Nanoscale solid-state single photon router

We demonstrate the triggered emission of polarization-entangled photon pairs from the biexciton cascade of a single InAs quantum dot embedded in a GaAs/AlAs planar microcavity. Improvements in the sample design blue shifts the wetting layer to reduce the contribution of background light in the measurements. Results presented show that >70% of the detected photon pairs are entangled. The high fidelity of the state that we determine is sufficient to satisfy numerous tests for entanglement. The improved quality of entanglement represents a significant step towards the realization of a practical quantum dot source compatible with applications in quantum information.

Introduction The advanced growth of semiconductor quantum dots (QDs) with high optical quality is one key to realizing novel devices in various research disciplines related to modern semiconductor technologies. Despite recent progress in the top-down lithographic fabrication of single semiconductor QD-like emitters [43], bottom-up fabrication methods are commonly applied for the realization of high quality light emitting QDs [7]. The exploitation of high-density QD arrays as an active material in laser diodes [17] and vertical cavity surface emitting lasers (VCSEL) resulted in a new class of devices featuring lower lasing thresholds and improved device performance (such as temperature stability) in comparison to devices with higher dimensional gain [2, 3]. Spintronics and quantum information processing are intensively studied fields in order to provide complementary or entirely novel routes to a future information technology [12, 5]. Single self-assembled quantum dots (QDs) grown in low-density arrays are promising candidates for realizing functional building blocks in these research fields as they allow to confine single charge carriers or spin carriers while providing a solid-state platform capable of electrical injection or manipulation. Extensive research has resulted in the realization of various devices relying on few or single QDs, for instance quantum light emitters such as single photon sources [23, 36], sources of pairs of entangled photons [1, 45], and fewto single-QD lasers [38]. Moreover, many milestone experiments have been performed with In(Ga)As QDs grown on GaAs, for instance the observation of the Purcell effect [10] or reaching the strong QD exciton–photon coupling regime [26], demonstration of the indistinguishability of photons emitted by a QD [29], or complete control of a single spin by ultrafast optical pulses [25].

We report on the magneto-optical study of spin polarized energetic fine structures for exciton complex in single CdSe quantum dot (QD) by using micro- photoluminescence (micro-PL) spectroscopy. The zero-field splitting of exciton luminescence peak arisen from the anisotropic exchange interaction of carriers in the QDs was observed. The g-factors for exciton and negatively-charged exciton, i.e. trion in a single QD were determined by fitting the magnetic field dependence of the corresponding PL peaks. By exciting the single QD with circularly polarized light of σ- and σ+ polarization, the spin-up and spin-down trions were selectively generated. The ratio, τ/τsf, of the exciton lifetime and the time constants for the spin-flipping process of trion in a single QD was estimated to be 0.13, which implies a long spin-lifetime in single CdSe QD.

Semiconductor quantum dots (QDs) are promising candidates to fulfill a wide range of applications in real-world quantum computing, communication, and networks. Their excellent optical properties such as high brightness, single-photon purity, and narrow linewidths show potential utility in many areas. In order to realize long term goals of integration into complex and scalable quantum information systems, many current challenges must be overcome. One of these challenges is accomplishment of all necessary computing operations within a QD, which might be enabled by coherent manipulation of single QD energy level structures. In the realm of scalability for quantum devices, a way to address problems posed by the inhomogeneous distribution of individual QD spectral properties is necessary. This dissertation presents record large AC Stark shifts applied to the energy level structure of a single semiconductor QD, as well as an automated spectroscopy method for efficient characterization of many QDs. The AC Stark shifts are spin-selective and fast, which are highly desirable qualities in a mechanism for energy level reconfiguration. The AC stark shifts are measured by resonant excitation spectroscopy; a coherent technique which explicitly shows the change in energetic structure. A novel optical filtering scheme is developed to discriminate the high-powered laser inducing the shifts from the weak QD photoluminescence, and a polarimeter device is constructed to accurately measure the polarization state of a laser beam. Machine vision is used to automate spectroscopy of many QDs in bulk by acquiring high-resolution emission spectra, which allows characterization of individual QD energy level structures prior to device fabrication.

