Mini WorldLaplace Energy, Flight & Sensing / Plate F42 — Frontier devices

Quantum Magnetometers in Diamond

A nitrogen beside a missing carbon in diamond: the pair’s spin, read at room temperature, measures faint fields.

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Chapters · 17
  1. 01Room-Temperature Picoteslas
  2. 02Every Carbon Holds Four
  3. 03A Nitrogen and a Gap
  4. 04Three Spin States
  5. 05Green In, Red Out
  6. 06Sweep the Microwaves
  7. 07One Dip Becomes Two
  8. 08Objective and Cube, Cut Open
  9. 09A Switch for the Green
  10. 10Microwave Source and Loop
  11. 11A Magnet and Four Axes
  12. 12One Pixel, One Magnetometer
  13. 13The Field of One Neuron
  14. 14Domains, Rocks and Orbit
  15. 15What Is Still Missing
  16. 16Taken Apart
  17. 17Measure a Field

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01

Room-Temperature Picoteslas

In 2015 an experiment used about 1.4 × 10¹¹ NV centres in one diamond to reach a sensitivity of 0.9 picotesla per root hertz at room temperature; the smallest field they measured was 100 femtotesla. The conditions matter: an oscillating field (spin echo), 400 mW of laser light, a sensing volume of 8.5 × 10⁻⁴ mm³. The Earth’s field at the surface is 22⁠–⁠67 microtesla.

A single centre is far less sensitive: in the best diamonds, a few to 10 nanotesla per root hertz for oscillating fields. Its strength is size: it is ångström-scale and can sit nanometres from a sample. In 2012 one centre in a diamond tip imaged magnetic domains 25 nm wide; in 2013 a centre about 7 nm below the surface detected nuclear magnetic resonance from a (5 nm)³ volume. NV sensors work at room temperature and pressure, with no cryogenics, vacuum or tesla-scale magnets.

0.9 pT/√Hz (ensemble, 2015) · 25 nm (single centre, 2012) · room temperature

02

Every Carbon Holds Four

Diamond is a crystal of carbon. Each atom is bonded to its four nearest neighbours, which sit at the corners of a regular tetrahedron: the bonds are 1.54 Å long and 109.5° apart. The cubic cell is 3.567 Å on a side, or 0.3567 nm; a cubic centimetre holds about 1.76 × 10²³ carbon atoms.

The model is a block two cells on a side, drawn about 170 million times larger. The diamond chip in the instrument is 4 mm square and 0.5 mm thick, and holds about 1.4 × 10²¹ carbon atoms.

bond 1.54 Å · angle 109.5° · cell 0.3567 nm

03

A Nitrogen and a Gap

Put a nitrogen atom in place of one carbon and leave the site beside it empty: that is a nitrogen-vacancy centre, or NV centre. The line from the nitrogen to the vacancy is its axis, along one of the crystal’s four ⟨111⟩ directions; a diamond holds centres in all four. They can grow in as the diamond is made by chemical vapour deposition, or be made by irradiation or ion implantation followed by annealing.

The centre comes as negative (NV⁻) or neutral (NV⁰). Magnetometers use NV⁻. In the usual bonding picture, the dangling bonds of the three carbons and the nitrogen round the vacancy give five electrons, and one more comes from a donor nearby: six in all. Their ground state is a spin triplet. NV⁰ has no magnetic resonance that can be detected.

nitrogen + vacancy · along ⟨111⟩ · NV⁻ does the sensing

04

Three Spin States

A spin triplet has three states along the NV axis: ms = 0, +1 and −1. Even with no magnetic field, ms = ±1 lies above ms = 0, by a frequency D of about 2.87 GHz at room temperature (about 2.88 GHz when cold). This zero-field splitting comes from the spin–spin interaction of the centre’s electrons, not from a field.

2.87 GHz is a microwave frequency. Electron paramagnetic resonance first measured the splitting in 1977⁠–⁠78; the triplet was taken then for an excited state, and is now known to be the ground state.

ms = 0, ±1 · D ≈ 2.87 GHz (room temperature)

05

Green In, Red Out

Shine green light on the centre (usually a 532 nm laser) and its electrons are excited; falling back, they give off red light, with a zero-phonon line at 637 nm and a glow across about 600⁠–⁠850 nm. A centre in ms = ±1, once excited, takes a detour through the singlet states about 55 % of the time; that path gives none of this red light and lingers in a singlet for about 140⁠–⁠220 ns. From ms = 0 the detour happens only 14⁠–⁠17 % of the time. So ms = ±1 glows less: for a single centre the contrast is about 20 %.

