A quantum search with ions

Among the much-touted abilities of quantum computers is a dramatic speedup in searching an unsorted database for a particular entry by using what is known as Grover’s algorithm (see Physics Today, October 1997, page 19 ). That algorithm has already been realized with photons, nuclei, and Rydberg atoms. In those cases, however, the needed resources grow exponentially with the number of qubits, according to Christopher Monroe (University of Michigan), whose group has now implemented Grover’s algorithm with cadmium ions. The physicists trapped two ions as qubits (storing four states) and used a variety of fields and optical and microwave sources to control the ions’ positions, states, entanglement, and measurement. With a single query, any initially marked state was successfully identified and recovered 60% of the time, whereas the classical result could be no better than 50%. The Michigan group has also trapped a single ion within an integrated gallium arsenide semiconductor chip, and sees no impediment to scaling the system up to large numbers of qubits. (K.-A. Brickman et al., Phys. Rev. A 72 , 050306, 2005 http://dx.doi.org/10.1103/PhysRevA.72.050306 ; D. Stick et al., Nat. Phys. 2 , 36, 2006 http://dx.doi.org/10.1038/nphys171 .)