A Quantum Dot Single-Photon Turnstile Device Michler, P.; Kiraz, A.; Becher, C. ...
Science (American Association for the Advancement of Science),
12/2000, Volume:
290, Issue:
5500
Journal Article
Peer reviewed
Quantum communication relies on the availability of light pulses with strong quantum correlations among photons. An example of such an optical source is a single-photon pulse with a vanishing ...probability for detecting two or more photons. Using pulsed laser excitation of a single quantum dot, a single-photon turnstile device that generates a train of single-photon pulses was demonstrated. For a spectrally isolated quantum dot, nearly 100% of the excitation pulses lead to emission of a single photon, yielding an ideal single-photon source.
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Thiovulum majus self-organize on glass surfaces into active two-dimensional crystals of rotating cells. Unlike classical crystals, these bacterial crystallites continuously rotate and reorganize as ...the power of rotating cells is dissipated by the surrounding flow. In this article, we describe the earliest stage of crystallization, the attraction of two bacteria into a hydrodynamically-bound dimer. This process occurs in three steps. First a free-swimming cell collides with the wall and becomes hydrodynamically bound to the two-dimensional surface. We present a simple model to understand how viscous forces localize cells near the chamber walls. Next, the cell diffuses over the surface for an average of 63 6 s before escaping to the bulk fluid. The diffusion coefficient D eff = 7.98 0.1 m 2 s − 1 of these 8.5 m diameter cells corresponds to a temperature of ( 4.16 0.05 ) × 10 4 K, and thus cannot be explained by equilibrium fluctuations. Finally, two cells coalesce into a rotating dimer when the convergent flow created by each cell overwhelms their active Brownian motion. This occurs when cells diffuse to within a distance of 13.3 0.2 m of each other.
We demonstrate that very few (2-4) quantum dots as a gain medium are sufficient to realize a photonic-crystal laser based on a high-quality nanocavity. Photon correlation measurements show a ...transition from a thermal to a coherent light state proving that lasing action occurs at ultralow thresholds. Observation of lasing is unexpected since the cavity mode is in general not resonant with the discrete quantum dot states and emission at those frequencies is suppressed. In this situation, the quasicontinuous quantum dot states become crucial since they provide an energy-transfer channel into the lasing mode, effectively leading to a self-tuned resonance for the gain medium.
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Tomographic analysis demonstrates that the polarization state of pairs of photons emitted from a biexciton decay cascade becomes entangled when spectral filtering is applied. The measured density ...matrix of the photon pair satisfies the Peres criterion for entanglement by more than 3 standard deviations of the experimental uncertainty and violates Bell's inequality. We show that the spectral filtering erases the "which path" information contained in the photons' color and that the remanent information in the quantum dot degrees of freedom is negligible.
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The fundamental optical excitation in semiconductors is an electron-hole pair with antiparallel spins: the "bright" exciton. Bright excitons in optically active, direct-bandgap semiconductors and ...their nanostructures have been thoroughly studied. In quantum dots, bright excitons provide an essential interface between light and the spins of interacting confined charge carriers. Recently, complete control of the spin state of single electrons and holes in these nanostructures has been demonstrated, a necessary step towards quantum information processing with these two-level systems. In principle, the bright exciton's spin could also be used directly as a two-level system. However, because of its short radiative lifetime, its usefulness is limited. An electron-hole pair with parallel spins forms a long-lived, optically inactive "dark exciton", and has received less attention as it is mostly regarded as an inaccessible excitation. In this work we demonstrate that the dark exciton forms a coherent two-level system that can fairly easily be accessed by external light. We demonstrate: optical preparation of its spin state as a coherent superposition of two eigenstates, coherent precession of its spin state at a frequency defined by the energy difference between its eigenstates, and readout of the spin by charge addition and subsequent polarized photon detection. PUBLICATION ABSTRACT
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Coulomb interactions between electrons lead to the observed multiplet structure and breakdown of the Aufbau principle for atomic d and f shells. Nevertheless, these effects can disappear in extended ...systems. For instance, the multiplet structure of atomic carbon is not a feature of graphite or diamond. A quantum dot is an extended system containing ∼106 atoms for which electron-electron interactions do survive and the interplay between the Coulomb energy, J, and the quantization energy, ΔE, is crucial to Coulomb blockade. We have discovered consequences of Coulomb interactions in self-assembled quantum dots by interpreting experimental spectra with an atomistic calculation. The Coulomb effects, evident in the photon emission process, are tunable in situ by controlling the quantum dot charge from +6e to −6e. The same dot shows two regimes: J≤ΔE for electron charging yet J ΔE for hole charging. We find a breakdown of the Aufbau principle for holes; clear proof of non-perturbative hole-hole interactions; promotion-demotion processes in the final state of the emission process, effects first predicted a decade ago; and pronounced configuration hybridizations in the initial state. The level of charge control and the energy scales result in Coulomb effects with no obvious analogues in atomic physics.
