Quantum Sensing

Quantum sensing is arguably the most immediately practical branch of quantum technology. While quantum computers require millions of near-perfect qubits and quantum networks require long-distance entanglement distribution, many quantum sensors work with single particles under ambient conditions and are already commercially deployed. The underlying idea is that quantum mechanical effects can make measurements more precise than any classical instrument could achieve.

The two limits of measurement precision

The standard quantum limit (also called the shot noise limit) applies when measurements are made with $N$ independent, unentangled probes. The measurement uncertainty scales as $1/\sqrt{N}$. To halve the uncertainty, you must quadruple the number of probes. This is the best any classical sensor can do.

The Heisenberg limit applies when $N$ probes are used in an entangled state. The uncertainty scales as $1/N$. To halve the uncertainty, you only need to double the number of probes. For large $N$, this is an enormous advantage. A sensor using 1,000 entangled probes achieves the same precision as a classical sensor using 1,000,000 independent probes.

Reaching the Heisenberg limit requires preparing and maintaining entangled states, which is technically demanding. A middle ground, quantum squeezing, redistributes uncertainty between two conjugate observables (trading worse precision on one for better precision on another) and can beat the standard quantum limit without full entanglement between all probes.

Atomic clocks

Atomic clocks are the most mature and widely deployed quantum sensors. They exploit the fact that atoms have extremely precise, reproducible resonance frequencies: when electromagnetic radiation is tuned to exactly the right frequency, it causes transitions between two internal energy levels of the atom.

Modern optical lattice clocks trap thousands of atoms in a standing wave of laser light and use a laser tuned to an optical transition, which oscillates roughly 100,000 times faster than the microwave transitions used in older cesium clocks. The NIST optical lattice clock based on strontium atoms is accurate to about one second of error per 300 million years, making it the most precise measurement instrument humans have built.

Atomic clocks underpin GPS navigation. Every GPS satellite carries atomic clocks, and the system depends on timing precision at the nanosecond level. Quantum-enhanced atomic clocks using entangled atoms could improve this precision further, with implications for navigation, geodesy, and tests of fundamental physics.

Gravitational wave detection

The LIGO and Virgo observatories detect gravitational waves by measuring changes in the length of 4-kilometer arms to a precision far smaller than a proton. This would be impossible without squeezed light. Vacuum fluctuations in the laser light create quantum noise that limits measurement precision. By injecting specially prepared squeezed light into the detector, LIGO suppresses this noise below the standard quantum limit in the measurement band most relevant for gravitational wave signals.

This is a direct application of quantum sensing in a deployed scientific instrument. LIGO has used squeezed light injection since 2019, and it measurably increases the detector’s sensitivity and the volume of space it can observe.

Magnetic field sensing with NV centers

Nitrogen-vacancy (NV) centers in diamond are point defects whose electron spin is exquisitely sensitive to magnetic fields. The spin state can be initialized and read out using laser light at room temperature, with no cryogenic cooling required.

The sensitivity arises because the energy splitting between spin states depends on the local magnetic field. By preparing the spin in a superposition and watching how quickly it dephases, or by applying pulse sequences that echo away slow drift, an NV center can detect magnetic fields at the level of picoTesla (trillionths of a Tesla) in nanometer-scale volumes.

Applications include: detecting the magnetic signals produced by individual neurons firing, mapping magnetic domains in materials at nanometer resolution, and finding defects in semiconductor devices. Compact NV-based magnetometers are moving from university labs into medical and industrial products.

Quantum gravimeters and inertial sensors

A quantum gravimeter uses the wave nature of atoms to measure gravitational acceleration. Cold atoms are launched upward in a fountain and undergo atom interferometry: the atom’s wavefunction is split, sent along two paths in a gravitational field, and recombined. The phase difference between the two paths encodes the gravitational acceleration with extraordinary precision.

Quantum gravimeters are more accurate and more stable than classical spring-based gravimeters. They are used in geodesy (mapping the shape of the Earth’s gravitational field), in surveys for oil and mineral deposits, and in navigation systems that do not depend on GPS satellites. A submarine equipped with quantum inertial sensors could navigate precisely without any external signal.

Quantum gyroscopes use atom interferometry to measure rotation rather than acceleration, with sensitivity that surpasses mechanical and fiber-optic gyroscopes.

Medical imaging

Quantum sensing has implications for medical imaging beyond what is achievable with classical devices. Magnetoencephalography (MEG) images brain activity by detecting the tiny magnetic fields produced by neuronal currents. Classical MEG requires bulky superconducting sensors cooled to liquid helium temperatures. Optically pumped magnetometers based on atomic vapors can achieve comparable sensitivity at room temperature, enabling lightweight, wearable brain scanners.

Longer term, enhanced MRI using hyperpolarization prepares nuclear spins in highly ordered, non-equilibrium states, boosting the signal-to-noise ratio of MRI scans and enabling faster imaging or imaging of metabolic processes in real time.

Why quantum sensing leads the field

Quantum sensing does not require error correction. A quantum computer needs thousands of error-free operations on many qubits; a quantum sensor might need only a single well-controlled qubit for microseconds. The technical requirements are far less stringent, which is why quantum sensors are already in commercial use while fault-tolerant quantum computers remain a future goal.

This also means the timeline for quantum sensing impact is now, not decades away. Quantum clocks, squeezed-light interferometers, and NV-center magnetometers are all either deployed or approaching deployment in near-term commercial and scientific instruments.

Connection to other quantum technologies

Quantum sensing and quantum networking intersect in quantum-enhanced telescope arrays. Two telescopes separated by a long baseline can, in principle, share entanglement distributed by a quantum network to achieve interferometric resolution equivalent to a telescope the size of the entire baseline. Classical radio telescope arrays (VLBI) achieve this with radio waves today; doing the same with optical telescopes requires a quantum network because the short wavelength of light makes the required timing precision unachievable with classical links.

Quantum sensors also provide diagnostic tools for other quantum technologies. NV-center magnetometers can image the magnetic fields produced by superconducting qubit processors, helping engineers identify crosstalk and leakage. Atom interferometers can test the gravitational environment around large quantum hardware installations that need vibration isolation.

See also

• Entanglement
• Superposition
• Decoherence
• Quantum Internet