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Electro-optic rotating half-waveplate for a quantum dot fine-structure eraser

Simon Daley

Published Aug 28, 2019• 1 citation
OpenAlex

Abstract

Quantum dots are a promising source of entangled photon pairs. Recent advances have \nshown nearly dephasing-free entanglement from quantum dots embedded in semiconductor \nnanowires. An outstanding challenge with these sources is the presence of the so-called \n fine-structure splitting, which is the lifting of spin degeneracy of the exciton state due to \nquantum dot potential asymmetry. This fine-structure splitting causes the output state \nto precess rapidly, which is degrades the quality of measured entanglement due to nite \ndetector temporal response and is undesirable for applications where preparation of a \nconsistent state is needed. The effects of fine-structure splitting can be \\erased" once the \nphotons have been emitted using a flexible all-optical approach. This optical fine-structure \neraser scheme requires a rapidly rotating half-waveplate, which cannot be implemented \nwith commercially available off-the-shelf systems. This thesis presents the operation of \nan electro-optic modulator which can emulate a rotating half-waveplate at the required \nspeeds, and demonstrates frequency conversion with an efficiency of 92%.

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Quantum dots are a promising source of entangled photon pairs. Recent advances have \nshown nearly dephasing-free entanglement from quantum dots embedded in semiconductor \nnanowires. An outstanding challenge with these sources is the presence of the so-called \n fine-structure splitting, which is the lifting of spin degeneracy of the exciton state due to \nquantum dot potential asymmetry. This fine-structure splitting causes the output state \nto precess rapidly, which is degrades the quality of measured entanglement due to nite \ndetector temporal response and is undesirable for applications where preparation of a \nconsistent state is needed. The effects of fine-structure splitting can be \erased" once the \nphotons have been emitted using a flexible all-optical approach. This optical fine-structure \neraser scheme requires a rapidly rotating half-waveplate, which cannot be implemented \nwith commercially available off-the-shelf systems. This thesis presents the operation of \nan electro-optic modulator which can emulate a rotating half-waveplate at the required \nspeeds, and demonstrates frequency conversion with an efficiency of 92%.
Entangled photons are an important resource for quantum optics. Quantum dots are a source of on-demand and highly entangled photon pairs at a high repetition rate. However, fine structure splitting (FSS) in the biexciton-exciton cascade causes the photons to be emitted in a time-dependant state instead of an ideal Bell state. Current techniques to remove the FSS include applying a strain, electric, or magnetic field and require post-processing of the quantum dots which reduces device yield. We use a novel all-optical approach implemented by emulating a fast-rotating half-wave plate in a Lithium Niobate waveguide using a electro-optic modulator. This method allows us to frequency shift single photons and produce a time-independant entangled photon.
Highly entangled photon sources play a crucial role in advancing the capabilities of quantum networks. In this context, we introduce an advanced scheme aimed at improving entanglement of photons emitted from quantum dots based on the framework proposed by Fognini et al. (2018). We propose a setup with reduced physical footprint which employs one electro-optic modulator strategically to enhance entanglement, mitigating the detrimental effects of fine structure splitting (FSS) observed in quantum dots that contribute to the degradation of entanglement.
We analyze the degree of entanglement measurable from a quantum dot via the biexciton-exciton cascade as a function of the exciton fine-structure splitting and the detection time resolution. We show that the time-energy uncertainty relation provides means to measure a high entanglement even in presence of a finite fine-structure splitting when a detection system with high temporal resolution is employed. Still, in many applications it would be beneficial if the fine-structure splitting could be compensated to zero. To solve this problem, we propose an all-optical approach with rotating waveplates to erase this fine-structure splitting completely which should allow obtaining a high degree of entanglement with near-unity efficiency. Our optical approach is possible with current technology and is also compatible with any quantum dot showing fine-structure splitting. This bears the advantage that for example the fine-structure splitting of quantum dots in nanowires and micropillars can be directly compensated without the need for further sample processing.
Entanglement is a rare resource only a select few people worldwide can create. Its fundamentally non-classical behavior seeds a tree of ideas from which transformative technologies in information processing, communication, and imaging grow. Particularly interesting is entanglement between photons in a pair made by a quantum dot, the most competitive contender among quantum photonic devices. \n \nA strength of semiconductor quantum dots is their deterministic reliability and potential for production at scale. The associated price is anchored in their inherent flaw: sensitivity to imperfections in their shape puts the entanglement they generate in motion. Thus, unless a specific entangled photon pair is caught at just the right moment, it will appear as though it is in fact not entangled at all. \n \nIngenious methods for making quantum dots perfect again have been devised,almost exclusively built around reshaping the dot by physical, electric, or magnetic means. One completely different proposal stands out, which instead stops the entanglement's motion once it has already been created. As a purely optical technique it can be used for any quantum dot and is completely non-intrusive. This is the method of the rotating half-wave plate. \n \nThe entanglement between photons in a pair revolves up to a few billion times per second and halting it requires a correction one-half that rate. Physically spinning a crystal waveplate at that tremendous speed is impossible, but if the properties of a crystal are spun in an identical way such quickly varying entanglement can be restored to its starting point with ease. This is the principle of electro-optic modulation. \n \nThis thesis shows the very first demonstration of slowing the frequency of single photons in a stream by using an electro-optical rotating half-wave plate prototype. Initial results show a reduction in frequency by 127.75 million oscillations per second. Any quantum dot whose entanglement precesses no more quickly than 255.5 MHz can thus directly make use of the technique contained herein.
Entanglement resources are key ingredients of future quantum technologies. If they could be efficiently integrated into a semiconductor platform, a new generation of devices could be envisioned, whose quantum-mechanical functionalities are controlled via the mature semiconductor technology. Epitaxial quantum dots (QDs) embedded in diodes would embody such ideal quantum devices, but a fine-structure splitting (FSS) between the bright exciton states lowers dramatically the degree of entanglement of the sources and hampers severely their real exploitation in the foreseen applications. In this work, we overcome this hurdle using strain-tunable optoelectronic devices, where any QD can be tuned for the emission of photon pairs featuring the highest degree of entanglement ever reported for QDs, with concurrence as high as 0.75 ± 0.02. Furthermore, we study the evolution of Bell's parameters as a function of FSS and demonstrate for the first time that filtering-free violation of Bell's inequalities requires the FSS to be smaller than 1 μeV. This upper limit for the FSS also sets the tuning range of exciton energies (∼1 meV) over which our device operates as an energy-tunable source of highly entangled photons. A moderate temporal filtering further increases the concurrence and the tunability of exciton energies up to 0.82 and 2 meV, respectively, though at the expense of 60% reduction of count rate.
We report on the observation of nearly maximally entangled photon pairs from semiconductor quantum dots, without resorting to postselection techniques. We use GaAs quantum dots integrated on a patterned piezoelectric actuator capable of suppressing the exciton fine structure splitting. By using a resonant two-photon excitation, we coherently drive the biexciton state and demonstrate experimentally that our device generates polarization-entangled photons with a fidelity of 0.978(5) and a concurrence of 0.97(1) taking into account the nonidealities stemming from the experimental setup. By combining fine-structure-dependent fidelity measurements and a theoretical model, we identify an exciton spin-scattering process as a possible residual decoherence mechanism. We suggest that this imperfection may be overcome using a modest Purcell enhancement so as to achieve fidelities >0.99, thus making quantum dots evenly matched with the best probabilistic entangled photon sources.