Centres coming back from the detour mostly land in ms = 0. After a little green light most centres (about 80 %, from single-centre data) sit in ms = 0: the same beam sets the spin and reads it out. All of it happens at room temperature, and the centre does not bleach the way dye molecules do.

532 nm in · zero-phonon line 637 nm · ms = ±1 about 20 % darker (single centre)

06

Sweep the Microwaves

Keep the green light on and sweep the microwave frequency slowly. When it matches D, the microwaves turn the spin from ms = 0 to ms = ±1, the red glow dims, and the fluorescence curve shows a dip. This is optically detected magnetic resonance (ODMR): the spin’s state read from how bright the light is.

In 1997 a confocal microscope at room temperature first saw single NV centres, and the dip was measured on a single centre. The curve shown is computed by the page, with a dip 5 % deep and 6.3 MHz wide, typical of an ensemble.

frequency meets D → the spin flips → the glow dims

07

One Dip Becomes Two

Add a field B∥ along the NV axis: ms = +1 rises, ms = −1 falls, and the dips move to D ± γB∥, with γ ≈ 28.03 GHz/T, or 28.03 MHz per millitesla. They are 2γB∥ apart: 56 MHz apart means 1 mT along the axis. D does not change with the field, so with the field along the axis the midpoint of the two dips is D.

As the magnet slides closer on its rod, the dips move further apart. A centre measures the field along its own axis: a field at an angle does more than shift the dips, and its sideways part lowers the contrast; at sideways fields of several tens of millitesla the method stops working well.

f = D ± γB∥ · γ ≈ 28.03 GHz/T · 1 mT ↔ 56 MHz

08

Objective and Cube, Cut Open

The microscope’s column, a corner cut away. Green light enters the cube from the left and a dichroic mirror turns it down through the objective onto the diamond; the centres’ red light comes back through the same objective and passes up through the dichroic. The dichroic reflects green and passes red; one teaching build uses a long-pass dichroic that cuts at 550 nm. The lenses inside the cut objective are a sketch.

Higher up, a long-pass filter removes what green is left (600 nm in that build), and the lens in the tube images the diamond onto the camera’s sensor. Collecting the light is hard: diamond’s refractive index is about 2.41, so light meeting the surface more than about 24.5° from the normal is totally reflected and stays trapped. Through a flat face an air objective (NA 0.95) collects only about 3.7 %, an oil objective (NA 1.49) about 10.4 %.

green reflected, red passed · index ≈ 2.41 · flat-face collection 3.7⁠–⁠10.4 %

09

A Switch for the Green

On the left is a 532 nm green laser; the job needs no narrow-linewidth laser, and a plain solid-state one will do. The beam first passes an acousto-optic modulator: a sound wave in a crystal bends the light a little, and only the bent beam gets through the iris behind it, so when the sound stops, the light is off. The periscope’s two mirrors lift the beam to the height of the microscope’s cube.

With a switch, measurements can be pulsed: green light puts the spin in ms = 0, the green goes off, a microwave pulse is sent, and the green comes back on to read. The reading pulse is usually set to about 300 ns, the length that gives the best contrast. The figure shows such a sequence, as a sketch.

532 nm · acousto-optic switch · reading pulse about 300 ns

10

Microwave Source and Loop

On the right the source sweeps near 2.87 GHz; its display shows the frequency now. The amplifier boosts the signal and a coaxial cable carries it to the board, where a gold line runs to a loop round the diamond; the microwaves’ magnetic field passes through the diamond and turns the centres’ spins. The simplest antenna is a piece of wire on the end of a coaxial cable.

One teaching build uses a copper wire 40 µm across and about 5 mm long, and a source of at most 20 dBm (0.1 W) swept over 2.7⁠–⁠3.1 GHz, which is enough to see clear dips. The 2013 cell-imaging set-up put a switch before the amplifier; strong microwaves heat the sample.

