Hanqing Liu

Entanglement is a central resource in quantum technologies, and the realization of photonic entanglement necessarily relies on interaction with matter. Resonance fluorescence, originating from the coherent interaction between a driving field and a two-level system, plays a pivotal role in quantum optics. Here, we experimentally demonstrate high-quality generation of post-selected temporal-mode entanglement using vacuum–one-photon number superposition states from a single quantum dot, an approach conceptually aligned with early proposals based on identical single photons, yet whose high-quality experimental realization has remained absent. By delocalizing this superposition via a beam splitter, we realize post-selected temporal-mode entanglement and observe a clear violation of the Clauser–Horn–Shimony–Holt Bell inequality using Franson-type interferometry. Our scheme removes the need for multiphoton generation, simplifies the experimental requirements, and establishes a scalable pathway toward solid-state entangled photon sources.

Generating light in well-defined photon-number states is central to photonic quantum technologies. While deterministic single-photon sources are well established, producing efficient two-photon states from individual emitters remains challenging. Here we demonstrate a high-efficiency two-photon emitter using the degenerate biexciton–exciton cascade in a Purcell-enhanced quantum dot–micropillar system. Leveraging polarization-selective p-shell excitation, we achieve effective biexciton loading and identify stimulated emission as a key mechanism enhancing two-photon temporal correlation. The emitter exhibits a two-photon correlation of g⁽²⁾(0) = 3,966(324), with a two-photon fraction of 0.983(1), and operates in a hybrid, predominantly cascade-dominated regime where cavity-stimulated two-photon emission coexists with the conventional biexciton–exciton cascade. These findings represent progress towards developing practical, on-demand, solid-state two-photon sources.

Quantum key distribution (QKD) can be used to establish a secret key between trusted parties. Many practical use-cases in communication networks, however, involve parties who do not trust each other. A fundamental cryptographic building block for such distrustful scenarios is quantum coin flipping, which has been investigated only in few experimental studies to date, all of which used probabilistic quantum light sources imposing fundamental limitations. Here, we experimentally implement a quantum strong coin flipping protocol using single-photon states and demonstrate a quantum advantage compared to both classical realizations and implementations using faint laser pulses. We achieve this by employing a state-of-the-art deterministic quantum dot light source in combination with fast, random polarization-state encoding enabling sufficiently low quantum bit error ratio. By demonstrating a single-photon quantum advantage in a cryptographic primitive beyond QKD, our work represents a major advance towards the implementation of complex cryptographic tasks in a future quantum internet.

Single photons with ultralong coherence time and high indistinguishability are key resources for interference-based photonic quantum technologies. While laser light exhibits excellent coherence, it cannot be attenuated to single photons through linear optics. Here, we demonstrate high-quality single photons coherently reflected from a single quantum dot (QD) coupled to a double-sided optical microcavity and driven by a continuous-wave single-frequency laser in the low-driving regime. Measuring the reflected light without cross-polarization filtering to suppress laser scattering, we observe a second-order correlation of g ⁽²⁾ (0)=0.030±0.002 and a two-photon interference visibility of V (0)=94.3%±0.2%, which remains robust for photon separations up to 8 km. Under weak driving, these photons exhibit an ultralong coherence time of 258±2µs—six orders of magnitude longer than the coherence time (115 ps) of spontaneously emitted photons—directly inherited from the driving laser. The observed anti-bunching arises from single-photon switching governed by the QD’s saturation nonlinearity and is further explained by quantum interference between the reflected driving field and incoherent scattering. By inheriting the laser’s ultralong first-order coherence and robust photon indistinguishability, coherently scattered single photons hold promise as valuable resources for quantum information science and photonic quantum technologies.

Topological photonics offers the potential to develop quantum light sources with inherent robustness against structural disorders. To date, topologically protected edge or corner states have been investigated for this purpose. Here, for the first time, we exploit a topological bulk state with vertical directionality to enhance the light emission from a single semiconductor quantum dot (QD). An irregular ‘Q’-shaped cavity is applied for establishing topological robustness. We experimentally demonstrate a 1.6-fold Purcell enhancement of single-photon emission in the topological bulk cavity, with tolerance to the emission wavelength or the positioning of the coupled QD. Simulations indicate that such a QD-cavity coupling system can retain a Purcell factor exceeding 1.6 under a broad spectral detuning range of 8.6 nm or a coverage area of 2.5 μm². Furthermore, the optimized cavity structure integrated with a reflector predicts a high single-photon extraction efficiency up to 92%. Our results offer a novel approach to develop topologically protected quantum light sources with high extraction efficiency and robust QD-cavity interaction against irregular cavity boundaries.

