Quantum Dot Single Photon Sources

Short answer: A single-photon source emits light one photon at a time, which quantum key distribution and quantum networks need. Semiconductor quantum dots are a leading candidate: a 2012 review said the fastest triggered single-photon sources to date used epitaxially grown quantum dots, and a 2025 review covers room-temperature quantum dot sources and their challenges. The key quality tests are antibunching and photon indistinguishability. These are review-level findings about laboratory devices.

What a single-photon source is

A 2025 review in Nanoscale describes a single-photon source as a device that emits photons one at a time, enabling precise control over quantum states, unlike classical light sources that produce streams that interfere. It says this is crucial for quantum key distribution and for quantum networks, where single photons carry quantum information. It lists several technologies: quantum dots, atom-like emitters and color centers in diamond. Quantum dots are semiconductor nanocrystals with size-dependent emission wavelengths, high photoluminescence efficiency, discrete energy levels, scalability and low background noise, according to the review.

Why quantum dots

A 2012 Reports on Progress in Physics review titled Engineered quantum dot single-photon sources says fast, high-efficiency, low-error sources are required for a number of quantum information applications. It states that the fastest triggered single-photon sources to date were demonstrated with epitaxially grown quantum dots, which can be conveniently integrated with optical microcavities. It adds that demonstrations of high-temperature and telecommunications-wavelength emission had made quantum dot sources more practical, and reviews progress against the requirements of different applications.

How quality is measured

A 2020 Applied Physics Reviews article explains the main tests. Single-photon emission is checked by the second-order correlation function at zero delay: a value between 0 and 0.5 indicates a single-photon emitter with some contamination, for example from imperfect isolation of the optical transition. Under pulsed excitation a pure emitter shows no peak at zero delay, and under continuous excitation it shows an antibunching dip. For most quantum information processing applications that need two-photon interference, single-photon emission is not enough, because photons must also be indistinguishable. That is confirmed by Hong-Ou-Mandel interference. The article notes indistinguishability is not strictly required for applications such as QKD.

Open challenges

The 2025 review concentrates on room-temperature quantum dot sources, discussing optical properties, performance metrics and the latest developments. It covers ways to reduce blinking and improve photon statistics, including plasmonic nanocavities and ligand exchange, and ends with the challenges and possible solutions.

Reading the numbers in the field

Two different questions sit behind most claims. The first is purity: how close the second-order correlation at zero delay is to zero, with the 2020 article treating values below 0.5 as a single emitter with some contamination. The second is indistinguishability, which matters when photons from different emission events must interfere, as in quantum networking schemes. A source can pass one test and fail the other, so a single headline figure rarely describes a source fully. The 2012 review organises its discussion around the requirements of each application for exactly this reason.

What this does not tell you

These reviews summarise many experiments and do not give one device's figures. Room-temperature operation and cryogenic high-performance operation are different regimes, and a source that does well on one metric may not on others.

FAQ

What is a single-photon source?

A device that emits photons one at a time.

Why are quantum dots used?

A 2012 review reports the fastest triggered sources used epitaxial quantum dots, which integrate with microcavities.

How do you test one?

By measuring antibunching in the second-order correlation, and indistinguishability by Hong-Ou-Mandel interference.

Is indistinguishability always needed?

No. The 2020 article says it is not strictly required for applications such as QKD.

Do they work at room temperature?

A 2025 review covers room-temperature quantum dot sources and lists remaining challenges such as blinking.