Quantum Coherence and Photon Correlation Techniques
Summary
Quantum coherence underpins the ability of light to exhibit wave-like interference effects and to maintain fixed phase relationships across spatial and temporal degrees of freedom. In quantum optics, coherence extends beyond classical descriptions to encompass non-classical superposition states of photons and emitters, enabling phenomena such as entanglement and single-photon generation. Photon correlation techniques probe these statistical properties by measuring the likelihood of detecting pairs or sequences of photons at defined time intervals, thereby revealing departures from classical behaviour.
First-order coherence is typically assessed via interferometric arrangements, while second-order and higher-order coherence are quantified through correlation functions, most notably g²(τ). Measurements of g² at zero delay distinguish thermal light (bunched photons), coherent laser emission (Poissonian statistics) and non-classical sources (antibunching). These methods have become indispensable in characterising single-photon emitters, superradiant ensembles and threshold-reduced nanolasers. Beyond fundamental studies, advances in photon correlation underpin applications in quantum communication, sub-shot-noise metrology and novel imaging modalities that exploit quantum fluctuations for enhanced resolution and sensitivity.
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Quantum Coherence and Photon Correlation Techniques publication trend
The graph below shows the total number of articles in quantum coherence and photon correlation techniques across all publications each year (not limited to Nature Index journals).
Technical terms
Quantum coherence: The maintenance of fixed phase relationships in quantum superpositions, enabling interference and entanglement phenomena.
Second-order correlation function g²(τ): A measure of the joint probability of detecting two photons separated by a time delay τ, used to classify light as bunched, coherent or antibunched.
Photon bunching: A statistical tendency for photons to arrive in clusters (g²(0) > 1), characteristic of thermal and chaotic light sources.
Photon antibunching: A signature of non-classical light whereby photons avoid simultaneous detection (g²(0) < 1), indicative of single-photon emission.
Superradiance: Collective and enhanced emission from an ensemble of coherently coupled emitters, yielding a burst of radiation with elevated intensity and temporal coherence.
References
- Isolating the classical and quantum coherence of a multiphoton system. PhotoniX (2024).
- Multiphoton quantum van Cittert-Zernike theorem. npj Quantum Information (2023).
- Thresholdless coherence in a superradiant laser. Light: Science & Applications (2024).
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