The first unambiguous proof that light from a thermal source behaves quantum mechanically has been claimed by physicists in China. Their demonstration involved interfering single photons from one distant thermal source – the Sun – with photons from a semiconductor quantum dot here on Earth. They say that their work could help enable teleportation, cryptography and other quantum technologies, as well as provide new insights into stellar astrophysics.

While many properties of light can be understood in terms of classical electromagnetic fields, others require a quantum-mechanical description based on discrete photons. Among these is the behaviour of two indistinguishable single photons that meet at a 50:50 beam splitter. Each photon is as likely to be reflected from the apparatus as it is to pass through, and therefore there are in principle four possible outcomes – two of which involve the photons leaving through the same output port while in the other two they make separate exits.

Classically, the photons’ behaviour is totally random and as such the particles are expected to leave together 50% of the time. However, physicists at the University of Rochester in the US showed in 1987 that that is not what happens. Chung Ki Hong, Zhe Yu Ou and Leonard Mandel found that, if perfectly distinguishable, the particles always exit the experiment together as a pair. Explainable using the statistics of bosons, such interference is described as yielding complete “visibility” – in other words, two detectors placed behind the beam splitter will never register signals at the same time.

Quantum sunshine

In this latest work, Chao-Yang Lu, Jian-Wei Pan and colleagues at the University of Science and Technology of China in Shanghai have shown that such quantum-mechanical behaviour even occurs when one of the photons comes from the Sun – a thermal light source 150 million kilometres from Earth. To do so, the researches tracked the Sun using an electrically driven mount and guided the light that they collected along a 50 m stretch of fibre-optic cable to their laboratory. There they interfered the solar photons with others from a quantum dot – in effect an artificial single atom made from semiconductor cooled to just a few degrees above absolute zero.

Lu explains that the photons from the quantum dot come essentially ready-made for the experiment, being intrinsically single and also identical – including having exactly the same energy, timing information and polarization. The sunlight, in contrast, is “dirty”, having a very broad and complex spectrum that only gets more complex after passing through the Earth’s atmosphere. To prepare those photons, the researchers filtered them spectrally, temporally and spatially, and also polarized them.

Interference between the two sets of photons then yielded a visibility of 0.796. This is far greater than the classical maximum of 0.5 and the researchers say this is an unambiguous hallmark of quantum behaviour. Lu explains that the visibility is less than the ideal value of one mostly because of thermal light’s “multi-photon contribution”. The team also measured clear signatures of entanglement and a violation of Bell’s inequality, so ruling out local realism.

“Highly non-classical”

The researchers point out that photons from independent light sources have previously been shown to interfere quantum mechanically, but those sources – such as single atoms or trapped ions – are manmade. Meanwhile, they argue, earlier demonstrations of interference using thermal light were either explainable “within the framework of classical coherence theory” or yielded visibilities around 0.5. “Our result is the first time that [a] thermal light – requiring only classical optics for its description – is involved in a highly non-classical quantum-optics experiment,” they write in a preprint recently uploaded to the arXiv server.

According to Lu, the team’s work could contribute to building large-scale hybrid quantum information networks by allowing independent photon sources to interact with one another. One such application that might benefit, he says, is quantum teleportation. That involves transferring quantum states over long distances without the movement of physical particles. Teleportation requires that the sender interfere one half of an entangled group of photons with another group – those bearing the states to be transmitted.

Indeed, the researchers are currently setting up a new experiment that will teleport the quantum states of solar photons using entangled photons from a quantum dot. What is more, says Lu, the experiments could be extended to larger scales by using a telescope several metres in diameter to collect the feeble light from distant stars (and combining that with better single-photon sources). This could provide information on stellar processes such as sudden changes in magnetic fields and a better understanding of space weather.

For Ronald Hanson, a quantum physicist at Delft University of Technology in the Netherlands, the latest work is interesting because of its novel confirmation of existing theory rather than its implications for quantum technology. “It is very appealing because it uses the Sun as a light source in a quantum experiment!,” he says.