Atlas of the Present Atlas · v0.1

As of 28 October 2026

Dossier 05

Quantum Sensing

v2.3-draft · unchecked

1. As of

Date
Version
v2.3-draft
Author / model
Atlas generator
Reviewer
unchecked (Josef)

2. In one sentence

Two countries, one second (Tofful, Filzinger, Schmidt … Godun, Huntemann; NPL/PTB arXiv:2610.29980, E2): two independently developed single-ion clocks on the Yb⁺ octupole transition in Teddington (NPL) and Braunschweig (PTB), compared over a 2220 km optical fibre, agree after 12.6 days of data (March–April 2025) to 1.0(4.8)×10⁻¹⁸ — per the authors the first international clock comparison below 5×10⁻¹⁸, the target before an optical redefinition of the second; the largest uncertainty comes not from the clocks but from the relativistic redshift between the sites (gravity potential, equivalent to just over 3 cm of height). Attotesla magnetometer hunts dark matter (Kang, Zhang, Rong, Rong; USTC/SIMIT/Zhejiang arXiv:2610.27764, E2): MnZn ferrite rods as a spin amplifier in front of a SQUID, superconducting-shielded at 7 mK, reach a noise floor below 100 aT/√Hz at tens of kHz (66 aT/√Hz at 24 kHz, nearly 3× better than before with a single SQUID per the authors); ten days of axion search reject all 1,728 candidates from run 1 and tighten terrestrial limits on the axion–electron coupling at 5–60 kHz by an order of magnitude — lab prototype, null result, not a field sensor. Barium fine-structure interferometer (Nagamori et al. Institute of Science Tokyo / JAE arXiv:2610.20168, E2): first Mach–Zehnder interferometer between two metastable fine-structure states of ¹³⁸Ba in an atomic beam, visibility above 10% at millimetre separations; the transition is about three orders of magnitude less sensitive to the quadratic Zeeman effect than rubidium, which by the authors’ calculation allows ~50× larger magnetic-field gradients for 100 m navigation drift per month — proof of principle ≠ navigation sensor. CW-ODMR (Okaniwa et al. Keio/Chuo/Kanazawa arXiv:2610.13153, E1): NV ensemble senses AC fields from 14.4–632 MHz without high-power pulse sequences; sensitivity 0.558 µT/√Hz at 14.4 MHz and 24.4 µT/√Hz at 632 MHz — lab ≠ clinic/GPS. Th-229 nuclear clock (Wang, Yin et al. USTC/TU Wien arXiv:2610.10056, E2): a solid-state thorium crystal runs locked as a clock for 30 hours, instability 1.24 × 10⁻¹³ at 10⁴ s — sustained operation shown, precision still far behind optical clocks. Araki et al. NV diamond (Institute of Science Tokyo / NIMS / QST arXiv:2610.05907, E1): dual feedback for double-quantum 4-Ramsey holds 7.0 pT/√Hz (5–100 Hz) for 4 h and against 2.8 K of temperature drift; ~4 h of averaging lowers the noise floor to 64 fT/√Hz, a 2.5 pT test field is recovered without distortion — inside a three-layer magnetic shield; authors: for real-time brain signals the intrinsic sensitivity must improve first — averaging ≠ real-time MEG. Parashar et al. MAS crystals (arXiv:2609.30899, E1): microspacing in-air sublimation — quantum-sensing crystals in minutes from commercial precursors; room-temperature ODMR; CT resonances <~4 MHz; P:PQ Hahn-echo T2=0.56 µs / CPMG 1.35 µs; lab crystal ≠ clinic/GPS. Luo et al. DBT antenna (arXiv:2609.29736, E1): cryogenic metallo-dielectric antenna — collection efficiency ~97%; ~10⁸ photons/s into the first lens; indistinguishability 91.2±2.3%; single-photon purity 97.7%; cryo lab ≠ room-temp deploy. Quantum sensing is already deployed as Cs fountains, GPS rubidium, and SERF OPMs in shielded rooms; Budker/Romalis (2007 review, arXiv) set SERF sensitivity >10⁻¹⁵ T/√Hz and near-zero-field <~10 nT; Kominis et al. 2003 (E1) measure 0.54 fT/√Hz in 0.3 cm³ SERF K vapor; Annirudh et al. 2026 (E1) show unshielded NV ensemble with noise factor 1.0±0.1 and ten-fold drift suppression; Gao et al. 2026 (E1) reach interaction-enhanced NV magnetometry up to 7.9(2) dB signal and 8.8(3) dB noise gain plus ~50 nm k-resolved correlation sensing — lab gain ≠ clinic/GPS; optical record clocks including Bothwell (E1) and atom gravimeters are lab/hybrid — not backpack 10⁻¹⁸ chronometers and not an ESA quantum satellite in 2026.

Established now · E1 / E2

3. What works today

First international clock comparison below 5×10⁻¹⁸: NPL and PTB Yb⁺ agree over 2220 km of fibre (Tofful et al.)

  1. E2

    Tofful, Filzinger, Schmidt … Godun, Huntemann (29 authors; NPL, PTB, LPL Paris Nord, Leibniz Universität Hannover, LNE-OP; arXiv:2610.29980v1, submitted 2026-10-21, 10 pp., PDF opened 2026-10-25): two independently developed single-ion clocks on the electric octupole (E3) transition of ¹⁷¹Yb⁺ — NPL-E3Yb+3 in Teddington and PTB-Yb1E3 in Braunschweig — compared over a 2220 km optical fibre link (operated in segments; the NPL–LPL section dominates the link uncertainty). 1.09×10⁶ s (12.6 days) of data within 24.7 days between 18 March and 12 April 2025 (availability NPL 92%, PTB 86%, combined including the link 51%); result: frequency ratio PTB/NPL = 1.000 000 000 000 000 001 0(48), i.e. a difference of 1.0(4.8)×10⁻¹⁸ (statistical 1.6×10⁻¹⁸). Per the authors the first international clock comparison below 5×10⁻¹⁸; until now agreement below 1×10⁻¹⁷ had almost only been seen between systems within one institute, and comparisons of other same-species clocks had revealed discrepancies well beyond the stated uncertainties.

    Tofful et al. NPL–PTB Yb⁺ comparison. International atomic clock comparison achieving 5 × 10⁻¹⁸ uncertainty. https://arxiv.org/abs/2610.29980. As of arXiv:2610.29980v1 (submitted 2026-10-21; physics.atom-ph; 10 pp.; paper dated 2026-10-22); PDF research/pdfs/arXiv2610.29980.pdf; Supplemental Material not separately opened. Checked 2026-10-25. Type: Paper (arXiv; PDF opened; measurement campaign of two metrology institutes, 1.09×10⁶ s of data).
  2. E2

    Same source, uncertainty budget (Table I, units of 10⁻¹⁸): the clocks themselves contribute 2.8 differentially (NPL 1.8, PTB 2.7; largest single items residual black-body radiation 1.3 and second-order Doppler 1.6), the international fibre 0.9, comb and local links 0.4 — but the relativistic redshift (static plus dynamic, including tides) 3.5, equivalent to a relative geopotential-height uncertainty of just over 3 cm; 4.6 systematic in total. Without the dynamic tidal correction the ratio shows a periodic variation at the sidereal period at the 2.6-sigma level. By-product: a search for Lorentz-invariance violation — α = −2.4(2.3)×10⁻⁹ in the Robertson–Mansouri–Sexl framework, an uncertainty about five times smaller than the previous best bound |α| < 1.1×10⁻⁸; Standard-Model Extension bounds about 4.5 times better than earlier comparisons of identical remote clocks.

    Tofful et al. NPL–PTB Yb⁺ comparison. International atomic clock comparison achieving 5 × 10⁻¹⁸ uncertainty. https://arxiv.org/abs/2610.29980. As of arXiv:2610.29980v1 (submitted 2026-10-21; physics.atom-ph; 10 pp.; paper dated 2026-10-22); PDF research/pdfs/arXiv2610.29980.pdf; Supplemental Material not separately opened. Checked 2026-10-25. Type: Paper (arXiv; PDF opened; measurement campaign of two metrology institutes, 1.09×10⁶ s of data).

Superconductor–spin hybrid magnetometer: 66 aT/√Hz at 24 kHz, ten days of axion search without a signal (Kang et al.)

  1. E2

    Kang, Zhang (USTC; Hefei National Laboratory), L. Rong (SIMIT, Chinese Academy of Sciences; UCAS; SIM United Superconducting Technology), X. Rong (USTC; Zhejiang University; arXiv:2610.27764v1 of 2026-10-20, PDF opened 2026-10-21): a pair of MnZn ferrite rods serves as spin sensor and field amplifier (gain G 5–10, from finite-element simulation with measured permeability); the amplified field is read out by a SQUID via a superconducting coupling circuit (transfer efficiency 7.7%, pickup coil 0.8 cm²); superconducting shields, dilution refrigerator at 7 mK. Noise floor below 100 aT/√Hz at tens of kilohertz, 66 aT/√Hz at 24 kHz — nearly three times better than the previous best result with a single SQUID per the authors; averaged from 210 raw 1-s spectra; the baseline is limited by readout-SQUID noise, peaks come from leaked environmental noise.

