arXiv subjects

M. S. Skolnick

Publications and source records attributed to M. S. Skolnick.

At least 91 records · Page 5Linked to original sources

arXiv2010

Optically tunable nuclear magnetic resonance in a single quantum dot

We report optically detected nuclear magnetic resonance (ODNMR) measurements on small ensembles of nuclear spins in single GaAs quantum dots. Using ODNMR we make direct measurements of the inhomogeneous Knight field from a photo-excited electron which acts on the nuclei in the dot. The resulting shifts of the NMR peak can be optically controlled by varying the electron occupancy and its spin orientation, and lead to strongly asymmetric lineshapes at high optical excitation. The all-optical control of the NMR lineshape will enable position-selective control of small groups of nuclear spins in a dot. Our calculations also show that the asymmetric NMR peak lineshapes can provide information on the volume of the electron wave-function, and may be used for measurements of non-uniform distributions of atoms in nano-structures.

cond-mat.mes-hall↗

arXiv2010

Damping of Exciton Rabi Rotations by Acoustic Phonons in Optically Excited InGaAs/GaAs Quantum Dots

We report experimental evidence identifying acoustic phonons as the principal source of the excitation-induced-dephasing (EID) responsible for the intensity damping of quantum dot excitonic Rabi rotations. The rate of EID is extracted from temperature dependent Rabi rotation measurements of the ground-state excitonic transition, and is found to be in close quantitative agreement with an acoustic-phonon model.

cond-mat.mes-hall↗

arXiv2009

Pumping of nuclear spins by the optical solid effect in a quantum dot

We demonstrate that efficient optical pumping of nuclear spins in semiconductor quantum dots (QDs) can be achieved by resonant pumping of optically "forbidden" transitions. This process corresponds to one-to-one conversion of a photon absorbed by the dot into a polarized nuclear spin, which also has potential for initialization of hole spin in QDs. Pumping via the "forbidden" transition is a manifestation of the "optical solid effect", an optical analogue of the effect previously observed in electron spin resonance experiments in the solid state. We find that by employing this effect, nuclear polarization of 65% can be achieved, the highest reported so far in optical orientation studies in QDs. The efficiency of the spin pumping exceeds that employing the allowed transition, which saturates due to the low probability of electron-nuclear spin flip-flop.

cond-mat.mes-hall↗

arXiv2009

Towards coherent optical control of a single hole spin: Rabi rotation of a trion conditional on the spin state of the hole

A hole spin is a potential solid-state q-bit, that may be more robust against nuclear spin induced dephasing than an electron spin. Here we propose and demonstrate the sequential preparation, control and detection of a single hole spin trapped on a self-assembled InGaAs/GaAs quantum dot. The dot is embedded in a photodiode structure under an applied electric-field. Fast, triggered, initialization of a hole spin is achieved by creating a spin-polarized electron-hole pair with a picosecond laser pulse, and in an applied electric-field, waiting for the electron to tunnel leaving a spin-polarized hole. Detection of the hole spin with picosecond time resolution is achieved a second picosecond laser pulse to probe the positive trion transition, where a trion is created conditional on the hole spin to be detected as a change in photocurrent. Finally, using this setup we observe a Rabi rotation of the hole-trion transition that is conditional on the hole spin, which for a pulse-area of $2π$ can be used to impart a phase-shift of $π$ between the hole spin states, a non-general manipulation of the hole spin.

cond-mat.mes-hall↗

arXiv2009

Coexisting Non-Equilibrium Condensates with Long-Range Spatial Coherence in Semiconductor Microcavities

Real and momentum space spectrally resolved images of microcavity polariton emission in the regime of condensation are investigated under non resonant excitation using a laser source with reduced intensity fluctuations on the timescale of the exciton lifetime. We observe that the polariton emission consists of many macroscopically occupied modes. Lower energy modes are strongly localized by the photonic potential disorder on a scale of few microns. Higher energy modes have finite k-vectors and are delocalized over 10-15 microns. All the modes exhibit long range spatial coherence comparable to their size. We provide a theoretical model describing the behavior of the system with the results of the simulations in good agreement with the experimental observations. We show that the multimode emission of the polariton condensate is a result of its nonequilibrium character, the interaction with the local photonic potential and the reduced intensity fluctuations of the excitation laser.

cond-mat.other↗

arXiv2009

Polarization freezing of 10000 optically-cooled nuclear spins by coupling to a single electron

