Quantum Photonics
Quantum photonics exploits the quantum mechanical properties of light to enable capabilities in computing, communication, and sensing that classical optics cannot reach. Where classical optics treats light as a continuous electromagnetic wave, quantum photonics operates in the regime where the discrete nature of photons and their quantum correlations become essential. A single photon serves as an ideal carrier of quantum information: it travels at the speed of light, interacts weakly with its environment, and therefore preserves quantum coherence over long distances, including across deployed optical fiber.
The field encompasses three core capabilities. The first is the generation of non-classical light, including single photons, entangled photon pairs, and squeezed states whose noise falls below the vacuum level in one quadrature. The second is the manipulation of these states using linear optical elements such as beam splitters and phase shifters, together with nonlinear processes for state conversion. The third is the detection of light with single-quantum sensitivity, where individual photons are counted and, in the best detectors, their number is resolved. Integrated quantum photonic circuits bring these functions onto chip-scale waveguide platforms, providing the phase stability and reproducibility that bulk free-space optics cannot sustain at scale.
Quantum photonics is a convergence of quantum physics, optical engineering, and materials science. Progress in the field drives photonic quantum processors, quantum networks for provably secure communication, and quantum sensors that exceed classical measurement limits. This category provides comprehensive coverage of the sources, components, and systems that generate, manipulate, and detect light at the quantum level.
Topics in This Category
Fundamental Concepts
Quantum States of Light
The quantum mechanical description of light differs fundamentally from the classical electromagnetic field. Photons are the elementary excitations of the quantized field, each carrying energy proportional to its frequency. Fock states, which contain a definite number of photons, form the basis for describing quantum light, although practical sources produce superpositions or statistical mixtures of these number states. Coherent states, the closest quantum analog of a classical laser field, carry Poissonian photon statistics, while squeezed states redistribute quantum noise so that one quadrature falls below the vacuum level at the expense of the other. Even the vacuum state, containing zero photons, exhibits quantum fluctuations that set fundamental limits on measurement precision.
Entanglement and Non-Classical Correlations
Quantum entanglement creates correlations between photons that exceed any possible classical explanation. Entangled photon pairs exhibit correlated outcomes across complementary measurement bases, the property that underlies entanglement-based quantum key distribution: any attempt to gain information about the shared state disturbs the correlations and reveals the intrusion. The violation of a Bell inequality, demonstrated in increasingly loophole-free experiments, confirms these correlations cannot arise from any local hidden-variable theory. Beyond two-photon entanglement, multiphoton states such as Greenberger-Horne-Zeilinger (GHZ) and cluster states provide the resources for measurement-based quantum computing and for sensing at the Heisenberg limit.
Photon Detection and Measurement
Detecting individual photons requires devices with single-quantum sensitivity and low dark-count noise. Single-photon avalanche diodes operating in Geiger mode register a photon through self-sustaining carrier multiplication but cannot resolve photon number. Superconducting nanowire single-photon detectors (SNSPDs) offer the best combination of detection efficiency above 90 percent, picosecond-scale timing jitter, and low dark counts, particularly at telecommunications wavelengths near 1550 nanometers. Transition-edge sensors add true photon-number resolution by measuring the heat deposited by absorbed photons, at the cost of slower response. Because measurement projects a quantum state onto a definite outcome, detection is not a passive readout but an active step in many photonic protocols.
Integrated Quantum Photonics
Integrating quantum photonic components onto a chip provides the stability, scalability, and miniaturization that practical quantum systems require. Silicon and silicon nitride support low-loss waveguides, directional couplers, and thermo-optic phase shifters fabricated with mature semiconductor processes, while thin-film lithium niobate adds fast electro-optic modulation and efficient nonlinear interactions for on-chip photon-pair generation. Co-integrating sources and detectors with these passive circuits moves toward complete quantum systems on a single die. The controlled fabrication environment yields reproducible, interferometrically stable devices that would be impractical to assemble and align from discrete bulk components.
Applications
Quantum Communication
Quantum key distribution (QKD) uses the laws of quantum mechanics to establish shared cryptographic keys whose secrecy is guaranteed by physics rather than by computational difficulty. Any attempt to intercept quantum-encoded information disturbs the underlying states in a detectable way. Prepare-and-measure protocols such as BB84 encode bits on single photons, while entanglement-based protocols such as E91 distribute correlated pairs. Fiber-based QKD is now a commercial technology over metropolitan distances, and satellite demonstrations have extended entanglement distribution across more than a thousand kilometers. Quantum repeaters, which rely on quantum memories and entanglement swapping, aim to extend secure links toward continental and global scales without trusted intermediate nodes.
Optical Quantum Computing
Photons serve as qubits, with information encoded in polarization, spatial path, or time-bin degrees of freedom. Linear optical quantum computing implements gates with beam splitters and phase shifters, using measurement and feed-forward to supply the effective nonlinearity needed for universal operation. Boson sampling provides a more specialized route to quantum computational advantage using only single photons or squeezed light, passive interferometry, and photon-number detection; programmable Gaussian boson sampling machines such as Xanadu's Borealis, operating on more than two hundred modes, have produced samples estimated to be intractable for classical supercomputers. Continuous-variable approaches instead encode information in the quadratures of squeezed states, offering distinct trade-offs for fault-tolerant error correction.
Quantum Sensing and Metrology
Non-classical states of light enable measurements with precision beyond classical limits. Squeezed vacuum injected into the dark port of the Advanced LIGO and Virgo interferometers reduces quantum noise and improves sensitivity to gravitational waves; frequency-dependent squeezing, implemented with long filter cavities, now suppresses both photon shot noise at high frequencies and radiation-pressure noise at low frequencies across the detection band. Path-entangled states allow phase estimation whose uncertainty scales as the inverse of the photon number, the Heisenberg limit, rather than the inverse square root that bounds classical measurements. Quantum illumination uses entangled photons to improve target detection against a bright thermal background, a principle explored for low-power radar and lidar.
Outlook
Quantum photonics sits at the frontier of quantum technology, translating fundamental physics into practical devices and systems. The intrinsic advantages of photons for quantum information, including room-temperature transmission, weak coupling to the environment, and compatibility with existing telecommunications infrastructure, make photonic approaches especially promising for near-term applications in secure communication and sensing. The central engineering challenges are deterministic single-photon generation, low-loss components, efficient photon-number-resolving detection, and large-scale integration. Progress on these fronts, surveyed across the topics in this category, defines how quantum properties of light will be harnessed in computing, communication, and metrology.