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arXiv:0707.4636 · Physical Review B

Coherent spin rotations in open driven double quantum dots

Rafael Sánchez, Carlos López-Monís, Gloria Platero

Published Apr 7, 2008• 24 citations
arXivHTMLPDF

Abstract

We analyze coherent spin rotations in a dc biased double quantum dot driven by crossed dc and ac magnetic fields. In this configuration, spatial delocalization due to interdot tunneling competes with intradot spin rotations induced by the time dependent magnetic field, giving rise to a complicated time dependent behavior of the tunneling current. When the Zeeman splitting has the same value in both dots and spin flip is negligible, the electrons remain in the triplet subspace performing coherent spin rotations and current does not flow. This electronic trapping is removed either by finite spin relaxation or when the Zeeman splitting is different in each quantum dot. In the last case, we will show that by applying a resonant bichromatic magnetic field, the electrons become trapped in a coherent superposition of states and electronic transport is blocked. Then, manipulating ac magnetic fields allows one to drive electrons to perform coherent spin rotations which can be unambiguously detected by direct measurement of the tunneling current.

DOI
10.1103/physrevb.77.165312

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We analyze coherent spin rotations in a dc biased double quantum dot driven by crossed dc and ac magnetic fields. In this configuration, spatial delocalization due to interdot tunneling competes with intradot spin rotations induced by the time dependent magnetic field, giving rise to a complicated time dependent behavior of the tunneling current. When the Zeeman splitting has the same value in both dots and spin flip is negligible, the electrons remain in the triplet subspace performing coherent spin rotations and current does not flow. This electronic trapping is removed either by finite spin relaxation or when the Zeeman splitting is different in each quantum dot. In the last case, we will show that by applying a resonant bichromatic magnetic field, the electrons become trapped in a coherent superposition of states and electronic transport is blocked. Then, manipulating ac magnetic fields allows one to drive electrons to perform coherent spin rotations which can be unambiguously detected by direct measurement of the tunneling current.
We analyze the charge and spin dynamics in a DC biased double quantum dot driven by crossed DC and AC magnetic fields. In this configuration, spatial delocalization due to inter-dot tunnel competes with intra-dot spin rotations induced by the time dependent magnetic field, giving rise to a complicated time dependent behavior of the tunnelling current. When the Zeeman splitting has the same value in both dots and spin flip is negligible, the electrons remain in the triplet subspace (dark subspace) performing coherent spin rotations and the current does not flow. This electronic trapping is removed either by finite spin relaxation or when the Zeeman splitting is different in each quantum dot. In the first case, our results show that measuring the current will allow to get information on the spin relaxation time. In the last case, we will show that applying a resonant bichromatic magnetic field, the electrons become trapped in a coherent superposition of states and electronic transport is blocked. Then, manipulating AC magnetic fields, electrons are driven to perform coherent spin rotations which can be unambiguously detected by direct measurement of the tunneling current.
We analyze coherent spin phenomena in triple quantum dots in triangular configuration under crossed dc and ac magnetic fields. In particular, we discuss the interplay between Aharonov-Bohm current oscillations, coherent electron trapping and spin blockade under two-electron-spin-resonance configurations. We demonstrate an unexpected antiresonant behavior in the current, allowing for both removal and restoration of maximally entangled spin-blockaded states by tuning the ac field frequency. Our theoretical predictions indicate how to manipulate spin qubits in a triangular quantum dot array.
We investigate the idea for probing spin states of two coupled quantum dots (CQDs) by the dc Josephson current flowing through them. This theory requires weak coupling between CQDs and electrodes, but allows arbitrary interdot tunnel coupling, intradot and interdot Coulomb interactions. We find that the Coulomb blockade peaks exhibit a nonmonotonous dependence on the Zeeman splitting of CQDs, which can be understood in terms of Andreev bound states. More importantly, the supercurrent in the Coulomb blockade valleys may provide information on the spin states of CQDs: for CQDs with total electron number N=1,3N=1,3N=1,3 (odd), the supercurrent will reverse its sign if the CQD becomes a magnetic molecule; for a CQD with N=2N=2N=2 (even), the supercurrent will decrease sharply around the transition between the spin singlet and triplet ground states of the CQD.
We study the pumping of spin-polarized electrons in a double quantum dot system with up to two electrons per dot, via an applied AC field and a constant magnetic field. The behaviour of the current through the double-dot system is studied as a function of the AC field and coupling to the leads, using a Markov master equation approach for the time evolution of the reduced density matrix. For up to two electrons in the system, we find that the formation of a spin-triplet state blocks the current through the device, and analyse possible solutions. When we incorporate three-and four-particle states, with up to two opposite spin electrons per dot, we find a regime where the pumping of spin-polarized electrons is realized through double occupancy states in each dot. This property is robust against spin-relaxation and decoherence processes which are taken into account phenomenologically. Finally we study the effects of applying a pulsed AC field and the possibility of the resolution of Rabi oscillations.
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.
We use quantum optimal control theory algorithms to design external electric fields that drive the coupled spin and orbital dynamics of an electron in a double quantum dot, subject to the spin-orbit coupling and Zeeman magnetic fields. We obtain time profiles of multifrequency electric field pulses which increase the rate of spin-flip transitions by several orders of magnitude in comparison with monochromatic fields, where the spin Rabi oscillations were predicted to be very slow. This precise (with fidelity higher than $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$) and fast (at the time scale of the order of 0.1 ns, comparable with the Zeeman spin rotation and the interdot tunneling time) simultaneous control of the spin and position is achieved while keeping the electron in the four lowest tunneling- and Zeeman-split levels through the duration of the pulse. The proposed algorithms suggest effective applications in spintronics and quantum information devices.