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Heralded Single PhotonsEdit

Heralded single photons are a foundational resource in modern quantum optics and quantum information. They arise when a nonlinear optical process produces photon pairs in a correlated state, and the detection of one member of the pair (the herald) signals the presence of its partner. The partner, or heralded photon, can then be used in experiments that require a well-defined quantum of light, such as quantum communication, quantum computing with linear optics, and precision metrology. The most common platforms for generating heralded photons are spontaneous parametric down-conversion in nonlinear crystals and four-wave mixing in optical fibers or waveguides, with ongoing advances in on-chip implementations that integrate generation, heralding, and processing in a single photonic circuit. Spontaneous parametric down-conversion Four-wave mixing Quantum information

Principles

  • Pair production and heralding

    • In many nonlinear media, a pump photon is converted into two lower-energy photons in a process called down-conversion or mixing. The two photons are emitted in correlated, but not independent, ways. When a detector registers the herald photon, the detection event conditionally projects the remaining photon into a state that can be used as a single-photon resource. The basic idea is conditional preparation: the presence of the herald tells you that the companion photon exists and is available for use. Spontaneous parametric down-conversion Four-wave mixing

  • State quality and purity

    • The heralded photon is not a perfect single photon in all practical situations. Spectral and temporal correlations between the two photons can reduce the purity of the heralded state. Achieving high purity often requires careful engineering of the source, including shaping the pump bandwidth, engineering phase-matching, and sometimes spectral filtering. The degree of spectral entanglement is quantified by concepts such as the Schmidt number, with a lower value (ideally K = 1) indicating a purer heralded state. Schmidt decomposition Indistinguishability (quantum mechanics) Spectral filtering

  • Metrics and characterization

    • Key performance metrics include the heralding efficiency (the probability that the heralded photon exists given a herald detection), the overall brightness or rate, the purity of the heralded state, and the second-order correlation at zero delay, g^(2)(0), when the herald is used to gate the measurement. A low g^(2)(0) close to zero signals sub-Poissonian statistics compatible with a near-ideal single-photon state. Other practical metrics include detector dark counts, timing jitter, and optical losses that affect the observed heralding performance. Klyshko efficiency Second-order correlation function Photon detector

  • Practical considerations

    • There is a trade-off between brightness (how many pairs are produced) and purity (how clean the single-photon state is). Narrow spectral filtering can improve purity but reduces rate; broadband sources may require more sophisticated shaping. Multiplexing multiple heralded sources is one strategy researchers pursue to approach on-demand single-photon behavior at higher rates. Multiplexing (photonic networks) Spectral filtering Indistinguishability (quantum mechanics)

Implementations

  • SPDC in crystals

    • In materials like periodically poled lithium niobate, beta-barium borate, or potassium titanyl phosphate, a pump photon is converted into a pair of lower-energy photons. Crystal engineering and waveguide geometries enable higher brightness and better mode matching to optical fibers or on-chip waveguides. The heralded photon inherits the spectral and spatial properties dictated by the phase-matching conditions and the pump. Spontaneous parametric down-conversion Periodically poled lithium niobate Waveguide (optics)

  • Four-wave mixing in fibers and waveguides

    • In optical fibers or photonic integrated circuits, nonlinear Kerr effects can produce correlated photon pairs via four-wave mixing. Fiber-based platforms are attractive for their long interaction lengths and compatibility with telecom wavelengths, enabling heralded photons for quantum communication over existing fiber networks. Four-wave mixing Fiber optic communication

  • On-chip and integrated sources

    • Recent progress combines heralded photon generation with routing, switching, and detection on a single chip, reducing loss and improving stability. Platforms include silicon nitride and lithium niobate on insulator, which support efficient SPDC or SFWM and can be integrated with on-chip detectors and interferometers. Integrated photonics Silicon nitride waveguide Lithium niobate on insulator

Characterization and performance

  • Purity and spectral-temporal mode

    • The degree of spectral factorization between the herald and heralded photons determines the purity of the heralded state. Techniques to characterize this include measuring the joint spectral amplitude and performing Hong–Ou–Mandel interference experiments to assess indistinguishability when heralded photons from different sources are made to interfere. Hong–Ou–Mandel effect Joint spectral amplitude

  • Heralding efficiency and brightness

    • Heralding efficiency reflects how reliably a herald detection implies a heralded photon exists. Brightness must be balanced against the likelihood of multi-photon events, which degrade the single-photon character. These aspects are crucial for scalable quantum networks and photonic quantum computing. Klyshko efficiency Brightness (optics)

  • Multiphoton probabilities

    • Even with heralding, there is a nonzero probability of producing more than one photon in the heralded mode, especially at higher pump powers. This is captured by the conditional multiphoton probability and by measurements of g^(2)(0) under heralding conditions. Reducing this probability is a central design goal for reliable single-photon sources. Second-order correlation function

Applications

  • Quantum communication and cryptography

    • Heralded single photons enable secure quantum key distribution and other protocols that rely on single-photon qubits transported over optical channels. The compatibility with telecom wavelengths and fiber networks makes them practical for metropolitan-scale quantum networks. Quantum key distribution Quantum communication

  • Photonic quantum computing

    • Linear optics quantum computing and boson-sampling experiments depend on reliable single-photon inputs. Heralded photons, when paired with high-fidelity interference and low-loss routing, form a core resource for these architectures. Linear optical quantum computing Boson sampling

  • Quantum metrology and sensing

    • Single-photon states improve certain metrological tasks, including phase measurements and imaging schemes, where heralding helps synchronize and validate photon arrival times. Quantum metrology

Controversies and debates

  • On-demand single photons vs heralded photons

    • A long-running discussion in the field centers on whether truly on-demand single-photon sources (which aim to emit exactly one photon per trigger with minimal loss and no multiphoton events) can outperform heralded sources in practical systems. Proponents of on-demand emitters emphasize deterministic operation and simplicity for scaling, while heralded sources are praised for their mature physics, relative ease of implementation, and strong performance in many settings. The best path often involves multiplexed heralded sources or hybrid approaches that blend the advantages of both methods. Heralded single-photon source Single-photon source

  • Filtering vs efficiency trade-offs

    • There is ongoing debate about the optimal balance between spectral/ppectral filtering and overall efficiency. Aggressive filtering improves purity but reduces rate, which can bottleneck applications requiring high throughput. Advances in engineering and multiplexing are aimed at mitigating these trade-offs. Spectral filtering Multiplexing (photonic networks)

  • Real-world deployment considerations

    • In practical networks, losses, detector dark counts, and timing jitter set hard limits on the utility of heralded photons. The community continues to optimize materials, waveguide designs, and detector technology to push performance toward reliable, scalable quantum information processing. Photon detector Loss (optical)

See also