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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

The maintaining of the exciton state in the two-QD system with dipole-dipole interaction and the intrinsic electron-hole exchange interaction

Shiang-Yu Huang, Guang-Yin Chen, Yueg-Nan Chen, Shun‐Jen Cheng

Published Jun 1, 2017• 0 citations
Publisher

Abstract

Summary form only given. The storage of the quantum bit, also called “qubit”, is a critical issue in the field of quantum information. As a promising candidate of being a quantum device, semiconductor quantum dots (QDs) are expected to play a central role in quantum information networks. Therefore, the study of quantum information storage in the QDs is really important. In this work, we employ the quantum master equation [1-2] to simulate the dynamics of the bright exciton, which is also used as a qubit, in coupled two QDs. In our model (shown in Fig. 1(a)), each QD is theoretically treated as a three-level subsystem and the exciton in this two-QD system is affected by the intrinsic electron-hole exchange interaction δ'ehxcand the dipole-dipole interactions for different exciton transfer δ'dd(change exciton spin after the transfer) and δdd(do not change exciton spin after the transfer). We assume that the incident photon is absorbed by the QD1, which creates an exciton. However, the initial exciton spin state will be disturbed by the intrinsic electron-hole exchange interaction, repeatedly changing the status from spin up to spin down during the exciton lifetime.

