Background and interest:

In the context of particle physics, a wide range of possibilities, covering low energy particle physics, precision metrology, but possibly also developments relevant for high-energy physics, has been opened by developments in quantum technologies and quantum sensing in the last decade or two. The breadth of the very dynamic developments in the area of quantum sensing far exceeds the focus of activities that can be imagined in an initial phase at CERN, although a number of areas of contact or even of overlap can already be identified at this stage. In the area of metrology, and particularly in the measurement of fields, as well as of temporal or spatial field changes, technologies such as SQUIDS, NV-diamonds, or graphene are well suited for magnetic field measurements; interferometric measurements sensitive to inertial forces may allow detecting (e.g. in the case of atom interferometers) magnetic or electric field gradients, vibrations (accelerations) or rotations (Sagnac effect). Low energy particle physics relies on the extreme sensitivity of quantum sensors to perturbations, such as very low energy deposits by microwave photons in kinetic inductance devices, TESs, or SNSPDs, or modifications of the properties of RF cavities due to interactions with a putative dark matter field. Similarly, superconducting amplifiers, electronics and detection systems allow very sensitively probing possible axion-photon conversions in strong magnetic fields. Finally, first attempts to apply quantum technologies to the needs of high energy physics, particularly in areas related to measurements of position or energy, but also more generally learning from advances and attempts at standardization in the related field of AMO, are underway. 

The following areas, mainly explored as part of physics-focused efforts at facilities like ISOLDE, the AD or non-accelerator based experiments, or as part of long-term detector R&D, provide the potential for compelling advances with, admittedly, partly still very speculative applications, particularly in the field of HEP.

1) Interferometric systems: Interferometric measurements are highly sensitive to any differential forces acting on the two branches of an interferometer. In the case of ongoing activities at CERN, beams of stable or sufficiently long-lived neutral atoms (specifically, positronium, but potentially also protonium, antiprotonic atoms or neutral radioisotopic atoms in the context of the AEgIS, ALPHA-g, GBAR and PUMA experiments at the AD, but also of precision trap experiments at ISOLDE) can be used to probe gravitational, electrical or magnetic potentials, as well as frame rotations or accelerations, with dephasing and/or phase shifting being a measure of the gradient strength.

2) Single-atom/ion systems: Beams of very weakly bound antiprotonic Rydberg atoms under development at the AD can either be investigated singly (e.g. in an interferometer) or else transported and trapped (in electric gradient traps). Such antiprotonic Rydberg atoms could evidence quantum state changes through annihilation, in contrast to Rydberg atoms of normal matter. Elementary particle, ionic and mixed-charge systems in traps are very sensitive to rotations or magnetic fields, for example. For sufficient sensitivity to compete with atomic systems, large numbers would have to be trapped, e.g. in multi-well surface (planar) traps. Such large-area planar traps are being evaluated as one of the possible developments for precision measurements at the AD (for trapping of antiprotons or positrons). Single trapped electrons or antiprotons (that form quantized atom-like “geonium” states in a Penning trap) are exceedingly sensitive to magnetic fields via detection of their cyclotron frequency, which the ATRAP and BASE experiments, for example, have measured to very high precisions, opening a novel window for axion searches. Devices based on this technology would naturally form the basis of NMR probes. 

3) Nuclear clocks based on production pathways for 229-Th: with a very long-lived excited state, and more importantly, a transition in the optical range (152 nm), 229-Th is a unique candidate for an ultra-stable nuclear clock, expected to ultimately achieve a relative time accuracy approaching 10−19. Different generation pathways for the excited state of the relevant isomer 229-Th (especially for the particularly useful 229-Th3+ ions trappable in Paul traps) are being explored widely; formation via the radioactive α decay of 233U (T1/2 = 1.6 105 years) or the β decay of 229Ac (T1/2 = 62.7 m) is under study at ISOLDE.

4) Applications of quantum sensing to HEP detectors: in order for quantum sensors to play a significant role, these must improve HEP detectors in the areas of signal yield (e.g. light output per interaction), reduction of existing noise (eliminating, e.g. back-flowing ions in a gaseous amplification stage), significantly better timing (e.g. to the ps level characteristic of processes in nanodots), or through the provision of novel information (e.g. detection of microwave photons produced via synchrotron radiation in the solenoidal magnetic fields of trackers, or chromatic readout - position-dependent scintillation light frequencies -  which could provide the additional benefit of contactless readout, and thus reduce the power consumption of detector elements). Several pilot projects focusing on nanodot scintillators and the improvements that graphene layers could bring to gaseous detectors are starting up.

5) Further systems (highly speculative): Position-sensitive detectors with a spatial resolution at a few nanometer-scale might be conceivable: a (local) quantum state change induced by a particle passing through a 2D array of, e.g. NV-nanodiamonds could be probed via lasers. Even more speculatively, ultra-sensitive calorimetry—reliant on detecting energy transitions in prepared atoms that are “stimulated” / “amplified” by coherence effects through the presence of low-energy electrons or photons—might be possible. Finally, the disappearance of quantum entanglement of pairs of atoms via state changes induced by atom-electron or atom-field interactions could possibly enable detection of ultra-low energy radiation, similar to the detection of single microwave photons in Rydberg atoms.

The long-term goals in the field of Quantum Sensing and Metrology are to:

  • Formalise and extend the existing catalogue of use cases and examples of possible applications of quantum sensing in areas relevant to HEP.
  • Establish links to similar initiatives starting up at university labs and form collaborations across the community.
  • Identify particularly promising technologies, focusing on a small number of developments with potential specifically for novel applications in HEP.
  • Set up common R&D projects and coordinate collaboration with other Institutes or companies to adapt or develop both the technologies themselves as well as new test systems for quantum sensing approaches and benchmark their current and potential performance.