photon pair sources (original) (raw)

Definition: light sources emitting photons in correlated or even entangled pairs

Category: quantum photonics

• light sources
  • quantum light sources
    * squeezed light sources
    * single-photon sources
    * photon pair sources

Related: single-photon sourcesquantum light sourcesnonclassical lightphotonsquantum noisequantum photonicsquantum information processing

DOI: 10.61835/fwy Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

Content quality and neutrality are maintained according to our editorial policy.

📦 For purchasing photon pair sources, use the RP Photonics Buyer's Guide — an expert-curated directory for finding all relevant suppliers, which also offers advanced purchasing assistance.

Contents

What Are Photon Pair Sources?

Photon pair sources are sources of photon pairs, i.e., pairs of photons with special quantum properties. Generating a kind of nonclassical light, they are considered as a kind of quantum light sources. Frequently, they exhibit quantum entanglement, but some sources only have classical correlations.

Photon pair sources underpin both foundational quantum optics experiments and support many quantum photonics applications by serving a key role in quantum information processing.

Types of Photon Pair Sources

Most implementations follow one of two technological routes:

Parametric Sources

One can use parametric nonlinear optical processes of different kinds, where spontaneous parametric downconversion (SPDC) occurs:

• The ($\chi^{(2)}$) nonlinearity of nonlinear crystal materials can be exploited, where one pump photon is converted into one signal and one idler photon. Mostly, one uses non-degenerate interactions, i.e., with distinguishable signal and idler photons. The interaction may occur in a bulk nonlinear crystal or in a nonlinear waveguide.
• Alternatively, a ($\chi^{(3)}$) nonlinearity can be utilized for four-wave mixing, often in optical fibers or other waveguides. Here, two pump photons (with identical or different wavelengths) are converted into one signal and one idler photon, having optical frequencies above and below the pump frequencies, respectively. Having not too dissimilar frequencies can be advantageous concerning waveguide optimization.

In any case, an intense pump laser beam is required in such a device. It is often used in continuous-wave operation, but pulsed lasers may also be employed.

Various details need to be considered for engineering such sources:

• The pump laser must have a suitable wavelength for obtaining appropriate signal and idler wavelengths, and should exhibit single-mode emission with low noise.
• Only a tiny part of the injected pump photons is converted. With increased pump intensity, one would obtain more parametric gain, resulting in a higher conversion efficiency, but that would eventually degrade the quantum light properties through increasingly likely multi-pair emission.
• Phase matching of the nonlinear interaction can be of different types, with temperature or angle tuning, and possibly using quasi-phase matching via periodic poling.
• There are cavity-enhanced sources, employing optical resonators of some types.
• For integration on photonic integrated circuits, technology platforms like thin-film LiNbO3 and Si/SiN can be utilized. This enables compact, fiber-compatible sources with high stability.
• Spectral engineering can involve group velocity matching, apodized poling and pump shaping.

Cascade Transitions

It is also possible to generate photon pairs with a cascade of optical transitions: An excited system emits a photon, dropping to an intermediate energy level, then emits a second photon. The biexciton–exciton cascade in quantum dots is the most prominent solid-state example for this operation principle. These devices may be pumped optically or electrically and can be placed in microcavities (typically in micropillars) to obtain efficient single-mode extraction via the Purcell effect.

Using cascade transitions generally leads to simpler and more compact photon pair sources compared with parametric sources. Further, on-demand operation (triggering with a high probability of pair generation) may be achieved.

Achieving indistinguishability and high entanglement fidelity requires careful engineering for control of fine-structure splitting, charge noise, and spectral diffusion (often via strain/electric-field tuning and resonant excitation).

Photon Pair Properties

The generated photon pairs can differ in many respects, which can be vital for applications:

