Over the past decade, researchers have become good at creating devices that emit single photons on demand, which is a key ingredient for quantum computing. However, reliably producing exactly two photons at the same time has remained a challenge.
Now, researchers in China have developed a device that behaves almost like a miniature photon-pair factory. In experiments, 98.3 percent of the emitted light from this device appeared as photon pairs, one of the purest results ever achieved with a solid-state device.
Photon pairs are highly valuable because they can be correlated or entangled, meaning they behave like synchronized partners.
For example, “entangled two-photon systems remain eternally synchronised in both time and energy. This property proves invaluable in precision measurements and quantum imaging,” Zhiliang Yuan, one of the study authors and the chief scientist at the Beijing Academy of Quantum Information Sciences (BAQIS), told SCMP.
Moreover, such pairs could enable ultra-secure communication, sharper quantum sensors, and highly advanced medical imaging techniques.
By demonstrating one of the most efficient solid-state two-photon sources yet, the researchers have moved quantum photonics a step closer to real-world applications.
Making two photons at once has been so hard
Producing pairs of photons from a single emitter is tricky. Traditional photon-pair sources rely on nonlinear crystals, where a powerful laser photon splits into two lower-energy photons.
Although widely used, “Non-linear crystal light sources are inherently probabilistic, sometimes emitting a single pair, sometimes two pairs or even multiple pairs,” Yuan said. This randomness introduces noise and reduces efficiency.
Scientists have been hoping that semiconductor quantum dots (tiny semiconductor particles that are often described as artificial atoms) could solve this problem. When excited by light, these nanoscale structures release photons as electrons fall back to lower-energy states.
In theory, a quantum dot could emit two photons through a process called a biexciton–exciton cascade, where two excited electrons recombine one after the other.
In practice, however, the process rarely works well. Once a single electron in the quantum dot is excited, it typically emits a photon immediately and relaxes, preventing the system from forming the two-electron state required for the cascade.
As a result, reliably generating photon pairs from a single quantum dot has remained extremely difficult.
Turning a dark state into a photon-pair generator
To overcome this limitation, the newly developed device places a single quantum dot inside a microscopic optical pillar cavity—a structure thinner than a human hair that traps and improves light emission.
The cavity boosts the process through the Purcell effect, which increases the rate at which photons can be emitted. The key innovation was steering the quantum dot into a long-lived quantum state called a dark exciton.
In simple terms, this state behaves like a temporary waiting room that does not easily emit light. Instead of immediately releasing a photon, the excited electron can remain there long enough for another electron to arrive.
The researchers used carefully tuned laser pulses and a technique known as polarization-selective p-shell excitation to guide electrons into this dark state. Once two electrons occupy the quantum dot simultaneously, they form a biexciton state.
This state then decays through a two-step cascade, releasing two photons in rapid succession. Since the quantum dot sits inside the optical cavity, the emission can also be strengthened by stimulated two-photon processes, further enhancing the correlation between the photons.
The results were eye-opening. Experiments showed that 98.3 percent of the collected light appeared as photon pairs. The pair-generation efficiency reached 29.9 percent, among the best reported for such systems.
The measured two-photon correlation value g²(0) was about 3.97, indicating strong pair emission. Overall, 98.3 percent of the emitted photons belonged to pairs. In other words, the system effectively behaves like a tiny two-photon factory, releasing paired photons with exceptional purity.
“By exploiting a dark-state pathway to biexciton population in a quantum-dot microcavity, bright high-purity photon pairs have been generated, offering a promising route towards practical two-photon sources,” the study authors note.
The technology must improve
Despite the impressive performance, the device still has limitations. It currently operates at extremely low temperatures below 10 kelvin, close to liquid-helium conditions.
For practical use, scientists hope to push the operating temperature closer to liquid-nitrogen levels (above 77 kelvin), which would make the technology far easier and cheaper to deploy.
The researchers now plan to improve the quality of the photon pairs further and explore new materials that could enable higher-temperature operation. If successful, their work could bring practical on-demand photon-pair sources closer to reality.
The study is published in the journal Nature Materials.