Initialization and read-out of spins in coupled core–shell quantum dots

Abstract

I n the field of quantum information science, semiconductor quantum dots (QDs) are of particular interest for their ability to confine a single electron for use as a qubit 1,2. However, to realize the potential offered by quantum information processing, it is necessary to couple two or more qubits. In contrast to coupling individual QDs, we demonstrate the integration of two coupled electronic states within a single QD heterostructure. These chemically synthesized nanocrystals, known as quantumdot quantum wells (QDQWs) 3-7 , comprise concentric layers of different semiconducting materials. We investigate carrier and spin dynamics in these structures using transient absorption and time-resolved Faraday rotation measurements. By tuning the excitation and probe energies, we find that we can selectively initialize and read out spins in different coupled states within the QDQW. These results open a pathway for engineering coupled qubits within a single nanostructure. The samples studied in this work are ensembles consisting of nanocrystals with a 5.5-nm-diameter, low-bandgap (E g = 1.74 eV) CdSe core, surrounded by a three-monolayer (ML), high-bandgap (E g = 3.68 eV) ZnS barrier and a 4 ML outer CdSe shell 7. A cutaway illustration of the sample structure is shown in Fig. 1a. Qualitatively similar results were also obtained on a sample with 6.4 nm core, 2 ML barrier and 4 ML shell. For comparison, a control sample of 6.8-nm-diameter CdSe QDs was also prepared. Figure 1a shows the radial potential of the core-shell structure along with the conduction-(c-) and valence-(v-) band states, 1S e , 2S e , 1S 3/2 and 2S 3/2 (discussed below). The band profile is analogous to a pair of coupled quantum wells, in which the core corresponds to one well and the shell to the other. Indeed, under 2.43 eV excitation the photoluminescence (PL) spectrum of these quantum-dot quantum wells (QDQWs) (Fig. 1b) shows two peaks at 2.18 and 1.92 eV, which have been previously attributed to radiative recombination from an electron-hole pair in the shell and in the core, respectively 7. When the excitation energy is tuned between the core and shell emission to 2.02 eV, only the lowerenergy (core) emission is observed. This behaviour of the PL indicates that two optically active, metastable exciton states exist in the QDQWs, and that by changing the pump energy either the core or both the core and the shell can be selectively excited. We have modelled the electronic states of the coupled core-shell QDQWs within k • p theory, extending our previous model of a single QDQW 8,9. (The theoretical modelling is discussed in more detail in the Supplementary Information.

Hole and trion spin dynamics in quantum dots under excitation by a train of circularly polarized pulses

“…Its optical initialization, readout, and manipulation have been the center of important investigations. [1][2][3][4][5][6][7][8] At the same time, theoretical and experimental studies have shown that the hyperfine coupling with the lattice nuclei spins is the ultimate limit, at low temperature, to the electron-spin relaxation or decoherence in QDs, leading to typical ensemble dephasing times of the order of 1 ns in III-V QDs. [9][10][11][12] Up to now, the studies have been centered on the spin of a conduction electron and only recently several experiments have evidenced the initialization and the readout of hole spins confined in QD nanostructures.…”

Nanospherical heterolayer in strong electrostatic field

“…They mainly consisted of II-VI, IV-VI, and III-V compounds and can be presented, such as CdS/HgS/CdS, ZnS/CdS/ZnS, CdSe/HgSe/CdSe, CdTe/ HgTe/CdTe, CdS/PbS/CdS, PbSe/CdS/PbSe, PbSe/CdSe/ PbSe, AlAs/GaAs/AlAs/, InP/InAs/InP, etc. heterostructures (see for e.g., (Balandin and Wang 2006;Schmid 2010;Rogach 2008;Hofman 2009;Eychmuller et al 1993;Schooss et al 1994;Mews et al 1996;Hause et al 1993;Bryant 1995;Little et al 1998;Braun et al 2001;Bryant and Jaskolski 2003;Perez-Conde and Bhattacharjee 2006;Berezowsky et al 2006;Weng et al 2009;Guo-Yi 2004;Royo et al 2007;Brovelli et al 2011;Jia 2011;SalmanOgli and Roatami 2011) and references therein).The QDQW structure involves an onion-like nanosystem composed of a quantum dot core surrounded by two or more shells of alternating narrower and wider band gap materials. The original feature of QDQW is that their physical properties can be controlled and can be tuned by changing the core radius, the thickness of the quantized layer and the size of the outermost shell.…”

“…So, it is clear that the study of states of charge carriers in the internal QW of such barrier/well/barrier structures under the different physical conditions is of a separate interest. From this point of view in several papers in the frameworks of various approximations are being discussed the electronic, hole (Eychmuller et al 1993;Schooss et al 1994;Mews et al 1996;Hause et al 1993;Bryant 1995;Little et al 1998;Braun et al 2001;Bryant and Jaskolski 2003;Perez-Conde and Bhattacharjee 2006), excitonic (Kim et al 2006;Cat et al 2009;Prasad 2004;Heng and Jun-Jie 2004;Kai and Xia (1998); Jun-Jie (2002)) and phonon (Berezowsky et al 2006;Weng et al 2009;Guo-Yi 2004;Royo et al 2007) states, as well as the optical absorption and luminescence spectra (Eychmuller et al 1993;Schooss et al 1994;Mews et al 1996;Hause et al 1993;Bryant 1995;Little et al 1998;Braun et al 2001;Kim et al 2006;Cat et al 2009;Prasad 2004;Porteanu et al 2001;Fang et al 2010) of QDQW structures.…”

Transient spectral dependence of photoinduced magneto-optical Faraday effect in CdTe quantum dots

“…The strong size-dependent electronic properties in QDs are compatible with modern optoelectronics. 1,2 Electron spins manipulation and coherent control in semiconductor QDs has been received increasing attention during the past decades due to both the fascinating physics [3][4][5][6][7] and the potential applications in spin-based nano-to micro-scale devices for spintronics and quantum computation. 8,9 The all-optical time-resolved magneto-optical (MO) Faraday effect (transmission geometry) and the time-resolved MO Kerr effect (reflection geometry) are well suited to investigate the spin dynamics in a broad spectrum, regardless of the photoluminescence efficiency.…”