Single-Photon Source Applications, Types & Future Directions
Single-photon emitters (SPEs) emit one photon. Several quantum technologies employ this to create accurate light quanta. Unlike traditional light sources, SPSs emit photons individually with sub-Poissonian characteristics.
Photons were hypothesised by Einstein in 1905 and quantised energy by Max Planck in 1900. Quantum physics explains photon superposition and entanglement. “The second quantum revolution,” which produced, manipulated, and detected photons, enabled quantum computing and communication.
Superior Single-Photon Sources
In quantum applications, single-photon sources must have numerous qualities:
High-efficiency photon release upon activation.
Pure: Guaranteeing one photon at a time with little risk of releasing more.
Quantum interference effects require strong indistinguishability since photons from the same source are nearly identical. Photon production on demand: Controlled, precise timing.
Brightness: Collecting most photons increases efficiency.
Solid-state sources allow integration and downscaling of many devices.
Easy implementation and robustness in simple contexts.
Different Single-Photon Sources
Among the many SPS varieties:
Quantum dot single-photon sources (QDs), sometimes known as “artificial atoms,” are tiny semiconductor nanostructures used in advanced single-photon technologies.
Working principle: Quantum dots catch electrons in a confined region to produce distinct energy states. Electrons gain energy from laser pulses. A specific photon is released when the electron returns to its ground state. Advanced cavity designs help direct photon emission for collection.
Solid-state QDs have deterministic emission, high purity, indistinguishability, brightness, and scalability. Size-dependent emission wavelength modification and high photoluminescence efficiency are possible using quantum confinement technologies' optical and electrical features.
Manufacturing: Quandela uses advanced epitaxial techniques to grow semiconductor quantum dots, position them deterministically at the core of micropillar optical cavities, maximise the cavity's dimensions to take advantage of the Purcell effect, incorporate electrical contacts, and couple fibres.
Asymmetric microcavities have been produced to balance indistinguishability and single-photon purity. A narrowband mode for collecting and a broadband cavity mode for excitation reduce two-photon emission in this design. Resonant excitation can achieve deterministic population inversion and help overcome photon efficiency loss under polarisation filtering. Indium arsenide QDs produce fundamental 1,550 nm telecom photons for fiber-optic transmission.
Colour centre single-photon sources include silicon carbide, boron nitride, and diamond nitrogen-vacancy (NV) centres. Diamond has highly researched NV centres.
Ionic/atomic Sources of single photons Individual atoms or ions can release light. Fluorescence from attenuated sodium atoms (1977) and mercury cascade transitions (1974) are early examples. In the mid-1980s, ion traps housed single ions as emitters. Blocking numerous excitations in a blockade volume allows Rydberg excitation in tiny atomic ensembles or crystals to produce single photons.
In p-terphenyl crystals, energised pentacene molecules release single photons.
Heralded Single-Photon Sources (Spontaneous Four-Wave Mixing and Parametric Down-Conversion): Higher-energy photons form pairs of single photons. The detection of one photon “heralds” the other's arrival, making it visible. This probabilistic, non-on-demand technique has been a “workhorse” for single photon research since the mid-1980s.
Laser beam attenuation was one of the original and most fundamental approaches. Although it has the probability ratio for a single photon, this source lacks antibunching, making it unsuitable for many quantum applications.
Applications of single photons
Single photons are effective quantum information carriers:
They are essential for linking quantum networks and safe quantum key distribution (QKD) protocols. Satellite and fibre communications have demonstrated QKD systems over long distances. Single photons could provide photonic qubits for quantum information processing in future quantum computers. Linear-optical quantum computing uses them. Quantum Metrology and Sensing: SPSs improve quantum-based sensor sensitivity and help measure quantum limits precisely.
These enable fundamental quantum physics and quantum optical investigations.
Struggles and Prospects
Even with advances, problems remain. Single photons in the telecommunication wavelength range are needed for low-loss quantum communication across optical fibres, but they are difficult to produce. High photon quantity and quality requirements make linear-optical quantum computing difficult.
Research aims to produce minuscule sources that release single photons at the highest rate, on demand, and indistinguishably. Innovations like the asymmetric microcavity for quantum dots provide “excellent performance” single-photon sources with increased brightness, near-unity purity, and indistinguishability without filtering in real-world experiments.