2.7⁠–⁠3.1 GHz, 0.1 W (teaching build)

11

A Magnet and Four Axes

The bias magnet sits on the turntable’s arm, aimed at the chip. It gives the centres a known field that pulls the two dips apart, into the range where the Zeeman shift follows the field in a straight line and the levels respond most to a change; the faint field to be measured rides on top. With the magnet along one centre direction, that direction’s pair of dips spreads furthest.

A diamond holds centres in all four directions, each measuring the field along its own axis, so an ensemble measures the whole field vector, with no blind spots. With the field along one ⟨111⟩ direction, the other three lie at 70.5° or 109.5° to it and their dips crowd in the middle. Turn the turntable and the four pairs move on their own.

four ⟨111⟩ directions · 70.5° or 109.5° apart · an ensemble measures the vector

12

One Pixel, One Magnetometer

A quantum diamond microscope images a thin layer of centres at the diamond’s surface onto a camera. Each pixel gathers the red light of its own patch of centres and has its own ODMR curve; read the dips pixel by pixel and they make a map of the field. Diffraction limits the resolution to about λ/(2NA): for red light at 700 nm and NA 0.7, about 500 nm.

In 2013 living bacteria were imaged with an sCMOS camera at about 400 nm resolution; from the paper’s two densities of centres, a 400 nm square holds about 480⁠–⁠1,600 of them. Without an image, one photodiode measuring the total glow is enough (the teaching build and the neuron experiment do this); a single centre is read with a single-photon-counting avalanche photodiode. The map shown is a toy drawn by the page.

one ODMR curve per pixel · λ/(2NA) ≈ 500 nm (NA 0.7)

13

The Field of One Neuron

In 2016 an experiment put a diamond about 10 µm from the sample and measured, at room temperature, the magnetic field of single neurons firing (action potentials): first giant axons cut from a marine worm and a squid, then from outside a whole live worm. The preprint gives a peak-to-peak signal of about 4.1 nT for the worm axon (150 averages) and about 1 nT outside the live worm. The method needs no labels or genetic changes, does not bleach, and senses through opaque tissue.

In 2013 a similar set-up imaged, at room temperature, the field of the magnetic particle chains inside living magnetotactic bacteria, at 400 nm resolution over a 100 × 30 µm field of view; in 2015 a quantum diamond microscope detected rare tumour cells carrying magnetic labels, at single-cell resolution over a field of view of about 1 mm². The centres also work as thermometers: in 2013 nanodiamonds mapped temperature at the subcellular level in single human cells.

about 4 nT (one neuron, preprint) · 400 nm (living bacteria, 2013)

14

Domains, Rocks and Orbit

A scanning single centre sees magnetism inside materials: in 2014, domain walls jumping between two pinning sites 50 nm apart in a ferromagnetic wire 1 nm thick; in 2017, at room temperature, the spin cycloid of a bismuth ferrite (BiFeO₃) film with a period of about 70 nm; in 2019, the magnetization of single layers of CrI₃, measured at 7 K because the material is magnetic only below 45 K.

Quantum diamond microscopes map rocks too: in 2017, at 5 µm resolution over a 4 mm field of view, sections of terrestrial rocks and of the Allende meteorite. A 2026 paper reports an NV magnetometer 1U in size (10 × 10 × 10 cm), 420 g and 5 W, that mapped the field vector aboard the International Space Station. Navigating with one, without satellite signals, is still at the stage of proposals and prototypes.

25 nm (domains) · 5 µm (rocks) · 1U (space station)

15

What Is Still Missing

Too little light. In common set-ups about one photon in a thousand is detected, and photon shot noise sets the limit; the best ensemble devices are still at least about 100 times above the spin-projection limit. The contrast is fixed by the centre’s own physics: about 20 % for one centre, 5⁠–⁠10 % for ensembles. Ensembles give more signal at the cost of spatial resolution. In absolute sensitivity, SQUIDs and atomic magnetometers reach femtotesla per root hertz and below, about 1000 times better than the best ensembles.

