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Quantum-enhanced Sensing via Quantum Control

Periodic Reporting for period 2 - QuSCo (Quantum-enhanced Sensing via Quantum Control)

Reporting period: 2019-11-01 to 2021-10-31

Quantum technologies (QT) aim to exploit quantum coherence and entanglement, the two essential elements of quantum physics. Successful implementation of quantum technologies faces the challenge to preserve the relevant nonclassical features at the level of device operation. It is thus deeply linked to the ability to control open quantum systems.
The currently closest to market quantum technologies are quantum communication and quantum sensing. The latter holds the promise of reaching unprecedented sensitivity, with the potential to revolutionise medical imaging and, for instance, structure determination in biology. Quantum control manipulates dynamical processes at the atomic or molecular scale employing specially tailored external electromagnetic fields. The purpose of QuSCo was to demonstrate the enabling capability of quantum control for quantum sensing and quantum measurement, advancing this field by a systematic use of quantum control methods. The work done within QuSCo shows that quantum control is vital for the progress of quantum technologies. We are also proud of our achievements concerning the educational aspects which can serve as a blueprint for PhD education in QT. QuSCo has exposed its students, at the same time, to fundamental questions of quantum mechanics and practical issues of specific applications. Albeit challenging, this reflects our view of the best possible training that the field of QT can offer. Training in scientific skills was based on the demonstrated tradition of excellence in research of the consortium and complemented by training in communication and commercialisation. The latter was built on active industry participation whereas the former on existing expertise on visualisation and gamification combined with more traditional means of outreach to realise target audience specific public engagement strategies. Our training has been very positively received by the students, as evident from the results of several surveys. We have summarized our conclusions in terms of best practice recommendations for PhD education in qT which will be submitted for publication as an opinion piece in a major peer-reviewed journal.
In conclusion, we believe that QuSCo represents an important milestone in both the development of quantum optimal control and in the creation of a cohesive plan for education in quantum technologies.
in WP 1 the partners successfully developed new theoretical tools for quantum optimal control (QOC) and improved upon existing ones. The results here obtained were also applied to the other, more experiment-drive. WP Highlights include findings on the controllability of rotating molecules with applications to the sensing of chiral molecules, and the improvement of the optimisation suite “RedCRAB” with applications to the generation of a 20-qubit entangled state of Rydberg atoms.
among the exploitable results, The work by Diaz et all (PRA 102 -2020) is relevant for the superiority of the quantum speed limit (QSL) and is a step forward in the understanding of the link between QSLs and optimal control.

WP2 explored how individual ensembles of quantum systems can be used to measure a classical quantity, and how quantum state engineering and QOC can be used to achieve this goal more efficiently. Open-loop control was used in magnetic resonance to overcome transients in quantum-limited EPR spectroscopy. QOC has been used to devise RF and microwave pulse sequences aiming at preparing Rydberg atoms in a non-classical superposition of states. QOC has been applied to design pulses that overcome the spatial inhomogeneity of the microwave field applied to the spins. These pulses extended the field of view of micrometer scale magnetic resonance imaging of nuclear spins deposited on diamond. Moreover, we improved the sensitivity of nuclear magnetic resonance imaging (MRI) down to the nanoscale using quantum sensors, observed genuine two atom interference, and used Single Microwave Photon Detector to demonstrate the first spin detection based on fluorescence
On the front of the exploitable results, The strongly hyperpolarized samples could be used to improve MRI measurements and increase the sensitivity of shallow NV centers.

in WP 3 we have been developing a quasi-particle tomography for quantum fields describing the evolution of many-body systems and a general pathway to extract the irreducible building blocks and its parameters of quantum field theoretical descriptions from experimental data.
We have developed an improved closed-loop tune-up method allowing for the simultaneous tune-up of an arbitrary set of pulse parameters to achieve high fidelity control pulses with multiple (correlated) parameters. Additionally, we have been developing a software package that will ultimately include both the calibration code as well as the simulation capabilities in an open-source software package, allowing for optimal control measurements as well as system identification.

For WP 4 all planned training events have been successfully implemented and the results of the surveys for each event analysed. On the basis of these activities, QuSCo has prepared a white paper on education in QT.

Concerning WP 5, our students have successfully developed outreach tools and tested them in specific events. QuSCo has also created 3 promotional videos (available on youtube). Finally, the games “Quantum Moves 2” and the App “Spindrop” have been published. All the results indicated have clear exploitation potential.

With 22 publications, 8 of which are in high impact journals and 12 more manuscripts currently under peer-review, QuSCo has fully achieved its scientific goals.
The emergence of quantum technologies from the firm foundation of quantum physics poses the challenge of bridging the labs to the industries. QuSCo demonstrated that the implementation of quantum optimal control into these endeavours is a viable route to the full potential of QT, representing an important milestone in establishing QOC as a practical tool for QT. in QuSCo, we implemented state-of-the-art algorithms into novel platforms as well as a significant extension of the state-of-the-art within QOC.
Over the last decade, the quantum control toolbox has been expanded significantly, tackling the issues of real-world applications such as noise and limited control. Improvements have already been demonstrated in the speed of operation and accuracy. Here, we have combined these individual efforts and focused them on quantum
sensing. The advanced stage of our physical platforms, together with our theory backbone holds the tangible promise for demonstrating unprecedented quantum-enhanced sensitivity.
Thanks to our systematic approach, we achieved the necessary progress beyond the current state of the art by investigating those quantum measurement and sensing processes that are susceptible to improvement via coherent control of experimental parameters; characterising those quantum control methods that have the highest potential for enhancing the performance of the above processes; realising such potential by applying promising quantum control methods to specific physical scenarios, and demonstrating the resulting advantage in laboratory experiments. The QuSCo project crossed the traditional boundaries, such as those between solid-state and atomic physics, physics, and control engineering, as well as theory and experiment as illustrated by the results of the project.
A truly entangled N-atom Greenberger-Horne-Zeilinger (GHZ) state is defined by a GHZ state fidelity