b'Henderson Byte: Quantum gravity surveysthe state of playA quantum gravity survey is soon to be conducted in Australia, more on that to follow. Firstly, quantum gravity is still only a theory. The main problem is that gravity is a very weak force, and its quantum effects are therefore going to be minuscule.Physicists are still debating the theory in meetings such as the Quantum Gravity Conference in Vancouver in August 2022. Seemore at en.wikipedia.org/wiki/Quantum_gravity.Regardless of this problem of physics, a practical method of gravity surveying has recently been developed, making use of the concept first hypothesised by Louis DeBroglie in 1924, that all matter has wave properties. In general, the method starts with a cloud of atoms in free fall in a vacuum chamber. The atoms are cooled to near absolute zero temperature by using lasers to slow their activity. At this temperature, the wave nature of atoms becomes apparent, and a further set of laser beams split and recombine the atom cloud into different paths like that of an optical interferometer. The interference pattern of these special waves, called matter waves, is contingent on the value of gravity. As the cloud continues to fall, the atoms quantum states accumulate a phase difference due to the value of gravity. This phase difference is interrogated and read out with a final set of laser pulses.Quantum gravity as employed for practical purposes can thus be described succinctly as gravity sensing using matter wave interferometry. It is also known as cold atom interferometry. Its advantages over classical gravity methods include excellent sensitivity leading to deeper penetration, and faster measurements with 10 Hz repetition rate making it suitable for moving platforms. Drawbacks of the method are related to experimental complexity, including the use of lasers and vacuum chambers. Applications are the same as those for classical methods.Initially, quantum gravity laboratory studies took place at various universities and institutions such as the Universities of Birmingham and Glasgow, several French universities, Humboldt University in Berlin, and Nanjing in China. Some of these studies have created survey companies, of which M-Squared (see below) and Muquans (see below) are examples.Some of the practical surveys conducted using the method include:University of Birmingham, Sensors and Timing Hub, UK https://quantumsensors.org A field survey has been carried out resulting in a 10 m profile over a shallow tunnel on the Birmingham campus. The sensitivity was about +/- 20 E (1 E = 109 s2).M-Squared of Glasgow, UK https://www.m2lasers.com/quantum-gravimeter.html M-Squared have developed a field deployable quantum sensor using cold atom interferometry as described above. The first-generation system has a footprint of 0.7 m x 0.7 m and a precision of 1.3 x 10 7 g. M-Squared are now working on their second-generation gravimeter, which will offer an increased sensitivity and enhanced repetition rate.Department of Physics, University of California, Berkeley, USA.The development at this site is described by Wu Xuiejian and colleagues in a paper entitled Gravity surveys using a mobile atom interferometer, Science Advances, 2019, v.3 (9). It includes a description of how they obtained a sensitivity of 37 Gal/Hz for the atomic gravimeter, with stability greater than 2 Gal within half an hour. The gravimeter, including an electronic system, vacuum system and laser unit, was installed in a cart of dimensions1.0 m x 0.8 m x 1.7 m, weighing 100 kg.Muquans, from the Institut dOptique dAquitaine, Talence, France, have developed an absolute quantum gravimeter (AQG).https://www.muquans.com/product/absolute-quantum-gravimeter/ The Muquans sensor has a sensitivity of 50 Gal/Hz, a measurement frequency of 2 Hz and a weight of 25 kg. Muquans have also developed shipborne quantum gravimeters for the French Ministry of Defence (2020) and had a fixed station on Mt. Etna in 2020.Onera, the French Aerospace Laboratory Consortium, describe their method and their cold atom accelerometer, Girafe, in a paper in May 2021 entitled Airborne and marine quantum gravimetry by Yannick Bidel. https://quge.iag-aig.org/doc/60b8918b52e2f.pdf The paper illustrates various marine and airborne surveys, with comparisons with conventional surveys. For measurements of gravity gradient by satellite, three pairs of electrostatic accelerometers are used with a resolution of 100 km. Absolute measurements are also achieved on ground surveys.Atomionics. Currently based in Singapore, Atomionics use a sensor called Gravio, based on cold atom interferometry as described above, which measures gravity, acceleration and rotation and is claimed to provide high spatial resolution in the shortest time. More information on the use of Gravio including instructive videos is at https://www.atomionics.comAtomionics is the first group using cold atom interferometry known to offer its surveys in Australia and has been engaged by Bridgeport Energy of Sydney in 2022. As to the intended purpose of this survey, the following quotes are selected from an abstract of a talk by Cameron Fink, Exploration Manager of Bridgeport, to the ASEG-NSW branch in August 2022to search for subterranean structures which provide the trapping mechanism for oil and gas. A data acquisition programme will be undertaken, initially with light vehicles, but with a promise of automated drone deployment in the future. If successful results are obtained over a number of field trials, a case may be made for replacing seismic altogether.The outcome of this Atomionics survey in Australia is eagerly awaited in 2023.Roger Henderson rogah@tpg.com.au11 PREVIEW FEBRUARY 2023'