Temperature and heat. D moves by −74.2 kHz per kelvin, so watching one dip, a change of 1 K looks like a field of 2.65 µT; watching both dips cancels it. Laser heating grows in step with power while the shot-noise limit improves only with its square root, and strong microwaves heat the sample too. Centres within 20 nm of the surface have poorer coherence and optics; yet the further the sample, the faster a dipole’s field falls (as the cube of the distance), and the blurrier the detail.

about 1 photon in 1000 · −74.2 kHz/K ↔ 2.65 µT · SQUIDs about 1000× better

16

Taken Apart

The camera, the dichroic and filter, the objective, the diamond, the microwave board, the bias magnet, the stage and turntable, the stand, the laser, the acousto-optic modulator, the iris and periscope, the microwave source and amplifier, one by one.

Rest the pointer on a part (tap it on a phone) and it lifts out on its own, named, while the rest fades.

17

Measure a Field

The lever has three stops: light, sweep, read. The green light puts the spin in ms = 0 and the chip glows red; the sweep runs the microwaves from 2.66 GHz to 3.08 GHz and draws the fluorescence curve; the reading takes the field along the NV axis from the gap between the two dips: B∥ = (f₊ − f₋) / 2γ.

Three sliders set the field (0⁠–⁠5 mT; the magnet slides along its rod), its angle to the NV axis (0⁠–⁠90°; the turntable carries the magnet round) and the temperature (10 K either way). With “four orientations” on, it is an ensemble: each direction gives its own pair of dips. Temperature moves both dips together and leaves the gap alone, so the field read from the gap does not change; watch only one dip and 1 K looks like a field of 2.65 µT. The readings are computed by the page.

Try thisSet the angle to 70°: is the field read from the two dips still the field along the NV axis? Now warm it by 10 K: does the reading change?

What the model leaves out

The table follows the arrangement papers and reviews describe: a 532 nm laser and an acousto-optic modulator, an iris and a periscope, a microscope objective, a dichroic mirror and a long-pass filter, a camera, a microwave antenna under the diamond, a bias magnet. It is no laboratory’s plans, and its light paths are shorter than real ones. The diamond is drawn 4 × 4 × 0.5 mm, the size used in the 2016 neuron experiment. The gold loop on the board and the magnet on its turntable arm are one way of doing it; real set-ups often use a piece of wire or a small loop, and a magnet placed by hand. The lattice model is about 170 million times larger than life: atoms as glazed spheres, bonds as steel rods, the vacancy as a clear sphere with a red rim, the centre’s red light drawn on the vacancy; the real electrons spread over the dangling bonds round the vacancy, with no such clean edges. The cones of green and red light and the violet shimmer round the microwave loop are signs. The fluorescence curves are computed by the page from the spin Hamiltonian with the zero-field splitting and Zeeman term only, no strain, hyperfine structure or power broadening; the chip’s red glow follows the curve, its change magnified ten times so it can be seen. The magnet’s place on its rod is estimated from the on-axis field of a magnet 12 mm across and 10 mm long, as an illustration; so is the 1 mT at the chip that the Zeeman chapter brings it in to. The level diagrams draw the Zeeman splitting larger than life (1 mT moves ±1 by only 28 MHz, a hundredth of 2.87 GHz). The cover’s loupe shows a small piece of lattice round the nitrogen and its vacancy, seen at a slant along a ⟨110⟩ direction, with the atoms spaced to scale (the bar is 0.2 nm). The field map in the camera chapter is a toy made of two magnetic dipoles.

Common mix-ups

The centre here is a sensor, not a quantum computer: the measurement uses the shift of the spin’s frequency. A centre measures the field along its own axis, not the total field; the whole vector comes from the centres in the diamond’s four directions. 2.87 GHz is the zero-field splitting, not a Zeeman splitting: it is there with no field, and a field pulls the two dips apart on either side of it. “Room temperature” means no cryogenics, vacuum or strong magnets, not that temperature does not matter: D drifts by −74.2 kHz per kelvin, and ensemble set-ups with watts of laser light need heat sinking. A single centre is not more sensitive than a SQUID; it wins on size, closeness to the sample and resolution. Nor does the centre turn green light into red one for one: each green photon absorbed gives at most one red photon, the rest of its energy goes into lattice vibrations, a detour through the singlets gives none, and most of the light that is given off stays trapped in the diamond. Navigating with NV magnetometers is still at the stage of proposals and prototypes. 1997 saw the first single centre; the defect itself and its 2.88 GHz splitting had been studied in bulk samples since the 1960s and 1970s.

Common questions · 4
What is the zero-field splitting D of an NV centre?

