Time has always had a special status in physics because of its fundamental role in specifying the regularities of nature and because of the extraordinary precision with which it can be measured. This precision enables tests of fundamental physics and cosmology, as well as practical applications such as satellite navigation. Recently, a regime of operation for atomic clocks based on optical transitions has become possible, promising even higher performance. We report the frequency ratio of two optical atomic clocks with a fractional uncertainty of 5.2 x 10(-17). The ratio of aluminum and mercury single-ion optical clock frequencies nuAl+/nuHg+ is 1.052871833148990438(55), where the uncertainty comprises a statistical measurement uncertainty of 4.3 x 10(-17), and systematic uncertainties of 1.9 x 10(-17) and 2.3 x 10(-17) in the mercury and aluminum frequency standards, respectively. Repeated measurements during the past year yield a preliminary constraint on the temporal variation of the fine-structure constant alpha of alpha/alpha = (-1.6+/-2.3) x 10(-17)/year.

Optical atomic clocks promise timekeeping at the highest precision and accuracy, owing to their high operating frequencies. Rigorous evaluations of these clocks require direct comparisons between them. We have realized a high-performance remote comparison of optical clocks over kilometer-scale urban distances, a key step for development, dissemination, and application of these optical standards. Through this remote comparison and a proper design of lattice-confined neutral atoms for clock operation, we evaluate the uncertainty of a strontium (Sr) optical lattice clock at the 1 × 10 –16 fractional level, surpassing the current best evaluations of cesium (Cs) primary standards. We also report on the observation of density-dependent effects in the spin-polarized fermionic sample and discuss the current limiting effect of blackbody radiation–induced frequency shifts.

Atomic parity violation has been observed in the 6s 2 1 S0 → 5d6s 3 D1 408-nm forbidden transition of ytterbium. The parity-violating amplitude is found to be two orders of magnitude larger than in cesium, where the most precise experiments to date have been performed. This is in accordance with theoretical predictions and constitutes the largest atomic parity-violating amplitude yet observed. This also opens the way to future measurements of neutron skins and anapole moments by comparing parity-violating amplitudes for various isotopes and hyperfine components of the transition. has not yet been possible to test an important prediction of the SM concerning the variation of Q W along a chain of isotopes. It has been suggested [4] that rare-earth atoms may be good candidates for APV experiments because they have chains of stable isotopes, and the APV effects may be enhanced due to the proximity of opposite-parity levels. While the accuracy of atomic calculations is unlikely to ever approach that achieved for atoms with a single valence electron, ratios of PV-amplitudes between different isotopes should provide ratios of weak charges, without involving, to first approximation, any atomic-structure calculations.The present experiment is inspired by the prediction [5] supported by further theoretical work of [6,7], that the PV-amplitude in the chosen transition is ≈100 times larger than that in Cs. The motivation for PVexperiments in Yb is probing low-energy nuclear physics by comparing PV-effects on either a chain of naturally occurring Yb isotopes, or in different hyperfine components for the same odd-neutron-number isotope. The ratio of PV amplitudes for two isotopes of the same element is sensitive to the neutron distributions within the nucleus. The difference between PV amplitudes measured on two different hyperfine lines belonging to the same transition is a manifestation of nuclear-spin-dependent APV, which is sensitive to the nuclear anapole moments (see, for example, reviews [8,9]) that arise from weak interactions between the nucleons. As the precision of the experiment increases, a sensitive test of the Standard Model may also become possible [10].Here we report on experimental verification of the predicted PV-amplitude enhancement in Yb using a measurement of the APV amplitude for 174 Yb. The idea of the experiment is to excite the forbidden 408-nm transition ( Fig. 1) with resonant laser light in the presence of a quasi-static electric field. The PVamplitude of this transition arises due to PV-mixing of the 5d6s 3 D 1 and 6s6p 1 P 1 states. The purpose of the electric field is to provide a reference transition amplitude due to Stark-mixing of the same states, interfering with the PV amplitude. In such interference method [11,12], one is measuring the part of the transition probability that is linear in both the reference Stark-induced amplitude and the PV amplitude. In addition to enhancing the PV-dependent signal, employing the Stark-PV interference technique provides for all-important reversals allow...

We report, for the first time, laser spectroscopy of the 1S0-->3P0 clock transition in 27Al+. A single aluminum ion and a single beryllium ion are simultaneously confined in a linear Paul trap, coupled by their mutual Coulomb repulsion. This coupling allows the beryllium ion to sympathetically cool the aluminum ion and also enables transfer of the aluminum's electronic state to the beryllium's hyperfine state, which can be measured with high fidelity. These techniques are applied to measure the clock transition frequency nu=1,121,015,393,207,851(6) Hz. They are also used to measure the lifetime of the metastable clock state tau=20.6+/-1.4 s, the ground state 1S0 g factor gS=-0.000,792,48(14), and the excited state 3P0 g factor gP=-0.001,976,86(21), in units of the Bohr magneton.

