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Quantum mechanics is one of the best-tested theories in science, and it's one of the few where physicists get to do experiments proving that Einstein was wrong.
That's what a team at Griffith University and the University of Tokyo in Japan did this week, showing that a weird phenomenon — in which the measurement of a particle actually affects its location — is real.
Back in the 1920s and 1930s, Albert Einstein said he couldn't support this idea, which he called "spooky action at a distance," in which a particle can be in two places at once and it's not until one measures the state of that particle that it takes a definite position, seemingly with no signal transmitted to it and at a speed faster than light. When the particle takes its definite position, physicists refer to this as its wave function collapsing.
The phenomenon was outside of contemporary experience in physics and seemed to violate the theory of relativity, which posits that the speed of light is an absolute limit on how fast any information can travel. Einstein proposed that the particle isn't in a superposition state, or two places at once; but rather it always has a "true" location, and people just couldn't see it.
Alice and Bob
The phenomenon is demonstrated with a thought experiment in which a light beam is split, with one half going to Alice and the other to Bob. Alice then indicates if she detected a photon and if so what state it is in — it might be the phase of the wave packet that describes the photon. Mathematically, though, the photon is in a state of "superposition," meaning it is in two (or more) places at once. Its wave function, a mathematical formula that describes the particle, seems to show the photon has no definite position.
"Alice's measurement collapses the superposition," meaning the photons are in one place or another, but not both, Howard Wiseman, director of Griffith University's Centre for Quantum Dynamics, who led the experiment, told Live Science. If Alice sees a photon, that means the quantum state of the light particle in Bob's lab collapses to a so-called zero-photon state, meaning no photon. But if she doesn't see a photon, Bob's particle collapses to a one-photon state, he said.
The experiment is described in the March 24 issue of the journal Nature Communications.
"Does this seem reasonable to you? I hope not, because Einstein certainly didn't think it was reasonable. He thought it was crazy," he added, referring to the fact that Alice's measurement looked like it was dictating Bob's.
The paradox was partially resolved years later, when experiments showed that even though the interaction between two quantum particles happens faster than light (it appears instantaneous), there is no way to use that phenomenon to send information, so there's no possibility of faster-than-light signals.
The team at Griffith, though, wanted to go a step further and show that the collapsing wave function — the process of Alice "choosing" a measurement and affecting Bob's detection — is actually happening. And while other experiments have shown entanglement with two particles, the new study entangles a photon with itself.
To do this they fired a beam of photons at a splitter, so half of the light was transmitted and half was reflected. The transmitted light went to one lab and the reflected light went to the other. (These were "Alice" and "Bob" of the thought experiment.)
The light was transmitted as a single photon at a time, so the photon was split in two. Before the photon was measured, it existed in a superposition state.
One lab (Alice) used a laser as a reference, to measure the phase of the photon. If one thinks of light as a repeating sine wave, phase is the angle one is measuring, from 0 to 180 degrees. When Alice changed the angle of her reference laser, she got varying measurements of the photon: Either her photon was in a certain phase or it wasn't present at all.
Then the other lab (or Bob) looked at their photons and found the photons were anti-correlated with Alice — if she saw a photon he did not, and vice versa. The state of Bob's photon depended on what Alice measured. But in classic physics that shouldn't happen; rather, the two particles should be independent of one another.