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Quantum-enhanced Sensing via Quantum Control

Periodic Reporting for period 1 - QuSCo (Quantum-enhanced Sensing via Quantum Control)

Reporting period: 2017-11-01 to 2019-10-31

Quantum technologies 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 or structure determination in biology or the controlled construction of novel quantum materials. Quantum control manipulates dynamical processes at the atomic or molecular scale employing specially tailored external electromagnetic fields.
The purpose of QuSCo is to demonstrate the enabling capability of quantum control for quantum sensing and quantum measurement, advancing this field by systematic use of quantum control methods. QuSCo will establish quantum control as a vital part for progress in quantum technologies.
QuSCo will expose 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 quantum technologies can offer. Training in scientific skills is based on the demonstrated tradition of excellence in research of the consortium. It will be complemented by training in communication and commercialisation. The latter builds on active industry participation whereas the former on existing expertise on visualisation and gamification and combines it with more traditional means of outreach to realise target audience specific public engagement strategies
In WP1, analytical as well as numerical results have been obtained, in relation with experiments, and resulting from collaborations within the consortium. Highlights include findings on the controllability of rotating molecules with applications to the sensing of chiral molecules, and the improvement of the open- and closed-loop optimisation suite “RedCRAB” with applications to the generation of a 20-qubit entangled state of Rydberg atoms.

In WP2 Open-loop control was used in magnetic resonance to overcome transients in quantum-limited EPR spectroscopy of donors in silicon at millikelvin temperatures. Quantum optimal control has been used to devise RF and microwave pulse sequences aiming at preparing Rydberg atoms in a non-classical superposition of states that would be useful for quantum sensing. Quantum sensing with an ensemble of NV centers in diamond is a central topic within QuSCO. Quantum optimal control has been applied to design pulses that overcome the spatial inhomogeneity of the microwave field applied to the spins. These pulses have successfully shown to extend the field of view of micrometer scale magnetic resonance imaging of nuclear spins deposited on diamond. Noise spectroscopy has been performed on dense ensembles of NV centers, which is necessary to design optimal control strategies to fight this noise. Optimal control has also been applied to target hyperpolarisation of large ensembles of carbon nuclear spins in diamond using the optical polarisation of the NV centers. Finally, the first steps have been taken to use the nuclear spin degree of freedom of the nitrogen atom inherent to the NV center to enhance the sensitivity by quantum logic spectroscopy.

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. The work is in progress towards the concrete implementation of repeated weak measurements. Efforts were made to develop optimal control sequences to prepare many-body systems in specific states.
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. The technique has been applied to single-qubit gates to reach the domain of maximally fast gates. Additionally, we have been developing a software package which 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 we have successfully organised the “QuSCo First School”, where we offered intensive training on all the subjects touched by our project. Furthermore, we have provided the opportunity to the student to organise a workshop, which took place in September 2019 in Padua, where the students invited the speakers that interested them the most. To keep the network-wide training going between the events, we have also implemented the “QuSCo on-air” webinars. During the Workshop in Padua and one of the QuSCo on-air session, the students have received training for the preparation of project proposals

Concerning the WP 5 outreach effort, our virtual presence is foremost represented by our website, which we are currently re-organizing to give more prominence to the students’ work, our twitter account and our youtube channel. In real life, our students have started carrying out outreach activities such as participating at “researchers’ night”, lectures for high-schoolers, and similar. Finally, the games “Quantum Moves 2” and the App “Spindrop” are both available to the broader public.
The emergence of quantum technologies from the firm foundation of quantum physics poses the challenge of bridging the labs to the industries. QuSCo seeks to demonstrate that successful implementation of quantum optimal control into these endeavours is a viable route to the full potential of quantum technologies, establishing quantum optimal control as a practical tool for quantum technologies. This entails both the implementation of state-of-the-art algorithms into novel platforms as well as a significant extension of the state-of-the-art within quantum optimal control.
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 combine these individual efforts and focus them on quantum
sensing. The advanced stage of our physical platforms, together with QuSCo’s theory backbone holds the tangible promise for demonstrating unprecedented quantum-enhanced sensitivity.
We will achieve the necessary progress beyond the current state of the art by a systematic approach, 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 will cross traditional boundaries, such as those between solid-state and atomic
physics, physics and control engineering, as well as theory and experiment.