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Correlation spectroscopy with epitaxial quantum dots

Single-photons alone in the dark.

Time: Fri 2020-06-05 09.00

Location: via Zoom, If you do not have access to a computer, you can contact for further information., (English)

Subject area: Physics, Optics and Photonics

Doctoral student: Lucas Schweickert , Kvant- och biofotonik

Opponent: Professor Richard J. Warburton, University of Basel, Department of Physics

Supervisor: Val Zwiller, Kvant- och biofotonik; Klaus D. Jöns, Kvant- och biofotonik

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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.