1. What quantum sensing is — quantum as a yardstick

First, an everyday sense

Think of a clock. A pendulum clock counts the swings of a pendulum to measure time. Now suppose every pendulum swings with exactly the same period. With no individual differences, a clock made anywhere would be the same clock. Atoms are close to this. Atoms of the same kind absorb light of exactly the same frequency (because their energy levels are values set by nature). An atomic clock uses that frequency as its “yardstick.”

This is the basic idea of quantum sensing. Things in the quantum world have discrete energy values, and the same values for the same kind of particle. If you read a slight shift in those values, you learn the influence of a magnetic field, gravity and so on.

A definition in words

The review by Degen and colleagues describes quantum sensing as the use of a quantum system, quantum properties or quantum phenomena to perform a measurement of a physical quantity, and lists SQUID and atomic-vapor magnetometers and atomic clocks as historical examples Sourced [Ref. 1]. They also divide quantum sensors into three types [Ref. 1].

• Type I: measuring with a quantum object that has discrete energy levels (spins, atoms, ions and so on)
• Type II: measuring with quantum coherence (wave-like superposition)
• Type III: using quantum entanglement to approach a sensitivity or precision that is impossible classically

The same review lists what a system needs to be a quantum sensor, in a form similar to the DiVincenzo criteria for quantum computers: (1) discrete energy levels, (2) it can be initialized and read out, (3) it can be manipulated coherently (and (4) it couples to the quantity to be measured) [Ref. 1]. The important point is that the same ingredients as a quantum computer are needed.

Quantum limits, and ordinary limits

When you measure with N identical quantum sensors, statistics improve the precision like 1/√N. This is called the standard quantum limit (SQL). The review by Degen and colleagues explains that this 1/√N scaling follows from the quantum Cramér-Rao bound, and that going beyond it requires states with quantum entanglement Sourced [Ref. 1]. With entanglement one can in theory approach 1/N (the Heisenberg limit). In experiments, however, noise (decoherence) is a major wall (our explanation).

2. Reading sensitivity — sizes of magnetic fields and sensor noise (our calculation)

What does “T/√Hz” mean?

The performance of a magnetometer is written as a “sensitivity” in units of T/√Hz (tesla per root hertz). It is a guide to the size of the magnetic field that cannot be told apart from noise after measuring for one second. The √Hz appears because noise averages down as the square root of time. If you lengthen the measurement time t (in seconds), the smallest measurable field becomes δB = η / √t (where η is the value in T/√Hz) (our explanation; for white noise).

For example, suppose you want to measure a field of 10 fT with a sensor of η = 1 pT/√Hz. The time needed is t = (η / δB)² = (10⁻¹² / 10⁻¹⁴)² = 10⁴ seconds, or about 2.8 hours Our calculation. If η is 1 fT/√Hz, the same 10 fT takes 0.01 seconds. A 1000-fold difference in sensitivity means a million-fold difference in measurement time. Barry and colleagues write that the roughly 1000-fold gap between NV ensemble sensors (pT/√Hz class) and SQUIDs or SERF magnetometers (fT/√Hz or below) corresponds to a roughly 10⁶-fold increase in required averaging time, which rules out many envisioned applications Sourced [Ref. 3].

3. NV centers in diamond — a defect that becomes a magnetometer

What an NV center is

Diamond is a crystal in which carbon atoms are lined up regularly. In it there is a defect in which one carbon atom has been replaced by nitrogen (N) and the neighboring carbon atom is missing, leaving a vacancy (V). This is called an NV center (nitrogen-vacancy center). Barry and colleagues describe it as a point defect made of a substitutional nitrogen fixed next to a vacancy within the rigid carbon lattice Sourced [Ref. 3]. There are three charge states, and the negatively charged NV⁻ is the one used for quantum sensing [Ref. 3].

This defect is like an artificial atom trapped inside diamond. According to Barry and colleagues, the ground state of NV⁻ is a spin-1 triplet with long spin lifetimes at room temperature (a longitudinal relaxation time T₁ of about 6 ms is typical, and coherence times T₂ up to a few ms are achievable) [Ref. 3].