• The generated signal and idler photons of a pair are usually distinguishable, e.g. via optical frequency, but in some cases (degenerate parametric downconversion) they are indistinguishable. In the former case, signal and idler photons are usually output through different ports, which are often single-mode waveguides or fibers.
• Another question is whether subsequent signal photons are indistinguishable. That is often the case, and required for some applications. Hong–Ou–Mandel interference is the common test method for this. Parametric sources, for example, may exhibit a substantial spectral bandwidth, with correlations between frequencies of signal and idler, and limited indistinguishability of subsequent signal photons.
• The optical frequencies or wavelengths matter for applications — for example, because photodetectors with high quantum efficiency are not available in all spectral regions. Popular wavelength bands include 800–900 nm (for detection with silicon APDs), the telecom C and L bands (1530–1625 nm, low fiber loss, detection with superconducting nanowire single-photon detectors (SNSPDs), and specific memory transitions (e.g., 795 nm for Rb).
• A related important quantity is the spectral bandwidth. Bandwidth spans from tens of kHz–MHz (cavity/atomic) to multiple THz (bulk SPDC); the right choice depends on detectors, channels, and interfacing components.
• The polarization may be a well-defined direction (e.g. with type-II processes for fixed polarizations of signal and idler) or random and then polarization-entangled.
• Quantum state fidelity: The purity of obtained quantum state can be quantified with a second-order correlation ($g^{(2)}(0)$), which is zero in the ideal case. In addition, the indistinguishability of photons can be quantified.
• Entanglement: Photon pairs are often quantum-entangled, although that property does not always matter for applications. The degree of entanglement depends not only on the used generation method but also on the detailed circumstances. Non-entangled photons are still correlated in their generation time.
• On-demand vs. probabilistic generation: Some sources based on quantum dots can generate photon pairs on demand — with a substantial (although not perfect) probability of emitting a photon pair when being triggered. The maximum repetition rate may be quite high, in some cases more than 1 GHz. Other sources generate a substantial (and possibly adjustable) number of photon pairs per second in irregular time intervals.
• Spatial modes: Most photon pair sources are emitting into a single spatial mode (although often separately for signal and idler), which allows coupling to a single-mode fiber or other waveguide. Note that incomplete coupling affects correlations, e.g. leading to signal photons with lost idler photons or vice versa.

Note that applications differ a lot in their requirements. For example, quantum entanglement is essential for some methods, while others require only simple correlation.

Applications of Photon Pairs

Photon pairs — whether entangled or merely correlated — are at the heart of multiple quantum technologies, enabling phenomena and capabilities fundamentally impossible with classical light. In the following, some typical applications are explained, spanning both research and practical quantum engineering.

Fundamental Physics

• Bell’s inequality tests use entangled photon pairs with polarization or time-bin entanglement. This allows experimental verification of quantum nonlocality and testing of hidden variable theories.
• Quantum teleportation relies on entangled photon pairs and Bell-state measurements for the transfer of an unknown quantum state from one location to another. This is both a fundamentally interesting phenomenon and a central proof-of-principle for quantum networks.

Quantum Random Number Generation

Quantum random number generation (QRNG) can be done with various techniques, some of which utilize entangled photon pairs. Measurements on those are inherently unpredictable and certified by the quantum nature of the process, generating high-quality, device-independent random numbers, as are crucial for secure communications (cryptography).

Note computation often uses quasi-random numbers, but these exhibit certain correlations which can be disturbing. True random numbers, as can be generated with quantum photonics, can avoid such problems.

Heralded Single-photon Sources

Some single-photon sources are obtained by generating photon pairs with distinguishable signal and idler outputs. One then detects the idler photons, for example, and uses the obtained electronic signals for “heralding” (announcing) the signal photons. For the applications of such heralded single photons, see the article on single-photon sources.

Quantum Communications

Various methods used in quantum communications rely on photon pairs. For example, some quantum key distribution (QKD) protocols such as Ekert's E91 leverage quantum nonlocality of photon pairs for unbreakable encryption. The security relies on the impossibility of eavesdropping without disturbing the quantum correlations in a detectable way. Heralded single photons (from photon pair sources like spontaneous parametric downconversion or quantum dots) support BBM92 and various free-space/fiber QKD protocols.

For such applications, one usually uses photon pairs in the 1.5-μm telecom wavelength window.

A fundamental challenge of quantum communications is that quantum properties of light tend to get lost due to propagation losses, e.g. in optical fibers. Quantum repeaters overcome this problem by using quantum entanglement over large distances. Here, entangled photon pairs are distributed between nodes of quantum networks, allowing quantum information transfer via entanglement swapping and quantum teleportation.

Photonic Quantum Computing

In some methods of photonic quantum computing, entangled and indistinguishable photon pairs are essential for quantum logic gates, cluster states, and boson sampling. Multi-photon interference effects (e.g., in Hong–Ou–Mandel setups) rely on indistinguishable photon pair sources for building large-scale quantum circuits.

Quantum encoding schemes often use polarization, spatial mode or time-bin entanglement among photon pairs for qubit representation and manipulation.

Quantum Sensing and Metrology

Entangled photon pairs offer improved sensitivity and resolution of certain measurements, surpassing the standard quantum limit for optical phase and timing, for example. Such methods may be used in quantum LIDAR, time standards (“quantum candela”), and super-resolution microscopy.

Techniques like ghost imaging leverage spatially correlated photon pairs to reconstruct images of objects never directly illuminated by the detector photon, enabling imaging through scattering and turbulence.

Suppliers

Bibliography

(Suggest additional literature!)

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.