杏吧原创

When a bubble pops in a liquid, it can produce a flash of light, which we now know is thanks to quantum mechanics.

Sonoluminescence is a phenomenon in which small bubbles, produced and fixed in place by an ultrasound wave in a liquid, collapse and make particles of light, or photons. Physicists have known about this process for decades, but the mechanisms behind it weren鈥檛 fully known.

Now, at the University of Ottawa, Canada, and his colleagues have measured the quantum nature of the photons produced for the first time, which means the underlying process is quantum mechanical.

Karimi and his team set up a quartz flask filled with water, which contained a single air bubble, and placed seven mirrors around the flask to reflect photons onto a special photon-detecting camera. When the bubble was made to collapse, it produced a burst of photons, which they measured sequentially.

In classical light sources, such as a lamp or the sun, photons are generated at random times and arrive at a destination 鈥� such as a camera 鈥� with no discernible pattern. But quantum processes produce an effect called anti-bunching, where photons arrive with a certain regularity. It is impossible to have anti-bunching if a system is obeying just the rules of classical physics, says Karimi. 鈥淚t requires quantum [mechanics] to be involved.鈥�

Karimi and his team used the camera to calculate statistical properties of the photons that are unique to quantum events. 鈥淲e did this for many different samples during different days with different timing,鈥� says Karimi. Doing so confirmed the photons were correlated with one another in a way that is consistent with quantum processes. 鈥淲hat we observed was that [these photons] were always different from [classical] thermal light and laser sources,鈥� he says.

The experiment, which took the researchers more than five years to complete, was particularly difficult because they didn鈥檛 know when the incredibly short photon pulse would be produced.

鈥淭his is like standing alongside a motorway and having one car a day and trying to find it, not knowing when the car will come, and being there at the right time,鈥� says at Loughborough University, UK.

While the new information doesn鈥檛 explain how the photons were created, it certainly helps narrow the possible sources. Karimi speculates that they could be the result of a form of the dynamical Casimir effect, a quantum mechanical process where particles are produced from a moving mirror, which might be created at the surface between the water and the bubble when the bubble collapses.

But nailing down sonoluminescence鈥檚 true origin is fiendishly difficult due to the number of different mechanisms and substances involved. 鈥淚t鈥檚 such a dirty system,鈥� says Eberlein. 鈥淣ormally, when you go into a quantum optics lab, people focus and adjust things and tune things to the nanometre, so they know exactly what鈥檚 happening.鈥�

Even if the photons鈥� origin is never discovered, their quantum nature could have real-world applications, says Karimi. To produce purely quantum photons for experiments, researchers need expensive lasers and tools that can cost millions of pounds. It is possible that a much cheaper set-up could be designed using a flask of water exhibiting sonoluminescence.

鈥淲e observed that there is a huge correlation [in our experiment],鈥� says Karimi. 鈥淓ven in a laboratory with high-tech, million-dollar facilities, I wouldn鈥檛 be able to generate such a quantum source.鈥�

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