Dannie Pengin Beijing

Published: 2:30pm, 8 Mar 2026

Chinese scientists have cracked a long-standing puzzle in quantum optics, creating a tiny device that spits out pairs of light particles on demand with unprecedented purity and efficiency – with potential implications for the quantum race.

Single-photon devices are maturing rapidly, but the field is now pursuing efficient two-photon sources that can pave the way for sharper medical imaging, unbreakable encryption and next-generation sensors. In precision measurement, for instance, using two photons can double the spatial resolution compared with a single photon.

The challenge with traditional quantum dots – tiny structures often called “artificial atoms” – is that getting them to emit just two photons at a time is like trying to balance two marbles on a needle. Now, a team of researchers from Beijing has found a way that delivers exceptionally strong two-photon efficiency.

Under pulsed excitation conditions – the process of applying a brief, intense force or impulse to a structure – their new emitter can achieve 98.3 per cent of emitted photons appearing in paired form, with a generation efficiency of 29.9 per cent. Such results represent the “international best-in-class” of its kind, according to its lead researcher.

The advance, unveiled this week in Nature Materials, comes from a team at the Beijing Academy of Quantum Information Sciences led by chief scientist Yuan Zhiliang, in collaboration with researchers from the Chinese Academy of Sciences’ Institute of Semiconductors.

In a research briefing in the same journal, Ding Fei, a scientist from Leibniz University in Germany, wrote that he was “truly impressed by the result”.

“This work, without any doubt, marks a major advance towards deterministic generation of photon-number states,” he added.

Yuan said: “Entangled two-photon systems remain eternally synchronised in both time and energy. This property proves invaluable in precision measurements and quantum imaging.”

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The go-to method for generating twin-photon pairs involves harnessing non-linear crystals: a high-energy “pump” laser hits the crystal, splitting one photon into two lower-energy ones.

These naturally entangled pairs sync in time, polarisation or momentum, powering quantum key distribution and entanglement experiments, but their random yield, low efficiency and multi-photon noise demand tricky optical filtering.

“Non-linear crystal light sources are inherently probabilistic, sometimes emitting a single pair, sometimes two pairs or even multiple pairs,” Yuan noted.

Semiconductor quantum dots have always had the potential to produce photon pairs. However, they are plagued by a persistent physical limitation: once an electron is excited, it becomes highly unstable. It rushes to radiate a single photon and escape before its partner can even arrive.

To address the puzzle, the team led by Yuan found an ingenious workaround by steering the quantum dot into a long-lived state called a “dark exciton”, which can be thought of as a secret waiting room.

Yuan likened it to a passageway where lone walkers zipped through instantly – most times, you see just one at a time. Now they have worked out a way by adding a gate with dual locks inside: it only opens when two arrive together, letting them pass as a pair.

They trapped a single quantum dot inside a super-thin optical pillar, narrower than human hair. Using laser pulses tuned just right, they guide electrons into the waiting zone that does not leak light easily. From there, the system efficiently loads a “biexciton” state that decays in a cascade, firing off two photons almost simultaneously.

Chinese scientists have cracked a long-standing puzzle in quantum optics that could upend the quantum race. Photo: Getty Images

Lab tests showed 98.3 per cent of collected light as pure photon pairs, with a record-marking generation efficiency of nearly 30 per cent.

While single photons already power prototype quantum networks, photon pairs are a key link for real-world leaps.

For example, in next-generation medical imaging, because these twin photons are temporally correlated, meaning the birth of one heralds the other, they can be used for noise-resistant imaging. This could allow for ultra-high-resolution biological scans that do not damage sensitive tissues.

In standard fibre-optic cables, information is sent as pulses of light that can be intercepted and copied. In a quantum network, entangled photon pairs can act as the security guard: if a hacker tries to “listen in” on one photon, the state of its partner is instantly disturbed.

But for Yuan, the scientist from Beijing Academy of Quantum Information Sciences, the journey has only just started.

For the next step, his team plans to further increase the quality of the photon pairs, and they will also work on seeking novel material systems to elevate the operating temperature of this newly discovered device from the liquid helium temperature range – below 10 kelvins – to the liquid nitrogen temperature range – above 77 kelvins – so it can unlock more application potential.

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Dannie Peng

Dannie joined SCMP in 2023 and focuses on science stories in China, with a particular interest in the scientific community and societal impacts of scientific advances.