Resonance fluorescence of a two-level emitter displays persistently anti-bunching irrespective of the excitation intensity, but inherits the driving laser’s linewidth under weak monochromatic excitation. These properties are commonly explained in terms of two disjoined pictures, i.e., the emitter’s single photon saturation or passively scattering light. Here, we propose a unified model that treats all fluorescence photons as spontaneous emission, one at a time, and can explain simultaneously both the spectral and correlation properties of the emission. We theoretically derive the excitation power dependencies, measurable at the single-photon incidence level, of the first-order coherence of the whole resonance fluorescence and super-bunching of the spectrally filtered, followed by experimental confirmation on a semiconductor quantum dot micro-pillar device. Furthermore, our model explains peculiar coincidence bunching observed in phase-dependent two-photon interference experiments. Our work provides an intuitive understanding of coherent light-matter interaction and may stimulate new applications.

Spontaneous two-photon emission (STPE) is a second-order quantum radiation process with implications in astrophysics¹, atomic physics² and quantum technology³. In particular, on-demand STPE from single quantum emitters has long been predicted to revolutionize photonic quantum science and technology^4,5. Here we report STPE with brightness comparable to that of competing single-photon radiation from a single semiconductor quantum dot deterministically coupled to a high-quality micropillar cavity. This is because of strong vacuum fluctuations in the microcavity, which drive a biexciton directly to the ground state. We show the quantum nature associated with STPE in the cavity quantum electrodynamics regime using photon statistics measurements. Furthermore, STPE is exploited to build unconventional entangled quantum light sources that can simultaneously achieve near-unity entanglement fidelity for spontaneous parametric down-conversion sources and on-demand photon emission for atomic quantum emitters. Our work provides insights into the two-photon process in the quantum regime, which could empower photonic quantum technology with nonlinear quantum radiation.

Due to the existence of flatbands within the band structure, twisting photonics introduces a possibility to enhance the interaction between excitons in single QDs and cavity photons because of the extremely high quality factor (Q) in theory. In this work, we report a Purcell effect between single QDs and moiré photonic crystal nanocavities. The moiré photonic crystal nanocavities in a GaAs slab with QDs embedded are formed by twisting two layer photonic crystal structures with specific angles. High Q, low mode volume and large overlap between QDs and cavity mode field have been achieved by optimizing the filling ratio of the single-layer photonic crystal, with which a Q of fundamental modes about 2000 is experimentally demonstrated. A photoluminescence intensity enhancement of a factor about 8.4 is observed when a single QD is in resonance with a cavity mode, with a Purcell factor of about 3.0 confirmed through the lifetime measurement. This result shows the potential of moiré photonics to implement solid-state cavity quantum electrodynamics for future optical quantum information processing.

In quantum cryptography, fundamental laws of quantum physics are exploited to enhance the security of cryptographic tasks. Quantum key distribution (QKD) is by far the most studied protocol to date, enabling the establishment of a secret key between trusted parties. Many practical use-cases in communication networks, however, involve parties who do not trust each other. The most fundamental quantum cryptographic building block in such a distrustful setting is quantum coin flipping, which has been studied only in few experimental studies to date, all of which used probabilistic quantum light sources facing fundamental limitations. Here, we experimentally implement a quantum strong coin flipping protocol using single-photon states and demonstrate an advantage compared to both classical realizations and implementations using faint laser pulses. We achieve this by employing a state-of-the-art deterministic single-photon source based on the Purcell-enhanced emission of a semiconductor quantum dot in combination with fast polarization-state encoding with sufficiently low quantum bit error ratio. The reduced multi-photon emission of the single-photon source yields a smaller bias of the coin flipping protocol compared to an attenuated laser implementation, both in simulations and in the experiment. By demonstrating a single-photon quantum advantage in a cryptographic primitive beyond QKD, our work represents a major advance towards the implementation of complex cryptographic tasks in a future quantum internet.