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Crystal growth anisotropy in molecular beam epitaxy usually prevents deterministic nucleation of individual quantum dots when a thick GaAs buffer is grown over a nanopatterned substrate. Here, we demonstrate how this anisotropy can actually be used to mold nucleation sites for single dots on a much thicker buffer than has been achieved by conventional techniques. This approach greatly suppresses the problem of defect-induced line broadening for single quantum dots in a charge-tunable device, giving state-of-the-art optical linewidths for a system widely studied as a spin qubit for quantum information.

The optical quality of site-controlled quantum dots is typically assessed by off-resonant photoluminescence spectroscopy, and emission linewidth is the most common figure of merit. Here, we combine photoluminescence and resonance fluorescence spectroscopy to obtain a more complete picture of site-controlled quantum dots embedded in a charge injection device. Although resonant and non-resonant linewidths are nearly as small as those of randomly nucleated dots, other optical properties show clear evidence of influence from defects introduced by the nanofabrication process. We demonstrate optical spin pumping and spin-flip Raman processes, which are important functions for use in quantum information applications. [ABSTRACT FROM AUTHOR]

Solid-state quantum emitters have shown strong potential for applications in quantum information, but the spectral inhomogeneity of these emitters poses a significant challenge. We address this issue in a cavity-quantum dot system by demonstrating cavity-stimulated Raman spin flip emission. This process avoids populating the excited state of the emitter and generates a photon that is Raman shifted from the laser and enhanced by the cavity. The emission is spectrally narrow and tunable over a range of at least 125 GHz, which is two orders of magnitude greater than the natural linewidth. We obtain the regime in which the Raman emission is spin dependent, which couples the photon to a long-lived electron spin qubit. This process can enable an efficient, tunable source of indistinguishable photons and deterministic entanglement of distant spin qubits in a photonic-crystal quantum network. [ABSTRACT FROM AUTHOR]

The effects of confinement on biexciton renormalization in self-assembled InAs and interfacial GaAs quantum dot (QD) ensembles are studied using two-dimensional Fourier-transform spectroscopy. We find that in thermally annealed InAs QDs, changes in the biexciton transition energy are strongly correlated with those of the exciton and that the biexciton binding energy is similar for all QDs in the ensemble. These results are in contrast to those from GaAs QDs formed from interfacial fluctuations of a narrow quantum well (QW). In both the GaAs QW and QDs, correlation is reduced and the biexciton binding exhibits a strong dependence on localization. Comparison with simulations reveals how confinement and Coulomb interactions modify biexciton renormalization. [ABSTRACT FROM AUTHOR]

A key ingredient for a quantum network is an interface between stationary quantum bits and photons, which act as flying qubits for interactions and communication. Photonic crystal architectures are promising platforms for enhancing the coupling of light to solid-state qubits. Quantum dots can be integrated into a photonic crystal, with optical transitions coupling to photons and spin states forming a long-lived quantum memory. Many researchers have now succeeded in coupling these emitters to photonic crystal cavities, but there have been no demonstrations of a functional spin qubit and quantum gates in this environment. Here, we have developed a coupled cavity-quantum dot system in which the dot is controllably charged with a single electron. We perform the initialization, rotation and measurement of a single electron spin qubit using laser pulses, and find that the cavity can significantly improve these processes. [ABSTRACT FROM AUTHOR]

A single hole spin in a semiconductor quantum dot has emerged as a quantum bit that is potentially superior to an electron spin. A key feature of holes is that they have a greatly reduced hyperfine interaction with nuclear spins, which is one of the biggest difficulties in working with an electron spin. It is now essential to show that holes are viable for quantum information processing by demonstrating fast quantum gates and scalability. To this end, we have developed InAs/GaAs quantum dots coupled through coherent tunnelling and charged with controlled numbers of holes. We report fast, single-qubit gates using a sequence of short laser pulses. We then take the important next step towards scalability of quantum information by optically controlling two interacting hole spins in separate dots. [ABSTRACT FROM AUTHOR]

The interaction between two quantum bits enables the creation of entanglement, the two-particle correlations that are at the heart of quantum information science. In semiconductor quantum dots, much work has focused on demonstrating control over single spin qubits using optical techniques. However, optical control of two spin qubits remains a major challenge for scaling to a fully fledged quantum-information platform. Here, we combine advances in vertically stacked quantum dots with ultrafast laser techniques to achieve optical control of the entangled state of two electron spins. Each electron is in a separate InAs quantum dot, and the spins interact through tunnelling, where the tunnelling rate determines how rapidly entangling operations can be carried out. We achieve two-qubit gates with an interaction rate of 30 GHz, more than an order of magnitude faster than demonstrated in any other system so far. These results demonstrate the viability and advantages of optically controlled quantum-dot spins for multi-qubit systems. [ABSTRACT FROM AUTHOR]