The nature of the nano-scale environment presents a major challenge for solid-state implementation of spin-based qubits. In this work, a single electron spin in an optically pumped nanometer-sized III-V semiconductor quantum dot is used to control a macroscopic nuclear spin of several thousand nuclei, freezing its decay and leading to spin life-times exceeding 100 seconds at low temperatures. Few-millisecond-fast optical initialization of the nuclear spin is followed by a slow decay exhibiting random telegraph signals at long delay times, arising from low probability electron jumps out of the dot. The remarkably long spin life-time in a dot surrounded by a densely-packed nuclear spin environment arises from the Knight field created by the resident electron, which leads to suppression of nuclear spin depolarization.

cond-mat.mes-hall↗

arXiv2009

Suppression of nuclear spin diffusion at a GaAs/AlGaAs interface measured with a single quantum dot nano-probe

Nuclear spin polarization dynamics are measured in optically pumped individual GaAs/AlGaAs interface quantum dots by detecting the time-dependence of the Overhauser shift in photoluminescence (PL) spectra. Long nuclear polarization decay times of ~ 1 minute have been found indicating inefficient nuclear spin diffusion from the GaAs dot into the surrounding AlGaAs matrix in externally applied magnetic field. A spin diffusion coefficient two orders lower than that previously found in bulk GaAs is deduced.

quant-ph↗

arXiv2009

Voltage controlled nuclear polarization switching in a single InGaAs quantum dot

Sharp threshold-like transitions between two stable nuclear spin polarizations are observed in optically pumped individual InGaAs self-assembled quantum dots embedded in a Schottky diode when the bias applied to the diode is tuned. The abrupt transitions lead to the switching of the Overhauser field in the dot by up to 3 Tesla. The bias-dependent photoluminescence measurements reveal the importance of the electron-tunneling-assisted nuclear spin pumping. We also find evidence for the resonant LO-phonon-mediated electron co-tunneling, the effect controlled by the applied bias and leading to the reduction of the nuclear spin pumping rate.

quant-ph↗

arXiv2008

Quantum-fluid dynamics of microcavity polaritons

Semiconductor microcavities offer a unique system to investigate the physics of weakly interacting bosons. Their elementary excitations, polaritons--a mixture of excitons and photons--behave, in the low density limit, as bosons that can undergo a phase transition to a regime characterised by long range coherence. Condensates of polaritons have been advocated as candidates for superfluidity; and the formation of vortices as well as elementary excitations with a linear dispersion are actively sought after. In this work, we have created and set in motion a macroscopically degenerate state of polaritons and let it collide with a variety of defects present in the sample. Our experiments show striking manifestations of a coherent light-matter packet that displays features of a superfluid, although one of a highly unusual character as it involves an out-of-equilibrium dissipative system where it travels at ultra-fast velocity of the order of 1% the speed of light. Our main results are the observation of i) a linear polariton dispersion accompanied with diffusion-less motion, ii) flow without resistance when crossing an obstacle, iii) suppression of Rayleigh scattering and iv) splitting into two fluids when the size of the obstacle is comparable with the size of the wavepacket. This work opens the way to the investigation of new phenomenology of out-of-equilibrium condensates.

cond-mat.mes-hall↗

arXiv2008

Fast optical preparation, control and read-out of single quantum dot spin

We propose and demonstrate the sequential initialization, optical control, and read-out of a single spin trapped in a semiconductor quantum dot. Hole spin preparation is achieved through ionization of a resonantly excited electron-hole pair. Optical control is observed as a coherent Rabi rotation between the hole and charged exciton states, which is conditional on the initial hole spin state. The spin-selective creation of the charged exciton provides a photocurrent read-out of the hole spin state.

cond-mat.mes-hall↗

arXiv2008

Overhauser effect in individual InP/GaInP dots

Sizable nuclear spin polarization is pumped in individual InP/GaInP dots in a wide range of external magnetic fields B_ext=0-5T by circularly polarized optical excitation. We observe nuclear polarization of up to ~40% at Bext=1.5T and corresponding to an Overhauser field of ~1.2T. We find a strong feedback of the nuclear spin on the spin pumping efficiency. This feedback, produced by the Overhauser field, leads to nuclear spin bi-stability at low magnetic fields of Bext=0.5-1.5T. We find that the exciton Zeeman energy increases markedly, when the Overhauser field cancels the external field. This counter-intuitive result is shown to arise from the opposite contribution of the electron and hole Zeeman splittings to the total exciton Zeeman energy.

cond-mat.mes-hall↗

arXiv2008

Two qubit conditional quantum logic operation in a single self-assembled quantum dot

The four-level exciton/biexciton system of a single semiconductor quantum dot acts as a two qubit register. We experimentally demonstrate an exciton-biexciton Rabi rotation conditional on the initial exciton spin in a single InGaAs/GaAs dot. This forms the basis of an optically gated two-qubit controlled-rotation (CROT) quantum logic operation where an arbitrary exciton spin is selected as the target qubit using the polarization of the control laser.

