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Correlation spectroscopy with epitaxial quantum dots: Single-photons alone in the dark.
KTH, School of Engineering Sciences (SCI), Applied Physics, Quantum and Biophotonics.ORCID iD: 0000-0002-1858-007x
2020 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Korrelationspektroskopi med epitaxiella kvantprickar (Swedish)
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 [en]
quantum dots, single--photons, indistinguishability, entanglement
National Category
Condensed Matter Physics
Research subject
Physics, Optics and Photonics
Identifiers
URN: urn:nbn:se:kth:diva-273213ISBN: 978-91-7873-554-9 (print)OAI: oai:DiVA.org:kth-273213DiVA, id: diva2:1429549
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)
Opponent
Supervisors
Note

QC 2020-05-14

Available from: 2020-05-14 Created: 2020-05-11 Last updated: 2020-06-03Bibliographically approved
List of papers
1. Bright nanoscale source of deterministic entangled photon pairs violating Bell's inequality
Open this publication in new window or tab >>Bright nanoscale source of deterministic entangled photon pairs violating Bell's inequality
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2017 (English)In: Scientific Reports, ISSN 2045-2322, 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
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-208820 (URN)10.1038/s41598-017-01509-6 (DOI)000400886100040 ()28490728 (PubMedID)2-s2.0-85019206401 (Scopus ID)
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: 2020-05-11Bibliographically approved
2. On-demand generation of background-free single photons from a solid-state source
Open this publication in new window or tab >>On-demand generation of background-free single photons from a solid-state source
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2018 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, no 9, article id 093106Article in journal (Refereed) Published
Abstract [en]

True on-demand high-repetition-rate single-photon sources are highly sought after for quantum information processing applications. However, any coherently driven two-level quantum system suffers from a finite re-excitation probability under pulsed excitation, causing undesirable multi-photon emission. Here, we present a solid-state source of on-demand single photons yielding a raw second-order coherence of g((2)) (0) = (7.5 +/- 1.6) x 10(-5) without any background subtraction or data processing. To this date, this is the lowest value of g((2)) (0) Peported for any single-photon source even compared to the previously reported best background subtracted values. We achieve this result on GaAs/AlGaAs quantum dots embedded in a low-Q planar cavity by employing (i) a two-photon excitation process and (ii) a filtering and detection setup featuring two superconducting single-photon detectors with ultralow dark-count rates of (0.0056 +/- 0.0007) s(-1) and (0.017 +/- 0.001) s(-1), respectively. Re-excitation processes are dramatically suppressed by (i), while (ii) removes false coincidences resulting in a negligibly low noise floor.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2018
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-225217 (URN)10.1063/1.5020038 (DOI)000427022500038 ()2-s2.0-85042689547 (Scopus ID)
Funder
Swedish Research Council, 638-2013-7152EU, European Research Council, 679183; 307687
Note

QC 20180404

Available from: 2018-04-04 Created: 2018-04-04 Last updated: 2020-05-11Bibliographically approved
3. A stable wavelength-tunable triggered source of single photons and cascaded photon pairs at the telecom C-band
Open this publication in new window or tab >>A stable wavelength-tunable triggered source of single photons and cascaded photon pairs at the telecom C-band
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2018 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, no 17, article id 173102Article in journal (Refereed) Published
Abstract [en]

The implementation of fiber-based long-range quantum communication requires tunable sources of single photons at the telecom C-band. Stable and easy-to-implement wavelength-tunability of individual sources is crucial to (i) bring remote sources into resonance, (ii) define a wavelength standard, and (iii) ensure scalability to operate a quantum repeater. So far, the most promising sources for true, telecom single photons are semiconductor quantum dots, due to their ability to deterministically and reliably emit single and entangled photons. However, the required wavelength-tunability is hard to attain. Here, we show a stable wavelength-tunable quantum light source by integrating strain-released InAs quantum dots on piezoelectric substrates. We present triggered single-photon emission at 1.55 mu m with a multi-photon emission probability as low as 0.097, as well as photon pair emission from the radiative biexciton-exciton cascade. We achieve a tuning range of 0.25 nm which will allow us to spectrally overlap remote quantum dots or tuning distant quantum dots into resonance with quantum memories. This opens up realistic avenues for the implementation of photonic quantum information processing applications at telecom wavelengths. 

