Novel quantum light sources of strongly ­correlated multiphotons

Work performed

The work conducted during the Fellowship followed two complementary lines.

First [1], we developed new schemes for the deterministic preparation of pure, indistinguishable single photons from semiconductor quantum dots. Building on the resonant two-photon excitation of the biexciton, we designed and analyzed a protocol where a precisely timed stimulation pulse selectively prepares the exciton state. Quantum-optical modeling predicted – and experiments confirmed – that this approach suppresses re-excitation, reduces multiphoton errors to ultralow levels, and strongly decreases timing jitter. An additional advantage is that the polarization of the stimulation pulse can be used to deterministically set the polarization (H or V) of the emitted photon, ensuring that all relevant emission occurs directly in the detection channel. Compared with cross-polarized resonant excitation, this method leads to significantly enhanced brightness while maintaining excellent indistinguishability.

Second [2], we investigated the fundamental quantum-optical mechanisms underlying single- and multiphoton emission in coherently driven two-level systems [3][4]. Contrary to the common assumption that multiphotons are merely accidental and independent of the single-photon process, we showed that virtual multiphoton fluctuations are intrinsic to the mechanism of coherent single-photon generation. We demonstrated theoretically and experimentally that these fluctuations can be actively controlled by introducing an external (laser) homodyne field that perturbs the emitter’s mean amplitude. By tuning this interference pathway, we observed a controlled transition from single-photon to multiphoton emission, including the appearance of measurable two- and three-photon correlations. A key conceptual result is that quantum fluctuations retain qualitative importance even when their net amplitude becomes vanishingly small − a manifestation of the paradoxical structure of quantum interference.

Throughout the project, the theoretical frame works of frequency-filtered detection, time-frequency-resolved correlations, and homodyne engineering played a unifying role. Together, they enabled a coherent approach to understanding and manipulating multiphoton emission processes across different experimental platforms. Outreach included invited seminars, integration of results into graduate-level lectures, and dissemination at international conferences in quantum optics and semiconductor photonics.
 

Main findings and impact

The Fellowship delivered two high-impact results.

(1) A new protocol for the deterministic generation of high-quality single photons, with advantages in suppression of multiphoton contamination, timing purity, and polarization programmability. This contributes directly to the development of deployable quantum communication hardware and SPS-based photonic architectures.

(2) A demonstration of multiphoton control via homodyne interference, revealing that engineered mean-field perturbations can sculpt the underlying quantum-interference landscape. This opens a previously unexplored route toward non-Gaussian, strongly correlated light sources based not on post-selection or multiplexing but on in situ engineering of multiphoton pathways.

Together, these results advance the central objective of the project: building a theoretical and conceptual basis for frequency-resolved homodyning as a practical tool for scalable multiphoton quantum technologies.

Use of outcomes and future directions

The advances made during this Fellowship open several new research directions. One promising line comes from a forthcoming work [5] showing that it is possible to deterministically shape the number of photons produced by a single-photon emitter using only linear optical tools. By exploiting basic interference effects, the setup can tune whether the emitter outputs mainly vacuum, a single photon, or a two-photon state. This provides a simple and flexible way to engineer few-photon resources without relying on complex nonlinear processes or probabilistic methods. The same framework suggests that using two identical emitters would even allow the controlled preparation of more sophisticated quantum states, such as NOON states, which are valuable for sensing and photonic computation.

These ideas naturally complement the core concepts developed in the project. Together, they point toward a broader strategy for generating non-Gaussian, strongly correlated states of light using a combination of homodyne control and interferometric engineering.

Future work will explore how these approaches can be scaled up, extended to multi-emitter systems, and applied to areas such as quantum metrology, photonic processing, and long-distance quantum networks. Overall, these directions strengthen the impact of the Fellowship by providing versatile and deterministic routes to the few-photon resources required for next-generation quantum technologies.