Integrated Photonics: Scalable Single Photon Sources Enable Quantum Computing Advances.

The pursuit of scalable photonic systems, essential for advancements in secure communication and quantum computation, relies heavily on the efficient generation and manipulation of single photons. Researchers are increasingly focused on integrated lithium niobate nanophotonics as a promising platform due to its ability to guide and process light on a chip, offering potential for miniaturisation and mass production. A team led by Tristan Kuttner, Alessandra Sabatti, Jost Kellner, Rachel Grange, and Robert J. Chapman, all from the Optical Nanomaterial Group at ETH Zurich’s Institute for Quantum Electronics, detail their progress in achieving multi-photon interference within such a system in their article, “Scalable quantum interference in integrated lithium niobate nanophotonics”. Their work demonstrates the integration of multiple heralded single-photon sources and the observation of bosonic interference, a key requirement for many quantum algorithms, bringing a fully scalable photonic platform closer to realisation.

Quantum interference between photon pairs originating from two integrated lithium niobate sources signifies progress towards scalable photonic quantum systems. This demonstration confirms the feasibility of combining photons from separate downconversion sources on a single chip and observing the resultant interference, a crucial requirement for many photonic computing protocols. Characterisation of each source reveals downconversion probabilities of 2.91% and 2.82% respectively, values which, while modest, sufficiently validate the principle of multi-source interference. The observed interference stems from the indistinguishability of the generated photons, achieved through precise control of experimental parameters and the inherent properties of the downconversion process.

Researchers fabricate a nanophotonic lithium niobate device capable of generating multiple heralded single photon sources via three-wave mixing, efficiently converting pump photons into pairs of signal and idler photons, forming the basis for quantum operations. Spontaneous parametric downconversion (SPDC), the process used, involves a nonlinear optical interaction where a photon splits into two lower-energy photons, the signal and idler, conserving energy and momentum. Crucially, the generated photon pairs exhibit spectral separability, allowing for precise control and manipulation, and by combining photons from these sources, the team demonstrates bosonic interference. Bosons, unlike fermions, can occupy the same quantum state, leading to constructive interference when identical bosons combine.

Researchers account for cross-talk between the sources, correcting for spurious coincidences arising from signal photons from one source coinciding with idler photons from the other. They achieve this through detailed data analysis, employing equations that accurately model the system’s behaviour, and furthermore, they quantify the impact of the directional coupler’s reflectivity on interference visibility, revealing a reduction in visibility due to coupler loss. Data presents in the form of bidirectional histograms, visually representing the distribution of coincidences between signal and idler photons for each source at zero time delay, confirming the rate of SPDC events. Graphs illustrate the calculated downconversion probabilities as a function of time delay, demonstrating the variation in probability with stage position, and the team demonstrates that the observed interference is indeed bosonic, confirming the indistinguishability of the photons generated within the integrated lithium niobate chip.

This work represents a significant step towards developing a truly scalable photonics platform, paving the way for more complex quantum photonic circuits and applications, and the detailed characterisation of the sources and careful consideration of system imperfections contribute to the robustness and reliability of the demonstrated platform.

Researchers account for system losses and the reflectivity of the directional coupler (R = 0.625) within their calculations, ensuring accurate determination of the downconversion probabilities and a precise understanding of the observed interference patterns. The equations employed relate coincidence rates to downconversion probability and account for reflectivity, providing a robust framework for analysing the experimental data.

The use of a time delay sweep enables the observation of the interference pattern, confirming the quantum nature of the interaction. Normalisation of the four-fold coincidence rate effectively mitigates variations in coupling efficiency and downconversion probability during this sweep, thereby enhancing the accuracy of the measurements.

Expanding the system to incorporate more sources represents a significant challenge, but unlocks the potential for implementing more complex quantum algorithms and architectures, and developing techniques for addressing and controlling individual photons within a multi-source system proves essential for realising the full capabilities of this scalable photonic platform. Ultimately, this research paves the way for building practical, integrated photonic quantum systems with the potential to revolutionise fields such as secure communication, enhanced sensing, and computational science.