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See Inside March 2006

Ion Power

Atomic ions prove their quantum versatility
ENTANGLED:



HARTMUT HAEFFNER University of Innsbruck

In their quest to build a computer that would take advantage of the weirdness of quantum mechanics, physicists are pursuing a number of disparate technologies, including superconducting devices, photon-based systems, quantum dots, spintronics and nuclear magnetic resonance of molecules. In recent months, however, teams working with trapped atomic ions have demonstrated several landmark feats that the other approaches will be hard-pressed to match.

A quantum computer operates on quantum bits, or qubits, instead of ordinary bits. A qubit can be not just 0 or 1 but also a superposition of the two, in which proportions of zero-ness and one-ness are combined in a single state.

An important class of multiqubit superpositions are entangled states. In these configurations, the state of each qubit is linked in a subtle way to the state of its companions, a linkage that Albert Einstein disparaged as “spooky action at a distance.” For example, in a so-called Schrödinger cat state, all the qubits will give the same result—0 or 1—on being measured, even though the choice between 0 and 1 is totally random. (The name comes from the famous thought experiment in which 0 and 1 correspond to the cat being dead or alive and the individual “qubits” are all the particles in the cat's body.)

Cat states are a fundamental building block of techniques for correcting errors in qubits. Such errors inevitably plague all the standard approaches to quantum computation, because states of qubits are exceedingly fragile.

Researchers at the National Institute of Standards and Technology in Boulder, Colo., led by David J. Wineland and Dietrich Leibfried, have now created cat states involving four, five and six beryllium ions. An electromagnetic trap holds the ions in a row in a vacuum, and lasers manipulate their states. The team estimates that their six-ion cat states last for approximately 150 microseconds.

In Austria, Rainer Blatt and Hartmut Haeffner of the University of Innsbruck and their colleagues relied on a similar technique to produce an entangled state of eight calcium ions. In this experiment a “W state” was created, not a cat state. A W state is in many ways more robust than a cat state. For example, an ion can be lost from a W state and the remaining ions will still be in a W state. Losing an ion from a cat state spoils the entire state.

An important feature of both experiments is that in principle the techniques can incorporate larger numbers of ions. An impediment to scaling up these approaches, however, was that the quality of the entangled state decreased as the number of ions increased. To reduce this error, the scientists might adjust the details of the laser pulses, use different states of the ions to represent 0 and 1, or work with a different ion species altogether.

For a quantum computer to be of use, one must not only create special qubit states but also manipulate them in ways that preserve their quantum characteristics. That is, one must run quantum algorithms on the computer. A group at the University of Michigan at Ann Arbor led by Christopher Monroe and Kathy-Anne Brickman has now demonstrated an algorithm known as Grover's quantum search on a system of two trapped cadmium ions.

The search algorithm rummages through a database with entries in random order. Searching for a particular item would usually demand the examination of every entry. The quantum search algorithm is magically faster because the quantum computer can poll all the database entries at once in a superposition. The speedup becomes more dramatic for larger databases. For example, a million-entry database would take only about 1,000 quantum lookups instead of the full million.

The Ann Arbor experiment operated on the equivalent of a four-entry database, the four entries being represented by two qubits. The researchers say that their system can be scaled up to larger numbers of qubits.

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