CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Section 371 National Stage Application of International Application No. PCT/GB2018/053630, filed Dec. 14, 2018, which is incorporated by reference in its entirety and published as WO 2019/116047 A1 on Jun. 20, 2019, in English.

FIELD OF THE INVENTION

The invention relates to a gravity gradiometer comprising a pair of magneto-optical traps for measuring a gravity gradient.

BACKGROUND

Gravity gradiometry involves measuring very small differences in the gravitational field, and can be used to determine variations in the density of subsurface materials, underground or undersea. Such measurements can be used to detect and characterise underground formations such as oil or gas reservoirs. For such measurements to be useful, however, the accuracy of the measurements needs to be extremely high, and any sources of interference or error need to be minimised as far as possible. Current technology allows very accurate measurements to be taken, but these measurements can be very slow due to the need to minimise sources of interference.

One known technique for making measurements of gravity gradients involves the use of a pair of cold atom traps, for example as described in WO 2014/106811 A2, in which a pair of magneto-optical atom traps are used to determine local measures of acceleration (being equivalent to gravity) at two points separated by a set distance. In this arrangement, a laser beam is directed to first and second spatially separated reflectors that generate a plurality of trapping laser beams directed towards first and second samples of cold atoms. A central hole is arranged within the first reflector that allows a portion of the laser beam to transmit through the first reflector and towards the second reflector. Local acceleration measurements are made by first switching off the laser beam and gradient coils, allowing the cold atom samples to undergo free-fall.

The laser beam is then changed to generate a Raman pulse sequence that creates two spatially separated cold atom interferometers. The samples are then interrogated to measure the fraction of atoms from each sample that are in one or more ground state sublevels, allowing a local acceleration experienced by each cold atom cloud to be computed. In common with other approaches, this arrangement has the advantage of using a common laser beam, which allows for a high common mode rejection, but has stringent optical requirements and requires a high vacuum common to both traps. Other problems associated with this type of arrangement relate to diffraction, alignment and beam quality of the laser beam between the first and second reflectors, which can limit the practical separation distance between the cold atom clouds, and optical shadowing in which the cooling beams are attenuated when passing through both clouds.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention there is provided a cold atom gravity gradiometer system comprising:first and second magneto-optical traps, each having a plurality of mirrored surfaces arranged to reflect an incident laser beam to trap respective first and second cold atom clouds separated from each other by a separation distance; andan optical subsystem arranged to transmit a first laser beam in a first direction along a first longitudinal axis towards the first magneto-optical trap and a second laser beam in an opposite second direction along a second longitudinal axis towards the second magneto-optical trap, the second longitudinal axis being parallel to the first longitudinal axis.

The first laser beam may comprise a first cooling laser beam and a first atom interferometry beam, while the second laser beam comprises a second cooling laser beam and a second atom interferometry beam. The atom interferometry beams may for example be beams for stimulating Raman or Bragg transitions in the cold atom clouds, and may therefore be alternatively termed Raman or Bragg beams. References herein to Raman beams may therefore be applied more generally to atom interferometry beams.

In some examples the first and second atom interferometry beams may be substantially coaxial with the respective first and second cooling laser beams.

The system may further comprise first and second intermediate reflection surfaces arranged to reflect the respective first and second laser beams back towards the respective first and second cold atom clouds.

Such a gradiometer enables cold atom gravity gradiometry over long and adjustable baselines in a more compact radial profile than in previous approaches, and also addresses several of the beam-related issues which may arise in previous approaches. The beam arrangement of the present gradiometer has been shown to enable gradiometry measurement with beam delivery through a single port, enabling a reduced number of windows and complexity for the physics package (in particular, having ramifications in reducing the cost, weight and size of the vacuum system and magnetic shielding). Furthermore, the first and second magneto-optical traps may be housed either in a single chamber, or in two separate chambers, enabling an adjustable baseline and the measurement of higher order terms through creating “kinks” in the measurement axis, as described in more detail below.

Each of the first and second magneto-optical trap, and associated cooling, Raman and detection systems, may be referred to as a gravimeter—i.e. the gradiometer comprises two separated gravimeters.

