We have studied the performance of a geometric phase gate with a quantized driving field numerically, and
developed an analytical approximation that yields some preliminary insight on the way the qubit becomes
entangled with the driving field.
We explore the entanglement between a single atom and a single, resonant field mode of a driven optical cavity, focusing on the strong driving regime. We show that, in the absence of spontaneous emission, there are special initial conditions that lead to approximately disentangled trajectories, whereas spontaneous emission results in coherent superpositions of such trajectories that may lead to (transient) near-maximally entangled atom-field states. We also discuss the possibility of using a special "asymmetric" field correlation function to track the time evolution of this entanglement.
Quantum error correction will be an indispensable ingredient of large-scale quantum computations. Conventional quantum error correction codes (QECC) have been devised with an independent-error model in mind, but one may expect that the noise affecting a system of qubits will, in general, exhibit nonzero correlations in time, or space, or both. This talk will present a brief introduction to the principles of quantum error correction, followed by a discussion of the performance of conventional QECCs in the presence of correlated noise.
A lower bound on the amount of energy needed to carry out an elementary logical operation on a qubit system, with a given accuracy and in a given time, has been recently postulated. This paper is an attempt to formalize this bound and explore the conditions under which it may be expected to hold. For a specific, important case (namely, when the control system is a quantized electromagnetic field) it is shown how one can extend this result to a generally stronger constraint on the minimum energy density required, per pulse.
This paper explores the limitations that interaction between the physical qubits making up a quantum computer may impose on the computer's performance. For computers using atoms as qubits, magnetic dipole-dipole interactions are likely to be dominant; various types of errors which they might introduce are considered here. The strength of the interaction may be reduce by increasing the distance between qubits, which in general will make the computer slower. For ion-chain based quantum computers the slowing down due to this effect is found to be generally more sever than that due to other causes. In particular, this effect alone would be enough to make these systems unacceptably slow for large-scale computation, whether they use the center of mass motion as the 'bus' or whether they do this via an optical cavity mode.
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