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A Quantum Boost for Atomic Sensors

Periodic Reporting for period 1 - QBAS (A Quantum Boost for Atomic Sensors)

Reporting period: 2016-08-09 to 2018-08-08

The goal of this project is to develop new precision sensing capability based on quantum technology, motivated by the desire to investigate fundamental physics as well as developing improved commercial and military technology.

Measurement is a cornerstone of the natural sciences, with increased measurement precision inevitably leading to the observation of previously unknown phenomena. Currently, atom interferometers provide physicists with a tool for making extremely sensitive measurements. However, the continued endeavour to increase the sensitivity of these devices is currently at a road-block: the limits set by quantum noise. The aim of this project is to use this fundamentally new approach to design practical sensing devices with unprecedented sensitivity.

Atom interferometry provides an important capability that other inertial sensing technologies lack, which is high-precision at low frequencies due to the lack of any long-term systematic drifts in the device. In terrestrial gravitational field and gradient detection, this can provide much higher sensitivities, allowing for new capabilities such as detection of underground structures and hydrology. Atom interferometry will also play an important role in inertial navigation (which is a crucial military capability in GPS denied environments). Here, the atomic sensor would supplement existing (usually high-bandwidth) mechanical sensing technology, by providing periodic re-calibration to correct for long-term drifts, which are currently a limiting factor. This project will apply quantum metrology to improve the sensitivity and bandwidth of these sensors.

There has recently been much interest in the use of quantum entanglement, or quantum squeezing, to enhance the sensitivity of atom interferometry, with recent proof of principle demonstrations in small ‘toy’ interferometers. While these experiments are of fundamental interest and may one day be useful as magnetometers, they lack any spatial path separation so are fundamentally incapable of measuring inertial quantities such as rotations, accelerations, or gravitational fields. It is not simply that these devices need to be refined until they are comparable to the current state-of-the-art. As the method of generating entanglement was based on atom-atom interactions, these devices will never be capable of out-performing the current state of the art precision atomic inertial sensors. The reasons for this are as follows:

- Strong atom-atom interactions are incompatible with the long interrogation times required for inertial sensing. This is because the same interactions that cause the quantum entanglement will inevitably cause an effect known as phase diffusion, which will drastically degrade the sensitivity.

- Increasing the number of particles in a strongly interacting system introduces increasingly complicated multi-mode dynamics, which will inhibit the visibility of the interference fringes.

- Strong interactions inevitably lead to an atomic sample with a broad momentum distribution, which is incompatible with the finely tuned large momentum transfer atomic beam-splitters required for ultra-precise atomic inertial sensors.

The current state of play in precision inertial sensing with atom interferometry is analogous to the situation where a new, much more efficient engine has been developed, but this engine can only be used in a child’s toy car, with no possibility of scaling the technology up to a useful transport solution. This project will design a method of utilising quantum entanglement compatible with current state-of-the-atomic sensors, which will pave the way for a measurement device with unprecedented sensitivity. This project will design a method of utilizing quantum entanglement compatible with current state-of-the-atomic sensors, delivering a measurement device with unprecedented sensitivity. This will be achieved by breaking away from the convention of purely atomic-based atom interferometers, and considering atomic-photonic hybrid devices.
This project has a number of significant results based on the theoretical investigation of quantum enhanced atom-interferoemeters.

Firstly, in collaboration with the quantum sensors group at the Australian National University (ANU) in Canberra, Australia, we have investigated how to generate quantum entanglement between atoms and photons in a way that is directly compatible with providing quantum enhancement to high-performance atomic gravimeter located at (ANU). In particular, PhD student Michail Kirtsotakis has investigated how to generate atom-light entanglement via a Quantum Non-Demolition (QND) interaction, in a way thats compatible with a free-space gravimeter. This work will be ready for publication by the end of the year.

We have also investigated how to increase the robustness of atom interferometers to the detrimental effects of additional detection noise (see publications 1,2, 4 and 5). Previously, it was found that detection noise would severly limit our ability to utilise quantum enhanced states. Here, we developed techniques that will allow quantum enhanced measurements with very noise detectors, which will drastically increase the performance of quantum enhanced sensors, including atom interferometers.

We have also investigated the use of atomic bright-solitons for rotation sensing (see publication 6). Contrary to previous studies, we found that the use of bright solitons introduces an unavoidable source of noise, and that there is no advantage to working in the soliton regime.

Finally, we investigated the fundamental limits to matter-wave gravimetry (see publication 3). We found that current gravimeters based on atom interferometer are not optimally extracting the sensitivity, and that improvements can be made by simply modifying the detection scheme.
This project made significant contributions to our understanding of matterwave gravimetry. In particular, the fundamental limits to the sensitivity of these devices. It also significantly enhanced our understanding to the effects of detection noise in quantum enhanced systems, and what can be done to combat these effects. In particular, a fundamental limit to sensitivity in the presence of detection noise was derived.

We also made significant progress towards understanding how to use atom-light entanglement to enhance the sensitivity of an atomic gravimeter in a way that is compatible with state of the art atom interferometery.

The outcomes from this project will potentially lead to new developments in quantum sensing which will lead to new technologies that will have a long term effect on the economy of the European Union. In particular, ultra-precise gravimeters based on atomic sensors will find application in geo-physics, and the mining industry through their potential to detect ore-bodies and oil and gas deposits, as well as providing environmental scientists with a new tool for hydrological mapping. These new tools and techniques will lead to commercialisation opportunities, and boost the economic productivity of the UK.