    Kang et al. hybrid magnetometer axion search. Searching for Axion Dark Matter via Superconductor-Spin Hybrid Magnetometers. https://arxiv.org/abs/2610.27764. As of arXiv:2610.27764v1 (2026-10-20; physics.ins-det); PDF research/pdfs/arXiv2610.27764.pdf; Supplemental Material not opened. Checked 2026-10-21. Type: Paper (arXiv; PDF opened; lab experiment).
  2. E2

    Same source, axion search: 10 runs of one day each (5,400 raw spectra per run). In run 1, 1,728 of 830,723 standardized power excesses exceed the threshold of 3; a real axion signal would have to appear in every run — after 10 runs all candidates are rejected. This yields 95% upper limits on the axion–electron coupling between 5 and 60 kHz, per the authors the most stringent terrestrial ones in this range and an order of magnitude better than the previous best (a fifth-force experiment). Tighter astrophysical limits from red giants remain; they assume axions are produced in and escape from stars.

    Kang et al. hybrid magnetometer axion search. Searching for Axion Dark Matter via Superconductor-Spin Hybrid Magnetometers. https://arxiv.org/abs/2610.27764. As of arXiv:2610.27764v1 (2026-10-20; physics.ins-det); PDF research/pdfs/arXiv2610.27764.pdf; Supplemental Material not opened. Checked 2026-10-21. Type: Paper (arXiv; PDF opened; lab experiment).

Barium: Mach–Zehnder between metastable fine-structure states, ~1,000× less Zeeman-sensitive (Nagamori et al.)

  1. E2

    Nagamori, Hatano, Hosoya, Miyazawa, Sato, Kozuma (Institute of Science Tokyo; Japan Aviation Electronics Industry; arXiv:2610.20168v1 of 2026-10-14, PDF opened 2026-10-18): by the authors’ account the first Mach–Zehnder interference between two metastable fine-structure states (³D₁ ↔ ³D₂ of ¹³⁸Ba) in a thermal atomic beam (most probable velocity ~360 m/s, oven 440 °C), driven by Raman lasers at 1,108 and 1,131 nm. Measured: visibility above 10% at the shortest beam separations (varied 0.4–8 mm), interference observable over several millimetres; a simulation without free parameters reproduces the level and the decay, which is explained mainly by the measured phase noise between the two lasers. Calculated: differential quadratic Zeeman coefficient 3.7×10⁷ Hz/T² versus 5.75×10¹⁰ Hz/T² for the usual ⁸⁷Rb clock transition — about three orders of magnitude less magnetic sensitivity.

    Nagamori et al. Ba fine-structure interferometry. Fine-structure Raman interferometry with metastable barium for magnetically resilient inertial sensing. https://arxiv.org/abs/2610.20168. As of arXiv:2610.20168v1 (2026-10-14; physics.atom-ph); PDF research/pdfs/arXiv2610.20168.pdf; data Zenodo 10.5281/zenodo.22200160 (not opened). Checked 2026-10-18. Type: Paper (arXiv; PDF opened; proposal + proof-of-principle experiment).

NV diamond: CW-ODMR senses AC fields from 14.4–632 MHz (Okaniwa et al.)

  1. E1

    Okaniwa, Suzuki, Matsuzaki, Tokuda, Ishi-Hayase (Keio / Chuo / Kanazawa; arXiv:2610.13153v1 of 2026-10-09; PDF opened 2026-10-14): NV ensemble in a nitrogen-doped CVD layer (~1.5 µm, preferential [111]; T₂ = 3.3 µs, T₂* = 231 ns) under a strong transverse bias; RF-dressed states; continuous-wave ODMR without high-power microwave pulse sequences. Detectable RF frequency is tuned via bias-field strength from 14.4 MHz to 632 MHz (Bx dominates; Table S1).

    Okaniwa et al. CW-ODMR AC. Continuous-wave AC magnetometry up to hundreds of megahertz using nitrogen-vacancy centers under a strong transverse magnetic field. https://arxiv.org/abs/2610.13153. As of arXiv:2610.13153v1 (2026-10-09); PDF research/pdfs/arXiv2610.13153.pdf. Checked 2026-10-14. Type: Paper (arXiv; PDF opened).
  2. E1

    Sensitivity 0.558 µT/√Hz at 14.4 MHz and 24.4 µT/√Hz at 632 MHz (~44× worse); ODMR contrast of the used f_{−0′} line falls about sixfold with bias, linewidth expands ~3.5× — authors name spin mixing and transverse noise as drivers. Lab CW-ODMR; pulsed DD bandwidth typically ~30 MHz limited by MW power.

    Okaniwa et al. CW-ODMR AC. Continuous-wave AC magnetometry up to hundreds of megahertz using nitrogen-vacancy centers under a strong transverse magnetic field. https://arxiv.org/abs/2610.13153. As of arXiv:2610.13153v1 (2026-10-09); PDF research/pdfs/arXiv2610.13153.pdf. Checked 2026-10-14. Type: Paper (arXiv; PDF opened).

Thorium-229: solid-state nuclear clock runs locked for 30 hours (Wang et al.)

  1. E2

    Wang, Yin et al. (USTC/Hefei National Laboratory with TU Wien; arXiv:2610.10056v1 of 2026-10-07; PDF opened 2026-10-10): in a ²²⁹Th:CaF₂ crystal, continuous-wave absorption at 148.4 nm resolves four quadrupole components of the D centre and a broader O-centre resonance. Feedback on the strongest narrow component is updated about every 10 s and stays active through a 30 h record; a hydrogen-maser-referenced frequency comb measures the output. Instability ≈ 1.29 × 10⁻¹¹ (τ/s)^−1/2, 1.24 × 10⁻¹³ at 10⁴ s; a separate 6.6 h run locks to a second component. Line centres consistent with Vienna and JILA measurements (e.g. b scan +11.6 ± 4.4 kHz vs Vienna). One lab, preprint.

    Wang et al. Th-229 clock. A solid-state nuclear clock based on VUV absorption spectroscopy of 229Th. https://arxiv.org/abs/2610.10056. As of arXiv:2610.10056v1 (2026-10-07); PDF research/pdfs/arXiv2610.10056.pdf. Checked 2026-10-10. Type: Paper (arXiv; PDF opened).

NV diamond: dual feedback holds pT sensitivity for hours (Araki et al.)

  1. E1

    Araki, Sekiguchi, Hatano et al. (Institute of Science Tokyo / NIMS / QST; arXiv:2610.05907v1 of 2026-10-05, PDF opened 2026-10-06): NV ensemble in ¹²C-enriched HPHT diamond (1.6 × 1.6 mm, 0.31 mm thick; NV 0.07 ppm) with a double-quantum 4-Ramsey sequence; two PI loops separately compensate common-mode (temperature) and differential-mode (magnetic field) detuning. Under deliberate heating the open-loop signal became undetectable at +2.5 K; with feedback sensitivity was maintained up to +2.8 K (end of measurement).

    Araki et al. NV DQ 4-Ramsey. Robust Diamond Quantum Sensing in Double-Quantum 4-Ramsey Interferometry by Dual-Feedback Scheme. https://arxiv.org/abs/2610.05907. As of arXiv:2610.05907v1 (2026-10-05); PDF research/pdfs/NV_DQ4Ramsey_DualFeedback_arXiv2610.05907.pdf. Checked 2026-10-06. Type: Paper (arXiv; PDF opened).
  2. E1

    With dual feedback 7.0 pT/√Hz in the 5–100 Hz band (photon-shot-noise limit 1.5 pT/√Hz); averaged over ~4 h (N = 14,449) noise floor 64 fT/√Hz, RMS noise follows 1/√T up to 4 h; a calibrated 2.5 pT test field (40 Hz) is recovered in amplitude and shape. Measured inside a three-layer magnetic shield box.

    Araki et al. NV DQ 4-Ramsey. Robust Diamond Quantum Sensing in Double-Quantum 4-Ramsey Interferometry by Dual-Feedback Scheme. https://arxiv.org/abs/2610.05907. As of arXiv:2610.05907v1 (2026-10-05); PDF research/pdfs/NV_DQ4Ramsey_DualFeedback_arXiv2610.05907.pdf. Checked 2026-10-06. Type: Paper (arXiv; PDF opened).

MAS: room-temperature ODMR crystals in minutes (Parashar et al.)

  1. E1

    Parashar, Chen, Druga, Tan, Reimer, Ajoy (arXiv:2609.30899v1, PDF opened 2026-09-28): microspacing in-air sublimation (MAS) — quantum-sensing materials in minutes from microgram commercial precursors on glass, in air, without vacuum/inert gas; optically clear stoichiometric single crystals including waveguiding plates/needles.

    Parashar et al. MAS crystals. Room-temperature quantum-sensing molecular crystals grown in minutes. https://arxiv.org/abs/2609.30899. As of arXiv:2609.30899v1 (2026-09-25); PDF research/pdfs/MAS_MolecularCrystals_arXiv2609.30899.pdf. Checked 2026-09-28. Type: Paper (arXiv; PDF opened).
  2. E1

    Donor–acceptor CT crystals (anthracene/biphenyl:TCNB) and localized-exciton P:PQ show room-temperature ODMR. CT resonances reach linewidths below ~4 MHz (motional narrowing of mobile triplet excitons); P:PQ Hahn-echo T2 = 0.56 µs; CPMG (4π) T2,CPMG = 1.35 µs; Ramsey T2* = 123 ns.