We present an experimental study of the lattice induced light shifts on the 1 S0 → 3 P0 optical clock transition (ν clock ≈ 518 THz) in neutral ytterbium. The "magic" frequency, νmagic, for the 174 Yb isotope was determined to be 394 799 475(35)MHz, which leads to a first order light shift uncertainty of 0.38 Hz on the 518 THz clock transition. Also investigated were the hyperpolarizability shifts due to the nearby 6s6p 3 P0 → 6s8p 3 P0, 6s8p 3 P2, and 6s5f 3 F2 two-photon resonances at 759.708 nm, 754.23 nm, and 764.95 nm respectively. By tuning the lattice frequency over the twophoton resonances and measuring the corresponding clock transition shifts, the hyperpolarizability shift was estimated to be 170(33) mHz for a linear polarized, 50 µK deep, lattice at the magic wavelength. In addition, we have confirmed that a circularly polarized lattice eliminates the J = 0 → J = 0 two-photon resonance. These results indicate that the differential polarizability and hyperpolarizability frequency shift uncertainties in a Yb lattice clock could be held to well below 10 −17 .

We consider the limitations due to noise (e.g., quantum projection noise and photon shot-noise) on the sensitivity of an idealized atomic magnetometer that utilizes spin squeezing induced by a continuous quantum nondemolition measurement. Such a magnetometer measures spin precession of N atomic spins by detecting optical rotation of far-detuned light. We show that for very short measurement times, the optimal sensitivity scales as N(-3/4); if strongly squeezed probe light is used, the Heisenberg limit of N-1 scaling can be achieved. However, if the measurement time exceeds tau(rel)/N(1/2) in the former case, or tau(rel)/N in the latter, where tau(rel) is the spin relaxation time, the scaling becomes N(-1/2), as for a standard shot-noise-limited magnetometer.

We report tests of local position invariance and the variation of fundamental constants from measurements of the frequency ratio of the 282-nm 199Hg+ optical clock transition to the ground state hyperfine splitting in 133Cs. Analysis of the frequency ratio of the two clocks, extending over 6 yr at NIST, is used to place a limit on its fractional variation of <5.8x10(-6) per change in normalized solar gravitational potential. The same frequency ratio is also used to obtain 20-fold improvement over previous limits on the fractional variation of the fine structure constant of |alpha/alpha|<1.3x10(-16) yr-1, assuming invariance of other fundamental constants. Comparisons of our results with those previously reported for the absolute optical frequency measurements in H and 171Yb+ vs other 133Cs standards yield a coupled constraint of -1.5x10(-15)

The weak force is the only fundamental interaction known to violate the symmetry with respect to spatial inversion (parity). This parity violation (PV) can be used to isolate the effects of the weak interaction in atomic systems, providing a unique, low-energy test of the Standard Model [see for example reviews 1, 2, 3]. These experiments are primarily sensitive to the weak force between the valence electrons and the nucleus, mediated by the neutral Z 0 boson and dependent on the weak charge of the nucleus, Q w . The Standard Model (SM) parameter Q w was most precisely determined in cesium (Cs) [4, 5] and has provided a stringent test of the SM at low energy. The SM also predicts a variation of Q w with the number of neutrons in the nucleus, an effect whose direct observation we are reporting here for the first time. Our studies, made on a chain of ytterbium (Yb) isotopes, provide a measurement of isotopic variation in atomic PV, confirm the predicted SM Q w scaling and offer information about an additional Z´ boson.The large PV observable in Yb was first predicted by DeMille [6], a prediction further supported by subsequent calculations [7,8,9] and confirmed by experiment [10,11]. The PV effect in Yb is approximately 100 times larger than that in Cs. Moreover, Yb has a chain of stable isotopes, allowing for an isotopic comparison of the effect [12]. Such a comparison has the potential to be a probe of neutron distributions in the Yb nuclei [13] and is sensitive to physics beyond the SM [14,15]. A related measurement, in which the PV effects are compared for different hyperfine components of isotopes with non-zero nuclear spin, is expected to improve the understanding of the weak interaction within the nucleus [3,16,17,18].The principle of our measurements is similar to that of the 1 st -generation experiment [10,11]. We optically excite Yb atoms in a beam, on the 6s 2 1 S 0 → 5d6s 3 D 1 transition ( fig. 1), in a region in which in addition to the applied optical field, static electric and static magnetic fields are applied to the atoms [19].The directions of the magnetic and static electric field and that of the optical-field polarization define the handedness for the experimental coordinate system. As the 1 S 0 and 3 D 1 states are of nominally same parity, an electric-dipole (E1) transition between them is forbidden by selection rules. In the presence of the weak interaction, however, mixing of the 1 P 1 state into 3 D 1 results in a E1 PV amplitude for the transition. The applied dc (or quasi-static) electric field results in additional mixing of these states, allowing for a larger and controlled Stark-induced E1 amplitude [20]. The Stark-induced and PV amplitudes will interfere with appropriate choice of field geometry. Field reversals flip the handedness of the field geometry, leading to a sign reversal of the Stark-PV interference term and a change in the transition rate. This change provides an experimental observable.We measured the PV effect in four nuclear-spin-zero isotopes ( 170 Yb, 172 Yb, 174 Yb a...