Quantum Nonlocality in Single-Photon Systems

Quantum nonlocality traditionally evokes images of two or more particles exhibiting correlations that defy classical intuition. However, a single photon may itself serve as a nonlocal resource when its wavefunction is coherently distributed across spatial or modal degrees of freedom. In such configurations, distinct field modes—or separate paths—act as subsystems that can be entangled despite the presence of only one quantum of excitation. By formulating Bell-type inequalities for mode correlations, researchers have demonstrated that local realism can be violated even in single-photon regimes. This avenue of inquiry has both foundational and technological import: it challenges our understanding of quantum reality and underpins device-independent quantum protocols, such as randomness generation and secure communications, which exploit nonlocal correlations without requiring multi-photon sources. Recent advances combine theoretical constructions of entanglement witnesses with scalable measurement techniques, facilitating tests of nonlocality in complex photonic networks and opening the door to new implementations of quantum information processing with minimal resources.

Research from Nature Portfolio

A recent development introduces a simplified form of polarisation observables that are particularly well suited to Bell-type tests in optical fields. By binning intensity differences into discrete outcomes, this framework yields operators that, while not fulfilling all formal properties of conventional Stokes operators, permit robust violations of Bell inequalities for a variety of states—including squeezed vacua with undefined photon number. Numerical evidence further suggests that four-mode squeezed vacuum states can exhibit nonlocal correlations across all gain regimes. The approach offers an intuitive realisation in terms of photon-counting and polarisation analysers, paving the way for experimental studies of nonlocality in complex single-photon and multi-mode systems.

Quantum Nonlocality in Single-Photon Systems publication trend

The graph below shows the total number of articles in quantum nonlocality in single-photon systems across all publications each year (not limited to Nature Index journals).

Technical terms

Single-photon entanglement: Entanglement realised between different modes or paths of a single photon, treating field modes as distinct subsystems.

Mode entanglement: Quantum correlations established between separate field modes, which may be spatial, temporal or polarisation channels.

Bell inequality: A constraint on correlation measurements that must be satisfied by any theory obeying local realism; its violation signals nonlocality.

Local realism: The assumption that physical properties exist prior to and independent of measurement (realism) and that no influence can travel faster than light (locality).

Homodyne measurement: A detection scheme in which a quantum signal is mixed with a strong coherent field (local oscillator) to measure quadrature amplitudes or photon-number distributions.

References

1. Local and scalable detection of genuine multipartite single-photon path entanglement. Quantum (2022).
2. Simplified quantum optical Stokes observables and Bell’s theorem. Scientific Reports (2022).
3. Can single photon excitation of two spatially separated modes lead to a violation of Bell inequality via weak-field homodyne measurements?. New Journal of Physics (2021).
4. Wave–particle complementarity: detecting violation of local realism with photon-number resolving weak-field homodyne measurements. New Journal of Physics (2022).