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Optical Fiber Communication Conference

Highly pure 4-qubit states fully integrated in a programmable silicon-photonic chip

Jong-Moo Lee, Jiho Park, Jeongho Bang, Young-Ik Sohn, Alessio Baldazzi, Matteo Sanna, Stefano Azzini, Lorenzo Pavesi

Published Jan 1, 2024• 0 citations
Publisher

Abstract

We demonstrate 98% Hong-Ou-Mandel (HOM) visibility, 82% Green-berger–Horne–Zeilinger (GHZ) fidelity, and Bell’s inequality violations by 4-photon coincident measurements using a silicon-photonic chip including photon-pair sources, filters, and linear-optic gates.
DOI
10.1364/ofc.2024.tu3c.1

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We present a programmable silicon photonic four-qubit integrated circuit for the generation and manipulation of diverse quantum states. The silicon photonic chip integrates photon-pair sources, pump-reducing filters, wavelength-division-multiplexing filters, Mach–Zehnder interferometer switches, and single-qubit arbitrary gates, enabling versatile state preparation and tomography. We measure Hong–Ou–Mandel interference with an impressive 98% visibility using four-photon coincidence, laying the foundation for high-purity qubits. Our analysis involves estimating the fidelity and purity of distinct quantum states through maximum-likelihood estimation applied to tomographic measurements. In our experimental results, we showcase the following achievements: a heralded single qubit achieving 98.2% fidelity and 98.3% purity, a Bell state reaching 95.2% fidelity and 94.8% purity, and a four-qubit system with two simultaneous Bell states exhibiting 87.4% fidelity and 84.6% purity. Finally, a four-qubit Greenberger–Horne–Zeilinger (GHZ) state demonstrates 85.4% fidelity and 81.7% purity. In addition, we certify the entanglement of the four-photon GHZ state through Bell’s inequality violations and a negative entanglement witness.
We present a programmable Silicon Photonic 4-qubit integrated circuit for the generation and manipulation of diverse quantum states. The Silicon Photonic chip integrates photon-pair sources, pump-removing filters, wavelength-division-multiplexing (WDM) filters, Mach-Zehnder Interferometer (MZI) switches, and single-qubit arbitrary gates, enabling versatile state preparation and tomography. We measure Hong-Ou-Mandel (HOM) interference with an impressive 98% visibility using 4-photon coincidence, laying the foundation for high-purity qubits. Our analysis involves estimating the fidelity and purity of distinct quantum states through maximum-likelihood estimation (MLE) applied to tomographic measurements. In our experimental results, we showcase the following achievements: a heralded single qubit achieving 98.2% fidelity and 98.3% purity, a Bell state reaching 95.2% fidelity and 94.8% purity, and a 4-qubit system with two simultaneous Bell states exhibiting 87.4% fidelity and 84.6% purity. Finally, a 4-qubit Greenberger–Horne–Zeilinger (GHZ) state demonstrates 85.4 % fidelity and 81.7% purity. Additionally, we certify the entanglement of the 4-photon GHZ state through Bell's inequality violations and a negative entanglement witness.
We present a programmable Silicon Photonic 4-qubit integrated circuit for the generation and manipulation of diverse quantum states. The Silicon Photonic chip integrates photon-pair sources, pump-removing filters, wavelength-division-multiplexing (WDM) filters, Mach-Zehnder Interferometer (MZI) switches, and single-qubit arbitrary gates, enabling versatile state preparation and tomography. We measure Hong-Ou-Mandel (HOM) interference with an impressive 98% visibility using 4-photon coincidence, laying the foundation for high-purity qubits. Our analysis involves estimating the fidelity and purity of distinct quantum states through maximum-likelihood estimation (MLE) applied to tomographic measurements. In our experimental results, we showcase the following achievements: a heralded single qubit achieving 98.2% fidelity and 98.3% purity, a Bell state reaching 95.2% fidelity and 94.8% purity, and a 4-qubit system with two simultaneous Bell states exhibiting 87.4% fidelity and 84.6% purity. Finally, a 4-qubit Greenberger–Horne–Zeilinger (GHZ) state demonstrates 85.4 % fidelity and 81.7% purity. Additionally, we certify the entanglement of the 4-photon GHZ state through Bell's inequality violations and a negative entanglement witness.
Photonic integrated circuits offer a scalable and robust route toward quantum information technologies by consolidating photon sources and linear optical networks onto compact, wafer-manufacturable chips. Although silicon photonics has enabled diverse discrete-variable quantum breakthroughs -- spanning multiphoton entanglement, quantum networking, and photonic qubit fusion for quantum computing -- scaling these platforms beyond proof-of-principle demonstrations remains severely constrained by a critical system-level bottleneck. Optical loss compounds rapidly across photon generation, routing, and state analysis, causing multiphoton generation probabilities to plummet exponentially as circuit depth and complexity grow. Here we overcome this rate-loss barrier by demonstrating a monolithic, ultralow-loss silicon nitride (Si3_33​N4_44​) integrated photonic platform engineered for high-performance discrete-variable quantum information processing. Our architecture seamlessly integrates narrowband photon-pair sources with low-loss qubit-fusion circuits and reconfigurable state-analysis interferometers. The on-chip sources prepare Einstein-Podolsky-Rosen (EPR) states with a fidelity of 0.9875(3) and exhibit near-unity photon indistinguishability, yielding a heralded Hong-Ou-Mandel interference visibility of 0.990(6). By executing on-chip fusion of two EPR states, we synthesize and characterize four-photon Greenberger-Horne-Zeilinger states with a record fidelity of 0.943(8) and a fourfold count rate of 27 Hz -- more than two orders of magnitude higher than previous silicon-photonic implementations. Combined with standard CMOS-compatible fabrication on 150-mm-diameter wafers, these results establish ultralow-loss Si3_33​N4_44​ integrated photonics as a definitive, manufacturable platform for deployable, large-scale quantum information processors.
Abstract Although holding great promise for low noise, ease of operation and networking 1 , useful photonic quantum computing has been precluded by the need for beyond-state-of-the-art components, manufactured by the millions 2–6 . Here we introduce a manufacturable platform 7 for quantum computing with photons. We benchmark a set of monolithically integrated silicon-photonics-based modules to generate, manipulate, network and detect heralded photonic qubits, demonstrating dual-rail photonic qubits with 99.98% ± 0.01% state preparation and measurement fidelity, Hong–Ou–Mandel (HOM) quantum interference between independent photon sources with 99.50% ± 0.25% visibility, two-qubit fusion with 99.22% ± 0.12% fidelity and a chip-to-chip qubit interconnect with 99.72% ± 0.04% fidelity, conditional on photon detection and not accounting for loss. We preview a selection of next-generation technologies: low-loss silicon nitride (SiN) waveguides and components to address loss, as well as fabrication-tolerant photon sources, high-efficiency photon-number-resolving detectors (PNRDs), low-loss chip-to-fibre coupling and barium titanate (BTO) electro-optic phase shifters for high-performance fast switching.