(Fig.1) External magnetic field B splits energy levels of NV center with electron's orbital magnetic moment = μB
Nitrogen vacancy (= NV ) center in diamond consisting of multiple electrons was often cited as a potential quantum magnetic sensor (= sensing small magnetic field ) 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.
The so-called quantum sensors use some atomic energy levels, which can be explained by quantized orbits by de Broglie wave, nuclear or orbital magnetic field, photon ( this-5th-paragraph ), as shown in NV-center in diamond, atomic clock, MRI, SQUID.., whose energy states can be easily determined by experiments that do Not need the useless quantum mechanics or unphysical spin which can Not predict any atomic energy states.
This (hyped) NV center in diamond as quantum magnetic sensing ( and quantum computer's qubit ) 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-p.2-2nd~3rd-paragraphs (10/2025) say -- Useless quantum sensor
"superconducting quantum interference devices ( SQUIDs)
and optically pumped magnetometers
(OPMs = using Zeeman effect irrelevant to spin ), both of which can detect magnetic fields with sensitivities surpassing a femto-Tesla.
However, these technologies typically suffer from significant limitations, such as the need for cryogenic operation, high power consumption, and limited spatial resolution"
"As an alternative, individual nitrogen-vacancy (NV) centers in diamond have emerged as highly promising magnetic field sensors,.. However, the sensitivity of NV centers to static (DC) magnetic fields is significantly lower—typically around 10 µT—and is fundamentally limited by the shorter dephasing time T∗"
This-middle Building an ecosystem-5th-paragraph says (2024)
"there is still not a stable production of very high-quality diamonds,... At conferences, you see that the industry is really moving toward commercialization, but there's still a long way to go" ← deadend ( this p.1-left-2nd-paragraph ).
This-p.16-right (in 2025) says -- Impractical sensors
"quantum
diamond sensors have not been free of challenges. For
instance, mass production of various types of diamond is still
lacking,...
Efficient conversion of NV centers after incorporating nitrogen
into diamond is a topic of further research" ← still useless, No commercialization
This-1st-paragraph (in 2025 ) says -- Useless quantum sensors
"Quantum sensors,.. However, these sensors face significant challenges when deployed in real-world settings" ← No real world application
This-2nd-last, 3rd-last-paragraphs (3/23/2025) say
"Fabricating high-quality diamond with precisely controlled NV centers at scale remains a significant hurdle. Quantum decoherence, the loss of quantum superposition (= some desirable energy states ), limits the coherence time of NV center spins, affecting the sensitivity and integration times of sensors."
"Environmental noise, such as fluctuations in temperature and electromagnetic fields, can also degrade sensor performance"
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 can be perfectly explained 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 (= μB, 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 ). Ordinary electron's orbital motion can perfectly explain this.
This-p.3-introduction-1st-paragraph says -- Uncertain spin
" a process that is essential for initialization and readout of
quantum states (or NV centers ), is still controversial"
(Fig.2) NV center in diamond is too unstable to be practical
This NV centers in diamond are 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 -- Noise problem
"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 -- Unstable sensor
"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.
This p.3-1st-paragraphs say -- Noisy quantum sensor
"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 -- Impractical NV sensor
"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.1-left-last-paragraph )"
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.
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 -- Inaccurate sensors
"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.
(Fig.3) Unreal quantum mechanical model cannot explain spin.
The current unphysical quantum mechanical model tries to express materials such as NV centers in diamond as one-pseudo-electron DFT or quasiparticle with artificially-chosen pseudo-potential ( this p.8-1st-paragraph ), which cannot predict electron spin.
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"
↑ This one pseudo-electron DFT model just artificially chooses fake exchange-correlation potential functionals, pseudo-potential ( this-p.7-left-methods, this-p.5-left-last-paragraph ) and basis set wavefunctions, which cannot predict nor prove the existent of spin.
This-p.8-right-1st-paragraph says -- DFT cannot predict
"choice of the functional is crucial. ... EC was shifted to reproduce
the experimental band gap, i.e., scissor correction was applied (= empirical correction without quantum mechanical prediction, this-middle-Nonlocal exchange-correlation functional )."
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