Photon Sorting, Efficient Bell Measurements and a Deterministic CZ Gate using a Passive Two-level Nonlinearity

Although the strengths of optical non-linearities available experimentally have been rapidly increasing in recent years, significant challenges remain to using such non-linearities to produce useful quantum devices such as efficient optical Bell state analysers or universal quantum optical gates. Here we describe a new approach that avoids the current limitations by combining strong non-linearities with active Gaussian operations in efficient protocols for Bell state analysers and Controlled-Sign gates

Large quantum dots with small oscillator strength

We have measured the oscillator strength and quantum efficiency of excitons confined in large InGaAs quantum dots by recording the spontaneous emission decay rate while systematically varying the distance between the quantum dots and a semiconductor-air interface. The size of the quantum dots is measured by in-plane transmission electron microscopy and we find average in-plane diameters of 40 nm. We have calculated the oscillator strength of excitons of that size and predict a very large oscillator strength due to Coulomb effects. This is in stark contrast to the measured oscillator strength, which turns out to be much below the upper limit imposed by the strong confinement model. We attribute these findings to exciton localization in local potential minima arising from alloy intermixing inside the quantum dots.Comment: 4 pages, 3 figures, submitte

Decay dynamics of quantum dots influenced by the local density of optical states of two-dimensional photonic crystal membranes

We have performed time-resolved spectroscopy on InAs quantum dot ensembles in photonic crystal membranes. The influence of the photonic crystal is investigated by varying the lattice constant systematically. We observe a strong slow down of the quantum dots' spontaneous emission rates as the two-dimensional bandgap is tuned through their emission frequencies. The measured band edges are in full agreement with theoretical predictions. We characterize the multi-exponential decay curves by their mean decay time and find enhancement of the spontaneous emission at the bandgap edges and strong inhibition inside the bandgap in good agreement with local density of states calculations.Comment: 9 pages (preprint), 3 figure

Optoelectronic cooling of mechanical modes in a semiconductor nanomembrane

Optical cavity cooling of mechanical resonators has recently become a research frontier. The cooling has been realized with a metal-coated silicon microlever via photo-thermal force and subsequently with dielectric objects via radiation pressure. Here we report cavity cooling with a crystalline semiconductor membrane via a new mechanism, in which the cooling force arises from the interaction between the photo-induced electron-hole pairs and the mechanical modes through the deformation potential coupling. The optoelectronic mechanism is so efficient as to cool a mode down to 4 K from room temperature with just 50 uW of light and a cavity with a finesse of 10 consisting of a standard mirror and the sub-wavelength-thick semiconductor membrane itself. The laser-cooled narrow-band phonon bath realized with semiconductor mechanical resonators may open up a new avenue for photonics and spintronics devices.Comment: 5 pages, 4 figure

Quantum properties of transverse pattern formation in second-harmonic generation

We investigate the spatial quantum noise properties of the one dimensional transverse pattern formation instability in intra-cavity second-harmonic generation. The Q representation of a quasi-probability distribution is implemented in terms of nonlinear stochastic Langevin equations. We study these equations through extensive numerical simulations and analytically in the linearized limit. Our study, made below and above the threshold of pattern formation, is guided by a microscopic scheme of photon interaction underlying pattern formation in second-harmonic generation. Close to the threshold for pattern formation, beams with opposite direction of the off-axis critical wave numbers are shown to be highly correlated. This is observed for the fundamental field, for the second harmonic field and also for the cross-correlation between the two fields. Nonlinear correlations involving the homogeneous transverse wave number, which are not identified in a linearized analysis, are also described. The intensity differences between opposite points of the far fields are shown to exhibit sub-Poissonian statistics, revealing the quantum nature of the correlations. We observe twin beam correlations in both the fundamental and second-harmonic fields, and also nonclassical correlations between them.Comment: 18 pages, 17 figures, submitted to Phys. Rev.

Microscopic theory of phonon-induced effects on semiconductor quantum dot decay dynamics in cavity QED

We investigate the influence of the electron-phonon interaction on the decay dynamics of a quantum dot coupled to an optical microcavity. We show that the electron-phonon interaction has important consequences on the dynamics, especially when the quantum dot and cavity are tuned out of resonance, in which case the phonons may add or remove energy leading to an effective non-resonant coupling between quantum dot and cavity. The system is investigated using two different theoretical approaches: (i) a second-order expansion in the bare phonon coupling constant, and (ii) an expansion in a polaron-photon coupling constant, arising from the polaron transformation which allows an accurate description at high temperatures. In the low temperature regime we find excellent agreement between the two approaches. An extensive study of the quantum dot decay dynamics is performed, where important parameter dependencies are covered. We find that in general the electron-phonon interaction gives rise to a greatly increased bandwidth of the coupling between quantum dot and cavity. At low temperature an asymmetry in the quantum dot decay rate is observed, leading to a faster decay when the quantum dot has a larger energy than to the cavity. We explain this as due to the absence of phonon absorption processes. Furthermore, we derive approximate analytical expressions for the quantum dot decay rate, applicable when the cavity can be adiabatically eliminated. The expressions lead to a clear interpretation of the physics and emphasizes the important role played by the effective phonon density, describing the availability of phonons for scattering, in quantum dot decay dynamics. Based on the analytical expressions we present the parameter regimes where phonon effects are expected to be important. Also, we include all technical developments in appendices.Comment: published PRB version, comments are very welcom

Single-photon nonlinear optics with a quantum dot in a waveguide

Strong nonlinear interactions between photons enable logic operations for both classical and quantum-information technology. Unfortunately, nonlinear interactions are usually feeble and therefore all-optical logic gates tend to be inefficient. A quantum emitter deterministically coupled to a propagating mode fundamentally changes the situation, since each photon inevitably interacts with the emitter, and highly correlated many-photon states may be created . Here we show that a single quantum dot in a photonic-crystal waveguide can be utilized as a giant nonlinearity sensitive at the single-photon level. The nonlinear response is revealed from the intensity and quantum statistics of the scattered photons, and contains contributions from an entangled photon-photon bound state. The quantum nonlinearity will find immediate applications for deterministic Bell-state measurements and single-photon transistors and paves the way to scalable waveguide-based photonic quantum-computing architectures

Teleportation of continuous quantum variables using squeezed-state entanglement

We report recent developments in our experiment to teleport light beams by utilizing Einstein-Podolsky-Rosen (EPR) entanglement for continuous quantum variables. We describe details of our experimental apparatus, including the generation of EPR entanglement from squeezed states of light. In addition, we have developed an explicit model for the teleportation of coherent states that includes the effect of diverse loss factors and limited degrees of entanglement, and that enables us to project the possibilities for achieving yet higher fidelities beyond the currently achieved value of 62% with our apparatus. Propects for other teleportation schemes will also be discussed

Entanglement properties of a quantum-dot biexciton cascade in a chiral nanophotonic waveguide

We analyse the entanglement properties of deterministic path-entangled photonic states generated by coupling the emission of a quantum-dot biexciton cascade to a chiral nanophotonic waveguide, as implemented by {\O}stfeldt et al. [PRX Quantum 3, 020363 (2022)]. We model the degree of entanglement through the concurrence of the two-photon entangled state in the presence of realistic experimental imperfections. The model accounts for imperfect chiral emitter-photon interactions in the waveguide and the asymmetric coupling of the exciton levels introduced by fine-structure splitting along with time-jitter in the detection of photons. The analysis shows that the approach offers a promising platform for deterministically generating entanglement in integrated nanophotonic systems in the presence of realistic experimental imperfections.Comment: 12 pages, 5 figure