The use of the first and second intermediate reflection surfaces in particular separates the dependencies of the cooling and interferometry beams, and has the advantage of allowing realisation of gravity gradient measurements in vibration environments, and facilitates longer term stability in gravity measurements when using the same system.

The optical subsystem may be arranged to transmit a cooling laser beam and one or more Raman laser beams in a first direction along a first longitudinal axis towards the first magneto-optical trap and in an opposite second direction along a second longitudinal axis towards the second magneto-optical trap. The one or more Raman laser beams may comprise Raman light of two different wavelengths. The gradiometer system may further comprise a detection system for detecting acceleration of atoms trapped in the first and second magneto-optical traps.

In some embodiments, each of the first and second magneto-optical traps may comprise four mirrored surfaces angled with respect to the respective first and second intermediate reflection surfaces and arranged around the respective first and second longitudinal axes to reflect the respective first and second incident laser beams towards the respective first and second cold atom clouds. The mirrored surfaces may be mirrored surfaces of one or more prisms. The mirrored surfaces provide the multi-directional laser beams necessary to cause trapping of atoms, in a compact arrangement.

In some embodiments, the first longitudinal axis may be substantially coincident with the second longitudinal axis. This arrangement may particularly be used if only the z-component of the gravity gradient is to be measured, and provides a compact gradiometer.

In some embodiments, the first and second intermediate reflection surfaces may be arranged on the respective first and second longitudinal axes.

In some embodiments, the second longitudinal axis may be laterally offset from the first longitudinal axis. Such a “kinked” arrangement may allow additional components of the gravity gradient to be determined, whilst maintaining an effectively single optical axis arrangement.

In some embodiments, the first and second intermediate reflection surfaces may be arranged on a lateral axis between the first longitudinal axis and the second longitudinal axis. The system may further comprise first and second deflection surfaces to deflect the laser beam onto the lateral axis from the first longitudinal axis and second longitudinal axis respectively.

In some embodiments, the optical subsystem may be further arranged to transmit a Raman beam along an atom interferometry axis towards the first and second magneto-optical traps, and wherein the angle between the atom interferometry axis and the first longitudinal axis is greater than 0°. Such an arrangement allows greater flexibility in the implementation of the optical subsystem.

In some such embodiments, the optical subsystem may be arranged to direct a first Raman beam towards the first magneto-optical trap in a first direction along the atom interferometry axis, and to direct a second Raman beam towards the second magneto-optical trap in an opposite, second direction along the atom interferometry axis.

In some such embodiments, the system may further comprise a Raman mirror positioned along the atom interferometry axis after the first and second magneto-optical traps, and arranged to reflect the Raman beam back along with atom interferometry axis towards the second and first magneto-optical traps.

In other embodiments, a Raman beam may be transmitted as part of the laser beam, or along a co-incident axis to the laser beam, to determine the gravity gradient, as would be understood by the person skilled in the art.

In some embodiments, the first and second intermediate reflection surfaces may be arranged to reflect the incoming laser beam away from a central region between the first and second magneto-optical traps. Centering the intermediate reflection surfaces provides improved noise cancellation between the two gravimeters.

In some embodiments, the first and second intermediate reflection surfaces form opposing first and second surfaces of an intermediate reflector.

According to a second aspect of the invention there is provided a cold atom gravity gradiometer system comprising:first and second magneto-optical traps, each having a plurality of mirrored surfaces arranged to reflect an incident laser beam to trap respective first and second cold atom clouds separated from each other by a separation distance; andan optical subsystem arranged to transmit a first laser beam in a first direction along a first longitudinal axis towards the first and second magneto-optical traps and a second laser beam in an opposite second direction along a second longitudinal axis towards the second and first magneto-optical traps,wherein the first magneto-optical trap is arranged on the first longitudinal axis and the second magneto-optical trap is arranged on the second longitudinal axis, and wherein the second longitudinal axis is parallel to and laterally offset from the first longitudinal axis.

