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Journal of Physics Photonics

Photonic quantum circuits with only synchronous directional couplers in ion-exchanged glass

Jesús Liñares, Carlos Montero-Orille, Xesús Prieto-Blanco, Javier Varela-Carballo, Daniel Balado

Published Oct 16, 2025• 0 citations
Publisher

Abstract

Abstract We show that photonic quantum circuits for implementing arbitrary SU(N) transformations can be made using only synchronous directional couplers. These simple elements provide both beam splitting and phase shifting so that, under a proper concatenation of such elements, unitary transformations of arbitrary dimension can be implemented. The circuits made in this way are robust and controllable, because only one type of element is used and the technology involved in their fabrication is very mature. We present several examples of original passive photonic circuits intended for quantum projective measurements and quantum state generation due to their interest in photonic quantum technology for quantum key distribution systems. Likewise, we have designed, fabricated, and characterized, on an ion-exchanged glass platform, different couplers, which form the basic elements or building blocks of these passive circuits highly compatible with optical fibres.

DOI
10.1088/2515-7647/ae142b

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Abstract We show that photonic quantum circuits for implementing arbitrary SU(N) transformations can be made using only synchronous directional couplers. These simple elements provide both beam splitting and phase shifting so that, under a proper concatenation of such elements, unitary transformations of arbitrary dimension can be implemented. The circuits made in this way are robust and controllable, because only one type of element is used and the technology involved in their fabrication is very mature. We present several examples of original passive photonic circuits intended for quantum projective measurements and quantum state generation due to their interest in photonic quantum technology for quantum key distribution systems. Likewise, we have designed, fabricated, and characterized, on an ion-exchanged glass platform, different couplers, which form the basic elements or building blocks of these passive circuits highly compatible with optical fibres.
Ion-exchanged glass as a platform for quantum photonics is proposed. Quantum projectors are implemented with integrated optical directional couplers fabricated by ion-exchange K+^++/Na+^++in soda-lime glass. We consider devices composed of concatenated directional couplers which implementNNN-dimensional quantum projective measurements, and concomitantly the production of 1-qudit states. The fundamental units of these devices are 2 × 2 directional couplers that are experimentally studied in order to obtain, through an optical characterization, empiric relationships between fabrication and optical parameters of such couplers. Likewise, a two-dimensional quantum projector is demonstrated so that projective measurements are obtained for the states of basesX(diagonal) andY(circular).
Abstract A design framework to implement non-unitary input–output operations to a practical unitary photonic integrated circuit is described. This is achieved by utilising the cosine-sine decomposition to recover the unitarity of the original operation. The recovered unitary operation is decomposed into fundamental unitary building blocks, forming a photonic integrated circuit network based on directional couplers and waveguide phase shifters. The individual building blocks are designed and optimised by three-dimensional full-wave simulations and scaled up using a circuit approach. The paper investigates the scalability and robustness of the design approach. Our study demonstrates that the proposed approach of performing unitary matrix completion can be applied to any arbitrary matrices. This design approach allows for implementation of non-unitary operations to perform various linear functions in neuromorphic photonics for computing, sensing, signal processing and communications.
Integrated photonic circuits with many input and output modes are essential in applications ranging from conventional optical telecommunication networks, to the elaboration of photonic qubits in the integrated quantum information framework. In particular, the latter field has been object in the recent years of an increasing interest: the compactness and phase stability of integrated waveguide circuits are enabling experiments unconceivable with bulk-optics set-ups. Linear photonic devices for quantum information are based on quantum and classical interference effects: the desired circuit operation can be achieved only with tight fabrication control on both power repartition in splitting elements and phase retardance in the various paths. Here we report on a novel three-dimensional circuit architecture, made possible by the unique capabilities of femtosecond laser waveguide writing, which enables us to realize integrated multimode devices implementing arbitrary linear transformations. Networks of cascaded directional couplers can be built with independent control on the splitting ratios and the phase shifts in each branch. In detail, we show an arbitrarily designed 5×5 integrated interferometer: characterization with one- and two-photon experiments confirms the accuracy of our fabrication technique. We exploit the fabricated circuit to implement a small instance of the boson-sampling experiments with up to three photons, which is one of the most promising approaches to realize phenomena hard to simulate with classical computers. We will further show how, by studying classical and quantum interference in many random multimode circuits, we may gain deeper insight into the bosonic coalescence phenomenon.
The design, fabrication, and detailed calibration of essential building blocks towards fully integrated linear-optics quantum computation are discussed. Photonic devices are made from silicon nitride rib waveguides, where measurements on ring resonators show small propagation losses. Directional couplers are designed to be insensitive to fabrication variations. Their offset and coupling lengths are measured, as well as the phase difference between the transmitted and reflected light. With careful calibrations, the insertion loss of the directional couplers is found to be small. Finally, an integrated controlled-NOT circuit is characterized by measuring the transmission through different combinations of inputs and outputs. The gate fidelity for the CNOT operation with this circuit is estimated to be 99.81% after post selection. This high fidelity is due to our robust design, good fabrication reproducibility, and extensive characterizations.
We present and discuss perspectives of current developments on advanced quantum optical circuits monolithically integrated in the lithium niobate platform. A set of basic components comprising photon pair sources based on parametric down conversion (PDC), passive routing elements and active electro-optically controllable switches and polarisation converters are building blocks of a toolbox which is the basis for a broad range of diverse quantum circuits. We review the state-of-the-art of these components and provide models that properly describe their performance in quantum circuits. As an example for applications of these models we discuss design issues for a circuit providing on-chip two-photon interference. The circuit comprises a PDC section for photon pair generation followed by an actively controllable modified mach–Zehnder structure for observing Hong–Ou–Mandel interference. The performance of such a chip is simulated theoretically by taking even imperfections of the properties of the individual components into account.
ABSTRACT Unitary operations lie at the heart of quantum computing, enabling the manipulation of quantum states in a reversible and coherent manner. In photonic integrated circuits, these operations are realized using photonic waveguide architectures composed of passive beam splitters and reconfigurable phase shifters. This review provides an overview of photonic circuit architectures designed to implement unitary transformations. We examine various design strategies, material platforms, and the current state‐of‐the‐art in realizing high‐fidelity photonic unitaries. In addition, we explore the theoretical underpinnings of photon transport in such circuits, shedding light on the quantum behavior of light in unitary photonic systems. Finally, we introduce a simulation framework for modeling and analyzing these photonic networks, offering a practical tool for research and development in this rapidly growing field.