Place, publisher, year, edition, pages
AMER INST PHYSICS, 2018
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-227769 (URN)10.1063/1.5021483 (DOI)000431072800036 ()2-s2.0-85046072975 (Scopus ID)
Funder
Swedish Research Council, 638-2013-7152VINNOVA
Note

QC 20180514

Available from: 2018-05-14 Created: 2018-05-14 Last updated: 2020-05-11Bibliographically approved
4. Resonance Fluorescence of GaAs Quantum Dots with Near-Unity Photon Indistinguishability
Open this publication in new window or tab >>Resonance Fluorescence of GaAs Quantum Dots with Near-Unity Photon Indistinguishability
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2019 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 19, no 4, p. 2404-2410Article in journal (Refereed) Published
Abstract [en]

Photonic quantum technologies call for scalable quantum light sources that can be integrated, while providing the end user with single and entangled photons on demand. One promising candidate is strain free GaAs/A1GaAs quantum dots obtained by aluminum droplet etching. Such quantum dots exhibit ultra low multi-photon probability and an unprecedented degree of photon pair entanglement. However, different to commonly studied InGaAs/GaAs quantum dots obtained by the Stranski-Krastanow mode, photons with a near-unity indistinguishability from these quantum emitters have proven to be elusive so far. Here, we show on-demand generation of near-unity indistinguishable photons from these quantum emitters by exploring pulsed resonance fluorescence. Given the short intrinsic lifetime of excitons and trions confined in the GaAs quantum dots, we show single photon indistinguishability with a raw visibility of V-raw = (95.0(-6.1)(+5.0))%, without the need for Purcell enhancement. Our results represent a milestone in the advance of GaAs quantum dots by demonstrating the final missing property standing in the way of using these emitters as a key component in quantum communication applications, e.g., as quantum light sources for quantum repeater architectures.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2019
Keywords
Semiconductor quantum dot, resonance fluorescence, indistinguishable photons, Al droplet etching
National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-251341 (URN)10.1021/acs.nanolett.8b05132 (DOI)000464769100028 ()30862165 (PubMedID)2-s2.0-85063372001 (Scopus ID)
Note

QC 20190521

Available from: 2019-05-21 Created: 2019-05-21 Last updated: 2020-05-11Bibliographically approved
5. GaAs Quantum Dot in a Parabolic Microcavity Tuned to Rb-87 D-1
Open this publication in new window or tab >>GaAs Quantum Dot in a Parabolic Microcavity Tuned to Rb-87 D-1
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2020 (English)In: ACS Photonics, E-ISSN 2330-4022, Vol. 7, no 1, p. 29-35Article in journal (Refereed) Published
Abstract [en]

We develop a structure to efficiently extract photons emitted by a GaAs quantum dot tuned to rubidium. For this, we employ a broadband microcavity with a curved gold backside mirror that we fabricate by a combination of photoresist reflow, dry reactive ion etching in an inductively coupled plasma, and selective wet chemical etching. Precise reflow and etching control allows us to achieve a parabolic backside mirror with a short focal distance of 265 nm. The fabricated structures yield a predicted (measured) collection efficiency of 63% (12%), an improvement by more than 1 order of magnitude compared to unprocessed samples. We then integrate our quantum dot parabolic microcavities onto a piezoelectric substrate capable of inducing a large in-plane biaxial strain. With this approach, we tune the emission wavelength by 0.5 nm/kV, in a dynamic, reversible, and linear way, to the rubidium D-1 line (795 nm).

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2020
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-267509 (URN)10.1021/acsphotonics.9b01243 (DOI)000508475800003 ()32025532 (PubMedID)2-s2.0-85077663113 (Scopus ID)
Note

QC 20200409

Available from: 2020-04-09 Created: 2020-04-09 Last updated: 2020-05-11Bibliographically approved
6. On-demand generation of entangled photon pairs in the telecom C-band for fiber-based quantum networks
Open this publication in new window or tab >>On-demand generation of entangled photon pairs in the telecom C-band for fiber-based quantum networks
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

On–demand sources of entangled photons for the transmission of quantum information in the telecom C–band are required to realize fiber–based quantum networks. So far, non–deterministic sources of quantum states of light were used for long distance entanglement distribution in this lowest loss wavelength range. However, they are fundamentally limited in either efficiency or security due to their Poissonian emission statistics. Here, we show on–demand generation of entangled photon pairs in the telecom C-band by an InAs/GaAs semiconductor quantum dot. Using a robust phonon–assisted excitation scheme we measurea concurrence of 91.4% and a fidelity of 95.2% to Φ+ . On–demand generation of polarization entangled photons will enable secure quantum communication in fiber–based networks.Furthermore, applying this excitation scheme to several remote quantum dots tuned into resonance will enable first on–demand entanglement distribution over large distances for scalable real–life quantum applications.