Align with light, rotate with microwaves, read with light

The spin of an NV center can be handled with light and microwaves alone. When a green laser (about 532 nm) shines on it, the spin is aligned into a definite state (ms = 0) (initialization), and the brightness of the red fluorescence differs with the spin state. Microwaves (around 2.87 GHz) rotate the state, and the fluorescence brightness reads it out Sourced [Ref. 3]. When a magnetic field is applied the energy levels split, so the microwave frequency at which the fluorescence dips gives the field (our explanation). The energy levels of an NV center are sensitive to magnetic fields, electric fields, strain and temperature, so one defect works as a multi-purpose sensor [Ref. 3]. In temperature, the zero-field splitting D changes by about −74 kHz/K [Ref. 3].

The advantage is that it works at room temperature, atmospheric pressure and low magnetic fields, and needs no cryogenic cooling, vacuum system or tesla-scale magnet. Barry and colleagues note that diamond is chemically inert and biocompatible, and that the sensor can be placed within about 1 nm of what is to be measured Sourced [Ref. 3].

How sensitive is it? — single and ensemble, and the gap to the theoretical limit

According to Wolf and colleagues, the room-temperature magnetic-field sensitivity of a single NV center is limited to about 10 nT/√Hz. With an ensemble of N centers, the sensitivity improves in proportion to √N. They used an ensemble of 10¹¹ NV centers and reached the photon-shot-noise-limited 0.9 pT/√Hz (effective sensor volume 8.5 × 10⁻⁴ mm³), which they describe as the best diamond magnetometer sensitivity so far Sourced [Ref. 4].

Let us do a simple calculation. 10 nT/√Hz / √(10¹¹) = 10⁻⁸ / 3.2 × 10⁵ = about 32 fT/√Hz. The measured 0.9 pT/√Hz is about 28 times larger (worse) than this simple estimate Our calculation (a rough comparison between experiments with different numbers and conditions, so only a guide). Barry and colleagues write that present NV ensemble devices have sensitivities orders of magnitude away from theoretical limits, and that the most promising avenues are the spin dephasing time, the readout fidelity and the properties of the host diamond material Sourced [Ref. 3]. They also state that while SQUIDs and SERF magnetometers reach fT/√Hz or below, NV ensembles are at the pT/√Hz level, a roughly 1000-fold gap [Ref. 3].

Materials set the performance — nitrogen and 13C in diamond

The sensitivity of an NV ensemble is set by the spin dephasing time T₂*, and T₂* is set by impurities and isotopes in the diamond (the review by Barry and colleagues) [Ref. 3]. There are two major factors.

• Substitutional nitrogen (nitrogen that did not become an NV): a magnetic noise source. Barry and colleagues express the dephasing of an NV ensemble by nitrogen with a coefficient of 101 ms⁻¹ ppm⁻¹ [Ref. 3].
• 13C (a carbon isotope with a nuclear spin): natural diamond contains 13C at 10,700 ppm (about 1.1%), which shifts the phase of the NV electron spin [Ref. 3]. This plays exactly the role of 29Si in silicon (see our explainer on silicon spin qubits).

Barry and colleagues give the limit that 13C sets on the dephasing time as 1/T₂*(13C) = A_13C × [13C], with coefficient A_13C ≈ 0.100 ms⁻¹ ppm⁻¹ Sourced [Ref. 3]. Using this, for natural 13C (10,700 ppm) the upper limit of T₂* is about 0.93 µs, and with 12C purified to 99.999% (13C at 10 ppm) it is about 1 ms Our calculation. Barry and colleagues also say that T₂*(13C) for natural-abundance diamond is about 1 µs [Ref. 3]. Isotope purification raises the limit more than a thousandfold.

As an example of 12C purification, Barry and colleagues note that Teraji and colleagues used methane with 99.999% 12C (13C of 10 ppm or less) to obtain CVD diamond whose 12C concentration measured by SIMS was 99.998% [Ref. 3]. The same review says that such isotopically enriched methane is 10³ to 10⁴ times more expensive than natural-abundance methane, and that 13C can also get in from other carbon sources in the CVD chamber Sourced [Ref. 3]. An overview of isotope control in diamond and silicon is given in the review by Itoh and Watanabe [Ref. 18].

