Quantum sensor is useless.

(Fig.1)  External magnetic field B splits energy levels of NV center with electron's orbital magnetic moment = μ_B

Nitrogen vacancy center (= NVC ) is a useless sensor not using (fictional) spin.

Nitrogen vacancy (= NV ) center in diamond consisting of multiple electrons was often cited as a potential quantum magnetic sensor based on (imaginary) electron spin, but actually, this NV center is too fragile to be practical, and it does Not prove the existence of (fictitious) electron spin.

This (hyped) NV center in diamond as quantum magnetic sensing has been useless ( except for experimental purpose ) despite extremely long years of researches since its discovery in 1950s, which fact proves NV center or quantum sensors are hopeless forever.

This challenges and future directions (2024) still says
"Building robust and scalable quantum sensors remains an ongoing effort. Additionally, integrating these sensors into practical devices requires further development."  ← deadend ( this p.1-left-2nd-paragraph )

NV center's (fictional) spin can be replaced by realistic electron's orbital motion

In NV center, physicists cannot directly measure the (fictional) electron spin.
Only its (spin) magnetic moment called Bohr magneton ( this p.2 ) can be measured, this magnetic moment is equal to and replaced by realistic electron's orbital motion.

So there is No evidence of electron spin, whose magnetic moment can be explained by electron's orbital motion in this NV center in diamond.

All physicists can measure in NV center is the light wave energy interacting with energy levels of NV center ( this p.4 ) whose energy levels' interval is the applied magnetic field B × an integer (= 2 due to 2 electrons involved ) × Bohr magneton (= μ_B,  this p.1-right-lower,  this p.2-left-[1]  this Fig.1,  this p.3-left-3rd-paragraph ).

This energy splitting related to Bohr magneton can be naturally explained by electron's orbital motion instead of unrealistic electron spin.

So this NV-center in diamond can Not prove the existence of (fictitious) electron spin (= quantum mechanics cannot explain its detailed mechanism even now, this p.3-2nd-paragraph,  this p.5-left-last, p.5-right. p.11-left,  this p.3-intro-1st-paragraph ).  Ordinary electron's orbital motion can perfectly explain this.

(Fig.2)  NV center in diamond is too unstable to be practical

Furthermore, this NV center is too fragile, and easily broken, degraded (= become inactive called decoherence, this p.2-2nd-paragraph,  this p.1-left,  this abstract,  this-1.introduction-3rd-paragraph ).
And due to its susceptibility to a lot of irrelevant noise, precise detection of small magnetic field by NV center is impossible ( this p.1-right-top,  this 2.3 ).

This p.2-upper says
"One of the current serious problems is noise, which reduces sensitivity of NV center diamond sensors by broadening of the spectral linewidth and reduces spectral resolution of the device"

This p.1-abstract says
"However, the promise of NV centers is hindered by a severe degradation of critical sensor properties, namely charge stability and spin coherence, near surfaces ( 10 nm deep)."  ← NV center is still useless due to its instability.

NV center is easily degraded by various noise.  ← impractical quantum sensor

This p.3-1st-paragraphs say
"While this makes the NV center a powerful sensor, it puts strict requirements for the surroundings. In fact, any noise or instability (and especially nearby paramagnetic defects) can strongly degrade its excellent as-fabricated properties."

This p.2-2nd-paragraph says
"Despite the high magnetic sensitivity of NVs and their very small size, measuring nano-scale magnetic field sources using NVs is still a challenge. Next to high magnetic sensitivity, proximity to the field source is essential for such sensors. Therefore, a high quality shallow layer of NVs is desirable. When using conventional implantation or growth techniques, shallow NVs (< 5 nm from the surface) are mostly stable in the neutrally charged state (= unusable as sensors, this 1.introduction-3rd-paragraph ). Even when a shallow NV maintains its charge, it usually has very short relaxation time leading to low magnetic sensitivity"

This p.7-right-4th-paragraph and p.11-3rd-paragraph says
"For NV centres within a few tens of nanometres of the diamond surface, surface-related charge instabilities and noise further degrade NV properties"

"Although there are promising opportunities, many challenges remain, probably calling for collaborations between multiple academic domains and industry"

See also This-p.2-I.introduction-left,  This p.2-right,  this p.18-8,   this introduction-3rd-paragraph.

Uncertain NV center positions is also why NV center is still useless as a sensor.

And it is impossible to produce NV centers in precise positions (= which affect their sensitivity to external magnetic fields ) inside diamond (= which means uncertain NV center sensitivities make it impossible to precisely measure the absolute value of the target magnetic field ), which fact makes the NV-center impractical as magnetometer.

↑ Because the sensitivity of NV center to magnetic field changes depending on the uncertain positions of NV centers or distance from diamond's surface ( this 1.introduction says distribution of NV centers at diamond is random,  this abstract ).

This p.6-right-1st-paragraph says
"Since there is No precise control of the NV center density and their proximity to the surface, NDs (= nano-diamonds ) suffer from variability in their sensitivity. Factors such as local strain anisotropy and crystal impurities also lead to spin and optical properties variations. This inhomogeneity results in inconsistent measurements among different NDs."

This-middle Challenges and road ahead says
"Despite their remarkable potential, NV centers still face certain challenges: (= still useless )"

"Scalability: Building large-scale quantum computers using NV centers requires precise placement and control of individual centers, which presents a significant technical hurdle."

"Fabrication Challenges: Engineering diamonds with well-defined NV centers at specific locations remains a complex process."

The fact that NV center is still useless despite longtime researches shows the idea that this too unstable NV center may become quantum magnetic sensor or qubit is unrealistic and just baseless hype.

Quantum mechanical unphysical model such as fictional spin, one-pseudo-electron DFT pseudo-potential hinders NV center's technological development.

The current unphysical quantum mechanical model trying to express multi-electron material as one-pseudo-electron DFT or quasiparticle with artificially-chosen pseudo-potential ( this p.8-1st-paragraph ) hampers the technological development in all fields including NV center.

Even this current mainstream (one-pseudo-electron) DFT (= density functional theory ) cannot explain NV center's energy levels or spin, as this p.4-2nd-paragraph says
"As mentioned above, DFT calculations of the excited states of the NV− center in diamond have given contradictory results"

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