We have investigated the possibility of using polarized single-photons as information carriers in an optical fiber with the aim of achieving error-free information transmission despite photon loss. To this end, a series of experiments are performed over a 20 km metro quantum link, where we compared the performance of three different error correcting and error detecting codes, based on three mutually orthogonal states |H〉, |V〉 and |0, alluding to two linearly polarized single photon states and the vacuum state, respectively. The experiment is carried out using a quantum dot single-photon source with g(2)(0) = 0.0053 ± 0.001, whose emitted photons are modulated and time-binned into code blocks. A 32 × 32 pixel black and white image is transmitted, demonstrating that error-correction can reduce errors significantly in this channel. Our method draws strength from the fact that the photon loss channel is asymmetric, allowing for loss errors to be efficiently pinpointed.

A new method for efficient, high-quality randomness extraction is presented. The method relies on quantum processes such as the emission of single photons and their subsequent detection, where each detection event has an associated detection time. By establishing a list of time differences between a fixed number of events, a unique order can be established. We note that, by utilising the number of ways to order the resulting list of time differences between the quantum events, the efficiency can be increased many-fold compared to current methods. The method delivers fundamentally uniform randomness and therefore, in principle, does not need debiasing.

Quantum communication networks are used for QKD and metrological applications. We present research connecting two nodes ≈ 20 kilometers apart over the municipal fiber network using semiconductor quantum dots emitting at 1550 nm.

Quantum communication networks are used for QKD and metrological applications. We present research connecting two nodes ˜ 20 kilometers apart over the municipal fiber network using semiconductor quantum dots emitting at 1550 nm.

Quantum communication networks are used for QKD and metrological applications. We present research connecting two nodes ≈ 20 kilometers apart over the municipal fiber network using semiconductor quantum dots emitting at 1550 nm.

Photonic integrated circuits (PICs) present significant benefits with respect to table-top optical systems regarding footprint, stability, and power consumption. Among the materials used to fabricate PICs, thin film lithium niobate (TFLN) is one of the most attractive ones, as its χ(2) nonlinearity and electro-optic properties allow to implement on-chip light generation and routing [1]. On-chip detection of light has also been demonstrated on TFLN, based on the waveguide integration of superconducting nanowire single photon detectors (SNSPDs) [1]. Combining efficient detectors with TFLN nanophotonic waveguides holds promises for the realization of quantum photonics experiments fully on-chip. On the other hand, the sensitivity of SNSPDs changes with the wavelength of the detected photons [2], setting a boundary to the longest detectable wavelength and limiting the use of the wide transparency window of TFLN. However, this wavelength dependency in the response of SNSPDs can be leveraged to achieve new on-chip functionalities. In this work, by performing a straightforward analysis of the light signal measured at different bias currents [2], we operate hairpin SNSPDs on TFLN as waveguide-integrated wavelength-meters in the telecom bandwidth.

Lithium niobate, because of its nonlinear and electro-optical properties, is one of the materials of choice for photonic applications. The development of nanostructuring capabilities of thin film lithium niobate (TFLN) permits fabrication of small footprint, low-loss optical circuits. With the recent implementation of on-chip single-photon detectors, this architecture is among the most promising for realizing on-chip quantum optics experiments. In this Letter, we report on the implementation of superconducting nanowire single-photon detectors (SNSPDs) based on NbTiN on 300 nm thick TFLN ridge nano-waveguides. We demonstrate a waveguide-integrated wavelength meter based on the photon energy dependence of the superconducting detectors. The device operates at the telecom C- and L-bands and has a footprint smaller than 300 × 180 μm2 and critical currents between ∼12 and ∼14 μA, which ensures operation with minimum heat dissipation. Our results hold promise for future densely packed on-chip wavelength-multiplexed quantum communication systems.

Quantum communication networks will connect future generations of quantum processors, enable metrological applications, and provide security through quantum key distribution. We present a testbed that is part of the municipal fiber network in the greater Stockholm metropolitan area for quantum resource distribution through a 20 km long fiber based on semiconductor quantum dots emitting in the telecom C-band. We utilize the service to generate random numbers passing the NIST test suite SP800-22 at a subscriber 8 km outside of the city with a bit rate of 23.4 kbit/s.

Optically active semiconductor quantum dots have proven to be excellent single- and entangled-photon sources, with applications in quantum optics and quantum photonics. These sources are considered crucial in the development of future photonic quantum technology, such as quantum communication, quantum computation and quantum metrology. In future quantum networks, they allow to share quantum information through optical fiber links and implement secure communication protocols based on quantum key distribution.

However, there are several challenges when developing quantum dot devices in order to unlock the full potential of these quantum emitters. The ideal quantum dot source efficiently generates triggered single- and entangled-photons on-demand. It provides further high collection-efficiency, low multi-photon probability, near-unity indistinguishability and high entanglement fidelity. Finally, it also offers compatibility with other systems by providing photons with the desired spectral properties and enabling efficient photon coupling.

