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arXiv:0907.0182 · Physical Review B

Control of spin blockade by ac magnetic fields in triple quantum dots

Maria Busl, Rafael Sánchez, Gloria Platero

Published Mar 15, 2010• 71 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevb.81.121306

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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 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.
A Pauli spin blockade in quantum dot systems occurs when the charge transport is allowed only for some spin states, and it has been an efficient tool in spin-based qubit devices in semiconductors. We theoretically investigate a Pauli spin blockade in a triple quantum dot molecule consisting of three identical quantum dots in a semiconductor in the presence of an external magnetic field through the molecule. When the three-electron state is on resonance with two- or four-electron states, the Aharonov–Bohm oscillation and the Zeeman splitting lead to a periodic spin blockade effect. We focus on the spin blockade at a two- and three-electron resonance and show that we can tune the magnetic field to selectively allow only either a spin-singlet or spin-triplet state to add an additional electron from tunnel-coupled leads. This spin blockade maintains the three quantum dots at the optimal sweet spot against the charge noise, demonstrating its potential as an efficient readout scheme for the qubits in quantum dot systems.
Abstract Coherence is a fundamental physical quantity that plays a key role in characterizing the superposition and entanglement of quantum states. In this study, we investigate the coherence properties of a triangular triple-quantum-dot system as functions of coupling strength t , Coulomb repulsion U , and temperature T . We compare the coherence in this triangular configuration with that in a series triple quantum dot structure, and find that the triangular arrangement exhibits significantly enhanced coherence. The effect of an external phase on coherence is also examined, revealing periodic modulation. Furthermore, we analyze the implementation of a CSWAP quantum gate within the triangular triple-quantum-dot platform. Leveraging the high-coherence regime, we decompose the CSWAP gate into a sequence of three CNOT gates and two Toffoli gates. By tuning inter-dot couplings and applying magnetic fields, we realize the CSWAP operation and estimate the required gate time. Triangular triple-quantum-dots are considered a promising platform for quantum computing due to their ability to host chiral spin states and maintain robust quantum coherence, which is essential for high fidelity qubit operations. Our results identify optimal parameter regimes for coherence enhancement and provide a theoretical foundation for implementing advanced quantum gates in chiral qubit architectures, paving the way toward scalable quantum computation.
The triple quantum dot energy level spectrum is far more complex than its double quantum dot counterpart. As a result it is a challenge to cleanly manipulate only the two required qubit states without invoking more complex multistate coherent evolution. In this talk I will describe experiments and modeling of lateral triple quantum dot devices where by suitable device gate (i.e. energy level spectrum) tuning and pulse characteristics we were able to characterize and manipulate various three spin qubit species. In particular I will describe measurements where the Landau-Zener –Stuckelberg approach previously demonstrated in double dots is extended to threeinteracting spin states permitting us to demonstrate phenomena such as pairwise exchange control. I will also demonstrate how by tuning the experimental parameters one can controllably switch to coherent oscillations originating from alternative potentially useful qubit states and how to distinguish them.