New waveguide design cuts wavelength fluctuations in quantum light by ten times

A new design using Su-Schrieffer-Heeger (SSH) waveguide arrays reduces fluctuations in the wavelength of generated quantum light by more than an order of magnitude, achieving ten times greater stability than conventional methods. Researchers from Université Paris Cité and Université Paris-Saclay demonstrated that these topological arrays uniquely maintain a stable resonance spectrum even with imperfections, unlike uniform or “trivial” designs.

The work addresses a previously unexplored area, the impact of topology on the spectral properties of spontaneous parametric down-conversion, a process important for building scalable quantum photonic circuits. These results establish nonlinear waveguide arrays as a promising platform for integrating topology and nonlinearity in quantum technologies.

Topological SSH Arrays Reduce Wavelength Fluctuations in SPDC

AlGaAs waveguide arrays now maintain stable photon-pair generation even with substantial coupling disorder, a feat achieved through the implementation of topological Su-Schrieffer-Heeger (SSH) arrays. Unlike conventional and “trivial” array designs, these topological configurations limit resonance wavelength fluctuations during spontaneous parametric down-conversion (SPDC), a process vital for creating scalable quantum photonic circuits. Experiments demonstrate that the topological arrays preserve a stable SPDC resonance spectrum, a critical improvement for on-chip quantum light sources.

The observed stability stems from the unique band-structure properties of the interacting modes within the SSH array, as revealed by both exact numerical simulations and analytical modeling. Disorder, which significantly impacts the SPDC resonance spectrum in uniform and trivial arrays, induces minimal fluctuations in the topological design, even at substantial disorder levels.

This resilience is not simply a matter of reduced fluctuation; the topological channel dominates the SPDC interaction, effectively stabilizing the resonance against coupling disorder and ensuring consistent performance. The team’s work, conducted with AlGaAs chosen for its strong nonlinear properties, highlights a pathway to robust on-chip SPDC. A systematic comparison of uniform, trivial and topological Su-Schrieffer-Heeger (SSH) arrays reveals that only the topological configuration preserves a stable SPDC resonance spectrum under coupling disorder, with fluctuations in the resonance wavelength reduced by more than an order of magnitude.

Beyond spectral stability, the research indicates that topological SSH arrays facilitate high-fidelity entangled state generation. Numerical analysis further shows that each SSH array can operate as a robust photon-pair source, enabling stable interference between multiple such sources and the generation of high-dimensional entangled states with high fidelity.

While perturbations like waveguide width fluctuations can break chiral symmetry and diminish perfect topological protection, the arrays still outperform non-topological designs. In contrast, uniform and trivial-mode arrays exhibit large spectral fluctuations, hindering the creation of predictable and reliable quantum light sources. This preservation of spectral properties is important for applications requiring precise control over the quantum state of the generated photons.

Su-Schrieffer-Heeger (SSH) Arrays Enable Robust Photon-Pair Generation

The ability to generate entangled photon pairs with stable spectral properties is critical for scalable quantum photonic circuits, and a new study demonstrates that Su-Schrieffer-Heeger (SSH) arrays significantly reduce fluctuations in the wavelength of generated photons. Numerical analysis detailed in the study reveals that the topological interface created within the SSH array by inserting an additional long spacing gives rise to a mode exponentially localized on the central waveguide.

This localized mode resides within the band structure, providing inherent protection against perturbations that would otherwise disrupt the SPDC resonance. The team’s analytical model confirms that this band structure is the origin of the observed robustness, offering a theoretical foundation for further optimization of these devices.

The study’s findings also demonstrate greater resilience in the generated biphoton properties compared to isolated waveguides, suggesting that SSH arrays offer a pathway to more reliable quantum light sources. The work builds on previous explorations of topological photonic lattices, which have demonstrated quantum interference of single-photon states and protection of polarization-entangled states.

However, this study represents the first experimental demonstration of topological protection of a parametric spectrum in a one-dimensional system, opening possibilities for simpler and more compact on-chip implementations. This intrinsic connection to condensed-matter models, they suggest, allows for optical simulations of phenomena like Anderson localization and Bloch oscillations.

Waveguide Disorder Impacts SPDC Resonance Spectrum Stability

This level of spectral stability is critical for building complex quantum photonic circuits. The work directly addresses a gap in understanding how topology, a property of materials relating to their band structure, influences the spectral characteristics of parametric processes. These processes, including SPDC, are essential for generating the entangled photon pairs used in scalable quantum computing and communication. The team systematically compared uniform arrays, those with trivial localized modes and the topological SSH arrays under varying degrees of coupling disorder.

