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Photonics for Quantum (PfQ)

Implementation of a universal fine-structure eraser for quantum dots

Maeve Wentland, Sonell Malik, Andreas Fognini, Val Zwiller, Dan Dalacu, Philip J. Poole, Michael E. Reimer

Published Jul 19, 2023• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.2673257

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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.
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%.
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.
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.
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.
To generate entangled photon pairs via quantum dots (QDs), the exciton fine-structure splitting (FSS) must be comparable to the exciton homogeneous linewidth. Yet in the (In,Ga)As/GaAs QD, the intrinsic FSS is about a few tens microeV. To achieve photon entanglement, it is necessary to cherry-pick a sample with extremely small FSS from a large number of samples or to apply a strong in-plane magnetic field. Using theoretical modeling of the fundamental causes of FSS in QDs, we predict that the intrinsic FSS of InAs/InP QDs is an order of magnitude smaller than that of InAs/GaAs dots, and, better yet, their excitonic gap matches the 1.55 microm fiber optic wavelength and, therefore, offers efficient on-demand entangled photon emitters for long distance quantum communication.
Generation of polarization-entangled photons from quantum dots via the biexciton-exciton recombination cascade is complicated by the presence of an energy spitting between the intermediate excitonic levels, which severely degrades the quality of the entangled photon source. In this paper we present a novel, conceptually simple and straightforward proposal for restoring the entanglement of said source by applying a cascade of time-dependent operations on the emitted photons. This is in striking contrast with the techniques usually employed, that act on the quantum emitter itself in order to remove the fine structure splitting at its root. The feasibility of the implementation with current technology is discussed, and the robustness of the proposed compensation scheme with respect to imperfections of the experimental apparatus is evaluated via a series of Monte Carlo simulations.