The effects of the dissipative decoherence on the quantum computation of the quantum kicked Harper (QKH) model are investigated using the quantum trajectory approach and the quantum Monte Carlo method. The simulation shows that the dynamical localization for the QKH model and the stochastic web in phase space are destroyed by moderate levels of dissipation. Taking the phase damping channel as the noise model, the universal law for fidelity decay and the timescales of reliable computation are analyzed. Compared with the static imperfections, the timescales for reliable computation of the dissipative system drop more quickly with the increase of the number of qubits when the decay rate is under a certain threshold. While above the threshold, the timescales drop more quickly in the presence of static imperfections.
An arbitrarily reliable quantum computer can be efficiently constructed from noisy components using a recursive simulation procedure, provided that those components fail with probability less than the fault-tolerance threshold. Recent estimates of the threshold are near some experimentally achieved gate fidelities. However, the landscape of threshold estimates includes pseudothresholds, threshold estimates based on a subset of components and a low level of the recursion. In this paper, we observe that pseudothresholds are a generic phenomenon in fault-tolerant computation. We define pseudothresholds and present classical and quantum fault-tolerant circuits exhibiting pseudothresholds that differ by a factor of
4 from fault-tolerance thresholds for typical relationships between component failure rates. We develop tools for visualizing how reliability is influenced by recursive simulation in order to determine the asymptotic threshold. Finally, we conjecture that refinements of these methods may establish upper bounds on the fault-tolerance threshold for particular codes and noise models.
Quantum computation has become a very active field ever since the discovery that quantum computers can be much more powerful than their classical counterparts. Decoherence of the qubits caused by the inevitable interaction with the noisy environment is the major obstacle to realization of quantum computation. A recent innovative discovery is that reliable quantum computation is possible provided that the error per operation caused by decoherence is below a threshold value. The estimated threshold error rate is about 10/sup -6/. With the present technology, the real noise seems necessarily beyond this threshold. Therefore, it is essentially important to further suppress noise before taking the procedure of fault-tolerant quantum error correction. We propose a noise suppression scheme which operates by applying a sequence of bit-flipping and phase-flipping pulses on the system, with the pulse period less than the noise correlation time.