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Ramification of stream networks Devauchelle, Olivier; Petroff, Alexander P.; Seybold, Hansjörg F. ...
Proceedings of the National Academy of Sciences - PNAS,
12/2012, Volume:
109, Issue:
51
Journal Article
Peer reviewed
Open access
The geometric complexity of stream networks has been a source of fascination for centuries. However, a comprehensive understanding of ramification—the mechanism of branching by which such networks ...grow—remains elusive. Here we show that streams incised by groundwater seepage branch at a characteristic angle of 2 π /5 = 72°. Our theory represents streams as a collection of paths growing and bifurcating in a diffusing field. Our observations of nearly 5,000 bifurcated streams growing in a 100 km ² groundwater field on the Florida Panhandle yield a mean bifurcation angle of 71.9° ± 0.8°. This good accord between theory and observation suggests that the network geometry is determined by the external flow field but not, as classical theories imply, by the flow within the streams themselves.
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The nonlinear Fano effect Govorov, A. O; Kroner, M; Remi, S ...
Nature (London),
01/2008, Volume:
451, Issue:
7176
Journal Article
Peer reviewed
The Fano effect is ubiquitous in the spectroscopy of, for instance, atoms, bulk solids and semiconductor heterostructures. It arises when quantum interference takes place between two competing ...optical pathways, one connecting the energy ground state and an excited discrete state, the other connecting the ground state with a continuum of energy states. The nature of the interference changes rapidly as a function of energy, giving rise to characteristically asymmetric lineshapes. The Fano effect is particularly important in the interpretation of electronic transport and optical spectra in semiconductors. Whereas Fano's original theory applies to the linear regime at low power, at higher power a laser field strongly admixes the states and the physics becomes rich, leading, for example, to a remarkable interplay of coherent nonlinear transitions. Despite the general importance of Fano physics, this nonlinear regime has received very little attention experimentally, presumably because the classic autoionization processes, the original test-bed of Fano's ideas, occur in an inconvenient spectral region, the deep ultraviolet. Here we report experiments that access the nonlinear Fano regime by using semiconductor quantum dots, which allow both the continuum states to be engineered and the energies to be rescaled to the near infrared. We measure the absorption cross-section of a single quantum dot and discover clear Fano resonances that we can tune with the device design or even in situ with a voltage bias. In parallel, we develop a nonlinear theory applicable to solid-state systems with fast relaxation of carriers. In the nonlinear regime, the visibility of the Fano quantum interferences increases dramatically, affording a sensitive probe of continuum coupling. This could be a unique method to detect weak couplings of a two-level quantum system (qubits), which should ideally be decoupled from all other states.
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Quantum dots or rings are artificial nanometre-sized clusters that confine
electrons in all three directions. They can be fabricated in a semiconductor
system by embedding an island of low-bandgap ...material in a sea of material
with a higher bandgap. Quantum dots are often referred to as artificial atoms
because, when filled sequentially with electrons, the charging energies are
pronounced for particular electron numbers; this is analogous
to Hund's rules in atomic physics. But semiconductors also have a valence
band with strong optical transitions to the conduction band. These transitions
are the basis for the application of quantum dots as laser emitters,
storage devices and fluorescence markers.
Here we report how the optical emission (photoluminescence) of a single quantum
ring changes as electrons are added one-by-one. We find that the emission
energy changes abruptly whenever an electron is added to the artificial atom,
and that the sizes of the jumps reveal a shell structure.
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We report the observation of a spin-flip process in a quantum dot whereby a dark exciton with total angular momentum L = 2 becomes a bright exciton with L = 1. The spin-flip process is revealed in ...the decay dynamics following nongeminate excitation. We are able to control the spin-flip rate by more than an order of magnitude simply with a dc voltage. The spin-flip mechanism involves a spin exchange with the Fermi sea in the back contact of our device and corresponds to the high temperature Kondo regime. We use the Anderson Hamiltonian to calculate a spin-flip rate, and we find excellent agreement with the experimental results.
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