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Determination of the gravitational constant G using laser-cooled atoms and quantum interferometry, a technique that gives new insight into the systematic errors that have proved elusive in previous experiments, yields a value that has a relative uncertainty of 150 parts per million and which differs from the current recommended value by 1.5 combined standard deviations. A new route to big G The Newtonian gravitational constant G , also known as the universal gravitational constant or 'big G ', is a fundamental physical constant that is used in the calculation of gravitational attraction between two bodies. There are several ways to measure G with high precision, but these measurements disagree, presumably because of the intervention of unknown errors in the different experiments. With the aim of identifying and ultimately removing the systematic errors that give rise to these discrepancies, Gabriele Rosi and colleagues have carried out a high-precision measurement of G using quantum interferometry with laser-cooled atoms, an experimental approach that differs radically from previous determinations. The authors obtain a value for G with a precision of ∼0.015% — approaching that of the traditional measurements, and with prospects for considerable further improvement. Although this result doesn't yet solve the problem of the discrepant measurements, the use of such a radically different technique holds promise for identifying the systematic errors that have plagued previous determinations. About 300 experiments have tried to determine the value of the Newtonian gravitational constant, G , so far, but large discrepancies in the results have made it impossible to know its value precisely 1 . The weakness of the gravitational interaction and the impossibility of shielding the effects of gravity make it very difficult to measure G while keeping systematic effects under control. Most previous experiments performed were based on the torsion pendulum or torsion balance scheme as in the experiment by Cavendish 2 in 1798, and in all cases macroscopic masses were used. Here we report the precise determination of G using laser-cooled atoms and quantum interferometry. We obtain the value G = 6.67191(99) × 10 −11  m 3  kg −1  s −2 with a relative uncertainty of 150 parts per million (the combined standard uncertainty is given in parentheses). Our value differs by 1.5 combined standard deviations from the current recommended value of the Committee on Data for Science and Technology 3 . A conceptually different experiment such as ours helps to identify the systematic errors that have proved elusive in previous experiments, thus improving the confidence in the value of G . There is no definitive relationship between G and the other fundamental constants, and there is no theoretical prediction for its value, against which to test experimental results. Improving the precision with which we know G has not only a pure metrological interest, but is also important because of the key role that G has in theories of gravitation, cosmology, particle physics and astrophysics and in geophysical models.

The Einstein equivalence principle (EEP) has a central role in the understanding of gravity and space–time. In its weak form, or weak equivalence principle (WEP), it directly implies equivalence between inertial and gravitational mass. Verifying this principle in a regime where the relevant properties of the test body must be described by quantum theory has profound implications. Here we report on a novel WEP test for atoms: a Bragg atom interferometer in a gravity gradiometer configuration compares the free fall of rubidium atoms prepared in two hyperfine states and in their coherent superposition. The use of the superposition state allows testing genuine quantum aspects of EEP with no classical analogue, which have remained completely unexplored so far. In addition, we measure the Eötvös ratio of atoms in two hyperfine levels with relative uncertainty in the low 10 −9 , improving previous results by almost two orders of magnitude. Atom interferometers can be used in precision measurements and to test the fundamental laws of physics. Here the authors test the weak equivalence principle in the quantum regime with high sensitivity using a Bragg atom interferometer with rubidium atoms in their hyperfine states.

How far can you stretch an atomic wavefunction? An experiment demonstrates that the wavefunction of an ensemble of ultracold atoms trapped in an optical lattice can be reversibly expanded and shrunk over a distance of 1.5 mm. The manipulation of matter waves had an important role in the history of quantum mechanics. The first experimental validation of matter-wave behaviour was the observation of diffraction of matter by crystals 1 , followed by interference experiments with electrons, neutrons, atoms and molecules using gratings and Young’s double slit 2 , 3 , 4 , 5 . More recently, matter-wave manipulation has become a building block for quantum devices such as quantum sensors 6 and it has an essential role in a number of proposals for implementing quantum computers 7 , 8 . Here, we demonstrate the coherent control of the spatial extent of an atomic wavefunction by reversibly stretching and shrinking the wavefunction over a distance of more than one millimetre. The quantum-coherent process is fully deterministic, reversible and in quantitative agreement with an analytical model. The simplicity of its experimental implementation could ease applications in the field of quantum transport and quantum processing.

We present a gradiometer based on matter-wave interference of alkaline-earth-metal atoms, namely 88Sr. The coherent manipulation of the atomic external degrees of freedom is obtained by large-momentum-transfer Bragg diffraction, driven by laser fields detuned away from the narrow 1S0-3P1 intercombination transition. We use a well-controlled artificial gradient, realized by changing the relative frequencies of the Bragg pulses during the interferometer sequence, in order to characterize the sensitivity of the gradiometer. The sensitivity reaches 1.5 × 10−5 s−2 for an interferometer time of 20 ms, limited only by geometrical constraints. We observed extremely low sensitivity of the gradiometric phase to magnetic field gradients, approaching a value 104 times lower than the sensitivity of alkali-atom based gradiometers, limited by the interferometer sensitivity. An efficient double-launch technique employing accelerated red vertical lattices from a single magneto-optical trap cloud is also demonstrated. These results highlight strontium as an ideal candidate for precision measurements of gravity gradients, with potential application in future precision tests of fundamental physics.

We have recently completed a measurement of the Newtonian constant of gravitation G using atomic interferometry. Our result is G=6.67191(77)(62)×10−11 m3 kg−1 s−2 where the numbers in parenthesis are the type A and type B standard uncertainties, respectively. An evaluation of the measurement uncertainty is presented and the perspectives for improvement are discussed. Our result is approaching the precision of experiments based on macroscopic sensing masses showing that the next generation of atomic gradiometers could reach a total relative uncertainty in the 10 parts per million range.

This paper presents the current status and future prospects of the Space Atom Interferometer project (SAI), funded by the European Space Agency. Atom interferometry provides extremely sensitive and accurate tools for the measurement of inertial forces. Operation of atom interferometers in microgravity is expected to enhance the performance of such sensors. Main goal of SAI is to demonstrate the possibility of placing atom interferometers in space. The resulting drop-tower compatible atom interferometry acceleration sensor prototype is described. Expected performance limits and potential scientific applications in a micro-gravity environment are also discussed.

Since atom interferometers were first realized about 20 years ago, atom interferometry has had many applications in basic and applied science, and has been used to measure gravity acceleration, rotations and fundamental physical quantities with unprecedented precision.

This paper reports the results from the accurate measurement of the acceleration of gravity g taken at two separate premises in the Polo Scientifico of the Florence University (Italy). In these laboratories, two separate experiments aiming at measuring the Newtonian constant and testing the measurement of forces with high spatial resolution are in progress. Both experiments require an independent knowledge on the local value of g . Gravity measurements were conducted using an FG5 absolute gravimeter, and accompanied by seismic recordings for evaluating the noise condition at the site. The absolute accelerations of gravity at the two laboratories are ( 980 492 160.6 ± 4.0 ) μ Gal and ( 980 492 048.3 ± 3.0 ) μ Gal for the European Laboratory for Non-Linear Spectroscopy (LENS) and Dipartimento di Fisica e Astronomia, respectively. Other than for the two referenced experiments, the data here presented will serve as a benchmark for any future study requiring an accurate knowledge of the absolute value of the acceleration of gravity in the study region.