◇Concept library

A physical parameter changes a quantum state, and a measurement converts that change into data.

The slope of a probability distribution relative to its noise determines information about a parameter.

Repeated independent measurements improve precision as the square root of sample count.

The probe state sets a measurement-independent upper limit on extractable local information.

Independent probes usually average down uncorrelated noise with shot-number scaling.

Correlated quantum probes can improve ideal phase sensitivity relative to independent probes.

A superposition accumulates relative phase while freely evolving.

A qubit sensor is a rotating state vector whose direction encodes the signal.

A stable atomic transition serves as a reference against which local oscillator drift is measured.

Diamond defects convert magnetic fields into shifts of spin resonance frequencies.

Spin-polarized atomic vapor precesses in magnetic fields.

A force displaces a trapped ion's quantized motion.

Matter waves accumulate phase along different trajectories.

Charge-sensitive superconducting levels shift with electric environment.

A superconducting loop turns magnetic flux into interference of Josephson currents.

Highly polarizable excited atoms respond strongly to electric fields.

Confined electronic levels are sensitive to neighboring charge configurations.

Photon counting resolves faint optical signals beyond analog intensity measurement.

Optical cavities can turn a spin state into a detectable photon response.

Tiny forces shift the motion of a resonator with quantized displacement.

Noise is frequency-dependent and should be matched to the sensing sequence.

Environmental coupling reduces useful phase information.

Control pulses reverse accumulated unwanted phase while retaining selected signals.

A synchronized pulse sequence selectively amplifies an oscillating signal.

Measuring a quantum system can disturb a conjugate observable.

Realistic sensors exchange information and energy with environments.

Each measurement updates what is known about an unknown parameter.

Collective quantum correlations reduce spin noise along a useful measurement axis.

One optical quadrature can fluctuate less than the vacuum reference.

Collective phase accumulation can amplify a small signal.

Correlated signal and idler modes can improve target discrimination in noisy environments.

A phase-referenced local oscillator converts optical field quadratures into electrical signals.

Two coherent paths turn a tiny path-length difference into an intensity imbalance.

Magnetic signals are mapped into precession phase or resonance frequency.

Atom interferometers can measure acceleration against an optical phase reference.

Quantum correlations can improve imaging under specific dose and noise constraints.

Temperature changes thermal occupation and transition populations.

Quantum probes can detect fields or local environmental changes associated with biological samples.

A credible sensor must quantify systematic as well as statistical uncertainty.

Longer phase accumulation helps until coherence loss outweighs the gain.

Claims of advantage require a fair reference under matched resources.

Networked probes can estimate spatially distributed parameters or weighted combinations.