An on-demand source of bright entangled photon pairs is desirable for quantum key distribution (QKD) and quantum repeaters. The leading candidate to generate entangled photon pairs is based on spontaneous parametric down-conversion (SPDC) in a non-linear crystal. However, a fundamental trade-off exists between entanglement fidelity and efficiency in SPDC sources due to multiphoton emission at high brightness, which limits the pair extraction efficiency to 0.1% when operating at near-unity fidelity. Quantum dots in photonic nanostructures can in principle overcome this trade-off; however, the quantum dots that have achieved an en- entanglement fidelity on par with an SPDC source (99%) have poor pair extraction efficiency of 0.01%. Here, we show a measured peak concurrence of 95.3% ± 0.5% and pair extraction efficiency of 0.65% from an InAsP quantum dot in an InP photonic nanowire waveguide. Additionally, we show that an oscillating two-photon Bell state generated by a semiconductor quantum dot can establish a secure key for peer-to-peer QKD while using all generated photon pairs. Using our time-resolved QKD scheme alleviates the need to remove the exciton fine structure splitting.
Membrane External-Cavity Surface-Emitting Lasers (MECSELs) are a new kind of vertically emitting semiconductor laser with enormous potential and versatility for tailoring the laser parameters. Part of their benefits is related to the fact that they do not need to employ integrated Distributed Bragg Reflectors (DBRs), which are known to hamper the heat transfer and limit wavelength versatility via strain and band-gap engineering constraints. Furthermore, the substrate on which the active region is grown on is removed and the resulting thin active region membrane is sandwiched between transparent Intra Cavity (IC) heat spreaders for improved thermal management. Initial characterization of room temperature operation of a new red emitting AlGaInP-based structure design containing 40 Quantum Wells (QWs) will be presented. Further, the main aspects of the design of active region membranes will be reviewed with respect to double-side pumping possibilities enabled by the absence of a DBR and the substrate. The comparably high cavity losses show future potential of a properly double-side pumped gain structure.
Quantum dots (QDs) embedded inside indium-phosphide (InP) nanowires have the potential to be bright, on-demand sources of perfect polarization-entangled photon pairs fabricated with near-unity yields. However, to date very high degrees of entanglement have not yet been measured from such devices. By performing quantum state tomography with state-of-the-art superconducting nanowire single-photon detectors (SNSPDs) and two-photon resonant excitation of the QD, we show that these sources are indeed capable of producing near-unity entangled photon pairs. We measure a raw peak fidelity of 97.5% +/- 0.8% and a lifetime-weighted fidelity of 0.94% +/- 0.04%. These results conclusively demonstrate that the majority of the degradation from unity-measured entanglement fidelity in earlier studies was not due to spin-spin dephasing from the large 9/2 nuclear spin of indium. These results solidify InP nanowire QDs as a promising platform for future quantum photonics applications.
We propose a scheme for the generation of highly indistinguishable single photons using semiconductor quantum dots and demonstrate its performance and potential. The scheme is based on the resonant two-photon excitation of the biexciton followed by stimulation of the biexciton to selectively prepare an exciton. Quantum-optical simulations and experiments are in good agreement and show that the scheme provides significant advantages over previously demonstrated excitation methods. Specifically, the scheme allows for ultra-low multi-photon error rates, high indistinguishability, high brightness and programmable linear polarization.
During recent years, quantum dots have become an increasingly established source of highly entangled photons 1. The main motivation for the development of this technology has resided in the expectation that a resonantly driven quantum emitter can offer a path towards on-demand photon pair generation 2. In fact, state-of-the-art sources relying on spontaneous parametric down-conversion intrinsically suffer from multipair emission at high pair generation rates, which causes a tradeoff between brightness and degree of entanglement 3. Despite the key importance of this aspect, the experimental study of how multiphoton emission affects the entanglement properties of quantum dot-based sources has received surprisingly little attention. In this paper we report the investigation of the multipair emission of the source under quasi-deterministic resonant two-photon excitation without filtering the excitation laser using polarization suppression. The focus is on measuring the real multipair emission entering in entanglement-based measurements, minimizing measurement artefacts from the setup and in particular from the excitation source. This is investigated by measuring the second-order correlation function at zero-time delay in several measurement conditions, including spectral filtering. Our work confirms that the multipair emission is provided also for entanglement-based measurement conditions and thus helps the design of efficient photon sources for quantum information and communication technologies.
We propose a scheme for the generation of highly indistinguishable single photons using semiconductor quantum dots and demonstrate its performance and potential. The scheme is based on the resonant two-photon excitation of the biexciton followed by stimulation of the biexciton to selectively prepare an exciton. Quantum-optical simulations and experiments are in good agreement and show that the scheme provides significant advantages over previously demonstrated excitation methods. Specifically, the scheme allows for ultra-low multi-photon error rates, high indistinguishability, high brightness and programmable linear polarization.
