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Optics Express

Low-loss, polarization-independent single-photon switch at telecom wavelength

Jin-Hun Kim, Youn-Chang Jeong

Published Aug 31, 2026• 0 citations
Publisher

Abstract

Dynamically routing single photons without disturbing their quantum state is an essential capability for photonic quantum information technology. Here, we experimentally demonstrate a heralded single-photon switch that combines low loss, strict polarization independence, and high process fidelity in a simple architecture based on a free-space Sagnac interferometer incorporating a single Pockels cell. The routing of the photon between two spatial output modes is governed solely by the phase modulation applied to the Pockels cell, and is therefore independent of the input polarization state, while the Sagnac geometry provides passive interferometric stability without active phase locking. Operating at the telecom wavelength of 1550 nm, the switch exhibits an insertion loss of approximately 0.4 dB and an average extinction ratio of 19.1 dB. Quantum process tomography yields process fidelities of 0.985 and 0.979 with respect to the ideal identity operation for the two output modes, demonstrating that the polarization information encoded in the single photons is preserved through the switch. These features make the device a practical building block for fundamental quantum optics experiments, photonic quantum information processing, and quantum networks at telecom wavelengths.

DOI
10.1364/oe.612558

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Dynamically routing single photons without disturbing their quantum state is an essential capability for photonic quantum information technology. Here, we experimentally demonstrate a heralded single-photon switch that combines low loss, strict polarization independence, and high process fidelity in a simple architecture based on a free-space Sagnac interferometer incorporating a single Pockels cell. The routing of the photon between two spatial output modes is governed solely by the phase modulation applied to the Pockels cell, and is therefore independent of the input polarization state, while the Sagnac geometry provides passive interferometric stability without active phase locking. Operating at the telecom wavelength of 1550 nm, the switch exhibits an insertion loss of approximately 0.4 dB and an average extinction ratio of 19.1 dB. Quantum process tomography yields process fidelities of 0.985 and 0.979 with respect to the ideal identity operation for the two output modes, demonstrating that the polarization information encoded in the single photons is preserved through the switch. These features make the device a practical building block for fundamental quantum optics experiments, photonic quantum information processing, and quantum networks at telecom wavelengths.
An essential component of future quantum networks is an optical switch capable of dynamically routing single photons. Here we implement such a switch, based on a fiber-optical Sagnac interferometer design. The routing is implemented with a pair of fast electro-optical telecom phase modulators placed inside the Sagnac loop, such that each modulator acts on an orthogonal polarization component of the single photons, in order to yield polarization-independent capability that is crucial for several applications. We obtain an average extinction ratio of more than 19 dB between both outputs of the switch. Our experiment is built exclusively with commercial off-the-shelf components, thus allowing direct compatibility with current optical communication systems.
Abstract Photon polarization serves as an essential quantum information carrier in quantum information and measurement applications. Routing of arbitrarily polarized single photons and polarization‐entangled photons is a crucial technology for scaling up quantum information applications. Here, a low‐loss, noiseless, polarization‐maintaining routing of arbitrarily polarized single photons and, crucially, multi‐photon entangled states is demonstrated where the entanglement is encoded in orthogonal polarization bases, at the telecom L‐band. The interferometer‐based router is constructed by optics with a low angle of incidence and cross‐aligned electro‐optic crystals, achieving the polarization‐maintaining operation with a minimal number of optical components. The routing of arbitrarily‐polarized heralded single photons with a 0.057 dB (1.3%) loss, a 22 dB switching extinction ratio, and 99% polarization process fidelity to ideal identity operation is demonstrated. Moreover, the high‐quality router achieves the routing of two‐photon N00N‐type entangled states with a highly maintained interference visibility of 97%. The demonstrated router scheme preserving multi‐photon polarization state paves the way toward polarization‐encoded photonic quantum network as well as multi‐photon entanglement synthesis via spatial‐ and time‐multiplexing techniques.
The ability to reliably inscribe information on single photons is a crucial requirement for quantum applications such as quantum communication and measurement-based photonic quantum computation. Experimental implementations generally employ phase modulators in single-pass or Mach-Zehnder-interferometer configurations to encode information on photonic qubits. However, these approaches are intrinsically sensitive to environmental influences, which limits the achievable quantum error rates in practice, posing a major bottleneck in the transition of quantum applications with deterministic sources from the laboratory to the field. Inspired by the adaptation of Sagnac interferometer encoders for laser-based applications, we report a demonstration of a polarization encoder for single-photon qubits based on a free-space Sagnac interferometer. The device showcases inherent phase stability and overcomes previous error-rate limitations: Telecom-wavelength photons emitted by a quantum dot are modulated at a rate of 152 MHz and with a quantum bit error rate of 0.69 ± 0.02 % , marking the lowest value reported to date for MHz-rate information encoding on single photons by a factor of > 3 . This combination of low error rate, high modulation speed, stability, and operation with telecom-wavelength sub-Poissonian photons is an essential requirement for bringing quantum applications based on deterministic photon sources from the laboratory to the field and a significant step toward unlocking the full potential of a future quantum Internet.
Abstract In photonic quantum applications, optical routers are required to handle single photons with low loss, high speed, and preservation of their quantum states. Single-photon routing with maintained polarization states is particularly important for utilizing them as qubits. Here, we demonstrate a polarization-maintaining electro-optic router compatible with single photons. Our custom electro-optic modulator is embedded in a configuration of a Mach–Zehnder interferometer, where each optical component achieves polarization-maintaining operation. We observe the performance of the router with 2%–4% loss, 20 dB switching extinction ratio, 2.9 ns rise time, and >99% polarization process fidelity to an ideal identity operation.
Fiber-based entangled photon sources are a key resource in emerging quantum technologies due to compactness, long-term stability and alignment-free operation. Fast control of the quantum state properties is also necessary in practical scenarios, where a universal source can be employed with different temporal and spectral configurations. Here, we report on a flexible source design that produces high-quality entanglement from continuous-wave up to GHz-pulsed operation modes. Our approach uses off-the-shelf optical components in a Sagnac configuration to generate polarization-entangled photon pairs at telecom wavelength. This design further allows phase modulation above MHz speeds with high fidelity. Together with its 60 nm spectral bandwidth, the proposed system is entirely suitable for wavelength-multiplexed and reconfigurable quantum networks, where state modulation or active bandwidth allocation is required.
A fully fibered source of entangled photon pairs based on a nonlinear Sagnac interferometer is reported. Operating at telecom wavelengths, the source relies exclusively on standard fiber-optic components and periodically poled lithium niobate (PPLN) waveguides, resulting in a compact, robust, and field-deployable architecture. The generation stage supports both polarization and energy-time entanglement without modification, enabling versatile operation depending on the targeted application. Broadband spontaneous parametric down-conversion allows dense wavelength-division multiplexing over the telecom C and L bands. High normalized brightness (10.3 kpairs/s/nm/mW2^22) is achieved on a standard 100 GHz ITU channel pair, together with high entanglement quality. Polarization and energy-time encodings are characterized through state tomography and two-photon interference measurements, yielding fidelities, purities, and visibilities exceeding 96 % over multiple wavelength channels. The stability and reproducibility of the source are further evaluated through long-duration operation in a network environment. These results demonstrate that the proposed Sagnac source constitutes a practical and scalable building block for future plug-and-play quantum communication and quantum networking platforms.