In this thesis the development and fabrication of bright and strain-tunable quantum dot devices for single- and entangled-photon generation has been studied. It covers highly-symmetric GaAs quantum dots emitting in the near-infrared, InAs quantum dots generating photons in the telecom C-band and InAsP quantum dots embedded in InP nanowires enabling deterministic integration into photonic circuits. The main aspects of operating these quantum dots in cryogenic micro-photoluminescence experiments are described, with focus on enhancing the collection efficiency using solid immersion lenses. For strain-tunability, the focus lies on the fabrication of piezoelectric actuators as substrates for the integration of quantum dot samples by polymer-based bonding. Finally, this thesis describes the simulation, fabrication and measurement of a novel device featuring quantum dots embedded in broad-band parabolic mirror microcavities for enhanced light collection.

Experimental results obtained with a variety of quantum dot devices are included: GaAs quantum dot devices featuring solid immersion lenses demonstrate record-low multi-photon probability and near-unity photon indistinguishability. Piezoelectric strain-tunable devices with InAs quantum dots emitting in the telecom C-band allow for on-demand generation of single- and entangled-photons with tunable quantum dot emission properties and high entanglement fidelity. Piezoelectric strain-tuning actuators enable further the realization of reconfigurable quantum photonic circuits featuring waveguide-integrated InAsP/InP nanowire quantum dots with tunable emission wavelength. Finally, GaAs quantum dots in microcavities with parabolic mirror integrated on piezoelectric actuators achieve an increase in brightness by one order of magnitude over planar structures while allowing to tune the emission wavelength to the atomic transition ⁸⁷Rb D₁ relevant for quantum memory applications.

Optiskt aktiva halvledarkvantprickar har visats vara utmärkta källor för enstaka och intraslade fotoner med tillämpningar inom kvantoptik och kvantfotonik. Denna typ av källor anses vara kritiska för utvecklingen av framtida fotonikbaserade kvantteknologier så som kvantkommunikation, kvantdatorer och kvantmetrologi. I framtida kvantnätverk kommer dessa källor tillåta utbytet av kvantinformation genom optiska fiberlänkar och de kommer även att möjligöra kryptografiskt säker kommunikation genom kvantnyckeldistribution.

Det finns emellertid flera utmaningar inom utvecklingen av dessa kvantprickkällor för att nå dess fulla potential. En ideal kvantprickkälla genererar effektivt enstaka och intrasslade fotoner på beställning. Den har också hög uppsamlingskoefficient, låg sannolikhet för multi-foton generation, oskiljbar intrassling och hög kvantfidelitet. Slutligen är en ideal källa kompatibel med andra system genom att generera fotoner med önskvärda spektral egenskaper och genom att möjligöra effektiv koppling av dessa fotoner.

I denna avhandling har utvecklingen och tillverkningen av ljusa och töjnings-justerbara kvantprickskällor för enstaka och intrasslade fotoner studerats. Avhandlingen täcker högsymmetriska GaAs kvantprickar som genererar fotoner i det nära infraröda spektrat, InAs kvantprickar som genererar fotoner i telekommunikations C-bandet och InAsP kvantprickar inbädade i InP nanotråd som möjliggör deterministisk integration med fotoniska kretsar. De kritiska aspekterna av att använda dessa kvantprickar i kryogena mikro-fotoluminescens-experiment beskrivs, med fokus på att öka insamlingseffektiviteten med hjälp av solida immersionslinser. Fokuset för möjligheten att justera kvantpricken med hjälp av töjning ligger på fabrikationen av piezoelektriska ställdon som substrat för integration med kvantprickar med hjälp av polymerbaserad bindning. Slutligen beskriver denna avhandling simulering, tillverkning och karakterisering av en ny enhet med kvantprickar inbäddade i bredbands-paraboliska speglar i mikrokaviteter för förbättrad ljusinsamling.

Experimentella resultat som erhållits från en mängd olika kvantpricksenheter ingår i avhandlingen: Enheter med GaAs kvantprickar och solid immersion linser visar rekordlåg multifoton-sannolikhet samt nästan enhetlig urskiljbarhet. Piezoelektriska töjnings-justerbara enheter med InAs kvantprickar som sänder ut fotoner i C-bandet för telekommunikation möjliggör generation av enstaka och intrasslade fotoner på begäran med stämmningsbara egenskaper och hög intrasslings-fidelitet. Piezoelektriskt töjningsbara ställdon möjligör också konfigurerbara kvantoptiska fotonik kretsar med InP/InAsP nanotråds kvantprickar med stämbar emmisionsvåglängd integrerade i vågledaren. Slutligen uppnår GaAs kvantprickar i mikrokaviteter med en parabolisk spegel integrerade på ett piezoelektriskt ställdon en ökning av ljusstyrkan med en storleksordning jämfört med plana strukturer samtidigt som den gör det möjligt att justera emissionsvåglängden till den atomövergången ⁸⁷Rb D₁ som är relevant för applikationer inom kvantminnen.

Rydberg excitons are, with their ultrastrong mutual interactions, giant optical nonlinearities, and very high sensitivity to external fields, promising for applications in quantum sensing and nonlinear optics at the singlephoton level. To design quantum applications it is necessary to know how Rydberg excitons and other excited states relax to lower-lying exciton states. Here, we present photoluminescence excitation spectroscopy as a method to probe transition probabilities from various excitonic states in cuprous oxide. We show giant Rydberg excitons at T = 38 mK with principal quantum numbers up to n = 30, corresponding to a calculated diameter of 3 mu m.