Charge Dephasing Research Articles

Dynamic generation of Greenberger-Horne-Zeilinger states with coupled charge qubits

In this paper, we present a proof-of-principle of the formation of pure maximally entangled states from the Greenberger-Horne-Zeilinger class, in the experimental context of charged quantum dots. Each qubit must be identified as a pair of quantum dots, sharing an excess electron, coupled by tunneling. The electron-electron interaction is accounted for and is responsible for the coupling between the qubits. The interplay between coherent tunneling events and many-body interaction gives rise to the formation of highly entangled states. We begin by treating the problem of encoding three-qubits in a system with three pairs of quantum dots, and the numerical analysis of the exact quantum dynamics to find the conditions for the generation of the GHZ states. An effective two-level model sheds light on the role of a high-order tunneling process behind the dynamics. The action of the main decoherence process, the charge dephasing, is quantified in the process. We then evaluate the physical requirements for the dynamical generation of GHZ states in a $N$ qubit scenario, and its challenges.

Abrupt enhancement of spin–orbit scattering time in ultrathin semimetallic SrIrO3 close to the metal–insulator transition

We report a magnetotransport study of spin relaxation in 1.4–21.2 nm epitaxial SrIrO3 thin films coherently strained on SrTiO3 substrates. Fully charge compensated semimetallic transport has been observed in SrIrO3 films thicker than 1.6 nm, where the charge mobility at 10 K increases from 45 cm2/V s to 150 cm2/V s with decreasing film thickness. In the two-dimensional regime, the charge dephasing and spin–orbit scattering lengths extracted from the weak localization/anti-localization effects show power-law dependence on temperature, pointing to the important role of electron–electron interaction. The spin–orbit scattering time τso exhibits an Elliott–Yafet mechanism dominated quasi-linear dependence on the momentum relaxation time τp. Ultrathin films approaching the critical thickness of metal–insulator transition show an abrupt enhancement in τso, with the corresponding τso/τp about 7.6 times of the value for thicker films. A likely origin for such unusual enhancement is the onset of strong electron correlation, which leads to charge gap formation and suppresses spin scattering.

Photon-assisted tunneling and charge dephasing in a carbon nanotube double quantum dot

We report microwave-driven photon-assisted tunneling in a suspended carbon nanotube double quantum dot. From the resonant linewidth at a temperature of 13 mK, the charge dephasing time is determined to be 280 +- 30 ps. The linewidth is independent of driving frequency, but increases with increasing temperature. The moderate temperature dependence is inconsistent with expectations from electron-phonon coupling alone, but consistent with charge noise arising in the device. The extracted level of charge noise is comparable with that expected from previous measurements of a valley-spin qubit, where it was hypothesized to be the main cause of qubit decoherence. Our results suggest a possible route towards improved valley-spin qubits.

Coherent Coupling of Two Dopants in a Silicon Nanowire Probed by Landau-Zener-Stückelberg Interferometry

We report on microwave-driven coherent electron transfer between two coupled donors embedded in a silicon nanowire. By increasing the microwave frequency we observe a transition from incoherent to coherent driving revealed by the emergence of a Landau-Zener-Stückelberg quantum interference pattern of the measured current through the donors. This interference pattern is fitted to extract characteristic parameters of the double-donor system. In particular we estimate a charge dephasing time of 0.3±0.1 ns, comparable to other types of charge-based two-level systems. The demonstrated coherent coupling between two dopants is an important step towards donor-based quantum computing devices in silicon.

DEPHASING OF QD EXCITON ORBITAL AND SPIN STATES DUE TO HYBRIDIZATION WITH BULK COLLECTIVE EXCITATIONS

We analyze theoretically a hybridization type dressing of orbital (i.e., charge) and spin degrees of freedom of excitations captured in semiconductor quantum dot with band phonons and magnons (the latter for magnetic semiconductor surroundings), as a mechanism of dephasing of rapidly excited dot exciton state. Within the Green function approach we derive our previously formulated heuristic general rule for estimation of corresponding dephasing time-rate. The pure dephasing (off-diagonal decoherence) of quantum dot exciton resulting due to this dressing is studied, for both orbital and spin states of the exciton. A significant difference between phonon-induced (for charge) and magnon-assisted (for spin) quantum dot exciton dephasing is indicated as the result of the spin conservation, which leads to the disappearance of the exciton spin pure-dephasing at T = 0 (freezing out of spin dephasing), in contrary to the exciton charge dephasing caused by phonons, which maintains strong even at T = 0.

Anisotropic conductivity of disordered two-dimensional electron gases due to spin-orbit interactions

We show that the disorder-averaged conductivity tensor of a disordered two-dimensional electron gas becomes anisotropic in the presence of both Rashba and Dresselhaus spin-orbit interactions (SOIs). This anisotropy is a mesoscopic effect and vanishes with vanishing charge dephasing time. Using a diagrammatic approach including zero-, one-, and two-loop diagrams, we show that a consistent calculation needs to go beyond a Boltzmann equation approach. In the absence of charge dephasing and for zero frequency, a finite anisotropy ${\ensuremath{\sigma}}_{xy}\ensuremath{\propto}{e}^{2}/{p}_{F}lh$ arises even for infinitesimal SOI, where ${p}_{F}$ is the Fermi momentum and $l$ is the mean free path of an electron.