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arXiv:2302.05359 · Physical Review Applied

Cross-Polarization-Extinction Enhancement and Spin-Orbit Coupling of Light for Quantum-Dot Cavity Quantum Electrodynamics Spectroscopy

Petr Steindl, J. A. Frey, Justin Norman, John Edward Bowers, Dirk Bouwmeester, W. Löffler

Published Jun 29, 2023• 13 citations
arXivHTMLPDF

Abstract

Resonant laser spectroscopy is essential for the characterization, operation, and manipulation of single-quantum systems such as semiconductor quantum dots. The separation of the weak resonance fluorescence from the excitation laser is key for high-quality single- and entangled-photon sources. This is often achieved by cross-polarization laser extinction, which is limited by the quality of the optical elements. Recently, it was discovered that Fresnel-reflection birefringence in combination with single-mode filtering counteracting spin-orbit-coupling effects enables a three-order-of-magnitude improvement of polarization extinction [Benelajla et al., Phys. Rev. X 11, 021007 (2021)]. Here, we demonstrate this method for cross-polarization-extinction enhancement in cryogenic confocal microscopy of a resonantly excited semiconductor quantum dot in a birefringent optical microcavity, and observe a $10\ifmmode\times\else\texttimes\fi{}$ improvement of the single-photon contrast.

