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arXiv:0808.3638

Decoherence induced entanglement

Colin Benjamin

Published Aug 27, 2008• 0 citations
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Abstract

Generating and detecting entangled states in solid state devices is attempted in the context of a non adiabatic pure spin pump. In contrast to previous works we show that decoherence can change the sign of spin noise cross-correlations from negative to positive, implying entanglement for spin-singlet electronic sources, a truly counterintuitive result. In addition we also focus on the full counting statistics of the problem, particularly the third moment.

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Generating and detecting entangled states in solid state devices is attempted in the context of a non adiabatic pure spin pump. In contrast to previous works we show that decoherence can change the sign of spin noise cross-correlations from negative to positive, implying entanglement for spin-singlet electronic sources, a truly counterintuitive result. In addition we also focus on the full counting statistics of the problem, particularly the third moment.
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The work contained in this thesis deals with two closely related topics. Papers I-III deal with charge transport statistics in nanoscale devices to investigate the presence of entanglement in the constituent electrons. We identify the contributions of entangled electron pairs to the current cross-correlations as a signature of entanglement. Paper IV treats the electric control of a quantum dot based spin qubit via Electric Dipole Spin Resonance.In Paper I, we present a spin entanglement detection model in a hybrid nanoscale device, with a general entangler connected to a set of quantum dots and anti-collinear ferromagnetic contacts. Our model allows us to study the implications of a non-ideal detector in an entanglement detection scheme.In Paper II, we investigate a scheme for generating and detecting orbitally entangled states in a device consisting of six quantum dots. The underlying process which enables our proposed scheme is sufficiently fast, even for the rapidly decaying quantum coherence of orbital states defined in a solid.In Paper III, we investigate a minimal entanglement detection scheme based on measurements of current cross correlations. We find a significant reduction in the number of measurements required for an entanglement test.In Paper IV, we investigate the optimisation of electrically controlling an electron spin in a gate defined semiconductor quantum dot. Specifically, we study how the shape of the quantum dot together with the direction of an in-plane magnetic field affect the spin relaxation rate and frequency of the induced Rabi oscillations.
In spin-based architectures of quantum devices, the hyperfine interaction between the electron spin qubit and the nuclear spin environment remains one of the main sources of decoherence. This paper provides a short review of the current advances in the theoretical description of the qubit decoherence dynamics. Next, we study the qubit-environment entanglement using negativity as its measure. For an initial maximally mixed state of the environment, we study negativity dynamics as a function of environment size, changing the numbers of environmental nuclei and the total spin of the nuclei. Furthermore, we study the effect of the magnetic field on qubit-environment disentangling timescales.
We propose a strategy to generate a many-body entangled state in a collection of randomly placed, dipolarly coupled electronic spins in the solid state. By using coherent control to restrict the evolution into a suitable collective subspace, this method enables the preparation of GHZ-like and spin-squeezed states even for randomly positioned spins, while in addition protecting the entangled states against decoherence. We consider the application of this squeezing method to improve the sensitivity of nanoscale magnetometer based on nitrogen-vacancy spin qubits in diamond.