Weak Measurement Techniques in Quantum Systems

Weak measurement techniques provide a nuanced window into quantum systems by minimally disturbing the state of a quantum particle during observation. Unlike conventional projective measurements that collapse the wavefunction, a weak measurement gently probes the system, yielding only partial information per trial but preserving coherence for subsequent measurements. When combined with appropriate preselection and postselection of states, this approach gives rise to “weak values”—anomalous expectation values that can lie outside the eigenvalue spectrum of the observable. Weak measurements have proven instrumental in resolving foundational paradoxes, such as the quantum Cheshire Cat, and in enhancing metrological protocols by redistributing information into rare events. These methods extend the toolkit of quantum control, enabling direct reconstruction of quantum processes, real-time characterisation of high-dimensional states and amplification of small signals against technical noise. Global interest has burgeoned around their capacity to reveal contextuality, probe nonclassical correlations and refine precision sensing in optical, atomic and solid-state platforms.

Research from Nature Portfolio

Recent studies have demonstrated the power of weak measurements in quantum state analysis and metrology. One work achieved direct process tomography of a qubit channel by sequentially measuring weak values of incompatible observables via two-photon interference, thereby bypassing the need for full state reconstruction. Another investigation harnessed postselection to amplify Fisher information per cost unit, revealing that negative quasiprobabilities underlie a genuine quantum advantage in parameter estimation. In a foundational experiment, neutrons traversing an interferometer were shown to exhibit spatial separation between particle trajectories and magnetic moments, exemplifying the quantum Cheshire Cat and validating weak values as probes of counter-intuitive phenomena.

Weak Measurement Techniques in Quantum Systems publication trend

The graph below shows the total number of articles in weak measurement techniques in quantum systems across all publications each year (not limited to Nature Index journals).

Technical terms

Weak measurement: A gentle measurement protocol that disturbs the quantum system only minimally, extracting limited information per trial while preserving coherence.

Weak value: The conditioned average outcome of a weak measurement obtained between a chosen initial (preselected) state and a final (postselected) state, which can lie outside the observable’s eigenvalue range.

Preselection and postselection: The preparation of an initial quantum state (preselection) and the selective consideration of outcomes in a desired final state (postselection) to define weak values.

Fisher information: A metric quantifying the amount of information that an observable random variable carries about an unknown parameter, used to assess measurement precision.

Quantum Cheshire Cat: A phenomenon in which a quantum property (for example, spin or polarisation) appears to be spatially separated from the particle to which it belongs, as revealed by weak measurements.

References

1. Experimental demonstration of separating the wave‒particle duality of a single photon with the quantum Cheshire cat. Light: Science & Applications (2023).
2. Observation of a quantum Cheshire Cat in a matter-wave interferometer experiment. Nature Communications (2014).
3. Quantum advantage in postselected metrology. Nature Communications (2020).
4. Direct quantum process tomography via measuring sequential weak values of incompatible observables. Nature Communications (2018).
5. Technical Advantages for Weak-Value Amplification: When Less Is More. Physical Review X (2014).
6. Scan-free direct measurement of an extremely high-dimensional photonic state. Optica (2015).