Weird! Quantum Entanglement Can Reach into the Past

Scientists have entangled particles in such a way that a future decision can affect the past states of the particles. CREDIT: Jon Heras, Equinox Graphics Ltd. View full size image

Spooky quantum entanglement just got spookier.

Entanglement is a weird statewhere two particles remain intimately connected, even when separated over vast distances, like two die that must always show the same numbers when rolled. For the first time, scientists have entangled particles after they've been measured and may no longer even exist.

If that sounds baffling, even the researchers agree it's a bit "radical," in a paper reporting the experiment published online April 22 in the journal Nature Physics.

"Whether these two particles are entangled or separable has been decided after they have been measured," write the researchers, led by Xiao-song Ma of the Institute for Quantum Optics and Quantum Information at the University of Vienna.

Essentially, the scientists showed that future actions may influence past events, at least when it comes to the messy, mind-bending world of quantum physics.

In the quantum world, things behave differently than they do in the real, macroscopic worldwe can see and touch around us. In fact, when quantum entanglement was first predicted by the theory of quantum mechanics, Albert Einstein expressed his distaste for the idea, calling it "spooky action at a distance."

The researchers, taking entanglement a step further than ever before, started with two sets of light particles, called photons. [Stunning Photos of the Very Small]

The basic setup goes like this:

Both pairs of photons are entangled, so that the two particles in the first set are entangled with each other, and the two particles in the second set are entangled with each other. Then, one photon from each pair is sent to a person named Victor. Of the two particles that are left behind, one goes to Bob, and the other goes to Alice.

But now, Victor has control over Alice and Bob's particles. If he decides to entangle the two photons he has, then Alice and Bob's photons, each entangled with one of Victor's, also become entangled with each other. And Victor can choose to take this action at any time, even after Bob and Alice may have measured, changed or destroyed their photons.

"The fantastic new thing is that this decision to entangle two photons can be done at a much later time," said research co-author Anton Zeilinger, also of the University of Vienna. "They may no longer exist."

Such an experiment had first been predicted by physicist Asher Peres in 2000, but had not been realized until now.

"The way you entangle them is to send them onto a half-silvered mirror," Zeilinger told LiveScience. "It reflects half of the photons, and transmits half. If you send two photons, one to the right and one to the left, then each of the two photons have forgotten where they come from. They lose their identities and become entangled."

Zeilinger said the technique could one day be used to communicate between superfast quantum computers, which rely on entanglement to store information. Such a machine has not yet been created, but experiments like this are a step toward that goal, the researchers say.

"The idea is to create two particle pairs, send one to one computer, the other to another," Zeilinger said."Then if these two photons are entangled, the computers could use them to exchange information."

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