An information and limitations concept describing quantitative bounds and distinguishability measures in quantum systems. It governs tradeoffs between incompatible measurements and how well states or processes can be inferred from finite data. It does not imply experimental impossibility in all cases and must be interpreted alongside the chosen measurement model and noise sources. It materially affects metrology and verification by bounding achievable precision and by quantifying similarity between states. The concept is generally stable, though tighter bounds and operational interpretations are refined over time.

Principle

Quantum probabilities derive from squared magnitudes of summed probability amplitudes; a single quantum carries an amplitude for each available path and its detection statistics, accumulated over many trials, reveal interference even though each detection is a localized event.

Demonstration

Use a heralded single-photon source or strongly attenuated pulsed laser to send photons through a Mach–Zehnder interferometer or a double-slit apparatus one by one. Individual detection events are localized, but the ensemble distribution after many photons yields high-visibility interference fringes when coherence is preserved.

Misapplication

Asserting that each photon physically splits into two classical pieces or that a single detection already 'shows' interference in isolation misreads the statistical character; conversely, assuming single-photon experiments cannot show interference because events are discrete is also incorrect.

Consequence

Single-photon interference demonstrates that interference is a property of single-particle quantum amplitudes and not merely a macroscopic wave phenomenon, underpinning foundational concepts and enabling quantum technologies such as single-photon interferometry and quantum cryptography.

Reversal

When photons are rendered distinguishable (timing jitter, polarization tagging, or which-path detection), the interference disappears and the detection histogram becomes the incoherent sum of single-path contributions.

Boundary

Applicable when photons are produced with sufficient temporal and spatial coherence and when detectors have single-photon sensitivity; chaotic thermal sources, multi-photon pulses, or loss of coherence remove the single-photon interference signature.

Semantic Tension

There is tension between classical wave explanations of fringes and quantum single-particle accounts; while ensemble fringes resemble classical interference, the quantum view emphasizes amplitude superposition and the indivisible detection event.

Synthesis

Single-photon interference unites the particlelike detection of individual quanta with wave-like buildup of interference: each photon carries amplitudes for alternative paths, and the statistical accumulation of localized detections reconstructs the interference pattern determined by amplitude superposition.