A single electron or hole spin trapped inside a semiconductor quantum dot forms the foundation for many proposed quantum logic devices. In group III–V materials, the resonance and coherence between two ground states of the single spin are inevitably affected by the lattice nuclear spins through the hyperfine interaction, while the dynamics of the single spin also influence the nuclear environment. Recent efforts have been made to protect the coherence of spins in quantum dots by suppressing the nuclear spin fluctuations. However, coherent control of a single spin in a single dot with simultaneous suppression of the nuclear fluctuations has yet to be achieved. Here we report the suppression of nuclear field fluctuations in a singly charged quantum dot to well below the thermal value, as shown by an enhancement of the single electron spin dephasing time T2*, which we measure using coherent dark-state spectroscopy. The suppression of nuclear fluctuations is found to result from a hole-spin assisted dynamic nuclear spin polarization feedback process, where the stable value of the nuclear field is determined only by the laser frequencies at fixed laser powers. This nuclear field locking is further demonstrated in a three-laser measurement, indicating a possible enhancement of the electron spin T2* by a factor of several hundred. This is a simple and powerful method of enhancing the electron spin coherence time without use of ‘spin echo’-type techniques. We expect that our results will enable the reproducible preparation of the nuclear spin environment for repetitive control and measurement of a single spin with minimal statistical broadening. [ABSTRACT FROM AUTHOR]

This paper concerns optically driven quantum logic devices based on semiconductor quantum dots. It provides a brief review of recent theoretical and experimental progress towards building such devices and a description of a possible direction of further research. We consider both the exciton and the electron spin as a potential qubit. Quantum dot fabrication and single dot spectroscopy studies are briefly discussed followed by a description of experimental demonstrations of basic quantum logic operations. A scheme for a scalable quantum computer based on optical control of electron spins localized in quantum dots is described in detail. Important lessons as well as challenges for future research are summarized. PACS: 78.67.Hc, 42.50.Md, 03.67.Lx, 42.50.Hz [ABSTRACT FROM AUTHOR]

Reviews optical studies of single quantum dots (QD). Properties of QD; Details on optical excitations; Ways of controlling electronic and nuclear spins.

The article offers information on the study about the impact of coherent tunnelling to a nearby sea of electrons to the optical spectrum of a single quantum dot. The study notes that when an electron combines with the hole, there is a shake-up of the electron sea that establishes rise to characteristic spectral lineshapes. The study infers that the quantum size effects can be tailored to engineer the functionality of semiconductor quantum-dot systems.

Focuses on semiconductor quantum dots. Optical studies of individual quantum dots; Interest in semiconductor quantum dots that interact strongly with light; Details of quantum confinement of electrons; Harnessing quantum mechanical properties.

We review spin properties of semiconductor quantum dots and their effect on optical spectra. Photoluminescence and other types of spectroscopy are used to probe neutral and charged excitons in individual quantum dots with high spectral and spatial resolution. Spectral fine structure and polarization reveal how quantum dot spins interact with each other and with their environment. By taking advantage of the selectivity of optical selection rules and spin relaxation, optical spin pumping of the ground state electron and nuclear spins is achieved. Through such mechanisms, light can be used to process spins for use as a carrier of information. [ABSTRACT FROM AUTHOR]

Charges and spins confined in quantum dots and quantum dot molecules are of great interest for new optoelectronic device applications. The strong confinement in quantum dot structures leads to unique interactions among electrons and holes. A detailed understanding of the magnitude and dynamics of these charge-carrier interactions will be essential to the development of future devices. We present experimental evidence of holes trapped in metastable higher-energy states of InAs/AlGaAs/InAs quantum dot molecules. We present a model for the kinetic pathways that lead to this dynamic hole trapping and analyze the consequences of dynamic hole trapping for carrier relaxation and optical emission. [ABSTRACT FROM AUTHOR]

The biexciton absorption spectrum of a pair of InAs/GaAs quantum dots is being studied by photoluminescence excitation spectroscopy. An absorption resonance with the characteristics of an instantaneous two-photon process reveals a coherent interdot two-photon transition. Pauli-selective tunneling is being used to demonstrate the transduction of the two-photon coherence into a nonlocal spin singlet state. The two-photon transition can be tuned spectrally by electric field, enabling amplification of its transition strength. [ABSTRACT FROM AUTHOR]

We demonstrate the suppression of nuclear-spin fluctuations in an InAs quantum dot and measure the timescales of the spin narrowing effect. By initializing for tens of milliseconds with two continuous wave diode lasers, fluctuations of the nuclear spins are suppressed via the hole-assisted dynamic nuclear polarization feedback mechanism. The fluctuation narrowed state persists in the dark (absent light illumination) for well over 1 s even in the presence of a varying electron charge and spin polarization. Enhancement of the electron spin coherence time (T2*) is directly measured using coherent dark state spectroscopy. By separating the calming of the nuclear spins in time from the spin qubit operations, this method is much simpler than the spin echo coherence recovery or dynamic decoupling schemes. [ABSTRACT FROM AUTHOR]