4. Atomic magnetometers — measuring the brain's field with alkali vapor

How it works — atoms in a glass container become tiny compasses

An atomic magnetometer puts vapor of an alkali metal such as rubidium or cesium in a glass cell. Kominis and colleagues describe it as detecting the Larmor spin precession (turning) of optically pumped atoms Sourced [Ref. 6]. The flow is as follows (our explanation).

1. A circularly polarized laser (the pump beam) aligns the spins of the vapor atoms in the same direction
2. In a magnetic field, the aligned spins begin to turn
3. A second laser (the probe beam) passes through the vapor, and the change in its polarization shows how the spins are turning

SERF magnetometers — the condition that nearly removes the noise

When atoms collide with each other and their spins are scrambled (spin-exchange relaxation), sensitivity drops. In the SERF (spin-exchange relaxation-free) regime this relaxation can be removed. Kominis and colleagues report that a SERF magnetometer reached 0.54 fT/√Hz with a measurement volume of only 0.3 cm³, and that their analysis puts the fundamental limit of the device below 0.01 fT/√Hz Sourced [Ref. 6]. However, Budker and Romalis explain that the SERF regime holds only for magnetic fields below about 10 nT, and that in a field such as the Earth's, a SERF magnetometer is used as a null detector with external feedback [Ref. 7]. That is why magnetic shielding or a way of cancelling the background field is needed.

A SQUID works at 4 K (liquid-helium temperature) and has a sensitivity of about 1 fT/√Hz, but it needs cooling. Kominis and colleagues write that atomic magnetometers could reach similar sensitivity without cryogenic cooling, opening new applications such as non-invasive mapping of the cortex [Ref. 6]. Budker and Romalis also list no need for cryogenic cooling and the potential for miniaturization as advantages of atomic magnetometers [Ref. 7].

Measuring the brain's field without cooling

Boto and colleagues reported a wearable magnetoencephalography (MEG) system that can be worn like a helmet, combining quantum sensors (optically pumped atomic magnetometers) with a way of cancelling the background field. They state that subjects could move naturally, for example nodding the head or playing with a ball, while brain activity was measured with millisecond resolution Sourced [Ref. 9]. Conventional SQUID-based MEG required the head to be held still. Declaration of competing interests: the paper states that one of its authors is the founding director of the company that sells the atomic magnetometers used, and that the company built the sensors and advised on the system design [Ref. 9]. Whether wearable MEG becomes practical still needs to be verified Not yet confirmed.

5. Atomic clocks and optical lattice clocks — time that measures height

How an atomic clock works

The first practical caesium atomic clock was built in 1955 (as stated at the opening of the paper by Takamoto and colleagues) Sourced [Ref. 11]. The definition of the second is tied to a microwave transition of the caesium atom; according to BIPM (the International Bureau of Weights and Measures) it is fixed by the microwave frequency characteristic of the Cs atom (Δν_Cs) [Ref. 34]. In their 2005 paper, Takamoto and colleagues write that the fractional uncertainty of this realization of the second is about 10⁻¹⁵ [Ref. 11].

The principle resembles a pendulum clock (our explanation). The frequency of a laser or microwave source is kept matched to a definite frequency that the atom absorbs, and the oscillations at that frequency are counted to make time. Every atom has the same frequency, so atoms give the same yardstick.

Optical lattice clocks — counting the oscillation of light, more finely

The frequency of light is much higher than that of microwaves (it oscillates faster), so the same time can be divided more finely (our explanation). To count the oscillations of light, a technology called the optical frequency comb links optical and radio frequencies [Ref. 11]. The group of Katori reported an optical lattice clock in 2005. In this scheme, many atoms are trapped in a “lattice” made by a standing wave of light and used as the quantum reference, with the potential to combine the accuracy of trapped ions with the stability of neutral atoms Sourced [Ref. 11].

Progress since has been large. In 2022, Bothwell and colleagues reduced the fractional frequency measurement uncertainty to 7.6 × 10⁻²¹, enough to measure the gravitational redshift within a single millimetre-scale sample of ultracold strontium Sourced [Ref. 13]. The review of optical clocks organizes in detail the measurement precision and systematic uncertainty of optical clocks that use trapped single ions and many neutral atoms [Ref. 10].

