We evaluate the sensitivity of a dual cloud atom interferometer to the measurement of vertical gravity gradient. We study the influence of most relevant experimental parameters on noise and long-term drifts. Results are also applied to the case of doubly differential measurements of the gravitational signal from local source masses. We achieve a short term sensitivity of 3 × 10 −9 g/ √ Hz to differential gravity acceleration, limited by the quantum projection noise of the instrument. Active control of the most critical parameters allows to reach a resolution of 5 × 10 −11 g after 8000 s on the measurement of differential gravity acceleration. The long term stability is compatible with a measurement of the gravitational constant G at the level of 10 −4 after an integration time of about 100 hours.

We report on the realization of a matter-wave interferometer based on single-photon interaction on the ultra-narrow optical clock transition of strontium atoms. We experimentally demonstrated its operation as a gravimeter and as a gravity gradiometer. No reduction of interferometric contrast was observed up to an interferometer time 2T = 10 ms, limited by geometric constraints of the apparatus. In the gradiometric configuration, the sensitivity approaches the shot noise limit. Singlephoton interferometers represent a new class of high-precision sensors that could be used for the detection of gravitational waves in so far unexplored frequency ranges and to enlighten the boundary between Quantum Mechanics and General Relativity. PACS numbers:Matter-wave interference enables the investigation of physical interactions at their fundamental quantum level and forms the basis of high-precision inertial sensors and for application in precision gravitational field sensing [1]. Today's best atom interferometers, based on multiphoton Raman/Bragg transitions and Bloch oscillations, can tailor matter-waves at will, up to macroscopic scales [2], preserving their coherence for extremely long times [3], allowing precision measurements of the Newtonian gravitational constant [4], Earth gravity acceleration [5][6][7][8], gravity gradients [9-11] and gravity curvature [12,13]. At the same time, optical spectroscopy of ultra-narrow optical transitions in atoms and ions have produced clocks with the highest relative frequency accuracy, approaching the 10 −19 level [14-17]. Thanks to these impressive results, schemes for gravitational waves detectors based on atom interferometers and optical clocks have been proposed [18][19][20][21][22].In this Letter, we demonstrate an atom interferometer based on the ultra-narrow 1 S 0 -3 P 0 optical clock transition of 88 Sr atoms. While atom interferometry with the 400 Hz-wide intercombination transition of calcium was reported already [23][24][25], the virtually infinite lifetime of the upper clock state in strontium is crucial for demanding applications as, for example, gravitational wave detectors. Based on a single-photon process, this novel sensor will indeed enable new fundamental tests, lying at the border of Quantum Mechanics and General Relativity [26,27], such as quantum interference of clocks, with the possible observation of the red-shift induced decoherence effects [28,29], light Dark Matter search [30] and tests of the Weak Equivalence Principle with quantum superpositions of states with large energy (∼eV) separation [31,32]. Precision measurements of gravity will also be necessary for the development of optical lattice clocks. The comparisons of clocks at the 10 −19 level will not only require a precise knowledge of the static gravitational field component at the atomic cloud location, but it will also demand a simultaneous measurement of time-varying gravitational potential effects down to the exceptional level of 10 −2 m 2 /s 2 [33]. Novel single-photon interferometers, based on t...

Spin squeezing can improve atomic precision measurements beyond the standard quantum limit (SQL), and unitary spin squeezing is essential for improving atomic clocks. We report substantial and nearly unitary spin squeezing in 171 Yb, an optical lattice clock atom. The collective nuclear spin of ∼ 10 3 atoms is squeezed by cavity feedback, using light detuned from the system's resonances to attain unitarity. The observed precision gain over the SQL is limited by state readout to 6.5(4) dB, while the generated states offer a gain of 12.9(6) dB, limited by the curvature of the Bloch sphere. Using a squeezed state within 30% of unitarity, we demonstrate an interferometer that improves the averaging time over the SQL by a factor of 3.7(2). In the future, the squeezing can be simply transferred onto the optical clock transition of 171 Yb.Optical lattice clocks (OLCs) employ ensembles of cold trapped atoms to reach unprecedented fractional accuracy at the level of 10 −18 [1][2][3][4][5]. Such clocks now operate near the standard quantum limit (SQL) set by quantum projection noise, where the precision of a sensor improves as √ N with the number of atoms N . Spin squeezed states (SSSs) [6-22] are many-body entangled states that can overcome the SQL [8,23]. They have simple Gaussian quasi-probability distributions with reduced (squeezed) and enhanced (antisqueezed) quantum noise, respectively, along two orthogonal directions of the collective atomic spin. While for fixed-bandwidth applications the precision depends on the squeezing alone, André et al. [24] have shown that for optimized clocks the antisqueezed direction eventually leaks into the measurement, reducing the gain in precision. In practice, the amount of antisqueezing typically far exceeds the squeezing, and this mechanism can dramatically reduce the precision gain to the point where, e.g., the state with the highest inferred squeezing of 20 dB (and an antisqueezing of 39 dB) [20] would improve the precision of a clock by a mere 2 dB [25]. Thus nearly unitary (area-preserving) squeezing is of high importance for future clock applications. Furthermore, of the most common OLC atoms, spin squeezing in Sr, Ca, Mg or Hg have not been demonstrated so far, and Yb has only been weakly squeezed by ∼ 2 dB [10].In this Letter, we demonstrate for the first time nearunitary optical spin squeezing, as well as the first substantial squeezing in an OLC atom. The observed metro-logical gain of up to 6.5(4) dB is limited by the state detection, while subtraction of the independently determined measurement noise implies that the generated SSSs offer 12.9(6) dB of metrological gain and 15.9(6) dB of spin noise suppression. Under conditions where the squeezing is unitary within 30%, and nearly optimal for clock applications, we demonstrate an interferometer with a factor of 3.7(2) reduction in averaging time over the SQL. In the future, the demonstrated squeezing between the two nuclear sublevels m = ± 1 2 of the electronic ground state 1 S 0 of 171 Yb can be directly used in the OL...