    Parashar et al. MAS crystals. Room-temperature quantum-sensing molecular crystals grown in minutes. https://arxiv.org/abs/2609.30899. As of arXiv:2609.30899v1 (2026-09-25); PDF research/pdfs/MAS_MolecularCrystals_arXiv2609.30899.pdf. Checked 2026-09-28. Type: Paper (arXiv; PDF opened).

What quantum sensing is — Type I/II deployed, Type III not

  1. E1

    Degen: (I) a quantized object as the probe, (II) coherence/superposition, (III) entanglement beyond the classical limit. Historical examples: SQUID, atomic-vapour magnetometers, atomic clocks. Type I/II often close to applications; Type III is the strict definition. Atomic clocks are explicitly excluded in the review — clock figures come from NIST/Marshall, not Degen.

    Degen, Reinhard, Cappellaro. Quantum sensing. https://arxiv.org/html/1611.02427. As of 2017-06. Checked 2026-08-28. Type: Review (RMP 89).

Atomic clocks: Cs defines the second; optical clocks are laboratory

  1. E1

    The SI second is 9,192,631,770 periods of the Cs-133 hyperfine transition. UTC(NIST) from maser+beam, calibrated by fountains. Optical clocks contribute but are not primary standards, “because the second is currently defined in terms of cesium.”

    NIST How Do We Know. How Do We Know What Time It Is?. https://www.nist.gov/atomic-clocks/how-do-we-know-what-time-it. As of 2025-05-28. Checked 2026-08-28. Type: Agency.NIST Fountain page. NIST’s Cesium Fountain Atomic Clocks. https://www.nist.gov/pml/time-and-frequency-division/time-realization/cesium-fountain-atomic-clocks. As of 2023-05-12. Checked 2026-08-28. Type: Agency.
  2. E2

    NIST-F4: type-B uncertainty 2.2×10⁻¹⁶ (abstract + news; body not opened). Fountains “very delicate”; <20 worldwide. F3+F4 ~90% uptime.

    NIST F4 News. New Atomic Fountain Clock Joins Elite Group That Keeps the World on Time. https://www.nist.gov/news-events/news/2025/04/new-atomic-fountain-clock-joins-elite-group-keeps-world-time. As of 2025-04-28. Checked 2026-08-28. Type: Agency.
  3. E1

    ²⁷Al⁺ quantum-logic clock: systematic uncertainty 5.5×10⁻¹⁹; instability 3.5×10⁻¹⁶/√(τ/s). 1 s probe via 3.6 km fibre to JILA’s cryogenic Si cavity. This is a metrology campus, not a field device.

    Marshall et al. High-stability single-ion clock with 5.5×10⁻¹⁹ systematic uncertainty. https://arxiv.org/html/2504.13071v1. As of 2025-04. Checked 2026-08-28. Type: Paper (PRL 135).NIST Ion Clock News. NIST Ion Clock Sets New Record for Most Accurate Clock in the World. https://www.nist.gov/news-events/news/2025/07/nist-ion-clock-sets-new-record-most-accurate-clock-world. As of 2025-07-14. Checked 2026-08-28. Type: Agency.
  4. E1

    BACON 2025: Al⁺/Sr, Al⁺/Yb, Yb/Sr uncertainties 2.2 / 3.2 / 3.1 ×10⁻¹⁸. Al⁺/Sr deviates ~14σ from BACON21; repeatability <10⁻¹⁷ remains an “outstanding issue” (authors). E3 for “milestone for redefinition” (Dimarcq not opened).

    Aeppli / BACON. Atomic clock frequency ratios with fractional uncertainty ≤3.2×10⁻¹⁸. https://arxiv.org/html/2512.21428v1. As of 2025-12. Checked 2026-08-28. Type: Paper.
  5. E1

    GPS: 31 satellites, rubidium clocks only onboard today; drift around a few nanoseconds per day; steered by ground clocks. CSAC: >100,000 units since 2011 (NIST; unit count vendor-via-NIST). CSAC is microwave Rb, not 10⁻¹⁸.

    NIST How Do We Know. How Do We Know What Time It Is?. https://www.nist.gov/atomic-clocks/how-do-we-know-what-time-it. As of 2025-05-28. Checked 2026-08-28. Type: Agency.NIST Clocks Galore. Clocks Galore. https://www.nist.gov/atomic-clocks/how-atomic-clocks-work/clocks-galore. As of 2026-08-21. Checked 2026-08-28. Type: Agency.
  6. E1

    Bothwell et al. Nature 2022 (NSF PAR AAM): ~100,000 ⁸⁷Sr at ~100 nK in a shallow large-waist 1D lattice. Linear frequency gradient consistent with gravitational redshift within a single millimetre-scale sample. Fractional frequency uncertainty 7.6×10⁻²¹ between two uncorrelated regions; optical atomic coherence 37 s; inferred single-region instability 3.1×10⁻¹⁸/√τ at 1 s. Predicted Earth-surface gradient −1.09×10⁻¹⁹ mm⁻¹; measured −9.8(2.3)×10⁻²⁰ mm⁻¹, consistent. Laboratory optical-lattice chronometry / metrology campus — not a field GPS replacement.

    Bothwell et al. Resolving the gravitational redshift across a millimetre-scale atomic sample. https://doi.org/10.1038/s41586-021-04349-7. As of 2022-02-16. Checked 2026-09-03. Type: Paper (Nature; NSF PAR AAM).

Magnetometers: SQUID clinical; SERF OPM research in an MSR; NV nano principle

  1. E2

    Budker & Romalis (Optical Magnetometry, arXiv physics/0611246 / Nat. Phys. 2007): SERF = spin-exchange-relaxation-free; demonstrated sensitivity >10⁻¹⁵ T/√Hz, projected fundamental limits <10⁻¹⁷ T/√Hz (citing Kominis et al. 2003). Atomic magnetometers need no cryogenics (unlike SQUIDs); measure field directly. SERF regime only for magnetic fields <~10 nT; above that, null-detector with feedback. E2 for review body (arXiv); not a clinical approval.

    Budker & Romalis. Optical Magnetometry. https://arxiv.org/pdf/physics/0611246. As of 2006-11-26 (arXiv; Nat. Phys. 2007 DOI 10.1038/nphys566). Checked 2026-09-04. Type: Paper (review; arXiv physics/0611246; Nat. Phys. 2007).
  2. E2

    Budker/Romalis: “no cryostat” ≠ “no shield/zero field”. SERF needs near-zero field; applications include bio-magnetics, NMR/MRI, space — review, no GPS-replacement claim.

    Budker & Romalis. Optical Magnetometry. https://arxiv.org/pdf/physics/0611246. As of 2006-11-26 (arXiv; Nat. Phys. 2007 DOI 10.1038/nphys566). Checked 2026-09-04. Type: Paper (review; arXiv physics/0611246; Nat. Phys. 2007).
  3. E1

    Kominis et al. Nature 2003 (DOI 10.1038/nature01484; full text opened 2026-09-13): SERF potassium magnetometer without cryogenics; sensitivity 0.54 fT Hz⁻¹/² at 0.3 cm³ measurement volume; intrinsic average 0.54 fT Hz⁻¹/² (28–45 Hz), <1 fT Hz⁻¹/² (10–150 Hz) after common-mode rejection.

    Kominis et al. A subfemtotesla multichannel atomic magnetometer. https://doi.org/10.1038/nature01484. As of Nature 422, 596–599 (2003-04-10). Checked 2026-09-13. Type: Paper (Nature; full-text PDF).
  4. E1

    Kominis: fundamental limits projected below 0.01 fT Hz⁻¹/²; multichannel operation; localization of magnetic sources at 2 mm resolution; channel spacing 3 mm. Lab SERF in shielded/near-zero conditions — not clinical approval, not GPS replacement (E0).

    Kominis et al. A subfemtotesla multichannel atomic magnetometer. https://doi.org/10.1038/nature01484. As of Nature 422, 596–599 (2003-04-10). Checked 2026-09-13. Type: Paper (Nature; full-text PDF).
  5. E1

    SERF OPM: femtotesla without cryogenic cooling, but in an MSR. Intrinsic noise well below 50 fT/√Hz (maker+PTB). Paper verbatim: “diagnostic and clinical relevance has not yet been demonstrated.” >100 institutes run SQUID MEG (citation, not independent).

    Elzenheimer et al. Characterizing timing parameters in commercial SERF-OPM multichannel systems for biomagnetic field sensing. https://arxiv.org/html/2509.22198v1. As of 2025-09. Checked 2026-08-28. Type: Paper.
  6. E2

    FieldLine HEDscan 128: empty-room after SSP 20±5 fT/Hz¹/². Closed-loop 350 Hz. MSR residual 150 nT → global coils to <30 nT. Higher amplitude than cryo MEG and higher background noise. Vendor coauthors.

