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Science

Exciton Storage in Semiconductor Self-Assembled Quantum Dots

T. Lundström, Winston Vaughan Schoenfeld, H. Lee, Pierre M. Petroff

Published Dec 17, 1999• 267 citations
Publisher

Abstract

Storage and retrieval of excitons were demonstrated with semiconductor self-assembled quantum dots (QDs). The optically generated excitons were dissociated and stored as separated electron-hole pairs in coupled QD pairs. A bias voltage restored the excitons, which recombined radiatively to provide a readout optical signal. The localization of the spatially separated electron-hole pair in QDs was responsible for the ultralong storage times, which were on the order of several seconds. The present limits of this optical storage medium are discussed.

DOI
10.1126/science.286.5448.2312

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Storage and retrieval of excitons were demonstrated with semiconductor self-assembled quantum dots (QDs). The optically generated excitons were dissociated and stored as separated electron-hole pairs in coupled QD pairs. A bias voltage restored the excitons, which recombined radiatively to provide a readout optical signal. The localization of the spatially separated electron-hole pair in QDs was responsible for the ultralong storage times, which were on the order of several seconds. The present limits of this optical storage medium are discussed.
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We demonstrate storage of excitons in a single nanostructure, a self-assembled quantum post. After generation, electrons and holes forming the excitons are separated by an electric field toward opposite ends of the quantum post inhibiting their radiative recombination. After a defined time, the spatially indirect excitons are reconverted to optically active direct excitons by switching the electric field. The emitted light of the stored exciton is detected in the limit of a single nanostructure and storage times exceeding 30 msec are demonstrated. We identify a slow tunneling of the electron out of the quantum post as the dominant loss mechanism by comparing the field dependent temporal decay of the storage signal to models for this process and radiative losses.
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.
Semiconductor Quantum dots are potentially an attractive source of non-classical light. They can confine bound electron-hole pairs, excitons, in all three spatial dimensions giving them discrete optical emission energies, much like the electronic states in atomic systems. The semiconductor nature of the system allows quantum dots to be integrated into complicated device structures including optical cavities [1] and electrical injection [2]. The radiative exciton emission from a semiconductor quantum dot can provide the simplest form of quantum light, a single photon source [2]. The finite re-excitation time following the radiative decay of an exciton in a quantum dot prevents the dot from emitting more than one photon if excited with a short optical or electronic pulse. In a similar fashion excitation of the biexciton state, consisting of two electrons and holes, in the quantum dot results in the emission of a pair of photons at the energy of the biexciton and exciton transitions. The biexciton state was, in fact, proposed as a possible source of polarisation-entangled photons in 2000 by Benson et al. [3] as it consists of two polarisation-correlated decay paths which could potentially be indistinguishable. The condition of indistinguishability is fundamental to the generation of entangled photons and is, in general, not true of the two decay paths from the biexciton state in a quantum dot; physical anisotropy causes the intermediate exciton states to hybridise and separate in energy providing ‘which-path’ information about the route taken through the biexciton decay. We demonstrate how the splitting between the intermediate exciton states in a single InAs quantum dot can be controlled by growth [4], rapid thermal annealing [5] and the application of an external magnetic field [6]. Each of these three techniques can be used to make the intermediate exciton states degenerate allowing the biexciton decay to produce singe pairs of entangled photons triggered by short laser pulses [7,8]. Entangled photon pair sources are essential for applications such as entanglement based quantum key distribution, and optical quantum computing. For these applications it is important that no more than one entangled photon pair is generated in a cycle, a requirement met by our quantum dot source which is not possible with traditional methods for generating entangled pairs of photons. Entanglement also enables quantum interferometry and metrology, the basis of image resolution beyond that possible with equivalent classical light. Biphoton interferometry using entangled emission from the biexciton decay reveals fringes with half the period of classical light [9], and less that that of the laser. In addition, interferometry suggests that this type of entangled light source is not limited by decoherence, we fine the biphoton is surprising robust against dephasing, in stark contrast to the devastating effect of decoherence upon single photons emitted by similar structures.