2D materials offer a wide range of perspectives for hosting highly localized 0D states, e.g. vacancy defects, that offer great potential for integrated quantum photonic applications. Here, we create individual defects that act as our single-photon emitters by highly local He-ion irradiation in a monolayer MoS2 van der Waals heterostructure. The defects show anti-bunched light emission at a characteristic energy of ~ 1.75 eV. The emission is highly homogeneous and background free due to the hBN encapsulation with a creation yield of > 70%. Spectroscopic investigation of individual single-photon emitters reveals a strongly asymmetric line shape resembling interaction with acoustic phonons in excellent agreement with an independent boson model. Moreover, emitters are spatially integrated and electrically controlled in field-switchable van der Waals devices. Our work firmly establishes 2D materials as a highly scalable material platform for integrated quantum photonics.
We investigate the indistinguishability of single photons generated from strain-free GaAs/AlGaAs quantum dots using pulsed resonance fluorescence techniques. In pulsed two-photon interference measurements we observe a single photon indistinguishability with a raw visibility of 95%. This can be traced back to the short intrinsic lifetime of excitons and trions confined in the GaAs quantum dots and demonstrates that for this material system the generation of single photons is possible with near-unity indistinguishability even without Purcell enhancement.
In recent years, two-dimensional semiconductor quantum emitters have gotten substantial attention from the solid-state quantum photonics community. Their potential for on-chip integration in silicon-based photonics makes them an ideal candidate to realize large-scale hybrid quantum photonic circuits.
Given the strain-induced quantum emitter formation in two-dimensional WSe2, coupling of such quantum emitters into a SiN photonic waveguides is very promising. However, demonstration of single-photon emission into a waveguide has been elusive so far. Here, we show single-photon emission of strain-induced quantum emitters in a 2D flake integrated into a SiN waveguide. We take advantage of the waveguide edges as nucleation sites for quantum emitters. We observe single-photon emission coupled into the waveguide with a g(2)(0) = 0.15±0.09. This result opens up the way towards large-scale 2D emitter integration in on-chip quantum photonic circuits.
Fiber-based quantum networks require on-demand sources of entangled photons in the telecom C-band for long distance information transfer. Historically, the field of in-fiber entanglement distribution has been dominated by photons provided via spontaneous processes. In recent years, semiconductor quantum dots have emerged as strong competitors in terms of generating single and entangled photons due to their promise of deterministic qubit generation in the NIR wavelength region. Here, we show the on-demand generation of polarization entangled photons in the telecom C-band based on InAs/GaAs quantum dots grown via metal-organic vapor-phase epitaxy. By employing a robust phonon-assisted two-photon excitation scheme, we are able to generate pairs of entangled photons with a concurrence of 91.4 ± 3.8 % and a maximum fidelity to the Bell state Φ+ of 95.2 ± 1.1 %.
We investigate the indistinguishability of single photons generated from strain-free GaAs/AlGaAs quantum dots using pulsed resonance fluorescence techniques. In pulsed two-photon interference measurements we observe a single photon indistinguishability with a raw visibility of 95%. This can be traced back to the short intrinsic lifetime of excitons and trions confined in the GaAs quantum dots and demonstrates that for this material system the generation of single photons is possible with near-unity indistinguishability even without Purcell enhancement.
We develop schemes to generate, manipulate and detect single photons at various frequencies including telecom wavelengths. With detectors based on superconducting nanowires we combine very high detection efficiency with high time resolution and very low noise levels. We demonstrate on-chip implementation of single photon techniques as well as long distance implementations using deployed optical fibers.
Quantum communication applications require a scalable approach to integrate bright sources of entangled photon-pairs in complex on-chip quantum circuits. Currently, the most promising sources are based on III/V semiconductor quantum dots. However, complex photonic circuitry is mainly achieved in silicon photonics due to the tremendous technological challenges in circuit fabrication. We take the best of both worlds by developing a new hybrid on-chip nanofabrication approach. We demonstrate for the first time on-chip generation, spectral filtering, and routing of single-photons from selected single and multiple III/V semiconductor nanowire quantum emitters all deterministically integrated in a CMOS compatible silicon nitride photonic circuit.
Nanowires offer new opportunities for nanoscale quantum optics; the quantum dot geometry in semiconducting nanowires as well as the material composition and environment can be engineered with unprecedented freedom to improve the light extraction efficiency.
Quantum dots in nanowires are shown to be efficient single photon sources, in addition because of the very small fine structure splitting, we demonstrate the generation of entangled pairs of photons from a nanowire.
Another type of nanowire under study in our group is superconducting nanowires for single photon detection, reaching efficiencies, time resolution and dark counts beyond currently available detectors. We will discuss our first attempts at combining semiconducting nanowire based single photon emitters and superconducting nanowire single photon detectors on a chip to realize integrated quantum circuits.
We demonstrate the generation of single photons as well as pairs of entangled photons with quantum dots in semiconducting nanowires, we show applications to quantum optics including generation, manipulation and detection of light at the nanoscale.
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