    Alem et al. An integrated full-head OPM-MEG system based on 128 zero-field sensors. https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2023.1190310/full. As of 2023-06-14. Checked 2026-08-28. Type: Paper (vendor+clinic).
  7. E1

    NV 2008: room temperature, optical polarisation. Single-NV DC with then collection ~1 µT/Hz¹/², not fT. Ensemble projection 3 fT/Hz¹/² and single proton = E3 (calculation under C=0.3).

    Taylor et al. High-sensitivity diamond magnetometer with nanoscale resolution. https://arxiv.org/html/0805.1367. As of 2008. Checked 2026-08-28. Type: Paper (proposal).
  8. E1

    Annirudh et al. (arXiv:2609.16901v1, PDF opened 2026-09-17): portable vector NV ensemble architecture — dynamic differential readout, second-derivative Lorentzian slope tracking, magnetic eigenvector transformation against bias-magnet thermal drift (SmCo ~40 nT/0.1 K cited).

    Annirudh et al. NV portable. Portable Vector NV-Diamond Magnetometer for Shot-Noise-Limited, Drift-Free Operation in Unshielded Environments. https://arxiv.org/abs/2609.16901. As of arXiv:2609.16901v1 (2026-09-15); PDF research/pdfs/NV_portable_magnetometer_arXiv2609.16901.pdf. Checked 2026-09-17. Type: Paper (arXiv; PDF opened).
  9. E1

    Unshielded ambient: off-resonance noise factor 1.0 ± 0.1 (fundamental limit); ten-fold long-term drift suppression (abstract); on-resonance Fon = 1.2 ± 0.1; calculated off-resonance Slimit ≈ 284.7 pT/√Hz at the reported operating point. pT-class lab/portable ≠ SERF fT clinic, ≠ GPS replacement.

    Annirudh et al. NV portable. Portable Vector NV-Diamond Magnetometer for Shot-Noise-Limited, Drift-Free Operation in Unshielded Environments. https://arxiv.org/abs/2609.16901. As of arXiv:2609.16901v1 (2026-09-15); PDF research/pdfs/NV_portable_magnetometer_arXiv2609.16901.pdf. Checked 2026-09-17. Type: Paper (arXiv; PDF opened).
  10. E1

    Gao, Put, Leitao et al. (Lukin/Park/Jayich; arXiv:2609.03039v1, PDF opened 2026-09-22): collective OAT dynamics in a ~10⁴ NV ensemble (bulk diamond, room temperature) with pulsed magnetic-field gradients and dressed-state encoding. Signal amplification up to 5.4(1)× (14.6(2) dB) for instantaneous rotation; practical metrological gains up to 7.9(2) dB (coherent AC signal) and 8.8(3) dB (noise), overheads fully accounted.

    Gao et al. NV many-body. Nanoscale magnetometry via collective many-body dynamics in diamond. https://arxiv.org/abs/2609.03039. As of arXiv:2609.03039v1 (2026-09-02); PDF research/pdfs/Gao_NV_manybody_arXiv2609.03039.pdf. Checked 2026-09-22. Type: Paper (arXiv; PDF opened).
  11. E1

    Momentum-space-resolved correlation sensing: gradient mapping shifts a k mode to k=0 for OAT amplification; sensitivity to spatially correlated noise down to ~50 nm. Lab interaction gain ≠ clinical MEG, ≠ GPS, ≠ field device without cryostat/photonics path. Prior NV squeezing/amplification reports ~0.5–0.56 dB under idealized conditions — here practical overhead accounting.

    Gao et al. NV many-body. Nanoscale magnetometry via collective many-body dynamics in diamond. https://arxiv.org/abs/2609.03039. As of arXiv:2609.03039v1 (2026-09-02); PDF research/pdfs/Gao_NV_manybody_arXiv2609.03039.pdf. Checked 2026-09-22. Type: Paper (arXiv; PDF opened).

Organic single-photon source: DBT antenna (Luo et al.)

  1. E1

    Luo, Hebenstreit, Shkarin, Renger, Utikal, Götzinger (MPL / FAU; arXiv:2609.29736v1, PDF opened 2026-09-27): cryogenic planar metallo-dielectric antenna for a single dibenzoterrylene (DBT) molecule in p-DCB; collection efficiency η_CE = (97±2)% into NA=0.77 first lens; detected broadband rate 21.3±0.3 Mcps → (99.5±12.5) Mcps into the first lens (~10⁸ photons/s).

    Luo et al. DBT antenna. 100 million photons per second from a single organic molecule. https://arxiv.org/abs/2609.29736. As of arXiv:2609.29736v1 (2026-09-24); PDF research/pdfs/DBT_Antenna_arXiv2609.29736.pdf. Checked 2026-09-27. Type: Paper (arXiv; PDF opened).
  2. E1

    0–0 ZPL: indistinguishability M_γ = (91.2±2.3)% (emitter-limited after pulsed HOM); single-photon purity per abstract 97.7% under strong CW and pulsed excitation; lifetime τ = 8.51±0.02 ns. Cryo (~1.4 K) lab antenna ≠ room-temperature deploy / network.

    Luo et al. DBT antenna. 100 million photons per second from a single organic molecule. https://arxiv.org/abs/2609.29736. As of arXiv:2609.29736v1 (2026-09-24); PDF research/pdfs/DBT_Antenna_arXiv2609.29736.pdf. Checked 2026-09-27. Type: Paper (arXiv; PDF opened).

Gravimeters: absolute and hybrid; not a GPS replacement

  1. E1

    GIRAFE marine: hybrid with Honeywell Q-Flex. Ship sea state 4–6: crossing error 0.4–0.9 mGal. Fringe period ≪ vehicle acceleration → ambiguity, hence hybrid. Dead times filled classically.

    Bidel et al. 2018. Absolute marine gravimetry with matter-wave interferometry. https://www.nature.com/articles/s41467-018-03040-2. As of 2018-02-12. Checked 2026-08-28. Type: Paper.
  2. E1

    GIRAFE airborne 2017: error 1.7–3.9 mGal. ESA AirQuantumGrav 2023 + Jensen 2025: accuracy 1–2 mGal; GIRAFE no tie, no drift. Filter 40–130 s → km resolution. Platform tilt up to 20 mGal.

    Bidel et al. 2020. Absolute airborne gravimetry with a cold atom sensor. https://arxiv.org/html/1910.06666. As of 2019-10. Checked 2026-08-28. Type: Paper (J. Geodesy).ESA AirQuantumGrav. AirQuantumGrav 2023. https://earth.esa.int/eogateway/campaigns/airborne-quantum-gravimetry-airquantumgrav-2023. As of 2024-07. Checked 2026-08-28. Type: Agency/dataset.Jensen et al. Airborne gravimetry with quantum technology: observations from Iceland and Greenland. https://essd.copernicus.org/articles/17/1667/2025/. As of 2025-04-17. Checked 2026-08-28. Type: Paper.
  3. E1

    AQG 2018: sensor head ~30 kg; sensitivity 500 nm s⁻² Hz⁻¹/²; long-term <10 nm s⁻². Accuracy budget 2018 incomplete; target <50 nm s⁻² = E3. Competing interests: Muquans.

    Ménoret et al. Gravity measurements below 10⁻⁹ g with a transportable absolute quantum gravimeter. https://www.nature.com/articles/s41598-018-30608-1. As of 2018-08-17. Checked 2026-08-28. Type: Paper.

Claimed · E3

4. What is claimed, not shown

An important step towards the optical redefinition of the second; further only with a better gravity field or a transportable clock (author reading)

  1. E3

    Tofful et al.: agreement below 5×10⁻¹⁸ is an important milestone towards an anticipated optical redefinition of the SI second, for which this target level is a precondition; even more accurate international comparisons require better knowledge of the gravity potential, or one clock would have to be transported to the remote site without losing performance. Author reading; the paper is no decision on the redefinition.

    Tofful et al. NPL–PTB Yb⁺ comparison. International atomic clock comparison achieving 5 × 10⁻¹⁸ uncertainty. https://arxiv.org/abs/2610.29980. As of arXiv:2610.29980v1 (submitted 2026-10-21; physics.atom-ph; 10 pp.; paper dated 2026-10-22); PDF research/pdfs/arXiv2610.29980.pdf; Supplemental Material not separately opened. Checked 2026-10-25. Type: Paper (arXiv; PDF opened; measurement campaign of two metrology institutes, 1.09×10⁶ s of data).

Hybrid magnetometer arrays: axion limits thousands of times to >10⁴ tighter (author projection)

  1. E3

    Kang et al.: with near-unit coupling (7.7% today), 100 spin sensors per refrigerator, 10 refrigerators, better shields in larger fridges and one year of data, the limits could improve “by several thousand times” or “by more than four orders of magnitude” (both phrasings appear in the paper). Projection with assumed parameters; no array built.

    Kang et al. hybrid magnetometer axion search. Searching for Axion Dark Matter via Superconductor-Spin Hybrid Magnetometers. https://arxiv.org/abs/2610.27764. As of arXiv:2610.27764v1 (2026-10-20; physics.ins-det); PDF research/pdfs/arXiv2610.27764.pdf; Supplemental Material not opened. Checked 2026-10-21. Type: Paper (arXiv; PDF opened; lab experiment).