DOI
10.1109/cleoe-eqec.2017.8087739

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Summary form only given. The storage of the quantum bit, also called “qubit”, is a critical issue in the field of quantum information. As a promising candidate of being a quantum device, semiconductor quantum dots (QDs) are expected to play a central role in quantum information networks. Therefore, the study of quantum information storage in the QDs is really important. In this work, we employ the quantum master equation [1-2] to simulate the dynamics of the bright exciton, which is also used as a qubit, in coupled two QDs. In our model (shown in Fig. 1(a)), each QD is theoretically treated as a three-level subsystem and the exciton in this two-QD system is affected by the intrinsic electron-hole exchange interaction δ'ehxcand the dipole-dipole interactions for different exciton transfer δ'dd(change exciton spin after the transfer) and δdd(do not change exciton spin after the transfer). We assume that the incident photon is absorbed by the QD1, which creates an exciton. However, the initial exciton spin state will be disturbed by the intrinsic electron-hole exchange interaction, repeatedly changing the status from spin up to spin down during the exciton lifetime.
Abstract Electron spins confined in semiconductor quantum dots (QDs) are one of potential candidates for physical implementation of scalable quantum information processing technologies. Tunnel coupling based inter exchange interaction between QDs is crucial in achieving single-qubit manipulation, two-qubit gate, quantum communication and quantum simulation. This review first provides a theoretical perspective that surveys a general framework, including the Helter−London approach, the Hund−Mulliken approach, and the Hubbard model, to describe the inter exchange interactions between semiconductor quantum dots. An electrical method to control the inter exchange interaction in a realistic device is proposed as well. Then the significant achievements of inter exchange interaction in manipulating single qubits, achieving two-qubit gates, performing quantum communication and quantum simulation are reviewed. The last part is a summary of this review.
Summary form only given. Over the last two years much effort has been devoted to experimentally explore spin electronics in semiconductors. These experiments also support the hope of implementing quantum information devices based on spins using the mature technology of semiconductors. A basic requirement for quantum information processing with spins is the control of the mutual interaction between single spins. We extend here the investigation of spin coherence to the case of exchange coupled electron hole spins within a single exciton. The sample under investigation is an intrinsic 3 nm GaAs/(AlGa)As multiple quantum well (MQW) with a large Stokes shift of 9 meV.
As a candidate scheme for controllably coupled qubits, we consider two quantum dots, each doped with a single electron. The spin of the electron defines our qubit basis and trion states can be created by using polarized light; we show that the form of the excited trion depends on the state of the qubit. By using the Luttinger-Kohn Hamiltonian we calculate the form of these trion states in the presence of light-heavy hole mixing, and show that they can interact through both the F\"orster transfer and static dipole-dipole interactions. Finally, we demonstrate that by using chirped laser pulses, it is possible to perform a two-qubit gate in this system by adiabatically following the eigenstates as a function of laser detuning. These gates are robust in that they operate with any realistic degree of hole mixing, and for either type of trion-trion coupling.
The computer industry has seen an immense development in the last decades. \nPersonal computers have become available for everybody living in industrialized \ncountries with rapidly increasing performance in terms of speed and storage capacities. \nHowever, the performance of nowadays' computers is fundamentally limited by the \nlaws of classical physics: a classical bit can only take on either of the two distinct \nvalues `0' or `1'. \nIn contrast, a quantum computer could, in principle, make direct use \nof quantum phenomena, such as state superpositions \n-- a quantum bit can be in both states `0' and `1' simultaneously --, \nto perform complex computational tasks much faster than any classical computer. \n \nThe idea of building computers that work according to the laws of quantum physics \nhas opened various fields of research, one of which is the search for the best \nphysical system to use as a quantum bit (qubit). One important criterion for determining \nthe optimal qubit system is the lifetime of state superpositions. \nTypically, once initialized, such superpositions are destroyed on remarkably short \ntimescales due to interactions with the environment -- a process which is referred to \nas decoherence --, posing the question which physical qubit candidate system might show a high-enough robustness against the \ninfluence of the `outside world' to allow for viable quantum computation. \n \nIn this thesis, we will consider three particular realizations of one specific and \nvery promising type of qubit candidate system: \nan electron (or hole) confined to a quantum dot -- a nanoscale structure \nwithin a (typically semiconducting) material --, where the spin states `down' \nand `up' of our electron (or hole) encode the logical states `0' and `1'. \nOur task will be to study the decay of spin-state superpositions in such quantum-dot \nsystems. The main objective of this thesis is to understand the most important physical \nprocesses that lead to spin decoherence and to show ways to suppress this undesirable effect. \nIt turns out that at low temperatures, the main source of decoherence \nis the coupling of the electron (hole) to the surrounding nuclear spins. \n \nThis thesis is divided into three logical parts, corresponding to the three qubit candidate \nsystems under consideration. First we will study electron-spin qubits in III-V semiconductor \nquantum dots, where the electron spin interacts with the nuclear spins of the semiconducting \nhost material \nvia the isotropic Fermi contact hyperfine interaction. Second we consider quantum-dot-confined \nheavy holes and the decoherence of their (pseudo-)spin states due to anisotropic interactions with the \nnuclear spins. Third and last, we study electron-spin qubits made from carbon-nanotube and \ngraphene quantum dots. Quantum dots made of carbon have the advantage of a low abundance \nof spin-carrying nuclear isotopes, therefore reducing decoherence effects significantly. \n \nFor each of the systems under consideration, we will carry out analytical calculations on \nthe nuclear-spin interactions and the spin dynamics of the qubit. Although one main goal of \nthis thesis is to show ways to extend spin decoherence times, we will also focus on physically more \nfundamental questions. Not only the timescale of the decay is relevant for \nthe system's applicability as a qubit, but also the form of the decay which can vary \nsignificantly from system to system. For example, the decay of spin-state superpositions \ncan follow an exponential, super-exponential or power-law decay, and can even pass through \nvarious stages. This is not only of academic interest, but also important for practical \npurposes, such as the implementation of quantum error-correction schemes in a potential \nquantum computer. \n
Currently there is strong interest in realizing implementations of quantum computation and quantum cryptography in a solid state environment. One of the systems that are actively studied are semiconductor quantum dots (QDs). Due to their discrete energy level structure, they are often called artificial atoms, and they attract immediately interest of quantum information science since they allow to mimic the design developed for atomic physics systems such as ions in traps or atoms in cavities. However, despite of the similarities, one has to keep in main that any elementary excitation in a QD has a generic many-body character. An essential building block of a quantum processor is a quantum gate which entangles the states of two quantum bits. Recently it has been proposed that a pair of vertically aligned QDs could be used as an optically driven quantum gate: The quantum bits are individual carriers either on dot zero or dot one. The different dot indices play the same role as a "spin," therefore we term them "isospin." Quantum mechanical tunneling between the dots rotates the "isospin" and leads to superposition of two quantum dot states. The quantum gate is built when two different particles, an electron and a hole, are created optically. The two particles form entangled isospin states. The entanglement can be controlled by application of an electric field along the heterostructure growth direction. Here we present spectroscopic studies on single quantum dot molecules (QDMs) with different vertical separation between the dots that support the feasibility of this proposal. The comparison of the evolution of the excitonic recombination spectrum with the results of calculations allows us to demonstrate coherent tunneling of electrons and holes across the separating barrier and the formation of entangled exciton states. For a given barrier width, we find only small variations of the tunneling induced splitting between the entangled states demonstrating a good homogeneity within the obtained QDM ensembles.
We present an exciton-spin memory scheme using a single QDM consisting of two InAs QDs separated by a thin GaAs tunnelling barrier. Electrical control of the electron-hole spatial separation allowed the exciton lifetime to be tuned in the 1-1000 ns range. Storage and read-out operations were implemented using sub nanosecond modulation of the lifetime and photon storage up to 1μs was demonstrated. Using resonant excitation the circular polarisation of photons was transferred and stored in the exciton Zeeman states of a QDM under a magnetic field. The delayed emission was clearly co-polarised with the excitation light and a combined fidelity of 80% for the write and read operations was achieved.