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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Two-photon interference from remote deterministic quantum dot microlenses

Peter Schnauber, Alexander Thoma, Jonas Böhm, Manuel Gschrey, Jan-Hindrik Schulze, A. Strittmatter, Sven Rodt, Tobias Heindel, Stephan Reitzenstein

Published Jun 1, 2017• 0 citations
Publisher

Abstract

A key requirement for implementing photonic quantum information networks is the mutual two-photon interference between photons emitted by spatially separated quantum light sources. In order to up-scale such networks, the deterministic fabrication of efficient sources of single and indistinguishable photons is of crucial importance. So far, few experiments reported on two-photon interference (TPI) from spatially separated QD-based single photon sources, which is required for entanglement swapping, and deterministic sources have been applied only recently [1]. However, narrowband enhancement of photon extraction in high-Q microcavities hinders their application for advanced quantum communication schemes based on polarization entangled photon pairs from the biexciton-exciton radiative cascade.

DOI
10.1109/cleoe-eqec.2017.8087339

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A key requirement for implementing photonic quantum information networks is the mutual two-photon interference between photons emitted by spatially separated quantum light sources. In order to up-scale such networks, the deterministic fabrication of efficient sources of single and indistinguishable photons is of crucial importance. So far, few experiments reported on two-photon interference (TPI) from spatially separated QD-based single photon sources, which is required for entanglement swapping, and deterministic sources have been applied only recently [1]. However, narrowband enhancement of photon extraction in high-Q microcavities hinders their application for advanced quantum communication schemes based on polarization entangled photon pairs from the biexciton-exciton radiative cascade.
We report on two-photon interference (TPI) experiments using remote deterministic single-photon sources. Employing 3D in-situ electron-beam lithography, we fabricate quantum-light sources at specific target wavelengths by integrating pre-selected semiconductor quantum dots within monolithic microlenses. The individual single-photon sources show TPI visibilities of 49% and 22%, respectively, under pulsed p-shell excitation at 80 MHz. For the mutual TPI of the remote sources, we observe an uncorrected visibility of 29%, in quantitative agreement with the pure dephasing of the individual sources. Due to its efficient photon extraction within a broad spectral range (>20 nm), our microlens-based approach is predestinated for future entanglement swapping experiments utilizing entangled photon pairs emitted by distant biexciton-exciton radiative cascades.
Quantum dots in cavities have been shown to be very bright sources of indistinguishable single photons. Yet the quantum interference between two such bright quantum dot sources, a critical step for photon-based quantum computation, still needs to be investigated. Here, we report on such a measurement, taking advantage of a deterministic fabrication of the devices. We show that cavity quantum electrodynamics can efficiently improve the quantum interference between remote quantum dot sources: Poorly indistinguishable photons can still interfere with good contrast with high quality photons emitted by a source in the strong Purcell regime. Our measurements and calculations show that cavity quantum electrodynamics is a powerful tool for interconnecting several quantum dot devices.
Entanglement swapping is a protocol that details how to create entanglement between previously uncorrelated particles. Its all-photonic version - mediated by the interference of photon pairs generated by separate quantum systems-finds disparate applications in quantum networks. So far, all-photonic entanglement swapping between remote systems has been implemented only using sources that operate probabilistically. However, the scaling up of quantum networks requires deterministic quantum emitters that do not suffer from a trade-off between degree of entanglement and photonpair generation rate. Here, we demonstrate all-photonic entanglement swapping using photon-pairs generated by two separate GaAs quantum dots. The emitters are deterministically embedded in hybrid semiconductor-piezoelectric devices that make the entangled-photons from two dissimilar quantum dots nearly identical. Entanglement swapping is demonstrated with a fidelity as high as 0.71(2), more than 10 standard deviations above the classical limit. The experimental data are quantitatively explained by a theoretical model that also suggests how to boost the protocol performances. Our work opens the path to the exploitation of quantum dot entangled-photon sources in quantum repeater networks.
A well controlled, triggered source of entangled photons is desirable for many applications in quantum information processing. The two-photon cascade from a biexciton state, two electrons and holes, confined in a quantum dot can be such a source of polarisation-entangled photons provided the two decay paths from the biexciton carry no “which-path” information. In recent years many techniques have been employed to make the two optical decay paths from the biexciton state indistinguishable. A number of these techniques will be discussed and entangled-photon emission from a single quantum dot with a high fidelity in the expected Bell-state from the cascade will be demonstrated. This source of entangled photons allows optical interferometry beyond the limits imposed by the photon wavelength. Interference fringes of the entangled biphoton state reveals a periodicity half of that obtained with the single photon, and much less than that of the pump laser. High fringe visibility indicates that biphoton interference is less sensitive to decoherence than interference of two sequential single photons. The effect of the exciton fine-structure splitting on our entangled photon source will be shown. Surprisingly the entanglement is found to persist despite relatively large separations between the intermediate energy levels of up to 4µeV. Measurements demonstrate that entanglement of the photon pair is robust to the dephasing of the intermediate exciton state responsible for the first order coherence time of either single photon. We distinguish between the first-order coherence time, and a parameter defined as the cross-coherence time, this is illustrated in the figure.
Long-distance quantum communication relies on the ability to efficiently generate and prepare single photons at telecom wavelengths. In many applications these photons must also be indistinguishable such that they exhibit interference on a beamsplitter, which implements effective photon-photon interactions. However, deterministic generation of indistinguishable single photons with high brightness remains a challenging problem. We demonstrate two-photon interference at telecom wavelengths using an InAs/InP quantum dot in a nanophotonic cavity. The cavity enhances the quantum dot emission, resulting in a nearly Gaussian transverse mode profile with high out-coupling efficiency exceeding 36% after multi-photon correction. We also observe Purcell enhanced spontaneous emission rate up to 4. Using this source, we generate linearly polarized, high purity single photons at 1.3 micron wavelength and demonstrate the indistinguishable nature of the emission using a two-photon interference measurement, which exhibits indistinguishable visibilities of 18% without post-selection and 67% with post-selection. Our results provide a promising approach to generate bright, deterministic single photons at telecom wavelength for applications in quantum networking and quantum communication.
Generating identical photons from remote emitter-based bright single-photon sources is an important step for scaling up optical quantum technologies. Here, we study the Hong-Ou-Mandel interference of photons emitted from remote sources based on semiconductor quantum dots (QDs). We make use of a deterministic fabrication technique to position the QDs in a spectrally resonant micropillar cavity and fine-tune their operation wavelength electrically. In doing so, we can match four pairs of sources between five distinct sources, study them under various excitation schemes, and measure their degree of indistinguishability. We demonstrate remote indistinguishabiltity between 44 ± 1% and 69 ± 1% depending on the pair of sources and excitation conditions, record values for QDs in cavities. The relative contribution of pure dephasing and spectral diffusion is then analyzed, revealing that the remaining distinguishability is mostly due to low-frequency noise.