Barium: standard field control instead of bespoke shielding for 100 m/month (author reading)

  1. E3

    Nagamori et al.: for an inertial-navigation target of 100 m drift per month (about an order of magnitude better than today’s high-end systems per the authors; implying bias limits of ~80 µm/s² and ~1 µdeg/h) the barium scheme would tolerate a magnetic-field gradient up to ~570 µT/m instead of ~12 µT/m for ⁸⁷Rb — a factor of ~50 — so that standard magnetic-field control would suffice instead of highly optimized shielding and compensation coils. Calculation for a stationary case with assumed parameters (bias field 10 µT, arm length 100 mm, 360 m/s); the predicted magnetic robustness has not yet been measured.

    Nagamori et al. Ba fine-structure interferometry. Fine-structure Raman interferometry with metastable barium for magnetically resilient inertial sensing. https://arxiv.org/abs/2610.20168. As of arXiv:2610.20168v1 (2026-10-14; physics.atom-ph); PDF research/pdfs/arXiv2610.20168.pdf; data Zenodo 10.5281/zenodo.22200160 (not opened). Checked 2026-10-18. Type: Paper (arXiv; PDF opened; proposal + proof-of-principle experiment).

Roadmaps and projections

  1. E3

    Chronometric geodesy “3 cm”: principle, not a field campaign. Marshall: 1×10⁻¹⁹ “feasible” with a cryostat = author extrapolation.

    Aeppli / BACON. Atomic clock frequency ratios with fractional uncertainty ≤3.2×10⁻¹⁸. https://arxiv.org/html/2512.21428v1. As of 2025-12. Checked 2026-08-28. Type: Paper.
  2. E3

    CNES CARIOQA: first space-capable atom accelerometer, launch planned 2030. Status “In development.” Not flown. Not: “ESA has a quantum sensor in orbit.”

    CNES CARIOQA. CARIOQA. https://cnes.fr/en/projects/carioqa. As of 2025-01-20. Checked 2026-08-28. Type: Agency.
  3. E3

    Araki et al. (discussion): diamond sensors at ~2 mm sensor-to-sample distance (vs. OPM 6 mm, SQUID 2–3 cm, cited from literature) would offer a spatial advantage for MEG — a projection; not shown on any biological signal in this work.

    Araki et al. NV DQ 4-Ramsey. Robust Diamond Quantum Sensing in Double-Quantum 4-Ramsey Interferometry by Dual-Feedback Scheme. https://arxiv.org/abs/2610.05907. As of arXiv:2610.05907v1 (2026-10-05); PDF research/pdfs/NV_DQ4Ramsey_DualFeedback_arXiv2610.05907.pdf. Checked 2026-10-06. Type: Paper (arXiv; PDF opened).
  4. E3

    Taylor 2008: ensemble few fT/Hz¹/²; “surpass SQUID … by more than an order of magnitude” — 2008 calculation, not a 2026 device.

    Taylor et al. High-sensitivity diamond magnetometer with nanoscale resolution. https://arxiv.org/html/0805.1367. As of 2008. Checked 2026-08-28. Type: Paper (proposal).

Constrained · Limit

5. Bottleneck and limit

NPL–PTB comparison: one same-species clock pair, one 2025 campaign, geodesy dominates, no field or navigation clock

  1. E2

    Tofful et al. 2026: a single pair of clocks on the same transition (Yb⁺ E3), the same two systems as in an earlier European campaign; one 24.7-day measurement campaign in spring 2025 with only 51% combined uptime; a ratio of one is a clear benchmark but does not show that clocks of different atomic species agree (BACON discrepancies, cited under established, remain open). The uncertainty is dominated by geodesy (gravity potential, tides), not by the clocks; without the dynamic tidal correction a 2.6-sigma residual signal would remain. These are stationary laboratory clocks with fibre between metrology institutes — not transportable, field or navigation clocks, no GPS replacement. Preprint; supplement not opened separately.

    Tofful et al. NPL–PTB Yb⁺ comparison. International atomic clock comparison achieving 5 × 10⁻¹⁸ uncertainty. https://arxiv.org/abs/2610.29980. As of arXiv:2610.29980v1 (submitted 2026-10-21; physics.atom-ph; 10 pp.; paper dated 2026-10-22); PDF research/pdfs/arXiv2610.29980.pdf; Supplemental Material not separately opened. Checked 2026-10-25. Type: Paper (arXiv; PDF opened; measurement campaign of two metrology institutes, 1.09×10⁶ s of data).

Attotesla at 7 mK inside a superconducting shield ≠ magnetometer for clinic or field

  1. E2

    Kang et al. 2026: lab prototype in a dilution refrigerator at 7 mK with superconducting shielding — not comparable with near-room-temperature OPM or NV sensors in this dossier; the sensitivity holds at tens of kilohertz, not at the low frequencies of brain or heart signals; the gain comes from simulation, the spectrum below 5 kHz is cut for lack of calibration; null result, no discovery; one group, supplement not opened, preprint.

    Kang et al. hybrid magnetometer axion search. Searching for Axion Dark Matter via Superconductor-Spin Hybrid Magnetometers. https://arxiv.org/abs/2610.27764. As of arXiv:2610.27764v1 (2026-10-20; physics.ins-det); PDF research/pdfs/arXiv2610.27764.pdf; Supplemental Material not opened. Checked 2026-10-21. Type: Paper (arXiv; PDF opened; lab experiment).

Barium: millimetre proof of principle ≠ navigation sensor

  1. E2

    Nagamori et al. 2026: the experiment uses co-propagating Raman beams, i.e. without the large momentum transfer needed for inertial sensing; the relative laser phase was not stabilized, pulse areas were non-ideal, ~29–34% of atoms were lost to spontaneous emission per interaction; beam separations in the millimetre range instead of the ~100 mm needed for sensing. No acceleration or rotation signal, no bias stability measured; counter-propagating setup in progress per the authors. Preprint; navigation target ≠ device, no GPS replacement.

    Nagamori et al. Ba fine-structure interferometry. Fine-structure Raman interferometry with metastable barium for magnetically resilient inertial sensing. https://arxiv.org/abs/2610.20168. As of arXiv:2610.20168v1 (2026-10-14; physics.atom-ph); PDF research/pdfs/arXiv2610.20168.pdf; data Zenodo 10.5281/zenodo.22200160 (not opened). Checked 2026-10-18. Type: Paper (arXiv; PDF opened; proposal + proof-of-principle experiment).

Hundreds of MHz ≠ pT MEG and ≠ field device

  1. E1

    Okaniwa et al. 2026: sensitivity stays in the µT/√Hz range and degrades sharply at high frequency — no warrant for the Araki pT MEG path, clinic, or GPS. CW-ODMR under a strong transverse field is a lab method for RF-current/EM characterisation, not a backpack sensor.

    Okaniwa et al. CW-ODMR AC. Continuous-wave AC magnetometry up to hundreds of megahertz using nitrogen-vacancy centers under a strong transverse magnetic field. https://arxiv.org/abs/2610.13153. As of arXiv:2610.13153v1 (2026-10-09); PDF research/pdfs/arXiv2610.13153.pdf. Checked 2026-10-14. Type: Paper (arXiv; PDF opened).

Nuclear clock ≠ better clock — not yet

  1. E2

    Same work: 1.24 × 10⁻¹³ after almost 3 hours of averaging is orders of magnitude behind optical lattice clocks. The authors name detection noise as the limit on short-term stability, and temperature and the local lattice environment as shifts; temperature-dependent lines require a common reference temperature (295.84 K) for lab comparisons. What is shown is sustained operation, not accuracy.

    Wang et al. Th-229 clock. A solid-state nuclear clock based on VUV absorption spectroscopy of 229Th. https://arxiv.org/abs/2610.10056. As of arXiv:2610.10056v1 (2026-10-07); PDF research/pdfs/arXiv2610.10056.pdf. Checked 2026-10-10. Type: Paper (arXiv; PDF opened).

What quantum sensing in 2026 is not

  1. E1

    Araki et al. 2026: 64 fT/√Hz is an averaging noise floor after ~4 h inside a three-layer shield, not real-time sensitivity (7.0 pT/√Hz). Authors: evoked MEG signals ≈100 fT; even OPM-MEG (~15 fT/√Hz) averages over dozens of trials — long integration does not make up for NV limits on real brain activity; intrinsic sensitivity must rise (at the shot-noise limit ~10 min instead of 4 h would be needed). Lab test coil ≠ patient, ≠ clinical MEG.

    Araki et al. NV DQ 4-Ramsey. Robust Diamond Quantum Sensing in Double-Quantum 4-Ramsey Interferometry by Dual-Feedback Scheme. https://arxiv.org/abs/2610.05907. As of arXiv:2610.05907v1 (2026-10-05); PDF research/pdfs/NV_DQ4Ramsey_DualFeedback_arXiv2610.05907.pdf. Checked 2026-10-06. Type: Paper (arXiv; PDF opened).
  2. E1

    Optical record clocks are not the SI second and not GPS. GPS payload is Rb microwave. CSAC ≠ optical.

