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arXiv:2605.25609 · Optics Letters

High fidelity preservation of photonic hyperentanglement in a free-space optical delay line

Yu Guo, Arya Chowdhury, Pranay Tiwari, Jia Boon Chin, Anindya Banerji, Alexander Ling

quant-phSubmitted May 25, 2026• Announced May 26, 2026• 0 citations• Revised Aug 19, 2026 (v2)
v1 · May 25, 2026v2 · Aug 19, 2026
arXivHTMLPDF

Abstract

Photonic hyperentanglement enables increased information capacity and enhanced functionality for quantum communication and networking. However, synchronization of hyperentangled photon pairs requires maintaining correlations simultaneously across multiple degrees of freedom (DOFs). The preservation of polarization and energy-time entanglement in hyperentangled photon pairs is demonstrated using a free-space optical delay line based on nested Herriott cells. After a delay of 647 ns, a two-photon interference visibility of 93.9(3)% is observed in the energy-time DOF, while a CHSH parameter of 2.758(5) is obtained in the polarization DOF. These results confirm that entanglement correlations in both DOFs are preserved after propagation through the delay line. They demonstrate that free-space optical delay lines are compatible with complex photonic quantum states and provide a promising route toward delay-based memories for synchronization and multiplexing in quantum networks.

Journal reference
Optics Letters Vol. 51, Issue 17, pp. 4781-4784 (2026)
DOI
10.1364/OL.606763
Comments
4 pages, 3 figures

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Photonic hyperentanglement enables increased information capacity and enhanced functionality for quantum communication and networking. However, synchronization of hyperentangled photon pairs requires maintaining correlations simultaneously across multiple degrees of freedom (DOFs). The preservation of polarization and energy-time entanglement in hyperentangled photon pairs is demonstrated using a free-space optical delay line based on nested Herriott cells. After a delay of 647 ns, a two-photon interference visibility of 93.9(3)% is observed in the energy-time DOF, while a CHSH parameter of 2.758(5) is obtained in the polarization DOF. These results confirm that entanglement correlations in both DOFs are preserved after propagation through the delay line. They demonstrate that free-space optical delay lines are compatible with complex photonic quantum states and provide a promising route toward delay-based memories for synchronization and multiplexing in quantum networks.
Hyperentanglement, the simultaneous and independent entanglement of quantum particles in multiple degrees of freedom, is a powerful resource that can be harnessed for efficient quantum information processing. In photonic systems, the two degrees of freedom (DOFs) often used to carry quantum and classical information are polarization and frequency, thanks to their robustness in transmission, both in free space and in optical fibers. Telecom-band hyperentangled photons generated in optical fibers are of particular interest because they are compatible with existing fiber-optic infrastructure and can be distributed over fiber networks with minimal loss. Here, we experimentally demonstrate the generation of telecom-band biphotons hyperentangled in both the polarization and frequency DOFs using a periodically poled silica fiber and observe entanglement concurrences above 0.95 for both polarization and frequency DOFs. Furthermore, by concatenating a Hong-Ou-Mandel interference test for frequency entanglement and full state tomography for polarization entanglement in a single experiment, we can demonstrate simultaneous entanglement in both the polarization and frequency DOFs. The states produced by our hyperentanglement source can enable protocols such as dense coding and high-dimensional quantum key distribution.
Multidimensional entangled photon states represent a useful resource for large-scale quantum communication networks. By leveraging multiple degrees of freedom (DoFs) of photons, the dimensionality of the Hilbert space can be expanded without increasing the number of particles, enhancing the efficiency of quantum information processing. Specifically, hyperentanglement, i.e. simultaneous entanglement across multiple DoFs, is robust against decoherence, thus providing strengthened performance in noisy quantum communication protocols with respect to n-photon quantum states [1].
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Abstract Exploiting the fantastic features of quantum mechanics, a hyperentangled quantum network encoded in multiple degree of freedoms (DOF), e.g., polarization and orbital angular momentum DOFs, can encode more qubits per transmitted photon and offers a promising platform for many dramatic applications. Here, we demonstrate such a hyperentangled multiuser network with a fully connected network architecture by using dense wavelength division multiplexing and entanglement transfer technique. Three hyperentangled states in polarization and time-energy DOFs are multiplexed to three single mode fibers to form the fully connected network architecture. Then, three interferometric quantum gates are utilized for transferring quantum entanglement from time-energy to orbital angular momentum DOF. The experimental results reveal a high quality of the hyperentanglement of the constructed network with the entangled state fidelity of higher than 96%. Our approach can provide a novel way to construct a large-scale hyperentangled network that can support various kinds of quantum tasks like superdense coding and teleportation.
Encoding many qubits in different degrees of freedom (DOFs) of single photons is one of the routes toward enlarging the Hilbert space spanned by a photonic quantum state. Hyperentangled photon states (that is, states showing entanglement in multiple DOFs) have demonstrated significant implications for both fundamental physics tests and quantum communication and computation. Increasing the number of qubits of photonic experiments requires miniaturization and integration of the basic elements, and functions to guarantee the setup stability, which motivates the development of technologies allowing the precise control of different photonic DOFs on a chip. We demonstrate the contextual use of path and polarization qubits propagating within an integrated quantum circuit. We tested the properties of four-qubit linear cluster states built on both DOFs, and we exploited them to perform the Grover's search algorithm according to the one-way quantum computation model. Our results pave the way toward the full integration on a chip of hybrid multi-qubit multiphoton states. Integrated optical chips performing operations with ‘hyperentangled photons’ have been fabricated by a team in Italy. Hyperentanglement, in which a photon is simultaneously entangled in several different degrees of freedom, is attracting interest because it offers a means to encode more than one quantum bit of information (qubit) onto the same photon, thus increasing the efficiency of quantum computation. However, for the approach to be practical, the hardware needed for hyperentanglement should ideally be an integrated chip platform. Professor Paolo Mataloni and co-workers have done just that using a chip that features high-quality optical waveguides written in a glass by a femtosecond laser. Their chip was used to engineer a four-qubit cluster state via hyperentanglement of the paths and polarizations of two photons. The researchers tested this cluster state by successfully performing Grover’s search algorithm.