cond-mat.mes-hall↗

arXiv2008

Electroluminescence emission from polariton states in GaAs-based semiconductor microcavities

The authors report the observation of electroluminescence from GaAs-based semiconductor microcavities in the strong coupling regime. At low current densities the emission consists of two peaks, which exhibit anti-crossing behaviour as a function of detection angle and thus originate from polariton states. With increasing carrier injection we observe a progressive transition from strong to weak coupling due to screening of the exciton resonance by free carriers. The demonstration that polariton emission can be excited by electrical injection is encouraging for future development of polariton lasers.

cond-mat.other↗

arXiv2007

Intersublevel Polaron Dephasing in Self-Assembled Quantum Dots

Polaron dephasing processes are investigated in InAs/GaAs dots using far-infrared transient four wave mixing (FWM) spectroscopy. We observe an oscillatory behaviour in the FWM signal shortly (< 5 ps) after resonant excitation of the lowest energy conduction band transition due to coherent acoustic phonon generation. The subsequent single exponential decay yields long intraband dephasing times of 90 ps. We find excellent agreement between our measured and calculated FWM dynamics, and show that both real and virtual acoustic phonon processes are necessary to explain the temperature dependence of the polarization decay.

cond-mat.mes-hall↗

arXiv2007

Long nuclear spin decay times controlled by optical pumping in individual quantum dots

Nuclear polarization dynamics are measured in the nuclear spin bi-stability regime in a single optically pumped InGaAs/GaAs quantum dot. The controlling role of nuclear spin diffusion from the dot into the surrounding material is revealed in pump-probe measurements of the non-linear nuclear spin dynamics. We measure nuclear spin decay times in the range 0.2-5 sec, strongly dependent on the optical pumping time. The long nuclear spin decay arises from polarization of the material surrounding the dot by spin diffusion for long (>5sec) pumping times. The time-resolved methods allow the detection of the unstable nuclear polarization state in the bi-stability regime otherwise undetectable in cw experiments.

cond-mat.mes-hall↗

arXiv2007

Bistability of optically-induced nuclear spin orientation in quantum dots

We demonstrate that bistability of the nuclear spin polarization in optically pumped semiconductor quantum dots is a general phenomenon possible in dots with a wide range of parameters. In experiment, this bistability manifests itself via the hysteresis behavior of the electron Zeeman splitting as a function of either pump power or external magnetic field. In addition, our theory predicts that the nuclear polarization can strongly influence the charge dynamics in the dot leading to bistability in the average dot charge.

cond-mat.mes-hall↗

arXiv2006

Single-photon nonlinearity of a semiconductor quantum dot in a cavity

A single atom in a cavity is the model system of cavity quantum electrodynamics (CQED). The strong coupling regime between the atom and cavity-confined photon corresponds to the reversible exchange of energy between the two modes, and underpins a wide range of CQED phenomena with applications in quantum information science, including for example as quantum logic gates and as sources of entangled states. An important advance was achieved recently when strong coupling between excitons and cavity photons was reported for the first time for localized quantum dots (QDs) in micron-size solid state cavities. This has significance in terms of scalability and integration with other optical devices, and could lead to the emergence of "quantum optics on a chip" technology. However the results presented so far for quantum dots are in the linear regime, corresponding to coupling to the vacuum field (vacuum Rabi splitting); they are not a true QED effect and can equally well be described by classical physics as the coupling between two oscillators. In this paper, we present evidence for a purely quantum phenomenon for the QD/cavity photon system, namely the increase in splitting of the levels when the mean number of photons in the cavity is increased. This corresponds to non-linearities on the single-photon scale: the presence of a single excitation in the cavity changes the level structure, affecting the emission energies for a second photon. Such results are a first step in demonstrating the promise of quantum dots for CQED applications.

cond-mat.mes-hall↗

arXiv2006

Nuclear Spin Switch in Semiconductor Quantum Dots

We show that by illuminating an InGaAs/GaAs self-assembled quantum dot with circularly polarized light, the nuclei of atoms constituting the dot can be driven into a bistable regime, in which either a threshold-like enhancement or reduction of the local nuclear field by up to 3 Tesla can be generated by varying the intensity of light. The excitation power threshold for such a nuclear spin "switch" is found to depend on both external magnetic and electric fields. The switch is shown to arise from the strong feedback of the nuclear spin polarization on the dynamics of spin transfer from electrons to the nuclei of the dot.

cond-mat.mes-hall↗