    NIST How Do We Know. How Do We Know What Time It Is?. https://www.nist.gov/atomic-clocks/how-do-we-know-what-time-it. As of 2025-05-28. Checked 2026-08-28. Type: Agency.Marshall et al. High-stability single-ion clock with 5.5×10⁻¹⁹ systematic uncertainty. https://arxiv.org/html/2504.13071v1. As of 2025-04. Checked 2026-08-28. Type: Paper (PRL 135).NIST Clocks Galore. Clocks Galore. https://www.nist.gov/atomic-clocks/how-atomic-clocks-work/clocks-galore. As of 2026-08-21. Checked 2026-08-28. Type: Agency.
  3. E1

    Bothwell millimetre lab redshift ≠ a GPS replacement. Cs still defines the SI second (sources 9, 11); Bothwell is laboratory optical-lattice chronometry (source 18).

    NIST How Do We Know. How Do We Know What Time It Is?. https://www.nist.gov/atomic-clocks/how-do-we-know-what-time-it. As of 2025-05-28. Checked 2026-08-28. Type: Agency.NIST Fountain page. NIST’s Cesium Fountain Atomic Clocks. https://www.nist.gov/pml/time-and-frequency-division/time-realization/cesium-fountain-atomic-clocks. As of 2023-05-12. Checked 2026-08-28. Type: Agency.Bothwell et al. Resolving the gravitational redshift across a millimetre-scale atomic sample. https://doi.org/10.1038/s41586-021-04349-7. As of 2022-02-16. Checked 2026-09-03. Type: Paper (Nature; NSF PAR AAM).
  4. E1

    Gao et al. 2026: 7.9/8.8 dB and ~50 nm are lab measurements on an NV ensemble with a gradient chip — no warrant for clinical MEG, GPS, or backpack Type-III sensing.

    Gao et al. NV many-body. Nanoscale magnetometry via collective many-body dynamics in diamond. https://arxiv.org/abs/2609.03039. As of arXiv:2609.03039v1 (2026-09-02); PDF research/pdfs/Gao_NV_manybody_arXiv2609.03039.pdf. Checked 2026-09-22. Type: Paper (arXiv; PDF opened).
  5. E1

    Parashar et al. 2026: minutes-scale MAS growth and RT ODMR/T2=0.56 µs apply to lab molecular crystals — no warrant for clinical MEG, GPS, backpack Type-III sensing, or NV-comparable coherence times.

    Parashar et al. MAS crystals. Room-temperature quantum-sensing molecular crystals grown in minutes. https://arxiv.org/abs/2609.30899. As of arXiv:2609.30899v1 (2026-09-25); PDF research/pdfs/MAS_MolecularCrystals_arXiv2609.30899.pdf. Checked 2026-09-28. Type: Paper (arXiv; PDF opened).
  6. E1

    Luo et al. 2026: ~10⁸ photons/s, 97% CE and 91.2% indistinguishability apply to a cryogenic (~1.4 K) antenna with SIL — no warrant for room-temperature deploy, fibre network, or commercial SPS modules.

    Luo et al. DBT antenna. 100 million photons per second from a single organic molecule. https://arxiv.org/abs/2609.29736. As of arXiv:2609.29736v1 (2026-09-24); PDF research/pdfs/DBT_Antenna_arXiv2609.29736.pdf. Checked 2026-09-27. Type: Paper (arXiv; PDF opened).
  7. E1

    SERF needs near-zero field. No fT MEG without a shield. Clinic not shown. “Without a cryostat” ≠ “without a shield.”

    Elzenheimer et al. Characterizing timing parameters in commercial SERF-OPM multichannel systems for biomagnetic field sensing. https://arxiv.org/html/2509.22198v1. As of 2025-09. Checked 2026-08-28. Type: Paper.Alem et al. An integrated full-head OPM-MEG system based on 128 zero-field sensors. https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2023.1190310/full. As of 2023-06-14. Checked 2026-08-28. Type: Paper (vendor+clinic).
  8. E1

    NV single-NV 2008 is microtesla, not femtotesla. fT is a volume-ensemble calculation. Nano-NMR/single proton is a projection.

    Taylor et al. High-sensitivity diamond magnetometer with nanoscale resolution. https://arxiv.org/html/0805.1367. As of 2008. Checked 2026-08-28. Type: Paper (proposal).
  9. E1

    The atom interferometer alone is fringe-ambiguous under vehicle acceleration. Airborne error 1–4 mGal, not µGal. Atom in orbit = CARIOQA plan, not flown.

    Bidel et al. 2018. Absolute marine gravimetry with matter-wave interferometry. https://www.nature.com/articles/s41467-018-03040-2. As of 2018-02-12. Checked 2026-08-28. Type: Paper.Bidel et al. 2020. Absolute airborne gravimetry with a cold atom sensor. https://arxiv.org/html/1910.06666. As of 2019-10. Checked 2026-08-28. Type: Paper (J. Geodesy).Jensen et al. Airborne gravimetry with quantum technology: observations from Iceland and Greenland. https://essd.copernicus.org/articles/17/1667/2025/. As of 2025-04-17. Checked 2026-08-28. Type: Paper.CNES CARIOQA. CARIOQA. https://cnes.fr/en/projects/carioqa. As of 2025-01-20. Checked 2026-08-28. Type: Agency.
  10. E2

    Type-III entanglement/Heisenberg is not the class of clocks/OPMs/gravimeters deployed today.

    Degen, Reinhard, Cappellaro. Quantum sensing. https://arxiv.org/html/1611.02427. As of 2017-06. Checked 2026-08-28. Type: Review (RMP 89).

6. Actors and incentives

Who keeps time, who measures fields

  1. E1

    NIST and BIPM (~450 clocks, ~90 labs) keep UTC; fountains calibrate, masers run 24/7.

    NIST How Do We Know. How Do We Know What Time It Is?. https://www.nist.gov/atomic-clocks/how-do-we-know-what-time-it. As of 2025-05-28. Checked 2026-08-28. Type: Agency.NIST F4 News. New Atomic Fountain Clock Joins Elite Group That Keeps the World on Time. https://www.nist.gov/news-events/news/2025/04/new-atomic-fountain-clock-joins-elite-group-keeps-world-time. As of 2025-04-28. Checked 2026-08-28. Type: Agency.
  2. E1

    PTB characterises commercial SERF OPMs (QuSpin, FieldLine) on timing; clinical relevance denied.

    Elzenheimer et al. Characterizing timing parameters in commercial SERF-OPM multichannel systems for biomagnetic field sensing. https://arxiv.org/html/2509.22198v1. As of 2025-09. Checked 2026-08-28. Type: Paper.
  3. E1

    The ONERA/CNES line (Bidel) and ESA AirQuantumGrav deploy atom gravimeters on ship and aircraft, hybrid.

    Bidel et al. 2018. Absolute marine gravimetry with matter-wave interferometry. https://www.nature.com/articles/s41467-018-03040-2. As of 2018-02-12. Checked 2026-08-28. Type: Paper.ESA AirQuantumGrav. AirQuantumGrav 2023. https://earth.esa.int/eogateway/campaigns/airborne-quantum-gravimetry-airquantumgrav-2023. As of 2024-07. Checked 2026-08-28. Type: Agency/dataset.

7. State of the dispute

Six things that are not “quantum sensing”

  1. E1

    Cs SI vs. optical lab clocks: more accurate ≠ defined. BACON repeatability hole against “redefinition done”.

    NIST How Do We Know. How Do We Know What Time It Is?. https://www.nist.gov/atomic-clocks/how-do-we-know-what-time-it. As of 2025-05-28. Checked 2026-08-28. Type: Agency.Marshall et al. High-stability single-ion clock with 5.5×10⁻¹⁹ systematic uncertainty. https://arxiv.org/html/2504.13071v1. As of 2025-04. Checked 2026-08-28. Type: Paper (PRL 135).Aeppli / BACON. Atomic clock frequency ratios with fractional uncertainty ≤3.2×10⁻¹⁸. https://arxiv.org/html/2512.21428v1. As of 2025-12. Checked 2026-08-28. Type: Paper.
  2. E1

    SQUID clinic vs. OPM research. Amplitude narrative vs. higher empty-room noise (Alem).

    Elzenheimer et al. Characterizing timing parameters in commercial SERF-OPM multichannel systems for biomagnetic field sensing. https://arxiv.org/html/2509.22198v1. As of 2025-09. Checked 2026-08-28. Type: Paper.Alem et al. An integrated full-head OPM-MEG system based on 128 zero-field sensors. https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2023.1190310/full. As of 2023-06-14. Checked 2026-08-28. Type: Paper (vendor+clinic).
  3. E1

    CARIOQA 2030 planned vs. “ESA quantum satellite 2026”.

    CNES CARIOQA. CARIOQA. https://cnes.fr/en/projects/carioqa. As of 2025-01-20. Checked 2026-08-28. Type: Agency.

8. Open questions

  1. Budker & Romalis Nature Physics HTML (arXiv physics/0611246 opened 2026-09-04 as source 19); Kominis 2003 Nature opened 2026-09-13 (source 20); Allred 2002 still closed.
  2. Barry RMP 2020 NV sensitivity review.
  3. Gerginov Metrologia 2025 body — NIST-F4 shift table.
  4. Dimarcq 2024 SI roadmap PDF.
  5. OSCAR-QUBE ISS NV — body missing.
  6. iXblue/Exail current AQG datasheet.
  7. CSAC / GPS-III Allan deviation from a maker paper.
  8. SQUID MEG noise floor 2026 (Elekta/CTF spec).
  9. Live inventory 2026: AQG devices, OPM-MEG installations, optical clocks in UTC.

