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Skadetålig programmering av klassiska och kvantmekaniska operationer på en fotonisk processor med hjälp av en digital tvilling

Scott Potter McHaffie

Published Jan 1, 2026• 0 citations
OpenAlex

Abstract

This thesis investigates whether a programmable photonic integrated circuit can remain useful even after some of its thermo-optic phase shifters have become permanently disabled. The approach is based on a digital twin: a differentiable model of the chip is trained from experimental current-output data and then used to solve an inverse-programming problem to recreate desired optical intensity transformations. The method is demonstrated on an 8 × 8 photonic circuit for both classical and quantum tasks. The trained twin accurately modeled the experimental setup, achieving a test fidelity greater than 0.99 for all disabled-phase-shifter configurations. Using the twin as a differentiable forward model to solve for currents, a mean fidelity of 0.9914 ± 0.0001 was measured over 100 target intensity transformations of the form T=|U|², where U is a Haar-random unitary. When any of five individual phase shifters were disabled this mean fidelity remained above 0.9739 ± 0.0002, and under cumulative disabled phase shifters the fidelity decreased to 0.8100 ± 0.0006 at 28 disabled phase shifters. A postselected dual-rail CNOT gate was also implemented and the highest postselected fidelity of 0.8730 ± 0.0097 was obtained for 20 disabled phase shifters, indicating that damage could act as a regularizer for two-photon interference.

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This thesis investigates whether a programmable photonic integrated circuit can remain useful even after some of its thermo-optic phase shifters have become permanently disabled. The approach is based on a digital twin: a differentiable model of the chip is trained from experimental current-output data and then used to solve an inverse-programming problem to recreate desired optical intensity transformations. The method is demonstrated on an 8 × 8 photonic circuit for both classical and quantum tasks. The trained twin accurately modeled the experimental setup, achieving a test fidelity greater than 0.99 for all disabled-phase-shifter configurations. Using the twin as a differentiable forward model to solve for currents, a mean fidelity of 0.9914 ± 0.0001 was measured over 100 target intensity transformations of the form T=|U|², where U is a Haar-random unitary. When any of five individual phase shifters were disabled this mean fidelity remained above 0.9739 ± 0.0002, and under cumulative disabled phase shifters the fidelity decreased to 0.8100 ± 0.0006 at 28 disabled phase shifters. A postselected dual-rail CNOT gate was also implemented and the highest postselected fidelity of 0.8730 ± 0.0097 was obtained for 20 disabled phase shifters, indicating that damage could act as a regularizer for two-photon interference.
Universal multiport interferometers based on Mach–Zehnder meshes are becoming a core building block for photonic neural-network accelerators and photonic quantum processors. This thesis presents the design, calibration, and control of such a programmable photonic integrated circuit (PIC). A fully automated software and hardware platform is developed to characterize and control the tunable elements of the PIC, enabling robust implementation of arbitrary linear optical transformations as well as target unitary operations. Using the calibrated device model, the control framework programs the PIC to realize specific unitaries and evaluates their performance through fidelity metrics computed from measured transfer matrices. By covering the complete stack from chip design to automated calibration and high-level unitary programming, this work establishes a scalable framework for design and operation of programmable photonic processors.
Integrated photonic technologies have recently shown significant advances, enabling the possibility to implement reconfigurable interferometers with increasing size. One of the main tasks to fully exploit the capabilities of reconfigurable integrated interferometers is the possibility to precisely program their operation to perform a desired target unitary. While recipes are known for circuit layouts based on a cascade of beam-splitter and phase-shifter operations, a methodology applicable for reconfigurable continuously-coupled waveguide arrays is currently missing. Here, we devise a machine learning based approach for this task, using a black box methodology that does not rely on precise a-priori modeling of the circuit internal architectures. We verify the effectiveness and the robustness of this approach via numerical simulations on different continuously-coupled waveguides layouts, either with planar or 3D structures. The proposed method makes use of a limited number of single- and two-photon measurements, making it suitable for optical quantum information processing. The obtained results open the perspective of employing this methodology as an effective tool to program the operation of integrated interferometers designed via different architectures.
Interest in quantum computing is rapidly growing in the scientific community as such technology could be the key to enable intensive use of machine learning and big data algorithms. Quantum computers can be conceived from basic photonic elements, such as Mach-Zehnder interferometers (MZI), but they still need control mechanisms and sensing elements from traditional VLSI technology. In this study, we present an ongoing study on a potential architecture for these basic photonic circuits. We realized two Photonic Integrated Circuit (PIC) test structures embedding metallic thermistors as phase shifters and silicon photodiodes as output detectors. We present a complete characterization of the phase shifter elements for an effective on-chip PIC interaction. To induce a consistent phase shift still retaining coherence of light, we act on the phase-shifters by means of a closed-loop control. By controlling through Pulse-Width Modulation the phase shifters, we drive the path of photons accurately in the PIC, demonstrating the effectiveness of the proposed configuration for the management of a photonic chip.
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.
General-purpose programmable photonic processors provide a versatile platform for integrating diverse functionalities on a single chip. Leveraging a two-dimensional hexagonal waveguide mesh of Mach–Zehnder interferometers, these systems have demonstrated significant potential in microwave photonic applications. Additionally, they are a promising platform for creating unitary linear transformations, which are key elements in quantum computing and photonic neural networks. However, a general procedure for implementing these transformations on such systems has not been established yet. This work demonstrates the programming of universal unitary transformations on a general-purpose programmable photonic circuit with a hexagonal topology. We detail the steps to split the light on-chip, demonstrate that an equivalent structure to the Mach–Zehnder interferometer with one internal and one external phase shifter can be built in the hexagonal mesh, and program both the triangular and rectangular architectures for matrix multiplication. We recalibrate the system to account for passive phase deviations. Experimental programming of 3 × 3 and 4 × 4 random unitary matrices yields fidelities >98% and bit precisions over five bits. To the best of our knowledge, this is the first time that random unitary matrices are demonstrated on a general-purpose photonic processor and pave the way for the implementation of programmable photonic circuits in optical computing and signal processing systems.
Programmable photonic integrated circuits have mainly been developed based on the single wavelength channel operation of fundamental building blocks consisting of Mach-Zehnder interferometers (MZIs) with tunable phase shifters. We propose and study optical circuit models consisting of cascaded optical resonators that enable the independent operation of multiple wavelength channels in a more compact footprint than the conventional MZIs. By adopting experimental values reported for silicon micro-ring resonators, the fidelities of different types of 2×2 unitary transformations and higher-dimensional unitary transformations are examined by employing the Reck algorithm and the Clements algorithm.