The researchers quantified this stability by calculating the overlap between disordered resonance spectra and a disorder-free spectrum, finding the topological configuration consistently outperformed the others. Analysis of 1000 disorder realizations demonstrated the resilience of the SSH array’s spectral features.

Further calculations showed that the standard deviation of the resonance peak position remained minimal even with increasing disorder. Results indicate that SSH arrays maintain higher fidelity than single-waveguide sources, even with waveguide-width fluctuations. The ability to generate robust photon pairs is a key step toward realizing more stable and scalable quantum information processing systems. The team’s findings suggest that topological protection can extend beyond simply preserving the overall spectrum to also safeguarding the spectral features important for complex quantum interference.

The work builds on the growing field of topological photonics, which seeks to harness the unique properties of topological insulators and apply them to the manipulation of light. By demonstrating topological protection of SPDC, this research opens new avenues for developing robust and efficient on-chip sources of entangled photons, essential components for future quantum technologies.

Band Structure Explains Topological Protection of SPDC

Unlike conventional designs, Su-Schrieffer-Heeger (SSH) arrays exhibit a unique resilience to coupling disorder, maintaining a stable resonance spectrum where uniform arrays fail. This preservation of spectral features is a direct consequence of the array’s band structure and how it interacts with the generated photons. Detailed analysis of the arrays demonstrates that the resonance profile and peak position in uniform designs are acutely sensitive to even minor disorder.

The introduction of a topological interface in SSH arrays created by alternating short and long spacings between waveguides fundamentally alters this behavior. This interface generates a localized topological mode residing within the band structure, providing a degree of protection against fluctuations in the coupling between waveguides.

The supermode model used in the study explains that this protection begins to diminish when disorder strength approaches the dimerization parameter, but even then, fluctuations remain significantly smaller than those observed in non-topological arrays. The dominance of the topological channel in the SPDC interaction is key to stabilizing the resonance, ensuring a more consistent output even with imperfections.

Simulations reveal that while disorder can shift or broaden the topological supermode, its continued prominence prevents the dramatic spectral instability seen in uniform arrays. Further investigation into the impact of waveguide width fluctuations, which modify the propagation constants of the photons, reveals that even these perturbations do not entirely eliminate the topological protection.

While such variations can break chiral symmetry and lift exact protection, the SSH array still demonstrates superior stability compared to its uniform counterparts. The implications of this work extend beyond simply improving the stability of individual photon-pair sources. The ability to generate and manipulate entangled states with high fidelity is important for a range of applications, from quantum computing to secure communication, and this new platform offers a promising pathway towards achieving those goals. The compact on-chip design, operating at room temperature and telecom wavelengths, further enhances its potential for practical implementation.

Evanescent Coupling Defines Waveguide Array Functionality

This level of spectral preservation is critical for scaling quantum photonic circuits, as even small wavelength shifts can diminish interference effects and reduce fidelity in complex systems. The team’s analysis reveals that the resilience stems from how light propagates within the array, specifically through the interplay of evanescent coupling and the engineered band structure. Coupled-mode equations describe the behavior of signal and idler photons generated via spontaneous parametric down-conversion (SPDC), demonstrating that the topological configuration effectively isolates the SPDC process from disruptive fluctuations in coupling constants.

This isolation is a preservation of the spectral shape itself, ensuring consistent photon-pair characteristics even with significant disorder, up to 40% in the coupling constants tested. The equations account for the pump field’s reduced coupling efficiency due to its halved wavelength, a detail important for accurate modeling of the SPDC process.

Further investigation focused on the impact of introducing a defect into the array, a wider central waveguide, creating a “trivial-mode array” intended to localize the generated photons. However, this design proved vulnerable to disorder, exhibiting significant fluctuations in the resonance spectrum. “As a result, the phase-matching condition associated with this mode remains stable in the SSH array, while it fluctuates with disorder in the trivial case,” the researchers note, highlighting the stark contrast in performance.

This instability arises because the localized mode lacks the inherent protection offered by the topological properties of the SSH array, making it susceptible to even minor variations in the waveguide network. The SSH array’s robustness extends beyond coupling disorder to include fluctuations in waveguide width, though the protection is not absolute. Simulations and experimental results confirm that the topological SSH array maintains a stable resonance spectrum even with significant disorder, while trivial arrays exhibit substantial spectral shifts.

More like this

Quantum Computing NewsPasqal and CNRS open joint lab to advance neutral-atom qubits

PhysicsPolitecnico di Milano joins $4M study of quantum reality’s foundations

Quantum HardwareEuropean Research Council funds Delft team to build scalable spin qubits over six years

Quantum PhysicsNew Physics Formulated Without Explicit Time. Research by David Deutsch and Chiara Marletto