Sounded

Atom interferometers measure g by laser-ranging a freely falling cloud of laser-cooled atoms; they are already commercial, not hypothetical. Exail's Absolute Quantum Gravimeter (developed with LP2N and LNE-SYRTE) is a turn-key field instrument at the 10 nm/s² (1 µGal) level, continuous for months, with no moving test mass and no drift — the only industry-grade device that can sit on a site and keep an absolute record (Antoni-Micollier et al., arXiv:2405.10844, sixteen fabricated units; Exail AQG-A/B specs). The Differential Quantum Gravimeter adds the vertical gradient at ~1 Eötvös, which rejects distant mass and sharpens near-surface voids. In October 2024 the EU FIQUgS campaign in Lisbon used a DQG to invert a Roman-era tunnel of about 5 m² under the historic centre (Sampietro, Capponi, Janvier, EGU 2025). An IEEE TIM 2025 study at Zhejiang University of Technology showed an atomic gravimeter resolving urban underground features at ~3 µGal. Reich, Janvier and Güntner (EGU 2024) forward-modelled soil water fluxes against DQG noise and argued the instrument can observe hydrological mass change — a slab, a basin, a seasonal integral — not which well-share took the kilogram. Gravity is an integral. The inverse from a lamp-post time-series to a parcel kilogram is underdetermined. This world names the regularisation: spatial smoothness, a well-log or piezometer prior, and a second instrument (existing well meters, InSAR, or a walking AQG). Without those, the number on a tablet is a political layer, not a unique measurement. Parcel law does not inherit uniqueness the physics does not have. The miniaturisation path is real and recent: Heine, Arnold, Herr, Rasel et al. (New J. Phys. 2025) loaded 1.0×10⁹ atoms in one second onto a grating atom chip — a single-beam MOT plus magnetic trap, the bottleneck for a portable BEC gravimeter. TerraQ project A03 at Leibniz Universität Hannover / DLR is explicitly building that chip-scale quantum sensor. Imperial College's Centre for Cold Matter has run chip-based BEC interferometers for years. A 2026 Applied Physics Letters compact grating-MOT gravimeter fits in <75 L, reaches 5 µGal stability at one hour, and needs no passive isolation. That package is still lasers, UHV, magnetic shielding, and an isolation stack — an atom source and MOT, not a street-furniture AQG by itself. AOSense has sold compact atom-optic gravimeters since 2010 for void detection and underground fluid monitoring. The speculative leap is not the physics. It is density: from a 100 kg tripod you book for a survey, to a chip-fed head you bolt to street furniture and leave running. Urban continuous µGal gravimetry is limited by traffic, HVAC, and seismic noise long after the physics package shrinks. A bus-route lamp post is a hostile site. This world does not pretend otherwise. Two instruments, two resolutions. A walking or van survey (Exail/AOSense class, operator present, can wait for a quiet window, can sit closer to the target) keeps Lisbon-class inversion: ~1–3 µGal, a ~5 m² tunnel, a room-scale basement, a large cistern. A lamp-post or manhole head (chip-fed atom source, still ~40–75 L) runs a named isolation stack — compact voice-coil or piezo active isolation under the head — plus a night-biased duty cycle on arterials (lock and integrate 01:00–04:30 local; discard rush-hour shots). Historic stone streets with little HVAC can run nearer-continuous. After stacking quiet nights, the furniture array sees aquifer slabs, large cisterns, room-scale basements, and connector tunnels at or above about 8–10 m². It loses the 5 m² Lisbon tunnel on a sunbelt arterial. A single grave is not a lamp-post object. Seasonal water mass is the dominant urban gravity signal. An archive good enough to meter aquifer kilograms will swamp a grave-scale residual unless hydrology is stripped: a dry-season-only stack plus a well-log or piezometer prior, then a walking survey on the residual. Persistent-across-years is not a filter if the aquifer is also persistent. When the lock is lost, a municipal gravity tech climbs: bucket truck or manhole, portable reference AQG on a short tripod, cold-atom lock kit in a hard case. That labor is part of the instrument. SandboxAQ's AQNav, often grouped with this literature, is a quantum magnetometer for GPS-denied navigation — adjacent sensing, not gravimetry — and is not used as a basis here.