Even with no magnetic field, ms = ±1 lies above ms = 0, by a frequency D of about 2.87 GHz at room temperature (about 2.88 GHz when cold).

04Three Spin States
What is optically detected magnetic resonance (ODMR)?

When it matches D, the microwaves turn the spin from ms = 0 to ms = ±1, the red glow dims, and the fluorescence curve shows a dip.

06Sweep the Microwaves
How does an NV centre read a magnetic field from its dips?

Add a field B∥ along the NV axis: ms = +1 rises, ms = −1 falls, and the dips move to D ± γB∥, with γ ≈ 28.03 GHz/T, or 28.03 MHz per millitesla.

07One Dip Becomes Two
Are diamond magnetometers more sensitive than SQUIDs?

In absolute sensitivity, SQUIDs and atomic magnetometers reach femtotesla per root hertz and below, about 1000 times better than the best ensembles.

15What Is Still Missing

Sources

  1. Doherty et al. (2013): The nitrogen-vacancy colour centre in diamond, Physics Reports 528, 1 (C3v nitrogen–vacancy pair along [111]; NV⁻ and NV⁰; six electrons; about 2.87 GHz at room temperature, 2.88 GHz cold; 1.54 Å bonds; 637 nm zero-phonon line; Loubser and van Wyk’s EPR; grown in, or irradiated or implanted and annealed)
  2. Rondin et al. (2014): Magnetometry with nitrogen-vacancy defects in diamond, Rep. Prog. Phys. 77, 056503 (only NV⁻; the projection on the axis; about 20 % contrast for one centre; reading pulse about 300 ns; detection efficiency about 10⁻³; index 2.4; coherence times; shallow centres; no bleaching)
  3. Barry et al. (2020): Sensitivity optimization for NV-diamond magnetometry, Rev. Mod. Phys. 92, 015004 (D ≈ 2.87 GHz; g ≈ 2.003; ν± = D ± γB; four directions and the vector; 532 nm; 600⁠–⁠850 nm; singlet lifetime about 140⁠–⁠220 ns; 3.7 % and 10.4 % through a flat face, n ≈ 2.41; about 100× above the spin-projection limit; SQUIDs and SERF about 1000× better; ambient operation; ensembles trade resolution for signal)
  4. Degen, Reinhard, Cappellaro (2017): Quantum sensing, Rev. Mod. Phys. 89, 035002 (what quantum sensing is; about 1 pT/√Hz; atomic vapour magnetometers)
  5. Acosta et al. (2010): Temperature Dependence of the Nitrogen-Vacancy Magnetic Resonance in Diamond, PRL 104, 070801 (dD/dT = −74.2 kHz/K over 280⁠–⁠330 K; watching both resonances cancels it)
  6. Robledo et al. (2011): Spin dynamics in the optical cycle of single nitrogen-vacancy centres in diamond, New J. Phys. 13, 025013 (chance of entering the singlets: about 14⁠–⁠17 % from ms = 0, about 55 % from ms = ±1; excited-state lifetimes; acousto-optic modulator; avalanche photodiode)
  7. Balasubramanian et al. (2009): Ultralong spin coherence time in isotopically engineered diamond, Nature Materials 8, 383 (0.3 % ¹³C; T₂ = 1.8 ms; 4 nT/√Hz)
  8. Shikata et al. (2018): Precise measurements of diamond lattice constant using Bond method, Jpn. J. Appl. Phys. 57, 111301 (3.567 095 Å)
  9. Loubser, van Wyk (1978): Electron spin resonance in the study of diamond, Rep. Prog. Phys. 41, 1201
  10. Gruber et al. (1997): Scanning Confocal Optical Microscopy and Magnetic Resonance on Single Defect Centers, Science 276, 2012 (single centres and their magnetic resonance at room temperature)
  11. Maletinsky et al. (2012): A robust scanning diamond sensor for nanoscale imaging with single nitrogen-vacancy centres, Nature Nanotechnology 7, 320 (magnetic domains 25 nm wide)
  12. Staudacher et al. (2013): Nuclear Magnetic Resonance Spectroscopy on a (5-Nanometer)³ Sample Volume, Science 339, 561 (a centre about 7 nm below the surface)
  13. Le Sage et al. (2013): Optical magnetic imaging of living cells, Nature 496, 486 (magnetotactic bacteria; 400 nm; 100 × 30 µm; sCMOS camera; source, switch, amplifier and wire; density of centres)
