Entangled PhotonEdit
Entangled photons are pairs (or larger groups) of light quanta whose quantum states are so interwoven that the result of measuring one photon is intimately connected to the result of measuring its partner, even when the photons are separated by large distances. This phenomenon is a cornerstone of quantum information science and a powerful resource for tasks such as quantum key distribution quantum key distribution, quantum teleportation quantum teleportation, and advanced sensing. In many setups, photons are entangled in properties such as polarization, momentum, or time-bin, and their joint states cannot be described as a simple product of individual states. The study of entangled photons spans foundations of physics as well as practical technologies, making it a central topic in both theory and experiment.
Entanglement challenges classical intuitions about how nature is supposed to work. It grew from discussions around the Einstein–Podolsky–Rosen paradox EPR paradox, which questioned whether quantum mechanics provides a complete description of physical reality. The decisive turn came with Bell’s theorem and associated Bell inequalities Bell inequality, which show that certain correlations predicted by quantum mechanics cannot be reproduced by any local hidden-variable theory. Over several decades, increasingly precise Bell test experiments with photonic systems confirmed violations of these inequalities, reinforcing the view that quantum states can exhibit nonlocal correlations that defy classical explanations. This area has progressed from early demonstrations to modern, loophole-aware and loophole-free tests, underscoring the robustness of photonic entanglement as a physical resource. See, for example, works involving spontaneous parametric down-conversion spontaneous parametric down-conversion as a reliable source of entangled photon pairs.
In most photonic platforms, entangled photons are generated via nonlinear optical processes, with spontaneous parametric down-conversion (SPDC) being the workhorse. In SPDC, a high-energy pump photon passing through a nonlinear crystal splits into two lower-energy photons whose polarizations, frequencies, or arrival times are correlated in a way that produces entanglement. The two primary flavors, type-I and type-II SPDC, differ in the polarization relationships of the produced photons and offer practical routes to create various entangled states. The engineering of entangled photons also extends to newer modalities such as time-bin encoding, frequency-bin entanglement, and hybrid approaches that combine multiple degrees of freedom. For readers exploring the underlying physics, the state of a two-photon system is described in a Hilbert space formalism, and specific entangled states (such as Bell states) serve as canonical examples of non-separable quantum states. See spontaneous parametric down-conversion and Bell test for more detail.
The distinctive feature of entangled photons is the way measurements reveal correlations that cannot be explained by classical theories of independent particles. If the two photons are measured in a polarization basis, the joint outcomes exhibit patterns that align with the prepared entangled state, yet each photon individually appears random. This nonclassical correlation does not enable faster-than-light signaling, because marginal statistics on either photon alone remain random; the nonlocal correlations only become evident when the results are compared after the fact. This no-signalling property is a fundamental constraint that keeps relativistic causality intact even in the presence of strong quantum correlations. See the entries on no-signalling and nonlocality for broader context.
Alongside foundations, entangled photons have become practical tools in a growing set of technologies. In quantum communication, entanglement-based protocols (for instance, versions of quantum key distribution that use entangled pairs, or the E91 protocol) aim to provide security rooted in quantum physics. In quantum information processing, entangled photons enable protocols such as quantum teleportation, which transfers a quantum state from one location to another using entanglement and classical communication, and superdense coding, which can double the amount of information sent with a given number of qubits. On the hardware side, advances in photonic integration, high-efficiency detectors (such as superconducting nanowire single-photon detectors), and efficient sources of entangled photons have driven the field toward scalable quantum networks and potentially practical quantum repeaters. See quantum teleportation, quantum key distribution, spontaneous parametric down-conversion, and single-photon detector for connected topics.
Experiments with photonic entanglement have evolved from sagacious demonstrations to sophisticated demonstrations that close various experimental loopholes and push distances and reliability. Early foundational experiments, such as those by Alain Aspect and collaborators, established clear violations of Bell inequalities using photon pairs. Contemporary efforts have realized long-distance entanglement distribution over fiber networks and free-space links, integrating sources and detectors into compact platforms. The field has also advanced to experiments that demonstrate entanglement swapping, multipartite entanglement (for example, GHZ states), and realistic quantum networking concepts. See Bell test, entanglement swapping, and quantum network for related developments.
Interpreting what entanglement says about the nature of reality remains a philosophical discussion with multiple viewpoints. Some schools of thought emphasize the operational accuracy of quantum predictions and the empirical success of no-signalling correlations, while others discuss deeper questions about locality, realism, and the meaning of the quantum state. Classical interpretations, such as the Copenhagen view, vs. alternatives like many-worlds Many-worlds interpretation or pilot-wave theories de Broglie–Bohm theory, offer different narratives about what entanglement implies about the fabric of reality. In practice, the success of entangled-photon experiments has sharpened both technical capabilities and conceptual inquiries into the foundations of quantum theory. See Copenhagen interpretation, Many-worlds interpretation, and de Broglie–Bohm theory for a broader sense of the interpretive landscape.
See also sections and cross-links to further explore the topic: - photon - quantum entanglement - Bell test - Bell inequality - EPR paradox - spontaneous parametric down-conversion - quantum teleportation - quantum key distribution - entanglement swapping - single-photon detector - quantum network - no-signalling - nonlocality - Alain Aspect - Copenhagen interpretation - Many-worlds interpretation - de Broglie–Bohm theory