What Makes Quantum Sensing So Special? Principles, Limits and Applications

Introduction

This article is part of our series on the First and Second Quantum Revolutions and how they shape modern technology. Quantum technologies have moved from theory into real applications, and quantum sensing stands out as one of the most transformative of them.

The advantage of quantum sensing comes from three quantum-mechanical properties: superposition, entanglement and quantum coherence. Together they enable a precision and sensitivity that classical sensors cannot reach. Below we explain the principles that give quantum sensors their edge over classical ones, and then walk through the applications where that edge already matters.

The science behind quantum sensing

Superposition and quantum coherence: the path to precision

Superposition lets a quantum system exist in several states at once. Because of it, a quantum sensor can interact with an external field, such as a magnetic or gravitational field, in ways a classical system cannot: the field shifts the relative phase between the superposed states, and that phase can be read out with great precision.

Quantum coherence is the system’s ability to keep this phase relationship over time. It is the basis of stable, precise measurement and matters most in atomic clocks. The coherence time T₂ tells how long the quantum state stays undisturbed, and it largely sets a sensor’s stability: the longer the phase can accumulate, the smaller the change that can be detected.

An atomic clock locks to the frequency of the transition between two energy levels of an atom:

ν = (E₂ − E₁) / h

where E₂ and E₁ are the energy levels and h is Planck’s constant. Because the transition frequency is fixed by nature, every atom of the same species ticks at the same rate; the SI second itself is defined by the caesium-133 hyperfine transition at 9,192,631,770 Hz. This level of precision underpins GPS and high-resolution spectroscopy.

Entanglement: the quantum advantage

Entangled particles share correlated quantum states: the result of a measurement on one is correlated with the result on the other, regardless of the distance between them. Sensors can use this correlation to go beyond the classical limits on measurement resolution.

With N independent (unentangled) atoms, the phase uncertainty of an atomic interferometer falls only as 1/√N, the standard quantum limit. Entangling the atoms allows the measurement to beat this limit and approach Heisenberg-limited precision:

Δθ = 1 / N

where Δθ is the measurement uncertainty and N is the number of entangled particles. For a million atoms, that is a thousand-fold improvement over the standard quantum limit. Classical techniques cannot reach this level, because noise and environmental interference limit them. In practice, entangled states are fragile, so real sensors operate between the two limits.

Quantum noise reduction: beating classical limits

Classical sensors are limited by thermal noise and shot noise. Quantum sensors can use squeezed states of light or atoms, which reduce the uncertainty in the measured quantity below the classical limit (at the cost of more uncertainty in a quantity that is not measured). Squeezed-light interferometry is already used in the LIGO gravitational-wave detectors to improve their sensitivity.

The noise reduction follows the squeezing relation:

ΔX = e^−r ΔX₀

where ΔX is the reduced quantum noise, r is the squeezing parameter and ΔX₀ is the original (vacuum or shot-noise) uncertainty. Squeezing is usually quoted in decibels: 20·log₁₀(e^r) ≈ 8.69·r dB, so 10 dB of squeezing (r ≈ 1.15) reduces the noise amplitude by a factor of about 3.2. This is what makes it possible to detect extremely small ripples in spacetime. Squeezed light is generated in nonlinear crystals, the same physics described in our article on nonlinear optics and its quantum impact.

Key advantages of quantum sensing

Real-world applications of quantum sensing

Magnetometry and biomedical imaging

Quantum magnetometers based on nitrogen-vacancy (NV) centres in diamond detect extremely weak magnetic fields at the nanoscale. Applications include non-invasive mapping of brain activity (quantum-enabled magnetoencephalography, MEG) and early disease detection. Our article on NV centers and Zeeman splitting explains how these sensors work.

Quantum clocks for navigation and timing

Atomic clocks based on quantum transitions are essential for GPS, secure communications and the synchronisation of financial transactions. Their accuracy also makes precise positioning possible in GPS-denied environments, where a vehicle must navigate on its own clock and inertial sensors.

Gravitational and geophysical sensing

Quantum gravimeters use atom interferometry to measure small variations in Earth’s gravitational field. A cloud of cold atoms is split, reflected and recombined by laser pulses, and the gravitational acceleration appears as a phase shift:

Δφ = k_eff g T²

where k_eff is the effective wavevector of the laser pulses (about 4π/λ for counter-propagating two-photon transitions), g is the gravitational acceleration and T is the interrogation time between pulses. Because the phase grows with T², a longer free fall gives a much larger signal. Uses include underground exploration, earthquake prediction and planetary science.

Defense and security

Quantum sensors can detect submarines, underground bunkers and stealth aircraft by sensing tiny gravitational and magnetic anomalies. Because this kind of sensing is passive, the sensor emits no signal that could reveal its own position, unlike active radar or sonar.

Quantum imaging and spectroscopy

Quantum-enhanced imaging enables ultra-sensitive spectroscopy for environmental monitoring and materials science. Quantum-enhanced lidar improves imaging in low-light or obscured conditions, where correlated photon pairs help separate the true return signal from background light. See understanding photon entanglement for how such correlations are measured.

Conclusion

Quantum sensing is a fundamental advance in measurement science. By using superposition, entanglement and quantum noise reduction, quantum sensors reach a precision and sensitivity once considered impossible, with applications across healthcare, defence, navigation, space exploration and more.

The author leads the Advanced Quantum Lab course at the Technion, and at IZAK Scientific we are working to bring these advances into real-world systems: stable optics and nonlinear-crystal alignment, deterministic timing electronics and complete electro-optical set-ups. Future articles in this series go deeper into specific quantum sensors, starting with NV centers and Zeeman splitting.

When this article was first published, IZAK Scientific was preparing to exhibit at OASIS 2025. The invitation still stands: if you work on quantum sensing or related technologies, we would be glad to talk.

IZAK products for this application

Instruments from our catalog that match the measurements and set-ups in this article.

Quantum opticsQuantumAlign NLO Kinematic StagePosition nonlinear crystals with X–Y translation, tip and tilt.View product →Laser metrologyLaser Beam ProfilersUV to LWIR families, packaged, OEM or software, with Test mode and reports.View product →

Need something that is not in the catalog? We design custom electro-optical systems →

References

1. Taylor, M. A., & Bowen, W. P. (2016). Quantum metrology and its application in biology. Physics Reports, 615, 1–59.
2. Pezzè, L., Smerzi, A., Oberthaler, M. K., Schmied, R., & Treutlein, P. (2018). Quantum metrology with nonclassical states of atomic ensembles. Reviews of Modern Physics, 90(3), 035005.
3. Giovannetti, V., Lloyd, S., & Maccone, L. (2011). Advances in quantum metrology. Nature Photonics, 5(4), 222–229.
4. Kitching, J., Knappe, S., & Donley, E. A. (2011). Atomic sensors – a review. IEEE Sensors Journal, 11(9), 1749–1758.

Frequently asked questions