In recent years interest arose into the study of transport through quantum dots. The interest is driven by two reasons. First, small devices like quantum dots and charging effects [1] In such devices open the possibility for future applications in electronics as transistors, memory cells, sensors etc. Small devices are especially interesting in high frequency applications due to their small capacitances. Although different schemes for applications have been proposed, none of it is under development or even in production and it is not clear what will be of real use in the end. The second reason for the interest into these systems is a basic physics point-of-view. These systems are extremely interesting since single quantum dots can be seen as a sort of artificial atom. In this manner-of-speaking coupled quantum dots can be called artificial molecules. These quantum dots are similar to atoms or molecules since a certain number of electrons is confined in a potential. But, in contrast to real atoms, in these artificial systems the number of electrons, the size of the system and the strength of the confinement potential can easily be changed. Therefore, in these quantum dots parameters, such as the ratio between cyclotron frequency and confinement energy or coupling strength between two quantum dots etc, can be easily varied, which is not achievable in real atoms or molecules.

We propose a scheme for generation of the stationary continuous-variable entanglement and Einstein-Podolsky-Rosen (EPR) steering between an optical cavity mode and a nanomechanical resonator (NMR) mode. The cavity and the NMR are commonly coupled with two separated quantum dots (QDs), where the two QDs are driven simultaneously by a strong laser field. By adjusting the frequency of the strong laser field, the two QDs are nearly trapped on different dressed states, which is helpful to generate the entanglement between the cavity mode and the NMR mode. Due to the combined resonant interaction of the two QDs with the NMR-cavity subsystem, the photon and the phonon created and (or) annihilated are correlated. In this regime, the optimal entanglement of the two modes is obtained and the purity of the state of the NMR-cavity subsystem is near to 1. Furthermore, the coupling strength between the cavity and two QDs is different from the dot-NMR coupling strength, which leads to the different mean occupation numbers of the cavity and the NMR. In this case, one-way EPR steering is observed. In addition, through analyzing the purity, we find the conditions of the existence for the different types of EPR steering.

A system of quantum dots based on Al x In1−xAs/Al y Ga1−yAs solid solutions is investigated. The use of Al x In1−xAs wide-gap solid solutions as the basis of quantum dots substantially extends the spectral emission range to the short-wavelength region, including the wavelength region near 770 nm, which is of interest for the development of aerospace systems of quantum cryptography. The optical characteristics of Al x In1−xAs single quantum dots grown by the Stranski–Krastanov mechanism were studied by cryogenic microphotoluminescence. The statistics of the emission of single quantum dot excitons was studied using a Hanbury Brown–Twiss interferometer. The pair photon correlation function indicates the sub-Poissonian nature of the emission statistics, which directly confirms the possibility of developing single-photon emitters based on Al x In1−xAs quantum dots. The fine structure of quantum dot exciton states was investigated at wavelengths near 770 nm. The splitting of the exciton states is found to be similar to the natural width of exciton lines, which is of great interest for the development of entangled photon pair emitters based on Al x In1−xAs quantum dots.

Colloidal semiconductor quantum dots (QDs) have strong light absorption, continuously adjustable narrowband emission, and high photoluminescence quantum yields, thereby making them promising materials for light-emitting diodes, solar cells, detectors, and lasers. Single-QD photoluminescence spectroscopy can remove the ensemble average to reveal the structure information and exciton dynamics of QD materials at a single-particle level. The study of single-QD spectroscopy can provide guidelines for rationally designing the QDs and giving the mechanism basis for QD-based applications. We can also carry out the research of the interaction between light and single QDs on a nanoscale, and prepare QD-based single-photon sources and entangled photon sources. Here, we review the recent research progress of single-QD photoluminescence spectroscopy and exciton dynamics, mainly including photoluminescence blinking dynamics, and exciton and multi-exciton dynamics of single colloidal CdSe-based QDs and perovskite QDs. Finally, we briefly discuss the possible future development trends of single-QD spectroscopy and exciton dynamics.

