000$ s. [Preview Abstract] |
Friday, June 7, 2013 9:48AM - 10:00AM |
T7.00007: Clock Spectroscopy of Interacting Fermions in a Harmonic Trap Andrew Koller, Ana Maria Rey We investigate the dynamics during Ramsey interrogation of interacting fermions in a harmonic trap. We consider the effect of both s-wave and p-wave collisions during the dynamics, including processes that change the spatial modes of particles. Prior theoretical treatments utilize the so-called spin model\footnote{K. Gibble, Phys. Rev. Lett. 103, 113202 (2009)}$^,$\footnote{A.M. Rey, A.V. Gorshov, C. Rubbo, Phys. Rev. Lett. 109, 260402 (2009)}$^,$\footnote{Z. Yu and C. J. Pethick, Phys. Rev. Lett. 104, 010801 (2010)} which includes processes that change the internal states of atoms, but leave the vibrational modes unchanged. We discuss how the inclusion of these mode-changing processes modifies the predicted density-dependent frequency shifts and contrast of Ramsey fringes, both of which are relevant for the precision of optical lattice clocks using fermionic alkaline-earth atoms. We also discuss how the frequency shifts and contrast depend on the pulse areas, interaction strength, and temperature - and how these dependences are affected by the inclusion of mode-changing collisions in the calculations. [Preview Abstract] |
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