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Bright nanoscale source of deterministic entangled photon pairs violating Bell's inequality
KTH, School of Engineering Sciences (SCI), Applied Physics, Quantum Nano Photonics. Delft University of Technology, Netherlands.ORCID iD: 0000-0002-5814-7510
KTH, School of Engineering Sciences (SCI), Applied Physics, Quantum Nano Photonics. Delft University of Technology, Netherlands.
KTH, School of Engineering Sciences (SCI), Applied Physics, Quantum Nano Photonics. Delft University of Technology, Netherlands.
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2017 (English)In: Scientific Reports, E-ISSN 2045-2322, Vol. 7, no 1, article id 1700Article in journal (Refereed) Published
Abstract [en]

Global, secure quantum channels will require efficient distribution of entangled photons. Long distance, low-loss interconnects can only be realized using photons as quantum information carriers. However, a quantum light source combining both high qubit fidelity and on-demand bright emission has proven elusive. Here, we show a bright photonic nanostructure generating polarization-entangled photon pairs that strongly violates Bell's inequality. A highly symmetric InAsP quantum dot generating entangled photons is encapsulated in a tapered nanowire waveguide to ensure directional emission and efficient light extraction. We collect similar to 200 kHz entangled photon pairs at the first lens under 80 MHz pulsed excitation, which is a 20 times enhancement as compared to a bare quantum dot without a photonic nanostructure. The performed Bell test using the Clauser-Horne-Shimony-Holt inequality reveals a clear violation (S-CHSH > 2) by up to 9.3 standard deviations. By using a novel quasi-resonant excitation scheme at the wurtzite InP nanowire resonance to reduce multi-photon emission, the entanglement fidelity (F = 0.817 +/- 0.002) is further enhanced without temporal post-selection, allowing for the violation of Bell's inequality in the rectilinear-circular basis by 25 standard deviations. Our results on nanowire-based quantum light sources highlight their potential application in secure data communication utilizing measurement-device-independent quantum key distribution and quantum repeater protocols.

Place, publisher, year, edition, pages
Nature Publishing Group, 2017. Vol. 7, no 1, article id 1700
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:kth:diva-208820DOI: 10.1038/s41598-017-01509-6ISI: 000400886100040PubMedID: 28490728Scopus ID: 2-s2.0-85019206401OAI: oai:DiVA.org:kth-208820DiVA, id: diva2:1108972
Funder
EU, European Research Council, 601126 210EU, FP7, Seventh Framework Programme
Note

QC 20170613

Available from: 2017-06-13 Created: 2017-06-13 Last updated: 2024-03-18Bibliographically approved
In thesis
1. Correlation spectroscopy with epitaxial quantum dots: Single-photons alone in the dark.
Open this publication in new window or tab >>Correlation spectroscopy with epitaxial quantum dots: Single-photons alone in the dark.
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Korrelationspektroskopi med epitaxiella kvantprickar
Abstract [en]

The advent of quantum computation promises exciting advances, not only in fields like medicine and metrology, but many industries that rely on parameter-heavy calculations or simulation of molecular interaction. At the same time Shor's algorithm for quantum computers presents a threat to current asymmetric encryption protocols used in everyday communication. Flying qubits, i.e. single-photons, can help mitigate this problem via quantum key distribution, which is insusceptible to an increase in computational power. In addition, they can link quantum computers, forming a quantum network, so that quantum states can be transmitted between them. Sources of flying qubits need good performance in key metrics like single--photon purity, repetition rate, indistinguishability and brightness to become useful in these applications. They should ideally emit strongly entangled pairs of photons and be matched to other quantum technologies in bandwidth and emission energy.

In this thesis the emission characteristics of single epitaxial quantum dots, the single-photon source of our choice, are investigated. Strongly entangled photon-pair emission is demonstrated for three different quantum-dot systems:

  • InAsP quantum dots embedded in nanowire waveguides, suitable for integration into photonic circuits, show emission of single photons and entangled photon pairs under non-resonant and quasi-resonant excitation. Violation of Bell's inequality is demonstrated using the traditional set of polarization angles.
  • GaAs quantum dots grown in droplet--etched nanoholes are tested with two resonant excitation methods: Using resonance fluorescence, near-unity indistinguishability and re-excitation limited single-photon purity, albeit not simultaneously with laser-inherited bandwidth, are measured. Using two-photon resonant excitation we set a new standard for single-photon purity, can generate pairs of entangled photons but suffer from reduced indistinguishability. In addition, nanofabrication of paraboloid shaped reflectors for enhanced extraction efficiency of photons and strain-tuning of the emission energy into resonance with the 87Rb D1-line are demonstrated.
  • Strain-tunable InAs quantum dots emitting in the telecom C-band are investigated under above-band excitation and two different resonant two-photon excitation techniques, all of which cause pure single-photon emission. Using the robust phonon-assisted two-photon excitation technique, close-to ideal entangled photon-pair emission is demonstrated.

