Quantum information processing is a rapidly emerging field, with development in atomic, photonic, and condensed matter systems underway. Classical computers (the standard computers of today) are inherently limited by memory storage and computational ability. For example, to store the complete quantum state of 300 interacting spin-1/2 particles in classical memory would require more bytes than the estimated number of protons in the universe. The pursuit of quantum computer technology is motivated by overcoming these limitations. The advantage of quantum computers over classical computers arises from the use of the qubit as a fundamental unit of information. Unlike a bit, which can be in one of two states (“0” or “1”), a qubit can be in a superposition of logic basis states. Potential applications for a quantum computer will be briefly discussed, including secure communication, factoring integers, and quantum simulation.
The basic properties of photons and atoms that make them ideal candidates for exploring quantum information science will be outlined. Photons, especially suited for quantum cryptography applications, travel at the speed of light, are mostly immune from the effects of decoherence, and have orthogonal polarization states which can be used for encoding information. Ultracold trapped atoms are an ideal system for quantum computation because deterministic state preparation and efficient state detection are possible and coherent manipulation of atomic systems is relatively advanced. I will briefly outline the state of the art in using optical and atomic systems for quantum information processing applications, and then focus on the NIST Boulder trapped atomic ion experiment.
In the NIST Boulder experiment, a few singly charged Be ions are confined by static and radio-frequency electric fields in a micro-machined linear Paul trap. The ions are cooled into the ground state of the confining potential using laser cooling, and optical pumping is used to prepare the ion in a particular spin state. By applying laser light and driving stimulated Raman transitions, the internal and motional states of the ions are coherently manipulated. In the scheme to build a quantum computer using trapped ions, the internal (spin) state of the ions are the qubits and the motional states are used as the “data bus.”