9. Changes

  • v2.3-draft2026-10-25: Tofful et al. NPL–PTB Yb⁺ clock comparison arXiv:2610.29980 (source 30, E2, PDF) — 2220 km fibre, 12.6 days of data 2025; difference 1.0(4.8)×10⁻¹⁸, first international below 5×10⁻¹⁸; budget: clocks 2.8, fibre 0.9, redshift 3.5 (just over 3 cm geopotential height); Lorentz α = −2.4(2.3)×10⁻⁹ (~5× better uncertainty); one pair, geodesy dominates; preprint.
  • v2.2-draft2026-10-21: Kang et al. superconductor–spin hybrid magnetometer arXiv:2610.27764 (source 29, E2, PDF) — MnZn ferrite + SQUID, 7 mK; <100 aT/√Hz, 66 aT/√Hz at 24 kHz (~3× better than single SQUID); 10 days, 1,728/830,723 candidates rejected; axion–electron limits 5–60 kHz ×10 tighter (terrestrial); array projection E3; lab prototype.
  • v2.1-draft2026-10-18: Nagamori et al. barium fine-structure interferometry arXiv:2610.20168 (source 28, E2, PDF) — first MZ between metastable fine-structure states, visibility >10%; Zeeman 3.7×10⁷ vs 5.75×10¹⁰ Hz/T²; gradient ~50× more tolerant (calculation, E3); co-propagating, mm ≠ sensor.
  • v2.0-draft2026-10-14: Okaniwa et al. CW-ODMR AC magnetometry arXiv:2610.13153 (source 27, E1, PDF) — 14.4–632 MHz; 0.558 µT/√Hz / 24.4 µT/√Hz; no pulse sequences; lab ≠ clinic.
  • v1.9-draft2026-10-10: Th-229 solid-state nuclear clock Wang, Yin et al. arXiv:2610.10056 (source 26, E2, PDF) — 30 h locked at 148.4 nm; 1.24 × 10⁻¹³ at 10⁴ s; sustained operation ≠ accuracy.
  • v1.8-draft2026-10-06: Araki et al. NV diamond DQ 4-Ramsey dual feedback arXiv:2610.05907 (source 25, E1, PDF) — 7.0 pT/√Hz for 4 h; +2.8 K compensated; 64 fT/√Hz after ~4 h averaging (N = 14,449) in a shield; 2.5 pT test field; averaging ≠ real-time MEG.
  • v1.7-draft2026-09-28: Parashar et al. MAS crystals arXiv:2609.30899 (source 24, E1, PDF) — minutes-scale growth; RT ODMR; CT <~4 MHz; P:PQ T2=0.56 µs / CPMG 1.35 µs; lab ≠ clinic/GPS.
  • v1.6-draft2026-09-27: Luo et al. DBT antenna arXiv:2609.29736 (source 23, E1, PDF) — CE 97±2%; ~10⁸ photons/s first lens; indist. 91.2±2.3%; purity 97.7%; cryo ≠ room-temp deploy.
  • v1.5-draft2026-09-22: Gao et al. NV many-body magnetometry arXiv:2609.03039 (source 22, E1, PDF) — up to 7.9(2) dB signal / 8.8(3) dB noise gain; amplification 5.4×; ~50 nm k-resolved; lab ≠ clinic/GPS.
  • v1.4-draft2026-09-17: Annirudh et al. portable NV magnetometer arXiv:2609.16901 (source 21, E1, PDF) — unshielded noise factor 1.0±0.1; ten-fold drift suppression; pT ≠ clinic/GPS.
  • v1.3-draft2026-09-13: Kominis et al. Nature 2003 SERF (source 20, DOI 10.1038/nature01484, E1) opened — 0.54 fT/√Hz in 0.3 cm³; limit <0.01 fT/√Hz; 2 mm localization; lab ≠ clinic/GPS. Allred 2002 still closed.
  • v1.2-draft2026-09-04: Budker & Romalis Optical Magnetometry (source 19, arXiv physics/0611246) opened — SERF >10⁻¹⁵ T/√Hz, limit <10⁻¹⁷, regime <~10 nT, no cryogenics. E2 review; Nature Physics HTML still historically 429.
  • v1.1-draft2026-09-03: Bothwell et al. Nature 2022 (source 18, DOI 10.1038/s41586-021-04349-7, NSF PAR AAM) opened — millimetre gravitational redshift in ~100k ⁸⁷Sr, 7.6×10⁻²¹, lab not field; Cs remains the SI definition.
  • v1.0-draftFirst version from the 2026-08-28 verification log.

