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arXiv:2401.06369 · Japanese Journal of Applied Physics

Low-loss polarization-maintaining optical router for photonic quantum information processing

Pengfei Wang, So-Young Baek, Keiichi Edamatsu, Fumihiro Kaneda

Published Mar 18, 2024• 5 citations
arXivHTMLPDF

Abstract

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.

DOI
10.35848/1347-4065/ad3533

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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.
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.
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.
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 ultra-compact optical quantum router (QR) consisting of a Mach–Zehnder interferometer (MZI) and waveguide tapers is proposed and numerically simulated, using silicon-on-insulator (SOI). The interferometer is designed to work at the center wavelength of 1550 nm with visibilities of 99.65% and 98.80% for TE and TM polarizations, respectively. Using the principle of phase compensation and self-image, the length of the waveguide tapers is shortened by an order of magnitude with the transmission above 95% for both TE and TM polarizations. Furthermore, polarization beam splitters (PBS) with an ultra-compact footprint of 1.4 × 10.4 μm2 with transmissions of 98% for bi-polarizations are achieved by introducing anisotropic metamaterials. The simulated results indicate that the interferometer facilitates low loss, a broad operating spectral range, and a large tolerance to size variation in fabrications. The optical switch possesses the routing function while maintaining the polarization states, which promises to pave the point-to-point BB84 protocol into applications of network-based quantum communication.
The router is a key element for a network. We describe a scheme to realize genuine quantum routing of single-photon pulses based on cascading of conditional quantum gates in a Mach-Zehnder interferometer and report a proof-of-principle experiment for its demonstration using linear optics quantum gates. The polarization of the control photon routes in a coherent way the path of the signal photon while preserving the qubit state of the signal photon represented by its polarization. We demonstrate quantum nature of this router by showing entanglement generated between the initially unentangled control and signal photons, and confirm that the qubit state of the signal photon is well preserved by the router through quantum process tomography.