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arXiv:2502.20069 · Advanced Quantum Technologies

High‐Fidelity Integrated Quantum Photonic Logic Via Robust Directional Couplers

Yonatan Piasetzky, Khen Cohen, Amit Rotem, Yuval Warshavsky, Yehonatan Drori, Yaron Oz, Haim Suchowski

Published Oct 1, 2026• 0 citations
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Abstract

ABSTRACT Scalable quantum information processing with integrated photonics requires quantum logic operations with low error probability and robustness. Directional couplers, the fundamental elements enabling quantum interference and logic operations, are inherently sensitive to fabrication imperfections and environmental fluctuations, leading to increased gate error probabilities. Here, we experimentally demonstrate a passive design strategy that mitigates these errors by exploiting a stationary geometrical configuration in uniform directional couplers, where first‐order variations in the coupling coefficient are intrinsically suppressed. The robust geometry is implemented in a silicon‐on‐insulator photonic chip hosting two‐photon controlled‐NOT (CNOT) quantum gates and its performance is directly compared to a non‐optimized design fabricated under the same process and measurement conditions. Measurements indicate a mean error probability of , lower than the non‐robust implementation value of , without active tuning or footprint increase. The remaining error is dominated by finite photon indistinguishability, with a corrected HOM‐limited error floor of . Monte Carlo simulations incorporating measured fabrication noise are consistent with the observed reduction and with suppression of gate‐level error rates. These results support the use of stationary‐geometry couplers as compact, fabrication‐tolerant building blocks for scalable photonic quantum circuits.

DOI
10.1002/qute.70507

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ABSTRACT Scalable quantum information processing with integrated photonics requires quantum logic operations with low error probability and robustness. Directional couplers, the fundamental elements enabling quantum interference and logic operations, are inherently sensitive to fabrication imperfections and environmental fluctuations, leading to increased gate error probabilities. Here, we experimentally demonstrate a passive design strategy that mitigates these errors by exploiting a stationary geometrical configuration in uniform directional couplers, where first‐order variations in the coupling coefficient are intrinsically suppressed. The robust geometry is implemented in a silicon‐on‐insulator photonic chip hosting two‐photon controlled‐NOT (CNOT) quantum gates and its performance is directly compared to a non‐optimized design fabricated under the same process and measurement conditions. Measurements indicate a mean error probability of , lower than the non‐robust implementation value of , without active tuning or footprint increase. The remaining error is dominated by finite photon indistinguishability, with a corrected HOM‐limited error floor of . Monte Carlo simulations incorporating measured fabrication noise are consistent with the observed reduction and with suppression of gate‐level error rates. These results support the use of stationary‐geometry couplers as compact, fabrication‐tolerant building blocks for scalable photonic quantum circuits.
Directional couplers are fundamental building blocks for integrated quantum photonic circuits but are highly sensitive to fabrication imperfections. We demonstrate that composite segmented directional couplers (CSDCs) enhance gate fidelity without active tuning. We design and fabricate fully integrated photonic controlled-NOT (CNOT) gates using both uniform and composite coupler variants and compare their performance via simulation, classical characterization, and quantum two-photon interference. The composite design reduces the average computational-basis error probability by nearly a factor of two and variability by a factor of five, with residual error primarily limited by photon indistinguishability. These results establish CSDCs as passive, foundry-compatible building blocks for scalable quantum photonic circuits.
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.
Integrated quantum photonic circuits are a key enabling technology for the scalability of quantum information systems. Among the available platforms, silicon photonics offers an unrivalled capability for the large-scale integration of photonic components within compact footprints. However, the strong index contrast that enables ultra-compact silicon devices also makes them highly sensitive to fabrication imperfections. As circuit complexity increases, active tuning is generally required to maintain spectral alignment among the different components, leading to significant power consumption that ultimately limit scalability. Here, we demonstrate a fully integrated silicon quantum photon-pair source operating without active tuning of any component. The circuit combines photon-pair generation in a micro-ring resonator, pump rejection using Bragg filters, and signal/idler demultiplexing through modal add-drop filters with building blocks engineered to minimize sensitivity to fabrication variations. The resulting circuit achieves excellent experimental quantum performance. Coincidence rates up to 4000 counts s^-1 with coincidence-to-accidental ratios as high as 100 are obtained across the generated spectrum, while separate two-photon interference measurements yield raw visibilities exceeding 93% for individually selected ITU wavelength-channel pairs. By eliminating the need for active spectral tuning while maintaining high quantum performance, this work addresses a major bottleneck in the scaling of complex silicon quantum photonic circuits.
We propose and analyze the design of a programmable photonic integrated circuit for high-fidelity quantum computation and simulation. We demonstrate that the reconfigurability of our design allows us to overcome two major impediments to quantum optics on a chip: it removes the need for a full fabrication cycle for each experiment and allows for compensation of fabrication errors using numerical optimization techniques. Under a pessimistic fabrication model for the silicon-on-insulator process, we demonstrate a dramatic fidelity improvement for the linear optics controlled-not and controlled-phase gates and, showing the scalability of this approach, the iterative phase estimation algorithm built from individually optimized gates. We also propose and simulate an experiment that the programmability of our system would enable: a statistically robust study of the evolution of entangled photons in disordered quantum walks. Overall, our results suggest that existing fabrication processes are sufficient to build a quantum photonic processor capable of high-fidelity operation.
Quantum photonic devices are typically based on either discrete variable (DV) or continuous variable (CV) principles. In either case, their performance must meet strict “fidelity” requirements, especially when scaled using integrated photonic circuitry. DV devices must be robust to imperfections such as timing delays between photons, unwanted spectral correlations present in photons from nonideal single photon sources and variations in circuit components. Furthermore, it becomes increasingly important to have a methodology for characterizing the performance of devices in the presence of such errors. Here, we present a method for simulating quantum photonic integrated circuits (PICs) and use it to model the behavior of a circuit constructed to model the evolution of a quantum state subject to a Bose-Hubbard Hamiltonian, introducing variations to the coupling gaps in the directional couplers inside the circuit. Input states consisting of both indistinguishable and distinguishable photons are modelled. Compact models for the directional couplers and other photonic circuit elements that are parameterized for a specific fabrication process can be incorporated into the circuit using a foundry-associated compact model library, ensuring consistency between circuit design and manufactured component. In CV applications, the degree of squeezing provided by some nonlinear element is often a key metric. Here we describe how the degree of squeezing produced by spontaneous four-wave mixing (SFWM) in a microring resonator can be modelled in the low-power limit, accounting for the effects of self-phase modulation (SPM), cross-phase modulation (XPM), and component losses.
This paper reviews recent advances in integrated waveguide circuits, lithographically fabricated for quantum optics. With the increase in complexity of realizable quantum architectures, the need for stability and high quality nonclassical interference within large optical circuits has become a matter of concern in modern quantum optics. Using integrated waveguide structures, we demonstrate a high performance platform from which to further develop quantum technologies and experimental quantum physics using single photons. We review the performance of directional couplers in Hong-Ou-Mandel experiments, together with inherently stable interferometers with controlled phase shifts for quantum state preparation, manipulation, and measurement as well as demonstrating the first on-chip quantum metrology experiments. These fundamental components of optical quantum circuits are used together to construct integrated linear optical realizations of two-photon quantum controlled logic gates. The high quality quantum mechanical performance observed at the single photon level signifies their central role in future optical quantum technologies.