A theoretical study reports a much stronger calculated response when a quantum detector has finite size, but only for one of the two motions it models: uniform linear acceleration. On the paper's dimensionless scale, a detector size of about 1 is linked to strong amplification in the low-acceleration regime, with peak sensitivity shifted toward smaller accelerations. The finding is a model result, so it comes without a reported statistical uncertainty.
A cavity that changes the detector's reach
The proposal uses a multimode optical cavity to connect a tweezer-trapped atom to a condensate. It describes a longitudinally pumped, near-planar cavity containing a one- or two-dimensional BEC of alkali atoms and a spatially separated two-level probe atom. By changing the cavity mode structure, the model gives the probe a tunable effective size, the feature meant to distinguish this detector from a fixed pointlike idealization.
To build that interaction, the authors derive an effective detector-condensate Hamiltonian in a far-detuned dispersive regime. They use adiabatic elimination, removing excited condensate states and fast cavity modes from the effective description, and assume contact interactions dominate the cavity-induced nonlocal interaction.
Filtering the part of the field the model can describe
That structure also acts as a momentum filter. The modeled Gaussian cavity profile suppresses momenta above the inverse detector size, which is intended to suppress nonphononic excitations and leave the acoustic, or sound-wave-like, relativistic sector. The authors say the pointlike limit exactly realizes an idealized Unruh-DeWitt detector coupled to the conjugate momentum of an emergent relativistic scalar field.
For a nonzero smearing width, which represents a finite spatial spread of the probe, the Fourier transform of the dimensionless correlator is evaluated numerically. At a small smearing width, the resulting response is reported to be virtually indistinguishable from the analytical Bose-Einstein distribution for the ideal pointlike limit.
Linear and circular motion part company
For uniform linear acceleration, a dimensionless detector size around 1 is reported to amplify the response at low acceleration and shift peak sensitivity toward smaller dimensionless accelerations. Because the result comes from the model, the paper reports no statistical error range around it.
Circular motion is different. Its modeled response increases with angular velocity and decreases with detector frequency, and the spectrum is not exactly thermal. In this case, increasing detector size is described as damping high-momentum harmonics rather than amplifying the response. The finite-size enhancement reported for linear acceleration therefore should not be treated as a prediction for circular motion.
The step from calculation to measurement
The paper's feasibility discussion says the setup is within current experimental capabilities and points to a realized platform based on a rubidium-87 condensate and a multimode cavity. It gives a minimum cavity-mediated interaction range of about 2 micrometres. The relevant acceleration, the authors state, is four orders of magnitude below what is currently possible.
That is a statement about the platform's ingredients, not an experimental validation of the predicted response. The response trends remain tied to assumptions in the construction, including far-detuned dispersive elimination and contact interactions dominating the cavity-induced nonlocal interaction, while the momentum filtering is described within the acoustic sector. Measurements would be needed to test whether the low-acceleration enhancement and filtering persist under real conditions.
The paper also checks a classical background drive. It produces bounded Rabi oscillations rather than an accumulating transition probability, giving a vanishing long-time rate. The authors say the analytical findings can be reconstructed from the formulas and derivations in the manuscript. The document is an arXiv preprint, and the authors report support from the Austrian Science Fund and additional support for AK from the Polish National Agency for Academic Exchange under the Polish Returns programme.
Paper data and sources
Original title: Tunable-Size Unruh-DeWitt Detector in a Multimode Cavity
Authors: Simon Brunner, Farokh Mivehvar, Helmut Ritsch, Arkadiusz Kosior
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-25
DOI: Not available
Original paper · Full text