A time shift is a height difference — our calculation

According to general relativity, a clock at a higher place runs faster than a clock at a lower place (gravitational redshift). The fractional frequency shift for a height difference Δh is approximately Δf/f = gΔh / c² (a general approximation; our explanation). Putting in g = 9.80665 m/s² and c = 299,792,458 m/s gives about 1.1 × 10⁻¹⁸ for 1 cm, about 1.1 × 10⁻¹⁶ for 1 m and about 4.9 × 10⁻¹⁴ for 450 m Our calculation. So a clock with an uncertainty at the 10⁻¹⁸ level can detect a height difference of about 1 cm as a time shift. Takamoto and colleagues make the same point [Ref. 12].

In 2020, Takamoto and colleagues built two transportable optical lattice clocks, placed them at the bottom and top of Tokyo Skytree (a height difference of 450 m), and made a frequency comparison with 18-digit precision. They write that the gravitational redshift was tested at (1.4 ± 9.1) × 10⁻⁵ relative to the prediction, an order of magnitude better than earlier ground-based clock comparisons Sourced [Ref. 12]. The same paper says optical clocks that can resolve centimetres are technically ready for field applications such as monitoring changes of geopotential caused by volcanoes or crustal deformation, and defining the geoid [Ref. 12].

Changing the definition of the second — a plan versus an achievement

According to BIPM, some optical frequency standards have reached systematic uncertainties two orders of magnitude smaller than caesium fountain clocks. The roadmap updated by the CCTF in 2025 treats 2030 as the decisive milestone, and at the CGPM in 2026 progress is to be reviewed and a draft resolution on continued work toward 2030 considered Sourced [Ref. 34]. The conditions for redefinition include an improvement of accuracy by 10 to 100 times, continuity with the caesium definition, continuous availability, and acceptability to national metrology institutes [Ref. 34]. We could not confirm the outcome of the 2026 CGPM. Redefining the second is a matter planned to be decided in 2030 Not yet confirmed.

6. Gravimeters and atom interferometers — falling atoms that probe underground

Using atoms as waves

In the quantum world, an atom is both a particle and a wave. An atom interferometer takes a cooled cloud of atoms and applies laser pulses three times, splitting the atomic wave in two and then recombining it. A phase difference due to gravity and other effects builds up along the two paths, and the interference seen on recombination reveals the acceleration (gravity). In 1999, Peters and colleagues used an atom interferometer based on a fountain of laser-cooled atoms to measure the acceleration of gravity g, and obtained an absolute uncertainty of Δg/g ≈ 3 × 10⁻⁹. They describe this as a million-fold improvement in absolute accuracy over earlier atom-interferometer experiments, and report that a macroscopic glass object falling in a Michelson-interferometer gravimeter fell with the same acceleration as a caesium atom to within 7 parts in 10⁹ Sourced [Ref. 14].

Multiplying Δg/g ≈ 3 × 10⁻⁹ by g ≈ 9.8 m/s² gives about 2.9 × 10⁻⁸ m/s² (about 2.9 µGal) Our calculation.

Finding an underground tunnel in the field

In 2022, Stray and colleagues reported field-survey results from a quantum gravity-gradient sensor. The instrument is designed to suppress micro-seismic and laser noise, temperature and magnetic-field variations, and instrument tilt. It reached a statistical uncertainty of 20 E (1 E = 10⁻⁹ s⁻²), carried out a survey with 0.5 m spatial resolution along an 8.5 m line, and detected a 2 m tunnel with a signal-to-noise ratio of 8. Using Bayesian inference, they determined the centre horizontally to ±0.19 m and the centre depth as 1.89 m (−0.59/+2.3 m) Sourced [Ref. 15]. The authors say that mapping aquifers, archaeology, determining soil properties, and reducing the risk of unforeseen ground conditions in infrastructure construction could be applications [Ref. 15].

The perspective by Bongs and colleagues says that atom interferometers are moving out of the laboratory to be used as ultraprecise quantum sensors in metrology, geophysics, space, civil engineering, oil and mineral exploration, and navigation, and discusses the scientific and technological challenges [Ref. 16].

