Cavity quantum electrodynamics (QED) studies the interaction between a quantum emitter and a single radiation-field mode. When an atom is strongly coupled to a cavity mode, it is possible to realize ...important quantum information processing tasks, such as controlled coherent coupling and entanglement of distinguishable quantum systems. Realizing these tasks in the solid state is clearly desirable, and coupling semiconductor self-assembled quantum dots to monolithic optical cavities is a promising route to this end. However, validating the efficacy of quantum dots in quantum information applications requires confirmation of the quantum nature of the quantum-dot-cavity system in the strong-coupling regime. Here we find such confirmation by observing quantum correlations in photoluminescence from a photonic crystal nanocavity interacting with one, and only one, quantum dot located precisely at the cavity electric field maximum. When off-resonance, photon emission from the cavity mode and quantum-dot excitons is anticorrelated at the level of single quanta, proving that the mode is driven solely by the quantum dot despite an energy mismatch between cavity and excitons. When tuned to resonance, the exciton and cavity enter the strong-coupling regime of cavity QED and the quantum-dot exciton lifetime reduces by a factor of 145. The generated photon stream becomes antibunched, proving that the strongly coupled exciton/photon system is in the quantum regime. Our observations unequivocally show that quantum information tasks are achievable in solid-state cavity QED.
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Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We present measurements of the buildup and decay of nuclear spin polarization in a single semiconductor quantum dot. Our experiment shows that we polarize the nuclei in a few milliseconds, while ...their decay dynamics depends drastically on external parameters. We show that a single electron can very efficiently depolarize nuclear spins in milliseconds whereas in the absence of the electron the nuclear spin lifetime is on the scale of seconds. This lifetime is further enhanced by 1-2 orders of magnitude by quenching the nonsecular nuclear dipole-dipole interactions with a magnetic field of 1 mT.
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.
Resonant laser scattering along with photon correlation measurements established the atom-like character of quantum dots. Here, we show that for a wide range of experimental parameters it is ...impossible to isolate elementary quantum-dot excitations from a strong influence of nuclear spins; the absorption lineshapes at magnetic fields exceeding 1 T indicate that the nuclear spins get polarized by an amount that ensures locking of the quantum-dot resonance to the incident laser frequency. In stark contrast to earlier experiments, this nuclear-spin polarization is bidirectional, allowing the combined electron-nuclear-spin system to track the changes in laser frequency dynamically on both sides of the resonance. This unexpected feature stems from a competition between two spin-pumping processes that attempt to polarize nuclear spins in opposite directions. We find that the confluence of laser excitation and nuclear-spin polarization suppresses the fluctuations in resonant absorption. A master-equation analysis suggests narrowing of the nuclear-spin distribution, pointing to applications in quantum information processing. PUBLICATION ABSTRACT
We demonstrate dynamical nuclear-spin polarization in the absence of an external magnetic field by resonant circularly polarized optical excitation of a single electron or hole charged quantum dot. ...Optical pumping of the electron spin induces an effective inhomogeneous magnetic (Knight) field that determines the direction along which nuclear spins could polarize and enables nuclear-spin cooling by suppressing depolarization induced by nuclear dipole-dipole interactions. Our experiments constitute a first step towards a quantum measurement of the Overhauser field.
We probe local charge fluctuations in a semiconductor via laser spectroscopy on a nearby self-assembled quantum dot. We demonstrate that the quantum dot is sensitive to changes in the local ...environment at the single-charge level. By controlling the charge state of localized defects, we are able to infer the distance of the defects from the quantum dot with ±5 nm resolution. The results identify and quantify the main source of charge noise in the commonly used optical field-effect devices.
Reliable preparation, manipulation and measurement protocols are necessary to exploit a physical system as a quantum bit. Spins in optically active quantum dots offer one potential realization and ...recent demonstrations have shown high-fidelity preparation and ultrafast coherent manipulation. The final challenge-that is, single-shot measurement of the electron spin-has proved to be the most difficult of the three and so far only time-averaged optical measurements have been reported. The main obstacle to optical spin readout in single quantum dots is that the same laser that probes the spin state also flips the spin being measured. Here, by using a gate-controlled quantum dot molecule, we present the ability to measure the spin state of a single electron in real time via the intermittency of quantum dot resonance fluorescence. The quantum dot molecule, unlike its single quantum dot counterpart, allows separate and independent optical transitions for state preparation, manipulation and measurement, avoiding the dilemma of relying on the same transition to address the spin state of an electron.
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Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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.
The nonlinear Fano effect Govorov, A. O; Kroner, M; Remi, S ...
Nature (London),
01/2008, Letnik:
451, Številka:
7176
Journal Article
Recenzirano
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.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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