Ion-trap quantum computing in the presence of cooling

This paper discusses ways to implement two-qubit gate operations for quantum computing with cold trapped ions within one step. The proposed scheme is widely robust against parameter fluctuations and its simplicity might help to increase the number of qubits in present experiments. Basic idea is to use the quantum Zeno effect originating from continuous measurements on a common vibrational mode to realise gate operations with very high fidelities. The gate success rate can, in principle, be arbitrary high but operation times comparable to other schemes can only be obtained by accepting success rates below 80%.Comment: 12 pages, 9 figures, submitted to Phys. Rev. A, revised version, new titl

Mollow triplet for cavity-mediated laser cooling

Here we analyse cavity-mediated laser cooling for an experimental setup with an external trap which strongly confines the motion of a particle in the direction of the cavity axis. It is shown that the stationary state phonon number exhibits three sharp minima as a function of the atom-cavity detuning due to a direct atom-phonon-photon coupling term in the system Hamiltonian. These resonances have the same origin as the Mollow triplet in the resonance fluorescence of a laser-driven atomic system. It is shown that a laser-Rabi frequency-dependent atom-cavity detuning yields the lowest stationary state phonon number for a wide range of experimental parameters.Comment: 10 pages, 7 figures, improved version, new titl

An efficient quantum filter for multiphoton states

We propose a scheme for implementing a multipartite quantum filter that uses entangled photons as a resource. It is shown that the success probability for the 2-photon parity filter can be as high as 1/2, which is the highest that has so far been predicted without the help of universal two-qubit quantum gates. Furthermore, the required number of ancilla photons is the least of all current parity filter proposals. Remarkably, the quantum filter operates with probability 1/2 even in the N-photon case, irregardless of the number of photons in the input state.Comment: 8 pages, 2 figures, revised version, accepted for publication in J. Mod. Op

Comparing cavity and ordinary laser cooling within the Lamb-Dicke regime

Cavity-mediated cooling has the potential to become one of the most efficient techniques to cool molecular species down to very low temperatures. In this paper we analyse cavity cooling with single-laser driving for relatively large cavity decay rates kappa and relatively large phonon frequencies nu. It is shown that cavity cooling and ordinary laser cooling are essentially the same within the validity range of the Lamb-Dicke approximation. This is done by deriving a closed set of rate equations and calculating the corresponding stationary state phonon number and cooling rate. For example, when nu is either much larger or much smaller than kappa, the minimum stationary state phonon number scales as kappa^2/16 nu^2 (strong confinement regime) and as kappa / 4 nu (weak confinement regime), respectively.Comment: 12 pages, 8 figures, final version accepted for publicatio

Composite quantum systems and environment-induced heating

In recent years, much attention has been paid to the development of techniques which transfer trapped particles to very low temperatures. Here we focus our attention on a heating mechanism which contributes to the finite temperature limit in laser sideband cooling experiments with trapped ions. It is emphasized that similar heating processes might be present in a variety of composite quantum systems whose components couple individually to different environments. For example, quantum optical heating effects might contribute significantly to the very high temperatures which occur during the collapse phase in sonoluminescence experiments. It might even be possible to design composite quantum systems, like atom-cavity systems, such that they continuously emit photons even in the absence of external driving.Comment: 4 pages, 1 figur

Dissipation-assisted quantum computation in atom-cavity systems

The principal obstacle to quantum information processing with many qubits is decoherence. One source of decoherence is spontaneous emission which causes loss of energy and information. Inability to control system parameters with high precision is another possible source of error. Strategies aimed at overcoming one kind of error typically increase sensitivity to others. As a solution we propose quantum computing with dissipation-assisted quantum gates. These can be run relatively fast while achieving fidelities close to one. The success rate of each gate operation can, at least in principle, be arbitrary close to one.Comment: proceedings for the SPIE conference on Fluctuations and Noise, June 2003 in Santa Fe, 12 pages, minor change