Atomic clocks have been instrumental in science and technology, leading to innovations such as global positioning, advanced communications, and tests of fundamental constant variation. Timekeeping precision at 1 part in 10(18) enables new timing applications in relativistic geodesy, enhanced Earth- and space-based navigation and telescopy, and new tests of physics beyond the standard model. Here, we describe the development and operation of two optical lattice clocks, both using spin-polarized, ultracold atomic ytterbium. A measurement comparing these systems demonstrates an unprecedented atomic clock instability of 1.6 × 10(-18) after only 7 hours of averaging.

Atomic clocks based on optical transitions are the most stable, and therefore precise, timekeepers available. These clocks operate by alternating intervals of atomic interrogation with 'dead' time required for quantum state preparation and readout. This non-continuous interrogation of the atom system results in the Dick effect, an aliasing of frequency noise of the laser interrogating the atomic transition 1,2 . Despite recent advances in optical clock stability achieved by improving laser coherence, the Dick effect has continually limited optical clock performance. Here we implement a robust solution to overcome this limitation: a zero-dead-time optical clock based on the interleaved interrogation of two cold-atom ensembles 3 . This clock exhibits vanishingly small Dick noise, thereby achieving an unprecedented fractional frequency instability of  6 10 17 /  for an averaging time  in seconds. We also consider alternate dual-atom-ensemble schemes to extend laser coherence and reduce 2 the standard quantum limit of clock stability, achieving a spectroscopy line quality factor  Q  4 10 15 .An optical atomic clock operates by tuning the frequency of a laser (optical local oscillator, OLO) into resonance with a narrowband, electronic transition in an atomic

The Stark shift due to blackbody radiation (BBR) is the key factor limiting the performance of many atomic frequency standards, with the BBR environment inside the clock apparatus being difficult to characterize at a high level of precision. Here we demonstrate an in-vacuum radiation shield that furnishes a uniform, well-characterized BBR environment for the atoms in an ytterbium optical lattice clock. Operated at room temperature, this shield enables specification of the BBR environment to a corresponding fractional clock uncertainty contribution of 5.5 × 10 −19 . Combined with uncertainty in the atomic response, the total uncertainty of the BBR Stark shift is now 1×10 −18 . Further operation of the shield at elevated temperatures enables a direct measure of the BBR shift temperature dependence and demonstrates consistency between our evaluated BBR environment and the expected atomic response.

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Despite being a canonical example of quantum mechanical perturbation theory, as well as one of the earliest observed spectroscopic shifts, the Stark effect contributes the largest source of uncertainty in a modern optical atomic clock through blackbody radiation. By employing an ultracold, trapped atomic ensemble and high stability optical clock, we characterize the quadratic Stark effect with unprecedented precision. We report the ytterbium optical clock's sensitivity to electric fields (such as blackbody radiation) as the differential static polarizability of the ground and excited clock levels α clock = 36.2612(7) kHz (kV/cm) −2 . The clock's fractional uncertainty due to room temperature blackbody radiation is reduced an order of magnitude to 3 × 10 −17 .PACS numbers: 32.10. Dk, 32.60.+i, 06.20.fb,44.40.+a An atom immersed in an electric field E a becomes polarized -the electronic wave-function is stretched into alignment with the field. Generally, energies of the lowest-lying electronic quantum states |i are reduced by − cold alkaline-earth atoms tightly confined in an optical standing wave potential so their intrinsically narrow, largely imperturbable,may establish stable and accurate frequency and time references [4,5]. In analogy to a pendulum's oscillation slowing due to thermal expansion, a ytterbium lattice clock slows when electrically stretched, or polarized, by thermal blackbody radiation (BBR) fields [6,7]. This phenomenon has been measured in the cesium fountain primary standard [8,9] and other optical transitions [1,10,11].No shield at finite temperature protects a clock atom from the time varying electric field of BBR, the electromagnetic energy absorbed and re-emitted by all matter in thermal equilibrium according to the Stefan-Boltzmann law [12]. Inside a hollow shell of opaque matter (a blackbody), the time-averaged electric field intensity depends strongly on temperature:where α ≈ 1/137 is the fine-structure constant, k B is Boltzmann's constant, and T is the blackbody's absolute temperature [13]. Near room temperature, the spectrum of radiation is peaked strongly near 9.6 µm-invisible to the eye and also far detuned from strong electronic transitions in ytterbium.The ytterbium clock frequency (ν ≈ 518 THz) is shifted by the net BBR Stark effect of the two clock states, which can be expressed aswhere αg,e are the static polarizabilities of ground and excited states ( 1 S 0 and 3 P 0 , respectively), and η clock (T ) [7] arXiv:1112.2766v1 [physics.atom-ph]