Coherent Manipulation of Coupled Electron Spins in Semiconductor Quantum Dots (original) (raw)
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Recent experiments have demonstrated quantum manipulation of two-electron spin states in double quantum dots using electrically controlled exchange interactions. Here, we present a detailed theory for electron spin dynamics in two-electron double dot systems that was used to guide these experiments and analyze experimental results. The theory treats both charge and spin degrees of freedom on an equal basis. Specifically, we analyze the relaxation and dephasing mechanisms that are relevant to experiments and discuss practical approaches for quantum control of two-electron system. We show that both charge and spin dephasing play important roles in the dynamics of the two-spin system, but neither represents a fundamental limit for electrical control of spin degrees of freedom in semiconductor quantum bits.
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Suppression of electron spin decoherence in a quantum dot
Journal of Modern Optics, 2007
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Physical Review Letters, 2006
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Physical Review B, 2008
We consider a double single-level quantum dot system with two embedded and nonaligned spin impurities to manipulate the magnitude and polarization of the electron-spin density. The device is attached to semiinfinite one-dimensional leads which are treated exactly. We provide a real-time description of the electron-spin dynamics when a sequence of ultrafast voltage pulses acts on the device. The numerical simulations are carried out using a spin-generalized modified version of a recently proposed algorithm for the time propagation of open systems ͓Kurth et al., Phys. Rev. B 72, 035308 ͑2005͔͒. Time-dependent spin accumulations and spin currents are calculated during the entire operating regime, which includes spin-injection and read-out processes. The full knowledge of the electron dynamics allows us to engineer the transient responses and improve the device performance. An approximate rate equation for the electron spin is also derived and used to discuss the numerical results.
Exchange Control of Nuclear Spin Diffusion in a Double Quantum Dot
Eprint Arxiv 0803 3082, 2008
Coherent two-level systems, or qubits, based on electron spins in GaAs quantum dots are strongly coupled to the nuclear spins of the host lattice via the hyperfine interaction. Realizing nuclear spin control would likely improve electron spin coherence and potentially enable the nuclear environment to be harnessed for the long-term storage of quantum information. Toward this goal, we report experimental control of the relaxation of nuclear spin polarization in a gate-defined two-electron GaAs double quantum dot. A cyclic gate-pulse sequence transfers the spin of an electron pair to the host nuclear system, establishing a local nuclear polarization that relaxes on a time scale of seconds. We find nuclear relaxation depends on magnetic field and gate-controlled two-electron exchange, consistent with a model of electron mediated nuclear spin diffusion.