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Yb Lattice-Based Optical Clock

 

 

 

Introduction

The Yb clock uses atoms that are laser-cooled and loaded into an optical lattice.  The lattice environment confers two important benefits to clock performance.  First, tight confinement of the atoms in the potential wells largely suppresses motion-related shifts of the clock frequency, which are a major source of concern in the clocks based on freely expanding neutral atoms.  Second, the lattice holds the atoms against gravity, so we can probe the atoms for long periods of time.  This enables resolution of extremely narrow lines on the 578 nm clock transition, which can serve as extremely high Q frequency references. Lattice-based clocks using the 1S0 → 3P0 transitions in the odd isotopes of alkaline earth-like atoms were first proposed and demonstrated in Sr by Katori et al [1].  Several groups are presently developing Sr based clocks, while we are pursuing the analogous transition in Yb.  Yb offers several advantages including large mass, low nuclear spin in the odd isotopes, a wide range of abundant isotopes, and straightforward second-stage cooling.  As an initial step toward a lattice clock, we performed the first absolute frequency measurements of the clock transition in two isotopes of Yb using atoms released from a second-stage magneto-optic trap.  These measurements reduced the uncertainty in the frequency of these lines one million-fold down to about 4 kHz, which made it easy to find these lines with lattice-trapped atoms.

                                       

In order to load the atoms into the shallow lattice trap, we first cool the atoms with two stages of laser cooling/trapping.   The initial stage loads several million atoms from a Yb beam into a magneto-optic trap using the strong 1S0→1P1 transition (Dn = 28 MHz) at 399 nm.  The cooling light is generated with violet laser diodes.  The trapped atoms have a temperature of a few mK, too warm for the lattice, so we transfer about 70 % of the atoms into a magneto-optic trap based upon the much narrower intercombination line (Dn = 182 kHz) at 556 nm.  In this trap the atoms are cooled to 30-50 mK (depending on the isotope), suitable for lattice loading.  We overlap a tightly focused lattice beam (1 W of power near 760 nm ) with the green magneto-optic trap for 30 ms, during which ~ 5000 atoms are loaded into the potential wells of the lattice.  To excite the clock transition, a well-stabilized probe laser beam at 578 nm is overlapped with the lattice beams (although it is only a traveling wave since it passes through the retro-reflecting mirror).  With the atoms trapped in the lattice, a probe pulse of 3-200 ms is used to excite the atoms.  The degree of excitation is detected by measuring depletion of the ground state population with a resonant pulse at 399 nm as shown in the following measurement sequence:

Spectroscopy of the Clock Transition

The probe laser light is generated by a dye laser whose frequency is locked tightly to high finesse Fabry-Perot cavity.  This laser system was originally developed for a high performance optical clock based on a single trapped Hg ion.  The laser has demonstrated linewidths as narrow as 0.2 Hz with very low drift rates (< 1 Hz/s).  Scanning the laser over the clock resonance shows the expected structure for lattice confined atoms [3]:

                                                           

In the center we see a narrow Doppler-free carrier with first-order sidebands located 90 kHz away.  Zooming in on just the central feature, we see a spectrum that has been resolved with linewidths as narrow as 4 Hz (line Q > 1014):

                                                               

Locking the frequency of the probe laser to this line would yield a fractional frequency instability of less than 2 x 10-15 for 1 s averaging time, competitive with the best existing frequency standards.  Implementation of normalized shelving detection should reduce the noise further, enabling an instability below 10-15 for 1 s averaging time.   

 

Using the Even Isotope                                           

One of the main concerns for lattice-based clocks is the magnetic field sensitivity of the odd isotopes.  It was first thought that only the odd isotopes could work for the forbidden 1S0→3P0 transition, since their non-zero nuclear spin enable mixing of the 3P0 state with other more allowed states to yield a non-negligible transition strength.  But the magnetic sensitivity of these states (100's of Hz/G) would require magnetic shielding at the 10's of mG level to reach projected absolute uncertainties.  To address this issue, two groups proposed multi-photon excitation methods for using the even isotopes, but these are experimentally quite difficult to implement.  In collaboration with A. Taichenachev and V. Yudin (affiliated with the Institute of Laser Physics of the Siberian Branch of the Russian Academy of Sciences and Novosibirsk State University) we have developed a new method that uses a small magnetic field (~ 1 mT or 10 G) to achieve the requisite level mixing but with much smaller magnetic field sensitivity for the clock transition [4].  This technique is easily applied to existing experiments using the odd isotopes and should accelerate progress with lattice-based optical clocks.  We are presently investigating the clock transition in both even and odd isotopes of Yb as well as preparing to work with multi-dimensional lattices for better Doppler suppression.

                                                                       

 

References (click on hyperlinks for pdf versions of the manuscripts) or visit the Division publication site for a updated listing of publications describing this work:

[1] Masao Takamoto, Feng-Lei Hong, Ryoichi Higashi, and Hidetoshi Katori,  “An optical lattice clock”, Nature 435, 321 (2005).

[2] C. W. Hoyt, Z. W. Barber, C. W. Oates, T. M. Fortier, S. A. Diddams, and L. Hollberg, "Observation and absolute frequency measurements of the 1S0→3P0 optical clock transition in neutral ytterbium", Phys. Rev. Lett. 95, 083003 (2005).

[3] Z. W. Barber, C. W. Hoyt, C. W. Oates, L. Hollberg, A. V. Taichenachev, and V.  I. Yudin, "Direct excitation of the forbidden clock transition in neutral 174Yb atoms confined to an optical lattice", Phys. Rev. Lett. 96, 083001 (2006).

[4] A. V. Taichenachev, V.  I. Yudin, C. W. Oates, C. W. Hoyt, Z. W. Barber, and L. Hollberg , "Magnetic field-induced spectroscopy of forbidden transitions with application to lattice-based optical clocks", Phys. Rev. Lett. 96, 083002 (2006).

Chris Oates, NIST (oates@boulder.nist.gov)