Studies of extreme cases within quantum mechanics have always been particularly attractive. How macroscopic can objects be and still demonstrate unique quantum features, such as entanglement? What are the real limits of measurement precision in quantum mechanics? I will review our experiments where macroscopic objects are driven deep into the quantum regime. Observation of a quantum trajectory in a quantum reference frame with, in principle, unlimited accuracy will be presented. A concept of a reference frame with an effective negative mass required for such observation will be introduced. Sensing of magnetic fields and of mechanical motion beyond standard quantum limits of sensitivity using this concept will be presented. Application of those ideas to gravitational wave detection, continuous variable quantum repeaters and e.-m. field sensing will be reported. References Hybrid quantum network for sensing in the acoustic frequency range. Novikov et al. Nature 643, 955 (2025). Entanglement between Distant Macroscopic Mechanical and Spin Systems. Thomas et al. Nature Physics 17, 228–233(2021). Overcoming the Standard Quantum Limit in Gravitational Wave Detectors Using Spin Systems with a Negative Effective Mass. Khalili and Polzik, Phys. Rev. Lett. 121, 031101 (2018).

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The seminar will provide a glimpse of some elements of the rapidly evolving field of quantum sensing, with a particular focus on applications in particle physics. Specific approaches involving quantum systems, such as low-dimensional systems or manipulations of ensembles of quantum systems, hold great promise for improving high-energy particle physics detectors, particularly in areas like calorimetry, tracking, and timing. The use of quantum sensors for high-precision measurements, such as precision spectroscopy of novel atomic, molecular or ionic systems, as well as the development of new quantum sensors based on superconducting circuits, ion and particle traps, crystals, and nanomaterials, are equally relevant for low energy measurements that rely on high energy physics infrastructures. Significant advances and improvements in existing or future quantum technologies will be necessary to address such topics related to the dark universe, the detection of relic neutrinos, precision tests of symmetries and of the standard model and probing general foundational issues in physics. The seminar will thus also feature discussions of the Quantum Sensing Initiatives at CERN and the ECFA R&D Roadmap on Quantum Sensing and Advanced Technologies and will discuss options for future collaborations in the context of the recently approved DRD5 implementation of the roadmap.

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