Quantum Photonics
Quantum photonics exploits the quantum mechanical properties of light to enable revolutionary capabilities in computing, communication, and sensing. Unlike classical optics where light behaves as continuous electromagnetic waves, quantum photonics operates in the regime where the discrete nature of photons and their quantum correlations become essential. Single photons serve as ideal carriers of quantum information, traveling at the speed of light with minimal decoherence and enabling secure communication over long distances.
The field encompasses the generation of non-classical light states including single photons, entangled photon pairs, and squeezed states; the manipulation of these quantum states through linear and nonlinear optical elements; and the detection of photons with single-quantum sensitivity. Integrated quantum photonic circuits bring these capabilities onto chip-scale platforms, enabling complex quantum systems with stability and scalability impossible in bulk optics.
Quantum photonics represents a convergence of quantum physics, optical engineering, and materials science. Progress in this field drives development of quantum computers using photonic qubits, quantum networks for secure communication, and quantum sensors exceeding classical measurement limits. This category provides comprehensive coverage of the technologies that generate, manipulate, and detect light at the quantum level.
Articles
Quantum Communication
Secure information transfer using quantum mechanical principles. This category covers quantum key distribution protocols, quantum cryptography systems, quantum random number generators, quantum digital signatures, quantum secret sharing, quantum repeaters and memories, quantum networks, satellite quantum communication, quantum internet protocols, device-independent and measurement-device-independent protocols, continuous-variable and discrete-variable systems, and commercial quantum communication systems.
Quantum Light Sources
Technologies for generating non-classical states of light including single photons, entangled photon pairs, and squeezed light. Coverage includes single-photon sources based on quantum dots, color centers in diamond, and molecules; entangled photon sources using parametric down-conversion and four-wave mixing; squeezed light from optical parametric oscillators; quantum frequency conversion for wavelength interfacing; heralded and deterministic source architectures; and comprehensive characterization methods including photon statistics, Hong-Ou-Mandel interference, and quantum state tomography.
Quantum Sensing and Metrology
Enhance measurement precision beyond classical limits using quantum mechanical properties of light. Coverage includes quantum interferometry, quantum imaging, quantum illumination, quantum radar, quantum lidar, quantum magnetometry, quantum gravimetry, quantum clock synchronization, quantum parameter estimation, quantum state discrimination, quantum sensing networks, biological quantum sensors, quantum-enhanced microscopy, sub-wavelength imaging, and quantum calibration standards.
Quantum Computing with Photons
Process information using quantum optics. Topics encompass linear optical quantum computing, measurement-based computing, boson sampling machines, quantum walks, quantum simulators, photonic quantum processors, integrated quantum circuits, quantum gates and operations, quantum error correction, cluster state generation, variational quantum algorithms, quantum machine learning, quantum advantage demonstrations, hybrid classical-quantum systems, and quantum software tools.
Fundamental Concepts
Quantum States of Light
The quantum mechanical description of light differs fundamentally from the classical electromagnetic field picture. Photons are the elementary excitations of the quantized electromagnetic field, with each photon carrying energy proportional to its frequency. Fock states containing definite numbers of photons form the basis for describing quantum light, though practical sources produce superpositions or statistical mixtures of these number states. The vacuum state, containing zero photons, still exhibits quantum fluctuations that set fundamental limits on measurements.
Entanglement and Non-Classical Correlations
Quantum entanglement creates correlations between photons that exceed any possible classical explanation. Entangled photon pairs exhibit perfect correlations in complementary measurement bases, enabling quantum key distribution where any eavesdropping attempt disturbs the correlations and reveals the intrusion. Bell inequality violations demonstrate these non-classical correlations experimentally. Beyond two-photon entanglement, multiphoton entangled states such as GHZ and cluster states provide resources for quantum computing and enhanced sensing.
Photon Detection and Measurement
Detecting individual photons requires devices with single-quantum sensitivity and low noise. Avalanche photodiodes operating in Geiger mode detect single photons through carrier multiplication but cannot distinguish photon numbers. Superconducting nanowire detectors achieve superior timing resolution and detection efficiency, particularly at infrared wavelengths. Transition edge sensors provide photon-number resolution through energy-sensitive detection. The measurement process itself affects quantum states, with detection generally projecting the quantum state onto a measurement outcome.
Integrated Quantum Photonics
Integrating quantum photonic components onto chips provides the stability, scalability, and miniaturization required for practical quantum systems. Silicon, silicon nitride, and lithium niobate platforms support waveguides, beam splitters, phase shifters, and modulators for photon manipulation. Integration of sources and detectors with these passive circuits creates complete quantum systems on chip. The controlled fabrication environment enables reproducible devices that would be impossible to achieve with bulk optical components.
Applications
Quantum Communication
Quantum key distribution uses the laws of quantum mechanics to guarantee secure communication. Any attempt to intercept quantum-encoded information disturbs the quantum states in detectable ways, alerting communicating parties to eavesdropping. Single-photon sources and entangled photon pairs implement various QKD protocols including BB84 and E91. Quantum networks extend secure communication across metropolitan and eventually global scales through quantum repeaters that overcome transmission losses without compromising security.
Optical Quantum Computing
Photons serve as qubits for quantum computing, with polarization, path, or time-bin encoding representing quantum information. Linear optical quantum computing uses beam splitters and phase shifters to implement quantum gates, with measurement-induced nonlinearity enabling universal computation. Boson sampling demonstrates quantum computational advantage using only linear optics and photon detection. Continuous-variable approaches encode information in the quadratures of squeezed states, offering different trade-offs for error correction and fault tolerance.
Quantum Sensing and Metrology
Quantum states of light enable measurements with precision exceeding classical limits. Squeezed light reduces quantum noise in gravitational wave detectors, improving sensitivity to spacetime ripples from cosmic events. Entangled photons enable phase measurements at the Heisenberg limit, with uncertainty scaling inversely with photon number rather than its square root. Quantum illumination uses entangled photons for target detection in noisy environments, potentially enabling radar with quantum advantage.
About This Category
Quantum photonics sits at the frontier of quantum technology, translating fundamental quantum physics into practical devices and systems. The inherent advantages of photons for quantum information, including room-temperature operation, low decoherence, and compatibility with existing telecommunications infrastructure, make photonic approaches particularly promising for near-term quantum applications. This category covers the sources, components, and systems that harness quantum properties of light, providing the technical foundation for understanding and contributing to this rapidly advancing field.