Semiconductor quantum dots (QDs) with excellent structural, optical and electronic properties can be easily fabricated by the self-assembled Stranski-Krastanow growth mode. However, in spite of numerous efforts and encouraging results, it has become clear that the bottom-up approach alone can not yield QDs with deterministically controllable electronic properties. Post-growth processing seems at present the only viable path to achieve this goal. Here we employ a focused laser beam both to characterize and to in-situ process single self-assembled QDs by heating them from cryogenic to high temperatures. The heat treatment allows us to blue-shift, in a broad range and with resolution-limited accuracy, the quantized energy levels of charge carriers confined in single QDs. We demonstrate the approach by bringing spatially separated QDs in mutual resonance. This processing method may open the way to a full control of nanostructures at the quantum level.

Summary form only given. The excitonic optical response of single quantum dots (QD) has recently been isolated by using high spatial resolution techniques. Photoluminescence (PL) magneto-optical studies of these confined systems have shown interesting physical phenomena such as Zeeman splittings, diamagnetic shifts and the Overhauser effect. Coherent nonlinear optical spectroscopy provides a powerful additional tool for the study of the interaction between magnetic substrates, as has been demonstrated for a single quantum dot. By directly probing the quantum dot resonances under the effect of a magnetic field we can obtain information about spin relaxation times and the presence of dark states. In this paper we show the effect of a magnetic field on single quantum dot resonances (i.e. Zeeman splitting and diamagnetic shift) and report the nonlinear optical response as a function of magnetic field.

Semiconductor quantum dots have long been described as artificial atoms because they have discrete energy states analogous to the energy levels of natural atoms. In recent years, it has become possible to create coupled pairs of quantum dots that are analogous to natural diatomic molecules. These artificial molecules have received a great deal of attention because of the potential applications in novel optoelectronic and spintronic devices, including the possibility of scalable implementations of quantum information processing. Just as in natural diatomic molecules, tunneling of electrons or holes between the two dots creates delocalized molecular orbitals. In natural diatomic molecules the molecular ground state has bonding orbital character and the first excited molecular state has antibonding character. Artificial quantum dot molecules were believed to behave in a similar way. However, recently Dr. Doty from the University of Delaware, Dr. Climente from Universitat Jaume I (Castellon, Spain), and their collaborators in Canada and the US, have experimentally verified and explained the existence of an antibonding molecular ground state for holes in artificial quantum dot molecules. The coherent coupling of quantum dots leads to the formation of delocalized molecular orbitals that appear in photoluminescence spectra under electric fields as ant crossings. The orbital character of the molecular states cannot be measured at zero magnetic field. However, a recent discovery by Doty and coworkers at the Naval Research Lab revealed that, when a magnetic field was applied, the resonant changes in the Zeeman splitting that depended on the orbital character of the molecular states appeared. Using these changes to identify the molecular orbital character, Doty and coworkers demonstrated that the orbital character of the molecular ground state reverses as a function of the distance between the dots. Holes confined in the molecular states of dots separated by 2 nm have the expected bonding molecular ground state; when the dots are separated by three or more nanometers, the molecular ground state becomes antibonding. “This was a surprising discovery for us,” said Doty. “Until we looked closely at the orbital character of the molecular states we had simply assumed that the molecular ground state was always a bonding orbital.” To explain this surprising result, Climente and coworkers utilized a four-bandk · papproximation which shows that the parity along the molecular axis is broken by the spin-orbit interaction in the valence band. “This leads to the mixing of bonding and antibonding heavy- and light-hole components of the spinor which destabilizes (stabilizes) the otherwise pure bonding (antibonding) states, leading to the state reversal” Climente told Nanospotlight. He continued explaining that the molecular ground states are found to have up to ~95% antibonding character as a result of this mixing, “This is about 10 times higher than the largest value observed in natural molecules, so we can speak about a novel kind of molecular state whose properties are still to be explored”. Asp3d5s* tight binding multimillion atom calculation was applied to probe theoretically a real case. “We considered self-assembled InGaAs/GaAs double quantum dot structures and included strain, structural asymmetries as well as vertical electric fields. The results are in qualitative agreement with thek · ptheory and predict a bonding-to-antibonding ground state reversal at interdot distances ofd ~ 2 nm” says Climente. This discovery drastically changed the previous conception of the single-particle ground state of holes in coupled quantum dots. “This paves the way for more accurate simulations of device performance for applications in optics, transport, or quantum information” Climente mentioned. He continued saying that the tunneling rate, which is an important parameter for the quantum computation and transport can now be flexibly tuned from large values down to zero using a magnetic field. “This is a consequence of the spinor nature of holes” says Climente, “and it shows that we can greatly manipulate the properties of artificial molecules through the spin-orbit interaction”. As a matter of fact, this discovery provides new tools for engineering the spatial distribution of molecular wavefunctions in regions with varying material parameters. “Wavefunction engineering can be used to control magnetic, spin, and optical properties, so this discovery will enable the design of wide variety of materials for novel optoelectronic applications,” said Doty (Fig. ​(Fig.11). Figure 1 Dissociation spectrum of a hole in an artificial molecule (solid line). Note the state reversal atd ~ 1.5 nm, implying a bonding-to-antibonding ground state transition. This never occurs for electrons in artificial or natural diatomic molecules, where ... This work is also featured in the OAtube nanotechnology journal,http://www.oatube.org/2008/09/jiclimente.html. This journal is a new kind of open access effort that offers video access to science.

