Quantum information processing with trapped Ca(+) ions

Simple, narrow, and robust atomic frequency reference at 993 nm exploiting the rubidium (Rb) 5S1/2 to 6S1/2 transition using one-color two-photon excitation

We experimentally demonstrate a one-color two-photon transition from the 5S_1/2 ground state to the 6S_1/2 excited state in rubidium (Rb) vapor using a continuous wave laser at 993 nm. The Rb vapor contains both isotopes (⁸⁵Rb and ⁸⁷Rb) in their natural abundances. The electric dipole-allowed transitions are characterized by varying the power and polarization of the excitation laser. Since the optical setup is relatively simple, and the energies of the allowed levels are impervious to stray magnetic fields, this is an attractive choice for a frequency reference at 993 nm, with possible applications in precision measurements and quantum information processing.

Universal instabilities of radio-frequency traps

Using standard tools of nonlinear dynamics we analyze recently discovered instabilities of radio-frequency charged-particle traps. In the cw-driven cylindrical Kingdon trap the instabilities occur at the two values eta*(3) =3.6130467...and eta*(4) =4.4311244...of the trap's control parameter eta. Analytical estimates based on the theory of Mathieu functions predict eta*(3) =pi square root of [(363-32 pi(2))/(66 pi square root of (6-48 pi(2))]=3.6923922...and eta*(4) = [(square root of pi)/2) x [(363-32 pi(2))/(square root of (1089+48 pi(2))-12 pi)](1/2) =4.4965466... The kicked Kingdon trap, an analytically solvable model, predicts eta*(3) = 1/3 square root of 105=3.4156502...and eta*(4) = square root of 17=4.1231056... We show that similar instabilities occur in the two-particle Paul trap and the cw-driven spherical Kingdon trap.

Quantum computing in the solid state: the challenge of decoherence

The current status of solid-state implementations of quantum computing is briefly described. There are numerous candidate proposals, but only comparatively recently have some of them begun to progress to the point of demonstrating coherent motion of the individual quantum bits (qubits), and the controlled coupling of more than one qubit remains a significant challenge. We also present a generalization of the fluctuation-dissipation theorem that provides a relationship between the coherent evolution entangling two spatially separated qubits, and the associated irreducible decoherence. This relationship may be used to bound the maximum attainable figures of merit in proposed two-qubit gates.

Scalable solid-state qubits: challenging decoherence and read-out

We review some basic facts about qubits and qubit processing. After a brief survey of solid-state qubits, we focus on quantized electrical circuits and superconducting qubits based on Josephson junctions. We review the general framework and indicate how the various qubits, such as the superconducting Cooper pair box charge qubit, the persistent current flux qubit, the hybrid charge-phase qubit, and the Andreev-level qubit, can be seen to appear due to different choices of design parameters. Finally, we consider multi-qubit systems and discuss some aspects of decoherence.