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The Philosophical Magazine A Journal of Theoretical Experimental and Applied Physics

Spin and charge localisation due to the interplay of Ac gate voltage and spin–orbit interaction

Kulsoom Rahim, Usman Hasan

Published Jan 6, 2021• 0 citations
Publisher

Abstract

We investigate the effects of a time-dependent gate voltage on a spin–orbit interaction (SOI) coupled interacting double quantum dot (DQD) system. For a tunnel coupled detuned DQD with two interacting electrons, SOI couples triplet T0 and hybridised singlet state (1,1)–(0,2) through spin nonconserving transition. In order to explore the effects of Ac gate voltage on charge localisation, we initiate the system in the vicinity of the singlet–triplet avoided crossing and investigate the interplay of the Ac gate voltage and SOI on the charge and spin dynamics of the system. We show that the triplet state (1,1) charge configuration can be frozen by tuning the Ac frequency leading to charge localisation and dynamical spin locking with a redefined spin–orbit field in terms of gate voltage.

DOI
10.1080/14786435.2020.1867325

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We investigate the effects of a time-dependent gate voltage on a spin–orbit interaction (SOI) coupled interacting double quantum dot (DQD) system. For a tunnel coupled detuned DQD with two interacting electrons, SOI couples triplet T0 and hybridised singlet state (1,1)–(0,2) through spin nonconserving transition. In order to explore the effects of Ac gate voltage on charge localisation, we initiate the system in the vicinity of the singlet–triplet avoided crossing and investigate the interplay of the Ac gate voltage and SOI on the charge and spin dynamics of the system. We show that the triplet state (1,1) charge configuration can be frozen by tuning the Ac frequency leading to charge localisation and dynamical spin locking with a redefined spin–orbit field in terms of gate voltage.
We have analyzed theoretically quenched dynamics of correlated double quantum dot (DQD) due to the switching "on" and "off" coupling to reservoirs. The possibility for controllable manipulation of charge and spin states in the double quantum dot was revealed and discussed. The proposed experimental scheme allows to prepare in DQD maximally entangled pure triplet state and to drive it to another entangled singlet state by tuning both applied bias and gate voltage. It was also demonstrated that the symmetry properties of the total system (double quantum dot coupled to electron reservoirs) allow to resolve the initially prepared two-electron states by detecting non-stationary spin-polarized currents flowing in both reservoirs and controlling the residual charge.
We characterize numerically and analytically the signatures of the spin-orbit interaction in a two-electron GaAs double quantum dot in the presence of an external magnetic field. In particular, we obtain the return probability of the singlet state by simulating Landau-Zener voltage detuning sweeps which traverse the singlet-triplet ($S\ensuremath{-}{T}_{+}$) resonance. Our results indicate that non-spin-conserving interdot tunneling processes arising from the spin-orbit interaction have well defined signatures. These allow direct access to the spin-orbit interaction scales and are characterized by a frequency shift and Fourier amplitude modulation of the Rabi flopping dynamics of the singlet-triplet qubits $S\ensuremath{-}{T}_{0}$ and $S\ensuremath{-}{T}_{+}$. By applying the Bloch-Feshbach projection formalism, we demonstrate analytically that the aforementioned effects originate from the interplay between spin-orbit interaction and processes driven by the hyperfine interaction between the electron spins and those of the GaAs nuclei.
Abstract We investigate the effect of spin–orbit interaction on the intra- and interdot particle dynamics of a double quantum dot (QD) under ac electric fields. The former is modeled as an effective ac magnetic field that produces electric-dipole spin resonance transitions, while the latter is introduced via spin-flip tunneling amplitudes. We observe the appearance of non-trivial spin-polarized dark states (DSs), arising from an ac-induced interference between photo-assisted spin-conserving and spin-flip tunneling processes. These DSs can be employed to precisely measure the spin–orbit coupling in QD systems. Furthermore, we show that the interplay between photo-assisted transitions and spin-flip tunneling enables the system to operate as a highly tunable spin filter. Finally, we investigate the operation of the system as a resonant flopping-mode qubit for arbitrary ac voltage amplitudes, allowing for high tunability and enhanced qubit control possibilities.
µmSpin-orbit effects in silicon have long been considered of minor importance for the operation of spin qubits.Contrary to this belief, recent and growing amounts of evidence in systems like spin 1/2 and atomic-precision devices seem to indicate otherwise, but the exact mechanisms and implications are not well understood.In this work, we use a metal-oxide-semiconductor and poly-Si (metal-free) double quantum dot singlet-triplet qubit as a sensitive probe of the spin-orbit effect.We first show that a strong magnetic field enables rotations between the singlet S and unpolarized triplet T 0 , an effect which amounts to an effective g-factor difference between the two quantum dots and is affected by gate voltages.Twoaxis control and single shot readout of the qubit is used to study AC resonant control in the deep detuning regime and achieve few microsecond Rabi flip times.Perhaps surprisingly, singlet-triplet qubits "just turn" without engineering feats.Secondly, we investigate a different effect of the spin-orbit interaction on the S-T -transition and its impact on schemes like dynamic nuclear polarization.Our results shed light on the implications of spin-orbit interaction for the operation of spin qubits in silicon.
Quantum dot-based quantum computation employs extensively the exchange interaction between nearby electronic spins in order to manipulate and couple different qubits. The exchange interaction, however, couples the qubit states to charge noise, which reduces the fidelity of the quantum gates that employ it. The effect of charge noise can be mitigated by working at noise sweetspots in which the sensitivity to charge variations is reduced. In this work we study the response to charge noise of a double quantum dot based qubit in the presence of ac gates, with arbitrary driving amplitudes, applied either to the dot levels or to the tunneling barrier. Tuning with an ac driving allows to manipulate the sign and strength of the exchange interaction as well as its coupling to environmental electric noise. Moreover, we show the possibility of inducing a second-order sweetspot in the resonant spin-triplet qubit in which the dephasing time is significantly increased.
Landau-Zener-Stuckelberg-Majorana (LZSM) physics has been exploited to coherently manipulate two-electron spin states in a GaAs double quantum dot (DQD) at a singlet (S)-triplet (T+) anti-crossing. The anti-crossing results from the hyperfine interaction with the nuclear spins of the host material [1,2]. However, the fluctuations of the nuclear spin bath result in spin dephasing within T ∗ 2 ∼ 10− 20 ns. As a consequence, the sweep through the anti-crossing would have to be performed on a timescale comparable to T ∗ 2 to achieve LZSM oscillations with 100% visibility. Moreover, the S-T+ anti-crossing is located near the (1, 1)− (2, 0) interdot charge transition, where (nl, nr) denotes the number of electrons in the left and right quantum dot. As a result the singlet state involved in the dynamics is a superpostion of (1, 1) and (2, 0) singlet states. Here we show that it is possible to increase the oscillation visibility while keeping sweep times less than T ∗ 2 using a tailored pulse with a detuning dependent level velocity. The pulse includes a slow level velocity portion that is chosen to coincide with the passage through the S-T+ anti-crossing and two fast level velocity portions. The latter minimize the time spent in regions where spin and charge degrees of freedom are entangled, which renders the qubit susceptible to charge noise. The slow level velocity portion of the pulse results in a stronger effective coupling between the spins states, which increases the oscillations visibility [3,4]. In particular, we were able to obtain a visibility of ∼ 0.5 for LZSM oscillations. This constitutes an important step towards the implementation of a Hadamard gate.