7. Examples of applications in materials, semiconductors and biology — avoiding exaggeration

Here we introduce applications of quantum sensors while stating what stage each report is at. “Reported” and “in use in the field” are different things. The studies that opened up nanoscale magnetic sensing with a single NV center are the 2008 papers by Maze et al., Balasubramanian et al. and Taylor et al. (as given in the titles) Sourced [Refs. 28, 29, 30]. A review for physics and biology is given by Schirhagl et al. [Ref. 17].

The table is our own organization (Our calculation). The “Stage” column is our reading of each paper's description, not a judgment of whether a product exists or how mature it is. No individual medical, food or safety advice is given.

8. How it differs from, and resembles, quantum computing

The table is our own organization (Our calculation). It does not argue which is better.

9. What we could not confirm, and a summary

What we could not confirm

• The outcome of the 2026 CGPM (which discusses progress on redefining the second): on BIPM's page we could confirm the planned meeting and the roadmap (2030 as the decisive milestone), but not the outcome of the meeting [Ref. 34].
• Records of NV magnetometer sensitivity since 2020: we stayed with the picture in the review by Barry and colleagues (2020) and did not comprehensively check later records.
• Performance, sales and adoption of quantum sensor products: we did not check company product specifications or market sizes against primary sources, and do not state them.
• Practical industrial use of NV sensors, such as semiconductor failure analysis: what we could confirm is an early laboratory experiment of 2015 [Ref. 22].
• Field use of quantum sensors that use entanglement (Type III): not confirmed.
• Standards specific to quantum sensing: not confirmed (Chapter 10).
• Domestic research programs and company efforts: not covered in this article (not checked).

10. How to read evaluations and standards

The performance of a quantum sensor is measured by sensitivity (how small a quantity it can measure; T/√Hz and so on), stability and uncertainty (fractional uncertainty and Allan deviation for clocks), accuracy (the correctness of absolute values; for example international comparisons of gravimeters), spatial resolution, size and the environments where it works. No single number tells the whole story. On the standards side there are definitions of frequency and time stability (IEEE 1139-2022), international comparisons of absolute gravimeters (BIPM), and the definition of the second (SI). But we could not confirm any evaluation standard specific to quantum sensing (checked 16 October 2026).

The table is our own organization (Our calculation). Numbers in brackets are reference numbers. Numbers, titles and status of standards and BIPM documents were checked on the official pages (16 October 2026). Standards texts are paywalled, so we limit ourselves to titles and scope.

Cautions in reading (for materials engineers)

• Sensitivity is a number with conditions: it changes with frequency band, integration time, sensor volume and environment (whether magnetic shielding is used). Even the same “pT/√Hz” cannot be compared if the premises differ
• A record in a paper and performance in the field are different: the 0.9 pT/√Hz of Wolf and colleagues is one laboratory's record, and the review by Barry and colleagues says that typical devices are orders of magnitude away from the theoretical limit [3, 4]
• Check the company's involvement: where a company that sells the sensor is involved, as in the paper of Boto and colleagues, competing interests are stated in the paper [9]. The performance of company products must be checked against primary sources
• Plans and achievements are different: redefining the second (a discussion toward 2030) and geodetic applications of optical clocks are Not yet confirmed. Standards are mostly recommendations, not requirements

11. Glossary

12. References (primary sources)

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38. ISO/IEC "IEC/ISO JTC 3 Quantum technologies" (committee page) — committee.iso.org/committee/10138914.html

13. Claim-to-source audit

Last updated: 16 October 2026. Sources are limited to primary ones (peer-reviewed papers, reviews, official pages of BIPM, NOAA, IEEE and ISO, and an international comparison report). Sensitivity and similar numbers come from different measurement conditions and do not show a ranking. Application examples are introduced mainly at the research-report or demonstration stage. We could not confirm, and therefore do not state, the outcome of the 2026 CGPM, NV magnetometer records since 2020, the performance and adoption of quantum sensor products, or standards specific to quantum sensing. All figures are explanatory concept drawings or drawings including our calculation; Figs. 1 and 10 are ImageGen-generated conceptual illustrations.