Albert Einstein may have been the greatest mind of the 20th century, but the great physicist famously disliked some of the weirder implications of quantum physics. Now, nearly a century after his protests, physicists may have proven one of the points that he doubted the most.
According to quantum mechanics, a particle can be described as a wave that spreads out over a great distance. Yet the particle is still just one particle. You can't detect it in two places at once. When physicists observe the particle in a particular location, they say that the wave function—the mathematics that describes how a particle could be in multiple places at once—has collapsed.
Einstein could not accept this. Or, at least, that he thought the quantum mechanics of his day could not adequately explain it, referring to the phenomenon with the now-iconic phrase "spooky action at a distance." But in new research published in Nature Communications, Griffith University's Howard Wiseman and colleagues use a single particle to show that the wave function really does collapse in this strange way. In so doing, their work backs up years of research into quantum entanglement, in which particles are connected in a mysterious way even when separated, so that observing or affecting one instantly affects the other.
Previous experiments had tested quantum entanglement with two particles, but the researchers wanted to get at Einstein's claim by entangling a single photon of light. They did this by firing a beam of photons into a splitter that cut each photon in two, sending half of the light to one lab and half to another lab.
Using a finely tuned homodyne detector—a tool used to measure the waves of these particles—Lab A tried to look for its photon and measure its phase. So did the scientists in Lab B. They found that if the Lab A researchers had detected the photon, then the Lab B researchers did not, and vice versa. Plus the photon state that Lab B detected depended upon what Lab A detected. That's exactly what you'd expect if the single split photon were entangled.
"Einstein's view was that the detection of the particle only ever at one point could be much better explained by the hypothesis that the particle is only ever at one point, without invoking the instantaneous collapse of the wave function to nothing at all other points," Wiseman says in a news release. "Through these different measurements, you see the wave function collapse in different ways, thus proving its existence and showing that Einstein was wrong."
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