Influence of Nuclear Spin Polarization on Quantum Wire Conductance

In this work, we study a possibility to measure the transverse and longitudinal relaxation times of a collection of polarized nuclear spins located in the region of a quantum wire via its conductance. The interplay of an external in-plane magnetic field, spin-orbit interaction, and the changing field of the spin-polarized nuclei cause the conductance of the quantum wire to evolve in time. We show that it is possible to extract the transverse and longitudinal relaxation times of the spin-polarized nuclei from the time dependence of the conductance.Comment: Presented at the 2004 IEEE NTC Quantum Device Technology Worksho

Decoherence in adiabatic quantum computation

We have studied the decoherence properties of adiabatic quantum computation (AQC) in the presence of in general non-Markovian, e.g., low-frequency, noise. The developed description of the incoherent Landau-Zener transitions shows that the global AQC maintains its properties even for decoherence larger than the minimum gap at the anticrossing of the two lowest energy levels. The more efficient local AQC, however, does not improve scaling of the computation time with the number of qubits $n$ as in the decoherence-free case. The scaling improvement requires phase coherence throughout the computation, limiting the computation time and the problem size n.Comment: 4 pages, 2 figures, published versio

Tunneling without tunneling: wavefunction reduction in a mesoscopic qubit

The transformation cycle and associated inequality are suggested for the basic demonstration of the wavefunction reduction in a mesoscopic qubit in measurements with quantum-limited detectors. Violation of the inequality would show directly that the qubit state changes in a way dictated by the probabilistic nature of the wavefunction and inconsistent with the dynamics of the Schr\"{o}dinger equation: the qubit tunnels through an infinitely large barrier. Estimates show that the transformation cycle is within the reach of current experiments with superconducting qubits.Comment: 5 pages, 2 figure

Correlated transport of FQHE quasiparticles in a double-antidot system

We have calculated the linear conductance associated with tunneling of individual quasiparticles of primary quantum Hall liquids with filling factors $\nu =1/(2m+1)$ through a system of two antidots in series. On-site Coulomb interaction simulates the Fermi exclusion and makes the quasiparticle dynamics similar to that of tunneling electrons. The liquid edges serve as the quasiparticle reservoirs, and also create the dissipation mechanism for tunneling between the antidots. In the regime of strong dissipation, the conductance should exhibit resonant peaks of unusual form and a width proportional to the quasiparticle interaction energy $U$. In the weakly-damped regime, the shape of the resonant conductance peaks reflects coherent tunnel coupling of the antidots. The Luttinger-liquid singularity in the rates of quasiparticle tunneling to/from the liquid edges manifests itself as an additional weak resonant structure in the conductance curves.Comment: 9 pages including 5 figure

Effect of Spin-Orbit Interaction and In-Plane Magnetic Field on the Conductance of a Quasi-One-Dimensional System

We study the effect of spin-orbit interaction and in-plane effective magnetic field on the conductance of a quasi-one-dimensional ballistic electron system. The effective magnetic field includes the externally applied field, as well as the field due to polarized nuclear spins. The interplay of the spin-orbit interaction with effective magnetic field significantly modifies the band structure, producing additional sub-band extrema and energy gaps, introducing the dependence of the sub-band energies on the field direction. We generalize the Landauer formula at finite temperatures to incorporate these special features of the dispersion relation. The obtained formula describes the conductance of a ballistic conductor with an arbitrary dispersion relation.Comment: will appear in Physical Review