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Journal of Lightwave Technology

Stable Signal Processing With a Photonic Processor

Igor A. Litvin, Gökhan Elmas, Paul R. Kohl, Janis Nötzel

Published Sep 4, 2025• 0 citations
Publisher

Abstract

A key objective in the development of integrated photonic circuits for large-scale optical quantum information processing is achieving both reconfigurability and stable output signals. One outcome of this development is the creation of multimode photonic processors, or reconfigurable multimode interferometers, which have broad applications in both quantum and classical information processing. However, active cooling systems introduce phase fluctuations in multi-port input signals, leading to instability in interference patterns and degrading signal processing precision. In this work, we implement an on-chip feedback control to mitigate input signal phase fluctuation. This approach reduces phase instability by more than 10-fold for multi-port input signals. This significantly enhances the precision and reliability of signal processing within the photonic processor. To validate the stability and repeatability of matrix multiplication using a feedback control scheme on a photonic processor, we successfully implemented Hadamard transformations of different orders with multi-port input signals.

DOI
10.1109/jlt.2025.3606027

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A key objective in the development of integrated photonic circuits for large-scale optical quantum information processing is achieving both reconfigurability and stable output signals. One outcome of this development is the creation of multimode photonic processors, or reconfigurable multimode interferometers, which have broad applications in both quantum and classical information processing. However, active cooling systems introduce phase fluctuations in multi-port input signals, leading to instability in interference patterns and degrading signal processing precision. In this work, we implement an on-chip feedback control to mitigate input signal phase fluctuation. This approach reduces phase instability by more than 10-fold for multi-port input signals. This significantly enhances the precision and reliability of signal processing within the photonic processor. To validate the stability and repeatability of matrix multiplication using a feedback control scheme on a photonic processor, we successfully implemented Hadamard transformations of different orders with multi-port input signals.
Achieving both reconfigurability and stable output signals is a critical challenge in the development of integrated photonic circuits for large-scale optical quantum information processing. This has led to the creation of multimode photonic processors, also known as reconfigurable multimode interferometers, which have wide-ranging applications in quantum and classical information processing. However, maintaining phase stability in multi-port input signals remains a significant hurdle, particularly due to the phase instabilities introduced by active cooling systems and temperature drifts in the photonic processor. In this study, we propose theoretical models to simulate phase instability in photonic processors and validate them against experimental results. Two distinct modeling approaches were employed: a Brownian random walk and phase reconstruction based on experimentally observed oscillating harmonics. Additionally, we verified and applied our model to a specific application for input phase correction using self-feedback control within the photonic processor.
Integrated photonics is an essential technology for optical quantum computing. Universal, phase-stable, reconfigurable multimode interferometers (quantum photonic processors) enable manipulation of photonic quantum states and are one of the main components of photonic quantum computers in various architectures. In this paper, we report the realization of the largest quantum photonic processor to date. The processor enables arbitrary unitary transformations on its 20 input modes with an amplitude fidelity of FHaar=97.4% and FPerm=99.5% for Haar-random and permutation matrices, respectively, an optical loss of 2.9 dB averaged over all modes, and high-visibility quantum interference with VHOM=98% . The processor is realized in Si3N4 waveguides and is actively cooled by a Peltier element.
Abstract : Photons play a central role in many areas of quantum information processing and quantum sensing, ranging from linear optics quantum computing and quantum simulation to quantum communications. A central problem in many of these applications is the need to control many spatial and temporal modes with high efficiency and precision. Photonic integrated circuits can contain closely-spaced and extremely phase-stable components that enable precision control of many spatial and temporal modes in dielectric waveguides. This program developed photonic integrated circuits (PICs) based on the silicon-on-insulator platform. We developed large-scale PICs with cascaded Mach-Zehnder interferometers (MZI) with precision electro-optic modulators. These PICs have very low internal losses (0.1 dB/MZI) and achieve exceptionally high contrast interference ( 80 dB) over tens of spatial modes. These fully programmable mode transformers have driven experimental and theoretical advances in quantum simulation, cluster-state quantum computing, all-optical quantum repeaters, neuromorphic computing, and other applications. In addition, we developed new schemes for ballistic quantum computation, new methods for high-efficiency single photon sources, a new approach for 3-photon cluster state generation that forms the essential ingredient for percolation-based generation of scalable cluster states, and quantum logic gates based on weak optical nonlinearities.
Programmable photonic integrated circuits are emerging as increasingly large and versatile interferometric processors operating at room temperature. However, their native operations are linear, while many computational tasks require nonlinear input-output transformations that typically rely on nonlinear optical materials or resonant devices. Here, we establish a direct mapping between quantum signal processing (QSP) and the native two-mode operations of programmable interferometric photonic integrated circuits, showing that the SU(2)SU(2)SU(2) structure required by QSP can be realized through programmable phase control and mode mixing. We experimentally demonstrate this correspondence using dual-rail single-photon encoding on a 24-mode programmable photonic integrated circuit, realizing QSP sequences up to depth L=11L=11L=11 and synthesizing STEP, ReLU and SELU functions through programmable phase control. The optical transformation remains linear, while repeated encoding of the input variable and coherent interference produce a nonlinear dependence of the output probabilities on the encoded variable. Across all accessible circuit depths, the measured responses follow the programmed QSP transformations, with hardware-induced mean squared errors between 10−310^{-3}10−3 and 10−210^{-2}10−2, all remaining below the intrinsic finite-depth approximation error. These results establish an algorithm-to-hardware mapping of QSP on programmable photonic processors. They also demonstrate an algorithmic route to programmable nonlinear functions of encoded variables on linear photonic hardware.
We report a joint-detection (JD) experiment between two reconfigurable photonic processors in which Hadamard codewords are generated on a transmitter processor and a Hadamard transform is performed on a receiver (decoder) processor. Crucially, the decoder executes multi-input phase stabilization concurrently with the optical Hadamard combining, ensuring the phase alignment required for coherent joint detection. We demonstrate JD using 2×2 and 4×4 Hadamard transformations and verify stable temporal outputs under active on-chip feedback control (FBC). The approach leverages our multi-input FBC scheme to suppress low-frequency inter-input phase fluctuations and to realize coherent combining consistent with Green-Machine style architectures. These results indicate a practical path to scalable, codeword-level optical joint detection on programmable photonic hardware and are compatible with larger-mode processors and higher-order code families.
Programmable photonics can find applications in myriad areas including the quantum information field, which encompasses communications, computing, sensing and tomography. Large-scale bulk optics setups previously prevented the development of more complex and scalable quantum optics configurations. Linear optic systems with the required fidelity require a strict control of interference through demanding phase stability mechanisms. Integrating a considerable number of photonic elements on a chip in order to implement multi-port interferometers has become the only viable technological path towards quantum information systems. This chapter introduces the applications of programmable photonics to quantum information systems. After introducing the general framework of a programmable quantum photonic system integrated on a chip and briefly describing the role of more external components such as sources and detectors, it covers the relationship between reconfigurable integrated optic circuits and linear optical quantum gates, quantum transport simulation, boson sampling and complex Hadamard and quantum Fourier transforms.