Gradiometer

Abstract: We describe an ultra-sensitive atomic magnetometer using optically-pumped potassium atoms operating in spin-exchange relaxation free (SERF) regime. We demonstrate magnetic field sensitivity of 160 aT/Hz$^{1/2}$ in a gradiometer arrangement with a measurement volume of 0.45 cm$^3$ and energy resolution per unit time of $44 \hbar$. As an example of a new application enabled by such a magnetometer we describe measurements of weak remnant rock magnetization as a function of temperature with a sensitivity on the order of 10$^{-10}$ emu/cm$^3$/Hz$^{1/2}$ and temperatures up to 420$^\circ$C.

Abstract: This review describes induction coil sensors, which are also known as search coils, pickup coils or magnetic loop sensors. The design methods for coils with air and ferromagnetic cores are compared and summarized. The frequency properties of coil sensors are analysed and various methods for output signal processing are presented. Special kinds of induction sensors, such as Rogowski coil, gradiometer sensors, vibrating coil sensors, tangential field sensors and needle probes are described. The applications of coil sensors as magnetic antennae are also presented.

Abstract: We report the demonstration of an atom interferometer-based gravity gradiometer. The gradiometer uses stimulated two-photon Raman transitions to measure the relative accelerations of two ensembles of laser cooled atoms. We have used this instrument to measure the gradient of the Earth's gravitational field.

Abstract: We present a new measurement of the Newtonian gravitational constant G based on cold-atom interferometry. Freely falling samples of laser-cooled rubidium atoms are used in a gravity gradiometer to probe the field generated by nearby source masses. In addition to its potential sensitivity, this method is intriguing as gravity is explored by a quantum system. We report a value of G � 6:667 � 10 � 11 m 3 kg � 1 s � 2 , estimating a statistical uncertainty of � 0:011 � 10 � 11 m 3 kg � 1 s � 2 and a systematic uncertainty of � 0:003 � 10 � 11 m 3 kg � 1 s � 2 . The long-term stability of the instrument and the signal-tonoise ratio demonstrated here open interesting perspectives for pushing the measurement accuracy below the 100 ppm level. The Newtonian constant of gravity G is one of the most measured fundamental physical constants and at the same time the least precisely known. Improving the knowledge of G has not only a pure metrological interest, but is also important for the key role that it plays in theories of gravitation, cosmology, and particle physics, in geophysical models, and in astrophysical observations. However, the extreme weakness of the gravitational force and the impossibility of shielding the effects of gravity make it difficult to measure G, while keeping systematic effects well under control. Many of the measurements performed to date are based on the traditional torsion pendulum method [1], direct derivation of the historical experiment performed by Cavendish in 1798. Recently, many groups have set up new experiments based on different concepts and with completely different systematics: a beam-balance system [2], a laser interferometry measurement of the acceleration of a freely falling test mass [3], experiments based on Fabry-Perot or microwave cavities [4,5]. However, the most precise measurements available today still show substantial discrepancies, limiting the accuracy of the 2006 CODATA recommended value for G to 1 part in 10 4 . From this point of view, the realization of conceptually different experiments can help to identify still hidden systematic effects and therefore improve the confidence in the final result. Cold-atom interferometry has demonstrated outstanding performances for the measurement of tiny rotations and accelerations, and it is widely used for many applications: precision measurements of gravity [6], gravity gradient [7], and rotation of the Earth [8,9], but also tests of Einstein’s weak equivalence principle [10], tests of Newton’s law at short distances [11], and measurement of fundamental physical constants [12,13]. Applications of these techniques for fundamental physics experiments in space are under study [14]. In this Letter, we present a new determination of the Newtonian constant of gravity based on cold-atom interferometry. An atomic gravity gradiometer is used to measure the differential acceleration experienced by two freely falling samples of laser-cooled rubidium atoms under the influence of nearby tungsten masses. The measurement is repeated in two different configurations of the source masses and modeled by a numerical simulation. From the evolution of the atomic wave packets and the distribution of the source masses, we evaluate the expected differential acceleration, having G as a unique free parameter. Avalue for Newton’s constant of gravity is finally extracted by comparing experimental data and numerical simulations. Proof-of-principle experiments with similar schemes using lead masses were already presented in [15,16]. In the present work, specific efforts have been devoted to the control of systematic effects related to atomic trajectories, positioning of source masses, and stray fields. In particular, FIG. 1 (color online). Schematic of the experiment showing the gravity gradiometer setup with the Raman beams propagating along the vertical direction. During the G measurement, the position of the source masses is alternated between configuration C1 (left) and C2 (right). PRL 100, 050801 (2008)