The University of Michigan researchers said their accomplishment marks an advance toward super-fast quantum computing and data transmission.
The scientists used light to establish what's called "entanglement" between two atoms, which were trapped one meter apart in separate enclosures. They described entangling as similar to controlling the outcome of one coin flip with the outcome of a separate coin flip.
David Moehring, the lead author of the paper who did this research as a U-M graduate student, says the most important feature of this experiment is the distance between the two atoms."This linkage between remote atoms could be the fundamental piece of a radically new quantum computer architecture," said Professor Christopher Monroe, principal investigator of the research who has since moved to the University of Maryland.
"Now that the technique has been demonstrated, it should be possible to scale it up to networks of many interconnected components that will eventually be necessary for quantum information processing," Monroe said. The research by Monroe, lead author David Moehring and colleagues is reported in the Sept. 6 issue of the journal Nature.
"The separation of the qubits in our entangled state is the most important feature," Moehring said. "Localized entanglement has been performed in ion trap qubits in the past, but if one desires to build a scalable quantum computer network (or a quantum internet), the creation of entanglement schemes between remotely entangled qubit memories is necessary."
In this experiment, the researchers used two atoms to function as qubits, or quantum bits, storing a piece of information in their electron configuration. They then excited each atom, inducing electrons to fall into a lower energy state and emit one photon, or one particle of light, in the process.
The atoms, which were actually ions of the rare-earth element ytterbium, are capable of emitting two different types of photon of different wavelengths. The type of photon released by each atom indicates the particular state of the atom. Because of this, each photon was entangled with its atom.
By manipulating the photons emitted from each of the two atoms and guiding them to interact along a fiber optic thread, the researchers were able to detect the resulting photon clicks and entangle the atoms. Monroe says the fiber optic thread was necessary to establish entanglement of the atoms, but then the fiber could be severed and the two atoms would remain entangled, even if one were "(carefully) taken to Jupiter".
Each qubit's information is like a single bit of information in a conventional computer, which is represented as a 0 or a 1. Things get weird on the quantum scale, though, and a qubit can be either a 0, a 1, or both at the same time, Monroe says. Scientists call this phenomenon "superposition." Even weirder, scientists can't directly observe superposition, because the act of measuring the qubit affects it and forces it to become either a 0 or a 1.
Entangled particles can revert automatically to the same position once measured, for example always ending in 0,0 or 1,1.
"When entangled objects are measured, they always result in some sort of correlation, like always getting two coins to come up the same, even though they may be very far apart," Monroe said. "Einstein called this 'spooky action-at-a-distance,' and it was the basis for his nonbelief in quantum mechanics. But entanglement exists, and although very difficult to control, it is actually the basis for quantum computers."
Scientists could set the position of one qubit and know that its entangled mate will follow suit. Here's a reasonable example.
There are 100 pennies. Someone breaks them into two piles and hands one pile to each of two people, who go into separate rooms. Person 1 counts their pennies and finds that he has 71 pennies, and knows, because there are 100 pennies, that Person 2 must have 29 pennies.
That's most of it. Now, imagine that Person 1 can change the number of pennies he has, but not the total number of pennies in the system, nor where the other pennies are. So if Person 1 chooses to have 69 pennies instead of 71 pennies, he would know instantly that Person 2 has 31 pennies now, and not 29. Likewise, person 2, seeing he has 31 pennies, knows person 1 must have changed from 71 to 69.
This is two people, who are entangled with respect to how many pennies they have.
Now, to make it more realistic, you have to add randomness. Let's say Person 1 wants to go from 71 pennies to 69 pennies. And goes to 69 pennies. Now, person 2, seeing they have 31 pennies now, is not sure that person 1 actually chose to go from 71 to 69 pennies. Just that Person 1 must now have 69 pennies. And of course, you can check. Person 2 opens the door and hollas, "I have 31 now - did you mean to drop from 71 to 69?"
So… we add distance. Let's make person 1 and person 2 a hundred miles apart, in a building with no outside means of communication (no phones, no Internet, etcetera). Now, when Person 2 sees he has 31 pennies instead of 29 pennies, he has no way to know if that was a random change, or the result of Person 1 choosing to have 69 pennies, but he knows how many pennies other person has.
Entanglement provides extra wiring between quantum circuits, Monroe says. And it allows quantum computers to perform tasks impossible with conventional computers. Quantum computers could transmit provably secure encrypted data, for example. And they could factor numbers incredibly faster than today's machines, making most current encryption technology obsolete (most encryption today is based on the inability for man or machine to factor large numbers efficiently).
Source: University of Michigan
