Periodic Reporting for period 3 - MEGANTE (MEasuring the Gravitational constant with Atom interferometry for Novel fundamental physics TEst)
Reporting period: 2022-02-01 to 2023-07-31
Starting from the original experiment performed by Henry Cavendish more than two centuries ago, the precision determination of the gravitational constant G remains a challenging endeavor. It has been measured about a dozen times over the last 50 years, but the results have varied much more than what would be expected from random and systematic errors. Likely, this is due to the fact that, so far, all the past experiments have relied on macroscopic classical instruments, which could all be governed by uncontrolled mechanical influences. On the other hand, a recent controversial study about correlations between the measured values of G and the variations of the length of day seems to suggest that some other not well-understood effects could be present.
The main aim of MEGANTE is to realize independent precision measurements of the Newtonian gravitational constant G using atom optics techniques at the state-of-art. The target relative accuracy is 10 ppm or below, striving to approach the ppm level, thus surpassing the state-of-art measurements based on torsion balance and simple pendulum. In addition, due to the different physical regime these determinations will be realized, high precision tests of gravity will be performed, in particular:
1. Verification of the Newton’s law of gravity at short distance (~10 cm).
2. Precision test of the gravitational red shift.
3. Search for dark energy signature due to Chameleon scalar fields.
The main aim of MEGANTE is to realize independent precision measurements of the Newtonian gravitational constant G using atom optics techniques at the state-of-art. The target relative accuracy is 10 ppm or below, striving to approach the ppm level, thus surpassing the state-of-art measurements based on torsion balance and simple pendulum. In addition, due to the different physical regime these determinations will be realized, high precision tests of gravity will be performed, in particular:
1. Verification of the Newton’s law of gravity at short distance (~10 cm).
2. Precision test of the gravitational red shift.
3. Search for dark energy signature due to Chameleon scalar fields.
In the first 30 months of activity we have been designing the experimental apparatus, which will consist of 4-5 m-tall atomic fountain based on cold atoms (Rubidium gas) in free-fall surrounded by a heavy source mass. The atomic probe will be employed to perform high-precision measurement of gravity and gravity gradients via atom interferometry while the source mass will provide the gravitation field to study.
In particular, the design of the source mass has been optimized by means of a dedicated computer simulation of the experiment, which will then be employed to extract the value of G from the experimental data. Furthermore, a trade-off study on several source mass materials has been carried out in order to identify the best solution in terms of signal-to-noise ratio and systematics effects on the G measurement.
At the same time, new concepts to enhance the precision of gravity and gravity gradients measurements in atom interferometers have been studied. More specifically, a novel high-power laser system to efficiently manipulate the Rubidium atoms in free-fall has been studied and realized and is currently being tested.
In particular, the design of the source mass has been optimized by means of a dedicated computer simulation of the experiment, which will then be employed to extract the value of G from the experimental data. Furthermore, a trade-off study on several source mass materials has been carried out in order to identify the best solution in terms of signal-to-noise ratio and systematics effects on the G measurement.
At the same time, new concepts to enhance the precision of gravity and gravity gradients measurements in atom interferometers have been studied. More specifically, a novel high-power laser system to efficiently manipulate the Rubidium atoms in free-fall has been studied and realized and is currently being tested.
Matter-wave atom interferometers are currently well established devices for extremely sensitive and accurate measurement of gravity and gravity gradients. The effectiveness of this method is based on a clever mechanism that allows atoms with different initial positions and velocities to acquire the same interferometric phase in presence of uniform forces. In general such condition does not hold in a G determination where the gravitational force produced by the source mass and probed by the atoms is not uniform over the interferometric region. Therefore, a deep characterization of the atomic sample size, trajectory and temperature is required, placing a limit on the final accuracy of the measurement in the 100 ppm range. To improve the accuracy by one order of magnitude and therefore be competitive with state-of-the-art measurements performed with classical methods, the clouds needs to be enclosed in a very small value during the ballistic flight. In principle, such condition can be achieved using an ultra-cold atomic source. However, such approach presents a major drawback: producing these atomic samples with a large atoms number is technically challenging and often the achieved atomic flux is much lower than can be obtained using simpler and standard methods. Less atoms implies larger shot noise at the interferometer output which in turn is detrimental for the G determination.
MEGANTE will get to the root of the problem making use of a completely different experimental strategy, based on a gravity-gradient compensation scheme. More specifically, the source mass field can be constructed in a way that the acceleration sensed by atoms is varying linearly with height. This constant gravity gradient can be
precisely compensated by properly changing the absolute frequency of the laser employed to realize the interferometer sequence. The obtained compensation point is almost independent by the spatial features of the atomic samples, thus genuinely relaxing the geometric constrains. In this way a determination of G at 10 ppm level is at hand and it looks feasible. Towards this final result, the project has defined a suitable source mass configuration (geometry and material employed) together with an optimized experimental sequence. Next goal will be to obtain the first interferometric signals in the new devoted infrastructure, performing also some preliminary G determinations.
MEGANTE will get to the root of the problem making use of a completely different experimental strategy, based on a gravity-gradient compensation scheme. More specifically, the source mass field can be constructed in a way that the acceleration sensed by atoms is varying linearly with height. This constant gravity gradient can be
precisely compensated by properly changing the absolute frequency of the laser employed to realize the interferometer sequence. The obtained compensation point is almost independent by the spatial features of the atomic samples, thus genuinely relaxing the geometric constrains. In this way a determination of G at 10 ppm level is at hand and it looks feasible. Towards this final result, the project has defined a suitable source mass configuration (geometry and material employed) together with an optimized experimental sequence. Next goal will be to obtain the first interferometric signals in the new devoted infrastructure, performing also some preliminary G determinations.