  14. Glenn et al. (2015): Single-cell magnetic imaging using a quantum diamond microscope, Nature Methods 12, 736 (about 1 mm² field of view, single-cell resolution)
  15. Kucsko et al. (2013): Nanometre-scale thermometry in a living cell, Nature 500, 54 (temperature mapped at the subcellular level)
  16. Barry et al. (2016): Optical magnetic detection of single-neuron action potentials using quantum defects in diamond, PNAS 113, 14133 (about 10 µm; single neurons of worm and squid, live worms; no labels, no bleaching)
  17. Barry et al. (2016): the same paper’s arXiv preprint 1602.01056 (4.1 nT peak to peak, 150 averages; about 1 nT outside a live worm; a 4 × 4 × 0.5 mm diamond; a photodiode; watts of laser light and a heat spreader)
  18. Wolf et al. (2015): Subpicotesla Diamond Magnetometry, PRX 5, 041001 (0.9 pT/√Hz, oscillating field, spin echo; about 1.4 × 10¹¹ centres; 400 mW; 8.5 × 10⁻⁴ mm³; 100 fT; single centres about 10 nT/√Hz)
  19. Tetienne et al. (2014): Nanoscale imaging and control of domain-wall hopping with a nitrogen-vacancy center microscope, Science 344, 1366 (a nanowire 1 nm thick, pinning sites 50 nm apart)
  20. Gross et al. (2017): Real-space imaging of non-collinear antiferromagnetic order with a single-spin magnetometer, Nature 549, 252 (BiFeO₃, about 70 nm, room temperature)
  21. Thiel et al. (2019): Probing magnetism in 2D materials at the nanoscale with single-spin microscopy, Science 364, 973 (monolayer CrI₃ at 7 K; Curie temperature 45 K)
  22. Glenn et al. (2017): Micrometer-scale magnetic imaging of geological samples using a quantum diamond microscope, Geochem. Geophys. Geosyst. 18, 3254 (5 µm, 4 mm; the Allende meteorite)
  23. Beerden et al. (2026): Diamond-based magnetometer aboard the International Space Station, Phys. Rev. Applied 25, 054017 (1U, 420 g, 5 W; vector field maps)
  24. Zhang et al. (2018): Little bits of diamond: Optically detected magnetic resonance of nitrogen-vacancy centers, Am. J. Phys. 86, 225 (teaching build: 532 nm, 550 nm dichroic, 600 nm long-pass, photodiode, 40 µm copper wire, 20 dBm, 2.7⁠–⁠3.1 GHz; ensemble contrast 5⁠–⁠10 %; fitted width 6.3 MHz)
  25. Levine et al. (2019): Principles and techniques of the quantum diamond microscope, Nanophotonics 8, 1945 (the quantum diamond microscope; λ/(2NA); wire antennas; laser heating and shot noise; 5 % contrast)
  26. British Geological Survey: The Earth’s magnetic field: an overview (22,000⁠–⁠67,000 nT at the surface)
  27. NIST, CODATA 2022: the Bohr magneton µB/h (γ = gµB/h ≈ 28.03 GHz/T, taking g = 2.003)

Data

The table follows the arrangement the papers and reviews describe (a green laser and a switch, a microscope objective, a dichroic mirror and a long-pass filter, a camera, a microwave antenna under the diamond, a bias magnet); it is no laboratory’s plans, and its light paths are shorter than real ones. The ODMR spectra in the HUD and the laboratory are computed by this page from the spin Hamiltonian (zero-field splitting and Zeeman term only); the field map and the magnet’s place on its rod are toys, not measurements. Material presets, parts and the softbox studio of the site’s model kit are ported from AirsupHQ/machine-skill (MIT, © 2026 Air Supply Technologies Inc.).

The text, pictures and models on this page are licensed CC BY-NC 4.0: free to use and adapt for teaching and other non-commercial purposes, with credit to world.lapla.si. Third-party material listed above keeps its own licence.

Cite this plate

Laplace’s Mini World, Plate F42, “Quantum Magnetometers in Diamond”, https://world.lapla.si/en/nvdiamond/, CC BY-NC 4.0, updated 2026-10-24.

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