Error Prevention Scheme with Four Particles

It is shown that a simplified version of the error correction code recently suggested by Shor exhibits manifestation of the quantum Zeno effect. Thus, under certain conditions, protection of an unknown quantum state is achieved. Error prevention procedures based on four-particle and two-particle encoding are proposed and it is argued that they have feasible practical implementations.Comment: 4 pages, RevTeX, references updated and improved protocol adde

State permutations from manipulation of near level-crossings

We discuss some systematic methods for implementing state manipulations in systems formally similar to chains of a few spins with nearest-neighbor interactions, arranged such that there are strong and weak scales of coupling links. States are permuted by means of bias potentials applied to a few selected sites. This generic structure is then related to an atoms-in-a-cavity model that has been proposed in the literature as a way of achieving a decoherence free subspace. A new method using adiabatically varying laser detuning to implement a CNOT gate in this model is proposed.Comment: 6 pages, 5 figures. Substantial revision and extension of the introduction and the atoms-in-a-cavity section

Quantum Zeno effect and parametric resonance in mesoscopic physics

As a realization of the quantum Zeno effect, we consider electron tunneling between two quantum dots with one of the dots coupled to a quantum point contact detector. The coupling leads to decoherence and to the suppression of tunneling. When the detector is driven with an ac voltage, a parametric resonance occurs which strongly counteracts decoherence. We propose a novel experiment with which it is possible to observe both the quantum Zeno effect and the parametric resonance in electric transport.Comment: 4 pages, 2 figure

Dynamics of Quantum Collapse in Energy Measurements

The influence of continuous measurements of energy with a finite accuracy is studied in various quantum systems through a restriction of the Feynman path-integrals around the measurement result. The method, which is equivalent to consider an effective Schr\"odinger equation with a non-Hermitian Hamiltonian, allows one to study the dynamics of the wavefunction collapse. A numerical algorithm for solving the effective Schr\"odinger equation is developed and checked in the case of a harmonic oscillator. The situations, of physical interest, of a two-level system and of a metastable quantum-well are then discussed. In the first case the Zeno inhibition observed in quantum optics experiments is recovered and extended to nonresonant transitions, in the second one we propose to observe inhibition of spontaneous decay in mesoscopic heterostructures. In all the considered examples the effect of the continuous measurement of energy is a freezing of the evolution of the system proportional to the accuracy of the measurement itself.Comment: 20 pages with figures, compressed and uuencoded ps fil