72 research outputs found
Distilling entanglement from cascades with partial "Which Path" ambiguity
We develop a framework to calculate the density matrix of a pair of photons
emitted in a decay cascade with partial "which path" ambiguity. We describe an
appropriate entanglement distillation scheme which works also for certain
random cascades. The qualitative features of the distilled entanglement are
presented in a two dimensional "phase diagram". The theory is applied to the
quantum tomography of the decay cascade of a biexciton in a semiconductor
quantum dot. Agreement with experiment is obtained
Radiative cascades in charged quantum dots
We measured, for the first time, two photon radiative cascades due to
sequential recombination of quantum dot confined electron hole pairs in the
presence of an additional spectator charge carrier. We identified direct, all
optical cascades involving spin blockaded intermediate states, and indirect
cascades, in which non radiative relaxation precedes the second recombination.
Our measurements provide also spin dephasing rates of confined carriers.Comment: 4 pages, 3 figure
Two-photon- photoluminescence excitation spectroscopy of single quantum-dots
We present experimental and theoretical study of single semiconductor quantum
dots excited by two non-degenerate, resonantly tuned variably polarized lasers.
The first laser is tuned to excitonic resonances. Depending on its polarization
it photogenerates a coherent single exciton state. The second laser is tuned to
biexciton resonances. By scanning the energy of the second laser for various
polarizations of the two lasers, while monitoring the emission from the
biexciton and exciton spectral lines, we map the biexciton photoluminescence
excitation spectra. The resonances rich spectra of the second photon absorption
are analyzed and fully understood in terms of a many carrier theoretical model
which takes into account the direct and exchange Coulomb interactions between
the quantum confined carriers.Comment: Accepted for publication in PR
Polarization memory in single Quantum Dots
We measured the polarization memory of excitonic and biexcitonic optical
transitions from single quantum dots at either positive, negative or neutral
charge states. Positive, negative and no circular or linear polarization memory
was observed for various spectral lines, under the same quasi-resonant
excitation below the wetting layer band-gap. We developed a model which
explains both qualitatively and quantitatively the experimentally measured
polarization spectrum for all these optical transitions. We consider quite
generally the loss of spin orientation of the photogenerated electron-hole pair
during their relaxation towards the many-carrier ground states. Our analysis
unambiguously demonstrates that while electrons maintain their initial spin
polarization to a large degree, holes completely dephase.Comment: 6 pages, 4 figure
Excitation spectroscopy of single quantum dots at tunable positive, neutral and negative charge states
We present a comprehensive study of the optical transitions and selection
rules of variably charged single self-assembled InAs/GaAs quantum dots. We
apply high resolution polarization sensitive photoluminescence excitation
spectroscopy to the same quantum dot for three different charge states: neutral
and negatively or positively charged by one additional electron or hole. From
the detailed analysis of the excitation spectra, a full understanding of the
single-carrier energy levels and the interactions between carriers in these
levels is extracted for the first time.Comment: 8 pages, 5 figure
Complete control of a matter qubit using a single picosecond laser pulse
We demonstrate for the first time that a matter physical two level system, a
qubit, can be fully controlled using one ultrafast step. We show that the spin
state of an optically excited electron, an exciton, confined in a quantum dot,
can be rotated by any desired angle, about any desired axis, during such a
step. For this we use a single, resonantly tuned, picosecond long, polarized
optical pulse. The polarization of the pulse defines the rotation axis, while
the pulse detuning from a non-degenerate absorption resonance, defines the
magnitude of the rotation angle. We thereby achieve a high fidelity, universal
gate operation, applicable to other spin systems, using only one short optical
pulse. The operation duration equals the pulse temporal width, orders of
magnitude shorter than the qubit evolution life and coherence times.Comment: main text: 4 pages, 3 figures Supplemental material: 3 pages, 1
figur
Radiative cascade from quantum dot metastable spin-blockaded biexciton
We detect a novel radiative cascade from a neutral semiconductor quantum dot.
The cascade initiates from a metastable biexciton state in which the holes form
a spin-triplet configuration, Pauli-blockaded from relaxation to the
spin-singlet ground state. The triplet biexciton has two photon-phonon-photon
decay paths. Unlike in the singlet-ground state biexciton radiative cascade, in
which the two photons are co-linearly polarized, in the triplet biexciton
cascade they are crosslinearly polarized. We measured the two-photon
polarization density matrix and show that the phonon emitted when the
intermediate exciton relaxes from excited to ground state, preserves the
exciton's spin. The phonon, thus, does not carry with it any which-path
information other than its energy. Nevertheless, entanglement distillation by
spectral filtering was found to be rather ineffective for this cascade. This
deficiency results from the opposite sign of the anisotropic electron-hole
exchange interaction in the excited exciton relative to that in the ground
exciton.Comment: 6 pages, 4 figure
Optically induced rotation of a quantum dot exciton spin
We demonstrate control over the spin state of a semiconductor quantum dot
exciton using a polarized picosecond laser pulse slightly detuned from a
biexciton resonance. The control pulse follows an earlier pulse, which
generates an exciton and initializes its spin state as a coherent superposition
of its two non-degenerate eigenstates. The control pulse preferentially couples
one component of the exciton state to the biexciton state, thereby rotating the
exciton's spin direction. We detect the rotation by measuring the polarization
of the exciton spectral line as a function of the time-difference between the
two pulses. We show experimentally and theoretically how the angle of rotation
depends on the detuning of the second pulse from the biexciton resonance.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
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