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First Entanglement of Two Separate Ions a Meter Apart: Photons Go the Distance

September 7, 2007

COLLEGE PARK , MD -- A team of physicists has exploited one of the most mysterious phenomena in nature to make a major advance toward the long-sought goal of super-fast quantum computing.

 Christopher Monroe and colleagues at the University of Maryland and the University of Michigan established a "spooky," intimate quantum-mechanical condition called "entanglement" between two completely unconnected individual atoms a meter apart in separate enclosures by carefully manipulating photons emitted by the atoms.

With these atoms, even though they have never come in physical contact, their properties are entangled: inextricably linked and giving precisely corresponding values if measured.

"Now that this technique has been demonstrated," Monroe says, "it should be possible to scale it up to networks of many interconnected components that will eventually be necessary for quantum information processing."

The team reports its results in the September 6 issue of the journal Nature.

Two Tracks Toward Quantum Computing

Research to develop quantum computers can be grouped into two categories. The first one consists of researchers, like Monroe and his team, working with atomic particles like atoms or electrons, for which a quantum nature and entangled states are inherent. A major question for these researchers is how to "scale up" from methods for manipulating individual or small numbers of such particles to actually building workable computers.

The second category, which includes leading work by other University of Maryland researchers, consists of research with solid-state electronic devices rather than subatomic particles. The leap from such devices to a working computer is potentially much easier. Here, a major challenge is achieving, at a macroscopic level, the quantum states naturally present at the atomic level. In 2003 a different team of University of Maryland physicists demonstrated the existence of entangled states between two quantum bits or "qubits," each created with a type of solid state circuit known as a Josephson junction. Entangled entities are doubly strange. First, they have a property normally peculiar to the atomic-scale world of quantum mechanics: Each exists in a "superposition" of different states at the same time like a coin with sides that are neither heads nor tails, but somehow both at once and remains that way until a measurement forces it to take on a specific state.

This aspect of quantum mechanics makes it very attractive for potential information processing. Conventional computer bits are stored in tiny capacitors, each of which can have only one of two values (on or off, 0 or 1) as determined by a measurable electrical charge.

But thanks to superposition, a qub it can be a0, a 1, or both at the same time . If arranged into a computer, qubits could exponentially increase the speed at which certain kinds of problems can be solved.

Quantum entanglement allows the "wiring" of qubits together and is the key to such massive parallelism. Even though they are physically unconnected, entangled qubits always have complementary characteristics. If the state of first is known, then the state of the second is known as well even when the second state is not measured.

In the coin analogy, an entangled pair would work like this: If one coin were flipped and came up heads, then the other would always come up tails, even if it were simultaneously flipped 10,000 miles away.

The problem with all this is that entangled quantum superpositions are incredibly fragile if any part of the entangled system interacts with its environment, then the quantum nature of the system is generally lost. Before the experiment reported today, entanglement of distant individual atoms had never been achieved.

"Atoms make ideal qubits," Monroe says, "because they can be trapped and maintained in the same condition for long time periods. But photons are the ideal medium for transferring and controlling information. Our work shows for the first time that it is possible to use emitted photons to entangle the fixed atoms that emitted them."

The researchers began by isolating two atoms of ytterbium, one in each of two chambers separated by about 1 meter. Ytterbium is one of the rare-earth elements, and conveniently has only two electrons in its outermost shell. Removing one of the electrons turns the atom into an ion with a net positive charge, allowing the scientists to "trap" the ion with electrical fields in a vacuum chamber.

Image removed.The state of the ions remaining outermost electron, which can be precisely controlled and measured with lasers, can then be used to store a qubit of quantum information. Once trapped, the ytterbium ions remain in their places for several days.

After preparing each of the two ions in a particular electronic qubit state, the team then excited the ions simultaneously with precisely tuned laser pulses so brief that each ion emitted at most a single photon as it fell back to one of the qubit states.

The structure of the ytterbium ion is such that it can emit one of two slightly different kinds of photons in response to the laser pulse. One photon has a somewhat longer wavelength than the other, and that binary (long-short, either-or) difference is conveyed to the resulting electronic state of the ion, so that both ion and photon function as qubits.

Each time the laser pulses strike the two ions, and each releases a single photon, the result is a quantum superposition of four outcomes: Both ions can emit the short photon. Both can emit the long photon. Or one can emit the short photon while the other emits the long one, and vice versa.

After the laser pulse, the resulting photon from each ion is routed through an optical fiber strand and the two photons are combined at a half-mirrored beamsplitter that reflects half of incoming photons into one light detector and allows half to pass into another. (See diagram, Figure 1.) If the simultaneously arriving photons are identical, they can only leave together along the same path; therefore no signal can simultaneously register on both detectors. But if each ion emits a different kind of photon, they can indeed emerge along separate paths, and both detectors can register a "hit."

At this point, the odd rules of quantum entanglement come into play. Because it is impossible to determine which ion emitted which kind of photon when a pair is detected, and because each photon represents the internal quantum state of its parent ion, the event produces entanglement of the widely separated ions.

This entanglement was verified by probing the trapped ions with specially tuned laser beams that directly measured the state of each qubit. Not only were correlations in the qubit states clearly visible, but the correlations persisted after each qubit state was scrambled in a particular way before measurement a proof of entanglement.

Because of the numerous sources of light-gathering inefficiencies (imperfect laser pulse excitation of the ions, imperfections in the optical fibers , inefficiencies in the filters and detectors, and so forth), the team observed the telltale signature of an entanglement event only about three times in every billion pulses. That's about once every nine minutes.

But "it's okay that it almost never works," says David Moehring, the lead graduate student on the project. "Once we receive the clicks from the detectors, we know right then that the two ions are entangled and ready for use." The entanglement events were measured with extremely high efficiency, and detected with sufficient fidelity to demonstrate the phenomenon and constitute a proof of concept for controlling future quantum networks. The group has identified a number of ways to increase the yield in subsequent experiments, and improvements are already underway.

Monroe and his group were at the University of Michigan when they conducted the work published in September 6 issue of Nature. However, they have since joined the Physics Department at the University of Maryland and the Joint Quantum Institute a new research center funded by the University of Maryland and the National Institute of Standards and Technology .

Their just published work was supported by the National Security Agency and the Disruptive Technology Office under Army Research Office contract, and the National Science Foundation Information Technology Research (ITR) and Physics at the Information Frontier (PIF) programmes.

Physicists at the University of Maryland have led many research discoveries on the road toward quantum computing and the university's Physics Departments is one of the nation's best with particular strengths in areas relevant to quantum technology research. The Physics Department of the University of Maryland was ranked 15th among all universities in 2007 by U.S. News and World Report and was ranked 10th in quantum physics and atomic, molecular and optical physics, and 12th in condensed matter/solid state physics, three key fields of quantum research.