The gravimeter according to the second aspect is thus similar to the “kinked” embodiments of the gradiometer according to the first aspect, and provides the same advantage of allowing measurement of additional components of the gravity gradient. In contrast to the gradiometer of the first aspect, the gradiometer of the second aspect does not comprise intermediate reflection surfaces arranged to reflect the laser beam back towards the respective first and second cold atom clouds. Cooling and Raman laser beams instead travel from one gravimeter of the gradiometer to the other gravimeter of the gradiometer. For example, a single cooling laser beam may be used, incident on the first magneto-optical trap, and the residual cooling beam may then be incident upon the second magneto-optical trap, forming the sixth cooling beam of the second magneto-optical trap. Raman beams may be transmitted from the first to the second magneto-optical trap, or a first Raman beam may be transmitted from the first to the second magneto-optical trap and a second Raman beam may be transmitted from the second to the first magneto-optical trap. Such gradiometers may yield reduced tilt noise due to enhanced common suppression of noise, and linked spatial dependencies on the Raman or interferometry beams.

In some embodiments the system may comprise a first deflection surface arranged to deflect the first laser beam from the first longitudinal axis onto a lateral axis between the first and second longitudinal axes, and to deflect the second laser beam from the lateral axis onto the first longitudinal axis; and a second deflection surface arranged to deflect the second laser beam from the second longitudinal axis onto the lateral axis, and to deflect the first laser beam from the lateral axis onto the second longitudinal axis.

In some embodiments of either aspect, the system may be rotatable around at least one rotation axis. For example, a rotation axis may pass through a central point between the first magneto-optical trap and the second magneto-optical trap. A rotation axis may be parallel to the first and second longitudinal axes. A rotation axis may be orthogonal to the first and second longitudinal axes. A rotation axis may be parallel to the lateral axis, and may be coincident with the lateral axis. A rotation axis may be orthogonal to both the lateral axis and the first and second longitudinal axes. The rotatable system allows the full gravity gradiometer tensor to be determined, with only two gravimeters.

In some embodiments of either aspect, the first and second magneto-optical traps may be disposed within respective first and second vacuum chambers. The first and second vacuum chambers may be connected to a vacuum system disposed at least partially between the first and second vacuum chambers. The first and second vacuum chambers may be isolated from each other, and so may be independent vacuum chambers, which may be simpler to manufacture.

In some embodiments of either aspect, the separation distance may be adjustable. In this way, the baseline of the gradiometer system may be adjusted to the match the needs of a particular experiment, whilst retaining a compact radial profile.

According to a third aspect of the invention there is provided a method of determining a gravity gradient using a gravity gradiometer system according to any embodiment of the first or second aspect, wherein the first longitudinal axis is offset from the second longitudinal axis, the method comprising:measuring a first gravity difference between the first magneto-optical trap and second magneto-optical trap of the gradiometer system with the gradiometer system in a first position;rotating the gradiometer system by substantially 180° about a rotation axis from the first position to a second position, the rotation axis being parallel to the first longitudinal axis and second longitudinal axis;measuring a second gravity difference between the first magneto-optical trap and second magneto-optical trap of the gradiometer system with the gradiometer system in the second position;determining a first component of the gravity gradient from the sum of the first gravity difference and the second gravity difference; anddetermining a second component of the gravity gradient from the difference between the first gravity difference and the second gravity difference.

This method allows multiple components of the gravity gradient to be determined from just two gravimeters.

In some embodiments, the method may further comprise:rotating the gradiometer system about an additional rotation axis to a third position, the additional rotation axis being orthogonal to the first longitudinal axis and second longitudinal axis; andmeasuring an additional gravity difference between the first magneto-optical trap and second magneto-optical trap of the gradiometer system with the gradiometer system in the third position.

In particular, rotating the gradiometer system about the additional rotation axis may comprise rotating the gradiometer system by a tilting angle about the additional rotation axis. The tilting angle may be less than 90°, or may be less than 30°, or may be less than 10°. The tilting angle may be determined by the maximum beam width of a Raman beam incident upon the first and/or second magneto-optical trap.

The additional rotation may allow more, or all, of the components of the gravity gradient to be determined.