For many of these findings photon arrival times were recorded over many hours with temporal precision on the order of 10 ps. We have developed a user-friendly, yet versatile piece of software in order to extract as much information as possible from this vast amount of data.

These results will facilitate integration of quantum dot based single- and entangled-photon sources into future quantum networks and quantum key distribution systems.

Abstract [sv]

Framstegen inom kvantberäkning förutspår spännande framsteg, inte enbart inom områden såsom medicin och metrologi, utan även inom många industrier som förlitar sig på parametertunga beräkningar eller simulering av molekylinteraktioner. Samtidigt utgör Shors algoritm för kvantdatorer ett hot mot aktuella asymmetriska krypteringsprotokoll som används i vardagskommunikation. Flygande kvantbitar, även känt som singelfotoner, kan hjälpa till att mildra detta problem genom att använda kvantnyckeldistribution, som är immuna mot en ökning av beräkningskraften. Dessutom tillåter de sammanlänkning av kvantdatorer till kvantnätverk, där kvanttillstånd kan överföras mellan kvantdatorerna. Ljuskällor som producerar flygande kvantbitar behöver påvisa högkvalitativa egenskaper i emittans av singelfotoner, repetitionsfrekvenens, oskiljbarhet och ljusstyrka för att kunna användas till dessa applikationer. Idealt vore om de emitterade starkt sammanflätade par av fotoner som kan anpassas i bandbredd och utsläppsenergi.

Denna avhandling går ut på att karakterisera den singelfotonkälla vi valt att fokusera på, nämligen enskilda epitaxiala kvantprickar. Emission av starkt sammanflätade fotonpar demonstreras för tre olika kvantpunktsystem:

  • InAsP-kvantprickar inbäddade i ledande nanotråd, redo att integreras i fotonkretsar, som visar singel-fotonemission under ickeresonant och kvasiresonant excitation. Brott mot Bells olikhet demonstreras genom att använda traditionella set av polarisationsvinklar.
  • GaAs kvantprickar som odlats med hjälp av droppetsning i hål av nanostorlek testas med två resonansexcitationsmetoder: Med hjälp av resonansfluorescens mäts fullständig oskiljbarhet och återexcitationsbegränsad renhet av singelfotoner, dock inte samtidigt som den nedärvda bandbredden från lasern. Med hjälp av tvåfotonresonant excitation sätter vi en ny standard hos singelfotoners renhet, där vi kan generera sammanflätade fotoner med reducerad oskiljbarhet. Dessutom påvisar vi hur paraboloidformade nanofabricerade reflektorer ger en förbättrad extraktion av fotoner samt att fotonenergin kan justeras genom att sträcka ut kvantpricken så att dess emission är resonant med 87Rb D1-linjen.
  • InAs kvantprickar som kan justeras genom sträckning och emitterar fotoner i telekom C-bandet undersöks, dels genom användning av excitation ovanför bandet och dels genom två olika resonanta tvåfotonexcitationstekniker, som alla orsakar ren singelfotonemission. Genom användning av den robusta fononassisterade tvåfotonsexcitationstekniken, kan emission av nära idealt sammanflätade fotonpar demonstreras.

För många av dessa upptäckter registreras fotonernas ankomsttider över flera timmar med tidsprecision i storleksordningen av 10 ps. Vi har utvecklat en användarvänlig och mångsidig mjukvara för att kunna extrahera så mycket information som möjligt från denna enorma datamängd.

Dessa resultat kommer att underlätta integration av kvantprickbaserade singelfoton-- och sammanflätningskällor i framtida kvantnätverk och kvantnyckeldistributionssystem.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2020. p. 59
Series
TRITA-SCI-FOU ; 2020:18
Keywords
quantum dots, single--photons, indistinguishability, entanglement
National Category
Condensed Matter Physics
Research subject
Physics, Optics and Photonics
Identifiers
urn:nbn:se:kth:diva-273213 (URN)978-91-7873-554-9 (ISBN)
Public defence
2020-06-05, via Zoom https://kth-se.zoom.us/webinar/register/WN_C4EGweeTS8S2K68zlRomow, If you do not have access to a computer, you can contact haviland@kth.se for further information., 09:00 (English)
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QC 2020-05-14

Available from: 2020-05-14 Created: 2020-05-11 Last updated: 2022-06-26Bibliographically approved

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Jöns, Klaus D.Schweickert, LucasVersteegh, Marijn A. M.Zwiller, Val

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