National Category
Condensed Matter Physics
Research subject
Physics, Optics and Photonics
Identifiers
urn:nbn:se:kth:diva-273203 (URN)
Note

QC 20200512

Available from: 2020-05-11 Created: 2020-05-11 Last updated: 2020-05-12Bibliographically approved
7. The crux of using the cascaded emission of a 3-level quantum ladder system to generate indistinguishable photons
Open this publication in new window or tab >>The crux of using the cascaded emission of a 3-level quantum ladder system to generate indistinguishable photons
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We investigate the degree of indistinguishability of cascaded photons emitted from a 3–level quantum ladder system; in our case the biexciton–exciton cascade of semiconductor quantum dots. For the 3–level quantum ladder system we theoretically demonstrate that the indistinguishability is inherently limited for both emitted photons and determined by the ratio of the lifetimes of the excited and intermediate states. We experimentally confirm this finding by comparing the quantum interference visibility of non–cascaded emission and cascaded emission from the same semiconductor quantum dot. Quantum optical simulations produce very good agreement with the measurements and allow to explore a large parameter space. Based on our model, we propose photonic structures too ptimize the lifetime ratio and overcome the limited indistinguishability of cascaded photon emission from a 3–level quantum ladder system.

National Category
Condensed Matter Physics
Research subject
Physics, Optics and Photonics
Identifiers
urn:nbn:se:kth:diva-273204 (URN)
Note

QC 20200512

Available from: 2020-05-11 Created: 2020-05-11 Last updated: 2020-05-12Bibliographically approved
8. Coherent scattering: either sub-natural linewidth or anti-bunched light
Open this publication in new window or tab >>Coherent scattering: either sub-natural linewidth or anti-bunched light
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Epitaxial quantum dots have emerged as one of the best single–photon sources, not only for applications in photonic quantum technologies but also for testing fundamental properties of quantum optics. One intriguing observation in this area is the emission of photons with subnatural–linewidth from a two-level system under resonant continuous wave excitation. In particular, an open question is whether these subnatural–linewidth photons exhibit simultaneously single–photon characteristics, i.e. show antibunching as a signature of single-photon emission. Here, we demonstrate that this simultaneous observation of subnatural–linewidth and single photoncharacter is not possible with simple resonant excitation. First, we independently confirm single–photon character and subnatural–linewidth by demonstrating antibunching in a Hanbury Brown and Twiss type setup and using high-resolution spectroscopy, respectively. However, when filtering the coherently scattered photons with filter bandwidths on the order of the homogeneous linewidth of the excited state of the two-level system, the antibunching dip vanishes in the correlation measurement. Our experimental work is consistent with recent theoretical findings, and can be explained by a fundamental model considering higher-order photon correlations.

National Category
Condensed Matter Physics
Research subject
Physics, Optics and Photonics
Identifiers
urn:nbn:se:kth:diva-273205 (URN)
Note

QC 20200512

Available from: 2020-05-11 Created: 2020-05-11 Last updated: 2020-05-12Bibliographically approved
9. Efficient toolbox for correlation of time tagged measurements
Open this publication in new window or tab >>Efficient toolbox for correlation of time tagged measurements
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Extracting correlations from time-series data is a wide-spread analysing method for large data sets, giving insights in temporal dynamics over several orders ofmagnitudes. However, the efficient correlation extraction and processing of big data is still a challenge widely encountered, independent of the application and research field. In optics, correlations among photon detection events can often yield insight into underlying physical processes. The recent advent of time-tagging techniques for photon detection events with timing resolution compa-rable to the coherence and lifetimes of quantum emitters offers an alternative to the well established start-stop histograms obtained directly with correlation electronics. Here we introduce a versatile toolbox for analysis of time tagged data, enabling extraction of a wide range of information from one measurement. A user of our software can specify the desired analysis method using a combination of graphical and traditional programming. Automatically selecting an appropriate algorithm, a just-in-time compiler combines these two inputs into an intermediate representation, which is then compiled into assembly code optimized for the target computer’s architecture. This procedure optimizes for fast analysis of large time tag files at the cost of upfront compilation time while maintaining flexibility. Our program finds uses in single molecule, LIDAR, quantum entanglement and fluorescence correlation spectroscopy measurements, as well as quantum key distribution protocols in which data from remote detectors needs to be synchronized and correlated. Our software is optimized and modular, offering high processing speed and extensibility.

National Category
Software Engineering
Research subject
Physics, Optics and Photonics
Identifiers
urn:nbn:se:kth:diva-273207 (URN)
Note

QC 20200512

Available from: 2020-05-11 Created: 2020-05-11 Last updated: 2020-05-12Bibliographically approved

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