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AIP conference proceedings

Optically-Generated Many Spin Entanglement in a Quantum Well

Jiming Bao

Published Jan 1, 2005• 0 citations
Publisher

Abstract

A many‐particle system may exhibit non‐local behavior in that measurements performed on one of the particles can affect a second particle that is far apart. These so‐called entangled states are essential for the implementation of quantum information protocols and gates for quantum computation. Here, we use ultrafast optical pulses and coherent techniques to create spin entangled states of non‐interacting electrons bound to donors in a CdTe quantum well.

DOI
10.1063/1.1994651

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•Sep 16, 2019•0 citations
A many‐particle system may exhibit non‐local behavior in that measurements performed on one of the particles can affect a second particle that is far apart. These so‐called entangled states are essential for the implementation of quantum information protocols and gates for quantum computation. Here, we use ultrafast optical pulses and coherent techniques to create spin entangled states of non‐interacting electrons bound to donors in a CdTe quantum well.
A quantum-mechanical many-particle system may exhibit non-local behavior in that measurements performed on one of the particles can affect a second one that is far apart. These so-called entangled states are crucial for the implementation of quantum information protocols and gates for quantum computation. Here, we use ultrafast optical pulses and coherent techniques to create and control spin entangled states in an ensemble of up to three non-interacting electrons bound to donors in a CdTe quantum well. Our method, relying on the exchange interaction between localized excitons and paramagnetic impurities, can in principle be applied to entangle an arbitrarily large number of spins.
A quantum-mechanical many-particle system may exhibit nonlocal behavior in that measurements performed on one of the particles can affect a second one that is far apart. These so-called entangled states are crucial for the implementation of quantum information protocols and gates for quantum computation. Here, we use ultrafast optical pulses and coherent pump-probe techniques to create and control spin entangled states in an ensemble of up to three non-interacting electrons bound to donors in a ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}$Te quantum well. Our method, relying on the exchange interaction between optically excited excitons and the paramagnetic impurities, can in principle be applied to entangle an arbitrarily large number of electrons. A microscopic theory of impulsive stimulated Raman scattering and a model for multi-spin entanglement are presented. The signature of entanglement is the observation of overtones of donor spin-flips in the differential reflectivity of the probe pulse. Results are shown for resonant excitation of localized excitons below the gap, and above the gap where the signatures of entanglement are significantly enhanced. Data is also presented on the generation of coherent excitations of antiferromagnetically coupled manganese pairs, folded acoustic phonons, exciton Zeeman beats and entanglement involving two Mn+2{\mathrm{Mn}}^{+2}Mn+2 ions.
Abstract Ultrafast laser pulses were used to control the spin state of a pair of entangled, donor‐bound electrons in a CdMnTe quantum well. The entanglement, mediated by an optically‐excited exciton, was characterized by pumpprobe spectroscopy. The data exhibits two dominant oscillations, one at the frequency of the spin‐flip of a donor electron and the second one at twice this frequency. Their amplitudes depend linearly on the pump power. The quantum state of the two‐electron spin system can be controlled by appropriate spatial and temporal shaping of the pump pulses. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
We report the experimental realization of coherent electron spin flip in a modulation-doped CdTe quantum well. Coherent spin rotation is realized with an off-resonant laser pulse, which induces a polarization-dependent optical Stark shift in the trion resonance. Complete electron spin flip is made possible by a laser pulse designed to avoid excessive excitations of nearby exciton resonances and minimizes detrimental many-body effects. These results demonstrate an effective approach for ultrafast optical spin control in a complex spin system.
Abstract We shall present a review of semiconductor spin‐based implementation schemes for the realization of quantum information/computation solid‐state devices. After briefly describing the fundamentals of quantum computation theory, we shall introduce and discuss potential implementation schemes based on the spin degrees of freedom in semiconductor nanostructures. More specifically, we shall describe an implementation scheme for quantum information processing in which the spin degrees of freedom of electrons confined to a quantum dot are the computational degrees of freedom, and spins are manipulated/controlled through interaction between trionic states created by interband optical transitions by ultrafast sequences of multicolor laser pulses. We will also review briefly an adiabatic method for operating the two‐qubit gate that avoids the main imperfections present in real quantum dots: exciton decay, hole mixing, and phonon decoherence.
In the first part of this thesis, we investigate the dynamics of open quantum systems coupled in a purely dephasing way to their environment, which is a paradigmatic example of decoherent dynamics. We focus more specifically on the emergence of non-Markovian effects in the dynamics of such open systems. After reviewing different measures of non-Markovianity, we obtain an analytical expression for the reduced density operator of a finite-dimensional open system coupled with its environment through time-dependent purely dephasing interactions. When the system is initially uncorrelated with its environment, we obtain the time-local master equation that describes its dynamics. In this case, we also derive an exact expression for the witness of non-Markovianity based on the volume of accessible states of the open system. These results are then applied to spin systems coupled to either a spin or a bosonic environment. For spin environments, we study non-Markovianity for different ranges of interaction and in different thermodynamical limits. Moreover, the generation of entanglement between the open system and its environment as well as between spins within the open system is investigated. For bosonic environments, we study the effect of the initial state of the environment on non-Markovianity. We also show that time-dependent modulations of the system-environment coupling can dramatically alter the non-Markovian behavior of the open system. Finally, we consider the case of an open quantum system simultaneously coupled to spin and bosonic baths. In that case, we study how the coupling between the different baths and of the presence of initial correlations in the environment affect the non-Markovianity displayed by the open quantum system. The second part of this thesis is a contribution to the investigation of how quantum systems can be used to process information. More precisely, we focus on the implementation of gate-based quantum computing using neutral atoms trapped in optical or magnetic lattices and interacting through dipole-dipole interactions when excited to Rydberg states. In this context, we present protocols implementing two-qubit quantum gates between atoms far apart in the lattice. To this end, we propose to use a chain of non-coding ancilla atoms to transfer the state of the control qubit to an atom near the target qubit. Before discussing entangling gates between distant qubits, we present two protocols that implement state transfer along a chain of two-level atoms relying respectively on single- and two-atom laser pulses combined with dipole blockade. After reviewing protocols implementing CZ and CNOT gates between nearby qubits using dipole-dipole interactions, we show how the state-transfer protocols previously presented can be used to perform entangling gates between distant qubits. The performance of the state-transfer and gate protocols are assessed using the process fidelity. Moreover, we provide a theoretical explanation for the behavior of the process fidelity with respect to dissipation and imperfect blockade.