Semiconductor Light Source Creates Anti-Bunched Photons for Advanced Technologies

Scientists are increasingly reliant on technologies capable of generating complex, non-classical states of light for diverse applications. David Theidel, Mackrine Nahra, and Ilya Karuseichyk, working with colleagues at the Laboratoire d’Optique Appliquée (LOA), CNRS, Ecole polytechnique, ENSTA, and Institut Polytechnique de Paris in Palaiseau, France, investigate the non-classical properties of high-harmonic generation in semiconductors, an emerging photonic platform. Their research demonstrates squeezed light emission from higher-order harmonics driven by a coherent laser and, crucially, the generation of anti-bunched photon statistics and a demonstrably non-Gaussian state through inter-order heralded measurements. This work establishes high-harmonic generation as a viable platform for creating valuable optical resources relevant to quantum information science.

Scientists have unlocked a new pathway for generating and manipulating quantum states of light using high-harmonic generation in semiconductors, demonstrating the creation of squeezed and entangled light directly from a semiconductor crystal, bypassing the need for complex external setups typically required for quantum photonics. Measuring the statistical properties of photons emitted across multiple high-harmonic orders, researchers definitively certified the non-classical nature of the generated light, establishing a foundation for engineering quantum states and achieving the generation of heralded states exhibiting anti-bunched photon statistics, a clear signature of non-classical behaviour. The research centres on high-harmonic generation (HHG), a non-linear process where intense laser light interacting with a material produces coherent radiation at higher frequencies; recent investigations have highlighted the potential of semiconductors for HHG, suggesting the possibility of intrinsic non-classical properties in the emitted light. The team employed an experimental setup involving ultrashort laser pulses focused onto a cadmium telluride (CdTe) crystal, generating harmonics ranging from the 8th to the 15th order. Single-photon avalanche photodiodes, arranged in a Hanbury-Brown and Twiss configuration, were used to measure the arrival times of photons, enabling the calculation of intensity correlation functions that reveal the underlying photon statistics. Results demonstrate a transition from photon bunching at low intensities to behaviour consistent with a coherent state at higher intensities, with a notable deviation indicating non-classical effects. Critically, evaluation of a specific witness operator confirms the generation of a quantum non-Gaussian state, solidifying the potential of semiconductor HHG as a versatile platform for quantum optical resource creation. Further analysis reveals the generation of a non-Gaussian state, a crucial resource for advanced quantum technologies like universal quantum computation and robust quantum error correction, representing a significant step towards miniaturizing and streamlining quantum optical systems. Measurements reveal that higher-order harmonics generated via high-harmonic generation in semiconductors are squeezed, demonstrating non-classical behaviour. The auto-correlation function, gH(i,i), initially indicates photon bunching at the lowest laser intensities, but transitions to a value of 1 for all harmonic orders at higher driving laser intensities and detector count rates. This trend mirrors observations from previous studies of lower-order harmonics, although a significant deviation from coherent state statistics remains apparent. Evaluation of non-classicality criteria further confirms these findings; the heralded intensity correlation function, gH(i|j), exhibits anti-bunched photon statistics, with values less than 1, when conditioning the detection of one harmonic on another. A non-classicality witness, denoted as WNC, consistently exceeds zero for all harmonics, H11, H12, and H13, certifying that the initial harmonic states cannot be described as mixtures of coherent states. The magnitude of WNC increases initially, then plateaus, a trend accurately reproduced by a generalised Gaussian state model developed as part of this work. Conditional measurements were then employed to engineer the emitted states. By heralding the detection of a photon in harmonic 13 to trigger measurement of harmonic 11, the study generated heralded states exhibiting distinct photon statistics. With mean photon numbers kept below 0.2 to ensure negligible three-photon emission, the heralded intensity correlation function, gh, consistently fell below 1 for all combinations of harmonics, confirming the non-classical nature of these heralded states. Notably, the degree of anti-bunching increased with increasing mean photon number, a behaviour contrasting with typical parametric photon pair sources and explained by high vacuum contributions and low single-photon detection probabilities at lower laser intensities. This technique, reliant on detecting individual photons, allows for precise characterisation of the quantum properties of the emitted light. The study employed a low mean number of photons, less than 0.2, in the detection arms to minimise the probability of three-photon emission, simplifying the approximation of the measured state as a superposition of vacuum, single-photon, and two-or-more photon events. Inter-order heralded measurements were then implemented to actively engineer the state of the emitted radiation, relying on detecting a photon in one harmonic to confirm the presence of a photon in another. Specifically, the measurement of harmonic 11 was conditioned on a detection event at harmonic 13, creating a heralded state denoted H(11|13). This conditional measurement scheme effectively projects the initial state onto a non-deterministic single-photon source, enhancing the observation of non-classical correlations. To further analyse the quantum nature of the generated states, a quantum non-Gaussian witness was evaluated; this operator, based on the probabilities of vacuum, single-photon, and multi-photon events, determines whether a state can be described by a Gaussian distribution, a key distinction in quantum information. The witness strength was then compared to the maximum value achievable by any Gaussian state, with a positive difference signifying a non-Gaussian state. A numerical model, based on a generalised two-mode Gaussian state, was employed to reconstruct an effective state reproducing the observed trends and non-classical features, solidifying the interpretation of the experimental data. Scientists are increasingly focused on harnessing the bizarre properties of quantum mechanics for practical technologies, but translating laboratory curiosities into robust devices remains a formidable challenge. This work represents a step forward in that endeavour, demonstrating a pathway to generate and control non-classical light using high-harmonic generation in semiconductors. For years, researchers have struggled to create reliable sources of squeezed light and other exotic quantum states outside of carefully controlled laboratory settings; this new approach offers a potentially scalable and solid-state alternative to traditional methods relying on bulky optical setups and fragile materials. The ability to engineer these states, and crucially, to verify their non-classical nature through clever measurement techniques, is not merely an academic exercise. Non-classical light is a vital ingredient for quantum communication, quantum sensing, and potentially even quantum computing. The demonstration of heralded states, where the properties of one photon are entangled with another, opens doors to more complex quantum protocols and enhanced security. Further investigation is needed to explore the full potential of this platform and to determine whether it can generate truly high-dimensional entangled states. The next phase will likely involve integrating this light source with other quantum components, testing its performance in realistic communication channels, and exploring different semiconductor materials to optimise the process. Ultimately, the goal is to move beyond proof-of-principle demonstrations and build practical quantum devices that can solve real-world problems.