The nonlinear and non-local effects in atomic Rydberg media under electromagnetically induced transparency (EIT) make it a versatile platform for fundamental studies and applications in quantum information. In this paper, we study the dynamics of a Rydberg-EIT system in an ensemble that allows for more than one Rydberg excitation in the propagation direction. The density of two-level atoms is such that transient superradiant effects occur. We experimentally observe a cross-over between coherent collective emission (‘flash’) of two-level atoms to a Rydberg dressed regime (‘dressed flash’) under EIT condition. The complex dynamics are characterised using both intensity and time correlation measurements. We show that while steady-state EIT gives a second order correlation g(2) = 0.79 ± 0.04, the Rydberg-dressed flash exhibits anti-bunching down to 0.2 ± 0.04.

The observation of photoluminescence spectra of self-assembled single InGaAs quantum dots at room temperature was performed under weak excitation conditions using a near-field scanning optical microscope. Operation in illumination-collection mode with a highly sensitive double-tapered optical fibre probe enabled detection of weak photoluminescence signals at room temperature with high efficiency and high spatial resolution. Each single quantum dot was imaged with a spatial resolution of about 250 nm, which corresponded to a quarter of the wavelength of the photoluminescence from quantum dots. The photoluminescence yields of individual quantum dots were widely distributed and were found to decrease with photoluminescence energy. This result serves as a clue to be pursued for better understanding of the thermal excitation of the carrier from confined states in quantum dots.

We demonstrate a straightforward way to obtain single well-isolated quantum dots emitting in the visible part of the spectrum and characterize the optical emission from single quantum dots using this method. Self-assembled InP quantum dots are grown using gas-source molecular-beam epitaxy over a wide range of InP deposition rates, using an ultralow growth rate of about 0.01 atomic monolayers/s, a quantum-dot density of 1 dot/μm2 is realized. The resulting isolated InP quantum dots embedded in an InGaP matrix are individually characterized without the need for lithographical patterning and masks on the substrate. Such low-density quantum dots show excitonic emission at around 670 nm with a linewidth limited by instrument resolution. This system is applicable as a single-photon source for applications such as quantum cryptography.

Quantum dots have been identified as potential sources of entangled photon pairs, essential for many applications in quantum information. However, in practice structural properties of the dots result in polarisation splitting of the emission lines, which has prevented the realisation of such a source. Here, we present two techniques developed to control the polarisation splitting in a quantum dot both irreversibly and reversibly. We find that the splitting is strongly dependent upon the emission energy, or lateral confinement, of the quantum dot. Thus by precise control of the InAs deposition thickness, it is possible to produce ensembles from which dots with zero polarisation splitting can be easily selected. Additionally, we demonstrate that by using modest in-plane magnetic fields to partially mix the bright and dark exciton states, the polarisation splitting can be reversibly tuned to zero for most dots of a certain type. Finally, we demonstrate the emission of triggered entangled photon pairs from a quantum dot with approximately zero splitting with fidelity > 70%.