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Science | DOI: 10.1145/1364782.1364787
Quantum computing
Researchers are optimistic, but a practical device is years away.

SINCE qUANTUM AlGORITHMS and architectures will ultimately need hardware on which to run, we’ve explored how the principal experimental efforts are striving to produce it. Even 15 years ago, a quantum computer was generally viewed by computer scientists and physicists alike as an intriguing but probably unattainable theoretical curiosity. But interest exploded in 1994 after Peter Shor, then at Bell Laboratories (now at MIT), published his famous quantum factoring algorithm capable of undermining widely used cryptosystems that relied on the difficulty of factoring large numbers. Today, several thousand physics, computer science, and engineering researchers in more than 100 groups in universities, institutes, and companies around the world are exploring the frontiers of quantum information, encompassing quantum computing, as well as recently commercialized quantum cryptography and quantum tele-portation communication techniques. Accelerating progress on virtually all fronts in this worldwide research community is yielding confidence that a practical quantum computer is indeed achievable.

Quantum computing’s potential has always been tantalizing: Exponentially scalable computing power that could solve problems beyond the capabilities of conventional computers. The key is exploiting the superposition of quantum-entangled information units, or qubits. But the research challenges are daunting: How to create and reliably compute with the qubits, which require the seemingly mutually exclusive conditions of exquisite classical control while being isolated from any external influences that could destroy the entanglement.

The computing power of a quantum computer grows exponentially with the number of qubits it uses. Dozens to hundreds of qubits will be needed for a

quantum computer to solve interesting problems using quantum algorithms (along with appropriate quantum er-ror-correction techniques needed to be sure the answer is correct). The qubits must also be connected by quantum communication channels into logic gates that can be manipulated to implement the algorithms.

However, merely having and connecting qubits is not sufficient for a quantum computer. They must remain entangled long enough to complete the number of gate operations required by

ing. Typically, the qubit is a two-level motion mode for a trapped ion. The modes are modulated by laser pulses. The ion motion acts like a data bus, and gates are implemented by modulating neighboring ions.

“Our decoherence times can be up to 10 minutes—very long compared with other quantum computing techniques,” says Dave Wineland of the National Institute of Standards and Technology, Boulder, CO. “But our gates are rather slow, about five microseconds for our two-qubit gates.” Since

Miniature ion trap manufactured by Sandia National Laboratories.

the algorithm and mandatory error correction. Faster gate operation, higher fidelity (percentage of gate operations completed correctly), and greater er-ror-correcting efficiency can speed the calculation or reduce the number of qubits needed to solve the problem.

More than a dozen different ways of creating qubits—each with its own strengths and challenges—have been developed to date. The following is a rundown of the leading candidates:

Ion traps use electrical and/or magnetic fields and laser-cooling to create a “pseudo-molecule” quantum register with micron-scale inter-ion spac-

factoring a 100-to-200-digit number would require a million operations, even error-free implementation would take far longer than the qubit could be maintained. Researchers, led by Rainer Blatt of the Institut fur Experimental-physik Universtität Innsbruck in Austria, recently set the record for qubit fidelity: 99.3%.

Integrating CMOS chips with ion traps is a recent innovation that permits quantum communication but uses classical control and measurement. One design created at Lucent by Richard Slusher (now at the Georgia Tech Quantum Institute) and Jungsang Kim