10. Sources

No. Source As of Checked Grade
1 Degen, C. L.; Reinhard, F.; Cappellaro, P.. Quantum sensing. https://arxiv.org/html/1611.02427. Type: Review (RMP 89). E1
2 Marshall, M. C.; Rodriguez Castillo, D. A.; Arthur-Dworschack, W. J.; Aeppli, A.; et al.. High-stability single-ion clock with 5.5×10⁻¹⁹ systematic uncertainty. https://arxiv.org/html/2504.13071v1. Type: Paper (PRL 135). E1
3 Aeppli, A.; et al. (BACON Collaboration). Atomic clock frequency ratios with fractional uncertainty ≤3.2×10⁻¹⁸. https://arxiv.org/html/2512.21428v1. Type: Paper. E1
4 Bidel, Y.; Zahzam, N.; Blanchard, C.; Bonnin, A.; Cadoret, M.; Bresson, A.; Rouxel, D.; Lequentrec-Lalancette, M. F.. Absolute marine gravimetry with matter-wave interferometry. https://www.nature.com/articles/s41467-018-03040-2. Type: Paper. E1
5 Bidel, Y.; Zahzam, N.; Bresson, A.; Blanchard, C.; Cadoret, M.; Olesen, A. V.; Forsberg, R.. Absolute airborne gravimetry with a cold atom sensor. https://arxiv.org/html/1910.06666. Type: Paper (J. Geodesy). E1
6 Ménoret, V.; Vermeulen, P.; Le Moigne, N.; Bonvalot, S.; Bouyer, P.; Landragin, A.; Desruelle, B.. Gravity measurements below 10⁻⁹ g with a transportable absolute quantum gravimeter. https://www.nature.com/articles/s41598-018-30608-1. Type: Paper. E1
7 NIST. NIST Ion Clock Sets New Record for Most Accurate Clock in the World. https://www.nist.gov/news-events/news/2025/07/nist-ion-clock-sets-new-record-most-accurate-clock-world. Type: Agency. E2
8 NIST. New Atomic Fountain Clock Joins Elite Group That Keeps the World on Time. https://www.nist.gov/news-events/news/2025/04/new-atomic-fountain-clock-joins-elite-group-keeps-world-time. Type: Agency. E2
9 NIST. How Do We Know What Time It Is?. https://www.nist.gov/atomic-clocks/how-do-we-know-what-time-it. Type: Agency. E1
10 NIST. Clocks Galore. https://www.nist.gov/atomic-clocks/how-atomic-clocks-work/clocks-galore. Type: Agency. E2
11 NIST. NIST’s Cesium Fountain Atomic Clocks. https://www.nist.gov/pml/time-and-frequency-division/time-realization/cesium-fountain-atomic-clocks. Type: Agency. E1
12 Elzenheimer, E.; Matz, H.; Zerfowski, J.; Anders, P.; Höft, M.; Rieger, R.; Soekadar, S. R.; Robinson, S.; Knappe-Grüneberg, S.. Characterizing timing parameters in commercial SERF-OPM multichannel systems for biomagnetic field sensing. https://arxiv.org/html/2509.22198v1. Type: Paper. E1
13 Alem, O.; Hughes, K. J.; et al.. An integrated full-head OPM-MEG system based on 128 zero-field sensors. https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2023.1190310/full. Type: Paper (vendor+clinic). E2
14 Taylor, J. M.; Cappellaro, P.; Childress, L.; Jiang, L.; Budker, D.; Hemmer, P. R.; Yacoby, A.; Walsworth, R.; Lukin, M. D.. High-sensitivity diamond magnetometer with nanoscale resolution. https://arxiv.org/html/0805.1367. Type: Paper (proposal). E1
15 CNES. CARIOQA. https://cnes.fr/en/projects/carioqa. Type: Agency. E3
16 ESA. AirQuantumGrav 2023. https://earth.esa.int/eogateway/campaigns/airborne-quantum-gravimetry-airquantumgrav-2023. Type: Agency/dataset. E1
17 Jensen, T. E.; Dale, B.; Stokholm, A.; Forsberg, R.; Bresson, A.; Zahzam, N.; Bonnin, A.; Bidel, Y.. Airborne gravimetry with quantum technology: observations from Iceland and Greenland. https://essd.copernicus.org/articles/17/1667/2025/. Type: Paper. E1
18 Bothwell, T.; Kennedy, C. J.; Aeppli, A.; Kedar, D.; Robinson, J. M.; Oelker, E.; Staron, A.; Ye, J.. Resolving the gravitational redshift across a millimetre-scale atomic sample. https://doi.org/10.1038/s41586-021-04349-7. Type: Paper (Nature; NSF PAR AAM). E1
19 Budker, D.; Romalis, M.. Optical Magnetometry. https://arxiv.org/pdf/physics/0611246. Type: Paper (review; arXiv physics/0611246; Nat. Phys. 2007). E2
20 Kominis, I. K.; Kornack, T. W.; Allred, J. C.; Romalis, M. V.. A subfemtotesla multichannel atomic magnetometer. https://doi.org/10.1038/nature01484. Type: Paper (Nature; full-text PDF). E1
21 Annirudh, K. P.; Rane, V.; Atakar, S.; Joshi, S.; Mangat, M.; Gharat, J.; Tallur, S.; Saha, K.. Portable Vector NV-Diamond Magnetometer for Shot-Noise-Limited, Drift-Free Operation in Unshielded Environments. https://arxiv.org/abs/2609.16901. Type: Paper (arXiv; PDF opened). E1
22 Gao, H.; Put, P.; Leitao, N. T.; Köylüoğlu, N. U.; Maccabe, A.; Mammen, M.; Dandavate, S.; Hughes Wyatt, L. B.; Martin, L. S.; Bleszynski Jayich, A. C.; Park, H.; Lukin, M. D.. Nanoscale magnetometry via collective many-body dynamics in diamond. https://arxiv.org/abs/2609.03039. Type: Paper (arXiv; PDF opened). E1
23 Luo, S.; Hebenstreit, T.; Shkarin, A.; Renger, J.; Utikal, T.; Götzinger, S. (Max Planck Institute for the Science of Light; FAU Erlangen-Nuremberg). 100 million photons per second from a single organic molecule. https://arxiv.org/abs/2609.29736. Type: Paper (arXiv; PDF opened). E1
24 Parashar, M.; Chen, G.; Druga, E.; Tan, L. Z.; Reimer, J.; Ajoy, A.. Room-temperature quantum-sensing molecular crystals grown in minutes. https://arxiv.org/abs/2609.30899. Type: Paper (arXiv; PDF opened). E1
25 Araki, Y.; Sekiguchi, T.; Hatano, Y.; Sekiguchi, N.; Li, Z.; Shinei, C.; Miyakawa, M.; Taniguchi, T.; Teraji, T.; Abe, H.; Onoda, S.; Ohshima, T.; Hatano, M.; Iwasaki, T. (Institute of Science Tokyo; NIMS; Univ. Tsukuba; QST; Tohoku Univ.). Robust Diamond Quantum Sensing in Double-Quantum 4-Ramsey Interferometry by Dual-Feedback Scheme. https://arxiv.org/abs/2610.05907. Type: Paper (arXiv; PDF opened). E1
26 Wang, P.; Yin, X.-F.; Wang, H.; Liu, J.; … Beeks, K.; … Schumm, T.; … Pan, J.-W.; Chen, Y.-A. (USTC / Hefei National Laboratory; SIOM CAS; TU Wien). A solid-state nuclear clock based on VUV absorption spectroscopy of 229Th. https://arxiv.org/abs/2610.10056. Type: Paper (arXiv; PDF opened). E2
27 Okaniwa, R.; Suzuki, R.; Matsuzaki, Y.; Tokuda, N.; Ishi-Hayase, J. (Keio University; Chuo University; Kanazawa University). Continuous-wave AC magnetometry up to hundreds of megahertz using nitrogen-vacancy centers under a strong transverse magnetic field. https://arxiv.org/abs/2610.13153. Type: Paper (arXiv; PDF opened). E1
28 Nagamori, M.; Hatano, H.; Hosoya, T.; Miyazawa, Y.; Sato, T.; Kozuma, M. (Institute of Science Tokyo; Japan Aviation Electronics Industry). Fine-structure Raman interferometry with metastable barium for magnetically resilient inertial sensing. https://arxiv.org/abs/2610.20168. Type: Paper (arXiv; PDF opened; proposal + proof-of-principle experiment). E2
29 Kang, R.; Zhang, J. (University of Science and Technology of China; Hefei National Laboratory); Rong, L. (Shanghai Institute of Microsystem and Information Technology, CAS; UCAS; SIM United Superconducting Technology Co.); Rong, X. (USTC; Hefei National Laboratory; Zhejiang University). Searching for Axion Dark Matter via Superconductor-Spin Hybrid Magnetometers. https://arxiv.org/abs/2610.27764. Type: Paper (arXiv; PDF opened; lab experiment). E2
30 Tofful, A.; Filzinger, M.; Schmidt, T.; … Godun, R. M.; Huntemann, N. (29 authors; National Physical Laboratory; Physikalisch-Technische Bundesanstalt; LPL Université Sorbonne Paris Nord/CNRS; Leibniz Universität Hannover; LNE-OP Observatoire de Paris). International atomic clock comparison achieving 5 × 10⁻¹⁸ uncertainty. https://arxiv.org/abs/2610.29980. Type: Paper (arXiv; PDF opened; measurement campaign of two metrology institutes, 1.09×10⁶ s of data). E2

11. Uncertainty log

Overall uncertainty of this entry, bound to the verification log of 2026-08-28 plus Bothwell Nature 2022 (NSF PAR AAM) 2026-09-03, Budker/Romalis arXiv 2026-09-04, Kominis Nature 2003 (2026-09-13, source 20), Annirudh 2026-09-17 (21), Gao 2026-09-22 (22) Luo DBT antenna 2026-09-27 (23), Parashar MAS 2026-09-28 (24) Araki NV dual feedback 2026-10-06 (25), Wang/Yin Th-229 2026-10-10 (26), Okaniwa CW-ODMR 2026-10-14 (27) Nagamori barium fine-structure interferometry 2026-10-18 (28, E2), Kang et al. hybrid magnetometer 2026-10-21 (29, E2) and Tofful et al. NPL–PTB clock comparison 2026-10-25 (30, E2). Openings incl. Bothwell AAM (E1), Budker/Romalis arXiv (E2), Kominis (E1), Annirudh/Gao/Luo/Parashar/Araki (E1); Bothwell/Budker-arXiv/Kominis removed from the hard unopened list. Not used as warrant: Wikipedia, vendor µGal without a paper, “quantum sensors replace GPS”, CARIOQA-2030 as hardware existence.

  • Established (layer 1): NPL–PTB Yb⁺ clocks over 2220 km fibre 1.0(4.8)×10⁻¹⁸, first international <5×10⁻¹⁸ (30, E2); Cs SI and UTC architecture; Al⁺ lab clock; BACON ratios including the repeatability hole; Bothwell millimetre redshift (18); SERF OPM timing/clinic-not-shown; Budker/Romalis SERF review (19); Kominis 2003 SERF lab 0.54 fT/√Hz (20, E1); Annirudh portable NV (21); Gao NV many-body (22); Luo DBT antenna ~10⁸ photons/s (23, E1); Parashar MAS RT ODMR / T2=0.56 µs (24, E1); Araki NV dual feedback 7.0 pT/√Hz for 4 h / 64 fT/√Hz averaged (25, E1); Nagamori Ba fine-structure Mach–Zehnder >10% visibility, Zeeman ~10³× smaller (28, E2); Kang hybrid magnetometer 66 aT/√Hz at 24 kHz, axion limits ×10 (29, E2); GIRAFE/AQG hybrid measurements.
  • Claimed (layer 2): NPL–PTB as a step towards redefining the second (30, E3); SI redefinition milestone; 3 cm chronometry; CARIOQA 2030; NV fT projection 2008; Araki MEG distance advantage (E3); Nagamori Ba gradient tolerance ~50× for 100 m/month (E3); Kang array projection >10⁴ (E3).
  • Constrained (layer 3): NPL–PTB one pair/one campaign, geodesy dominates, lab clock ≠ navigation (30); optical ≠ GPS (including Bothwell lab redshift ≠ GPS replacement); OPM ≠ clinic; NV ≠ fT field device (Annirudh 2026 pT noise-factor ≠ clinic; Gao 2026 dB-gain lab ≠ clinic/GPS); Luo DBT cryo antenna ≠ room-temp deploy; Parashar MAS lab crystal ≠ clinic/GPS; Araki 64 fT/√Hz averaged ≠ real-time MEG; Kang 7 mK prototype ≠ field/clinical magnetometer; Nagamori mm proof of principle ≠ navigation sensor; gravimeter ≠ µGal flight without filters; Type III not deployed.

Not opened (not a warrant)

  • Budker, Romalis Nat. Phys. 2007 HTML — HTTP 429 history; arXiv body source 19.
  • Barry et al. RMP 2020 — review body not opened.
  • Allred et al. PRL 2002 full text — still closed (OA closed). Kominis 2003 Nature opened 2026-09-13 (source 20).
  • Gerginov Metrologia 2025 body.
  • Dimarcq Metrologia 2024 roadmap PDF.
  • OSCAR-QUBE ISS; compact NV magnetometers 2025 (PRA 23, 034008; Dai) — snippets, not opened.
  • iXblue/Exail / Muquans AQG datasheet — timeout.
  • QuSpin/FieldLine spec PDFs; Microchip CSAC datasheet.
  • Boto NeuroImage 2017; Peters/Chu Nature 1999; Kasevich & Chu 1991.