Tunable exciton relaxation rates are observed in individual vertically coupled semiconductor quantum dots (CQDs). An applied electric field is used to tune the energy difference between the spatially direct (SD) and indirect (SI) excitons in InAs CQDs. The intensity and lifetime of the SI exciton is found to vary as a result of wave-function distribution, carrier tunneling, and phonon-mediated relaxation effects. This includes a modulation of the phonon relaxation rate between the SI and SD excitons, a consequence of momentum space restrictions resulting from the structure factor of the CQD. [ABSTRACT FROM AUTHOR]

Abstract: We review the investigation of a single quantum dot driven by a strong optical field. By coherent pump-probe spectroscopy, we demonstrate the Autler–Townes splitting and Mollow absorption spectrum in a single neutral quantum dot. Furthermore, we also show the typical Mollow absorption spectrum by driving a singly charged quantum dot in a strong optical coupling regime. Our results show all the typical features of an isolated atomic system driven by a strong optical field, such as the AC stark effect, Rabi side bands and optical gain effect, which indicate that both neutral and charged quantum dots maintain the discrete energy level states even at high optical field strengths. [Copyright &y& Elsevier]

Abstract: Here we review the basic optical spectra of quantum dot molecules. We apply a simple and straightforward model to calculate charge stability regions in vertically coupled double dot molecules that are embedded in a Schottky diode. This model allows us to relate features in the optical spectrum to the diode structure. The underlying concepts allow one to design quantum dot molecules functionalized for optical operations. [Copyright &y& Elsevier]

Abstract: Using polarization-sensitive photoluminescence and photoluminescence excitation spectroscopy, we study single InAs/GaAs self-assembled quantum dots. The dots were embedded in an n-type, Schottky diode structure allowing for control of the charge state. We present here the exciton, singly charged exciton (positive and negative trions), and the twice negatively charged exciton. For non-resonant excitation below the wetting layer, we observed a large degree of polarization memory from the radiative recombination of both the positive and negative trions. In excitation spectra, through the p-shell, we have found several sharp resonances in the emission from the s-shell recombination of the dot in all charged states. Some of these excitation resonances exhibit strong coulomb shifts upon addition of charges into the quantum dot. One particular resonance of the negatively charged trion was found to exhibit a fine structure doublet under circular polarization. This observation is explained in terms of resonant absorption into the triplet states of the negative trion. [Copyright &y& Elsevier]

Coherent optically driven semiconductor quantum dot systems are proposed for the implementation of quantum information processing devices. Qubits defined in the exciton and spin bases are presented. In particular, single qubit rotation and a two-qubit logic gate have been demonstrated experimentally in the model exciton system. The experimental work done on the exciton system serves as a foundation for future work on spin-based qubits, which possess a much longer coherence lifetime. [Copyright &y& Elsevier]

Optical spin rotations and cycling transitions for measurement are normally incompatible in quantum dots, presenting a fundamental problem for quantum information applications. Here we show that for a hole spin this problem can be addressed using a trion with one hole in an excited orbital, where strong spin-orbit interaction tilts the spin. Then, a particular trion triplet forms a double Λ system, even in a Faraday magnetic field, which we use to demonstrate fast hole spin initialization and coherent population trapping. The lowest trion transitions still strongly preserve spin, thus combining fast optical spin control with cycling transitions for spin readout. [ABSTRACT FROM AUTHOR]

We demonstrate the extension of coherence between all four two-electron spin ground states of an InAs quantum dot molecule (QDM) via nonlocal suppression of nuclear spin fluctuations in two vertically stacked quantum dots (QDs), while optically addressing only the top QD transitions. Long coherence times are revealed through dark-state spectroscopy as resulting from nuclear spin locking mediated by the exchange interaction between the QDs. Line shape analysis provides the first measurement of the quieting of the Overhauser field distribution correlating with reduced nuclear spin fluctuations. [ABSTRACT FROM AUTHOR]

Spins confined in quantum dots are a leading candidate for solid-state quantum bits that can be coherently controlled by optical pulses. There are, however, many challenges to developing a scalable multibit information processing device based on spins in quantum dots, including the natural inhomogeneous distribution of quantum dot energy levels, the difficulty of creating all-optical spin manipulation protocols compatible with nondestructive readout, and the substantial electron-nuclear hyperfine interaction-induced decoherence. Here, we present a scalable qubit design and device architecture based on the spin states of single holes confined in a quantum dot molecule. The quantum dot molecule qubit enables a new strategy for optical coherent control with dramatically enhanced wavelength tunability. The use of hole spins allows the suppression of decoherence via hyperfine interactions and enables coherent spin rotations using Raman transitions mediated by a hole-spin-mixed optically excited state. Because the spin mixing is present only in the optically excited state, dephasing and decoherence are strongly suppressed in the ground states that define the qubits and nondestructive readout is possible. We present the qubit and device designs and analyze the wavelength tunability and fidelity of gate operations that can be implemented using this strategy. We then present experimental and theoretical progress toward implementing this design. [ABSTRACT FROM AUTHOR]