Nonclassical States of Light in Quantum Optics

Nonclassical states of light are quantum electromagnetic fields whose behaviour defies any description in terms of classical wave or particle theories. These states exhibit features such as squeezing, antibunching and entanglement that arise from genuine quantum fluctuations and correlations. They are typically characterised by negative or highly singular quasiprobability distributions in phase space, sub-Poissonian photon statistics, or coherence properties that exceed classical limits. Nonclassical light underpins precision measurement beyond the standard quantum limit, enables secure quantum communication and forms the basis of optical quantum computing. Generation techniques range from parametric down-conversion and four-wave mixing to engineered atom-cavity interactions. Detection and certification employ homodyne tomography, photon-number-resolving detectors and recently developed criteria that remain robust under realistic noise and coarse-graining. Theoretical frameworks, notably resource theories of nonclassicality, provide quantitative measures that constrain allowable transformations under passive linear optics and point the way to optimising the distribution of quantum advantage in networks. Ongoing efforts aim to scale these concepts to multimode systems, integrate them with on-chip photonic devices and harness them for applications in metrology, imaging and secure communications.

Research from Nature Portfolio

Recent studies have introduced an invariant quantity for two-mode Gaussian states that remains constant under any global photon-number conserving transformation. This nonclassicality invariant naturally separates into an entanglement monotone and local squeezing measures, revealing how entanglement can be reversibly converted into single-mode nonclassicality and vice versa. Twin-beam experiments processed at beam splitters validate the framework by producing squeezed outputs with predictable nonclassical characteristics. Extensions to three-mode systems have further demonstrated the versatility of this invariant, offering a unified approach to quantify Gaussian resources under experimentally relevant unitary operations.

Nonclassical States of Light in Quantum Optics publication trend

The graph below shows the total number of articles in nonclassical states of light in quantum optics across all publications each year (not limited to Nature Index journals).

Technical terms

Nonclassical state: A quantum optical state lacking any classical counterpart, typically identified by negative or singular phase-space distributions.

Gaussian state: A state whose Wigner function is a Gaussian distribution, fully characterised by first and second moments of field quadratures.

Glauber–Sudarshan P function: A quasiprobability distribution whose non-positivity signals nonclassicality.

Squeezed state: A state in which fluctuations in one field quadrature are reduced below the vacuum level at the expense of increased fluctuations in the orthogonal quadrature.

Fock state: An eigenstate of the photon-number operator with a definite number of quanta.

Quadrature: One of two conjugate field observables analogous to position and momentum, used to describe amplitude and phase fluctuations.

Entanglement monotone: A quantitative measure of entanglement that does not increase under local operations and classical communication.

References

1. Rigorous results on approach to thermal equilibrium, entanglement, and nonclassicality of an optical quantum field mode scattering from the elements of a non-equilibrium quantum reservoir. Quantum (2024).
2. Continuous-variable nonclassicality certification under coarse-grained measurement. Physical Review Research (2023).
3. Operational Resource Theory of Continuous-Variable Nonclassicality. Physical Review X (2018).
4. Nonclassicality detection from few Fock-state probabilities. npj Quantum Information (2022).
5. Nonclassicality Invariant of General Two-Mode Gaussian States. Scientific Reports (2016).