DOI
10.1103/physrevapplied.19.064082

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Resonant laser spectroscopy is essential for the characterization, operation, and manipulation of single-quantum systems such as semiconductor quantum dots. The separation of the weak resonance fluorescence from the excitation laser is key for high-quality single- and entangled-photon sources. This is often achieved by cross-polarization laser extinction, which is limited by the quality of the optical elements. Recently, it was discovered that Fresnel-reflection birefringence in combination with single-mode filtering counteracting spin-orbit-coupling effects enables a three-order-of-magnitude improvement of polarization extinction [Benelajla et al., Phys. Rev. X 11, 021007 (2021)]. Here, we demonstrate this method for cross-polarization-extinction enhancement in cryogenic confocal microscopy of a resonantly excited semiconductor quantum dot in a birefringent optical microcavity, and observe a 10\ifmmode×\else\texttimes\fi10\ifmmode\times\else\texttimes\fi{}10\ifmmode×\else\texttimes\fi improvement of the single-photon contrast.
Combining resonant excitation with Purcell-enhanced single quantum dots (QDs) stands out as a prominent strategy for realizing high performance solid-state single photon sources. However, optimizing photon efficiency requires addressing challenges associated with effectively separating the excitation laser from QDs' emission. Traditionally, this involves polarization filtering, which limits the achievable polarization directions and the scalability of photonic states. In this study, we have successfully tackled this challenge by employing spatially-orthogonal resonant excitation of QDs, deterministically coupled to monolithic Fabry-Perot microcavities. Leveraging the membrane cavity structures, we have achieved filter-free single photon resonant fluorescence. The resulting source produces single photons with a simultaneous high extraction efficiency of 0.87, purity of 0.9045(4), and indistinguishability of 0.963(4).
Combining resonant excitation with Purcell-enhanced single quantum dots (QDs) stands out as a prominent strategy for realizing high-performance solid-state single-photon sources. However, optimizing photon extraction efficiency requires addressing the challenge of effectively separating the excitation laser from the QDs’ emission. Traditionally, this involves polarization filtering, limiting the achievable polarization directions and the scalability of polarized photonic states. In this study, we have successfully tackled this challenge by employing spatially orthogonal resonant excitation of QDs, deterministically coupled to monolithic Fabry–Perot microcavities. Leveraging the planar microcavity structure, we have achieved spectral filter-free single-photon resonant fluorescence. The resulting source produces single photons with a high extraction efficiency of 0.87 and an indistinguishability of 0.963(4).
Next-generation information technology is expected to rely on discrete two-state quantum systems that can deterministically emit single photons. Quantum dots are mesoscopic (~10,000 atoms large) semiconductor islands grown in a host crystal of larger band-gap that make well-defined two-level quantum systems and are very attractive due to stability, record coherence times, and the possibility of integrating them into larger structures, such as optical microcavities. This work presents experimental progress towards understanding the coherent optical processes that occur in single quantum dots, particularly such phenomena that might be one day utilized for quantum communication applications. High resolution low temperature optical spectroscopy is used in conjunction with first order (amplitude) and second-order (intensity) correlation measurements of the emitted field. A novel technique is introduced that is capable of harvesting the fluorescence of single dots at the same frequency as the laser, previously impossible due to insurmountable scattering. This technique enables the observation, for the first time, of single quantum dot resonance fluorescence, in both the weak and strong excitation regimes, which forms the basis for deterministic generation of single photons. Guided by the rich theoretical description available from quantum optics with atoms we obtain insight into the complex dynamics of this driven system. Quantum dots confined to novel optical microcavities were further investigated using micro photoluminescence. An optical microcavity properly coupled to a two-level system can profoundly modify its emission characteristics via quantum electrodynamical effects, which are highly attractive for single photon sources. The all-epitaxial structures we probe are distinguished by a bulk morphology that overcomes the fragility problems of existing approaches, and provides high quality factors as well as small mode volumes. Lasing is obtained with larger strucutres. Additionally, isolation of individual dots is further realized in smaller cavities and the Purcell effect observed in time-resolved photon counting experiments.
Semiconductor quantum dots (QDs) coupled with optical microcavities provide an excellent platform for generating high-performance single-photon sources. However, the current microcavity structures are difficult to solve the tradeoff between single-photon purity and indistinguishability without additional filtering, due to challenges in suppressing broad phonon sideband spectrum and two-photon emission in the practical experiments. To address these issues, we have developed a novel asymmetric microcavity that supports two nondegenerate polarized modes, both resonant with a single quantum dot. One cavity mode is broadband and is used for excitation to reduce two-photon emission intensity, while the other is narrowband and is used for collection to enhance the indistinguishability of single-photon sources. This asymmetric microcavity also has the ability to overcome at least half of the photon efficiency loss under polarization filtering. Moreover, our model has employed an excitation pulse that resonates with cavity modes eliminating spectral distortion of intracavity fields, and efficiently achieving deterministic population inversion of QDs compared to an elliptical cavity. Our scheme paves the way to develop the excellent performance single-photon sources with higher brightness, near-unity purity, and indistinguishability without any additional filtering in practical experiments.
We develop high-efficiency single- and entangled-photons sources that simultaneously combine high purity, efficiency, and indistinguishability. We develop elliptical microcavities to boost the polarized single-photon extraction efficiency over 50%. We coherently excite a quantum dot with a dichromatic pulsed laser to circumvent the 50% loss issue in resonance fluorescence due to the polarization-extinction configuration. We directly observe intensity squeezing of 0.59 dB in resonance fluorescence, and a corrected squeezing of 3.29 dB at the first lens. Quantum interference between quantum-dot single photons and sunlight is also demonstrated in our experiments.
Entangling a single spin to the polarization of a single incoming photon, generated by an external source, would open new paradigms in quantum optics such as delayed-photon entanglement, deterministic logic gates or fault-tolerant quantum computing. These perspectives rely on the possibility that a single spin induces a macroscopic rotation of a photon polarization. Such polarization rotations induced by single spins were recently observed, yet limited to a few 10(-3) degrees due to poor spin-photon coupling. Here we report the enhancement by three orders of magnitude of the spin-photon interaction, using a cavity quantum electrodynamics device. A single hole spin in a semiconductor quantum dot is deterministically coupled to a micropillar cavity. The cavity-enhanced coupling between the incoming photons and the solid-state spin results in a polarization rotation by ± 6° when the spin is optically initialized in the up or down state. These results open the way towards a spin-based quantum network.