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arXiv:2401.01485 · Optica

Single-shot characterization of photon indistinguishability with dielectric metasurfaces

Jihua Zhang, Jinyong Ma, Neuton Li, Shaun Lung, Andrey A. Sukhorukov

Published Apr 11, 2024• 11 citations
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Abstract

Characterizing the indistinguishability of photons is a key task in quantum photonics, underpinning the tuning and stabilization of the photon sources and thereby increasing the accuracy of quantum operations. The protocols for measuring the degree of indistinguishability conventionally require photon-coincidence measurements at several different time or phase delays, which is a fundamental bottleneck towards fast measurements and real-time monitoring of indistinguishability. Here, we develop a static dielectric metasurface grating without any reconfigurable elements that realizes a tailored multiport transformation in the free-space configuration without the need for phase locking and enables single-shot characterization of the indistinguishability between two photons in multiple degrees of freedom including time, spectrum, spatial modes, and polarization. Topology optimization is employed to design a silicon metasurface with polarization independence, high transmission, and high tolerance to measurement noise. We fabricate the metasurface and experimentally quantify the indistinguishability of photons in the time domain with fidelity over 98.4%. We anticipate that the developed framework based on ultrathin metasurfaces can be further extended for multi-photon states and additional degrees of freedom associated with spatial modalities.

DOI
10.1364/optica.516064

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Characterizing the indistinguishability of photons is a key task in quantum photonics, underpinning the tuning and stabilization of the photon sources and thereby increasing the accuracy of quantum operations. The protocols for measuring the degree of indistinguishability conventionally require photon-coincidence measurements at several different time or phase delays, which is a fundamental bottleneck towards fast measurements and real-time monitoring of indistinguishability. Here, we develop a static dielectric metasurface grating without any reconfigurable elements that realizes a tailored multiport transformation in the free-space configuration without the need for phase locking and enables single-shot characterization of the indistinguishability between two photons in multiple degrees of freedom including time, spectrum, spatial modes, and polarization. Topology optimization is employed to design a silicon metasurface with polarization independence, high transmission, and high tolerance to measurement noise. We fabricate the metasurface and experimentally quantify the indistinguishability of photons in the time domain with fidelity over 98.4%. We anticipate that the developed framework based on ultrathin metasurfaces can be further extended for multi-photon states and additional degrees of freedom associated with spatial modalities.
With recent advances in nanophotonics, metasurfaces based on nano-resonators have facilitated novel types of optical devices. In particular, the interplay between different degrees of freedom, involving polarization and spatial modes, boosted classical polarization measurements and imaging applications. However, the use of metasurfaces for measuring the quantum states of light remains largely unexplored. Conventionally, the task of quantum state tomography is realized with several bulk optical elements, which need to be reconfigured multiple times. Such setups can suffer from decoherence, and there is a fundamental and practical interest in developing integrated solutions for measurement of multi-photon quantum states. We present a new concept and the first experimental realization of all-dielectric metasurfaces with no tuneable elements for imaging-based reconstruction of the full quantum state of entangled photons. Most prominently, we implement multi-photon interferometric measurements on a sub-wavelength thin optical element, which delivers ultimate miniaturization and extremely high robustness. Specifically, we realize a highly transparent all-dielectric metasurface, which spatially splits different components of quantum-polarization states. Then, a simple one-shot measurement of correlations with polarization-insensitive on-off click detectors enables complete reconstruction of multi-photon density matrices with high precision. In our experiment, we prepare sets of polarization states and reconstruct their density matrices with a high fidelity of over 99% for single photon states and above 95% for two-photon states. Our work provides a fundamental advance in the imaging of quantum states, where multi-photon quantum interference takes place at sub-wavelength scale.
Summary form only given. Measurements of quantum states of photons are conventionally performed with series of optical elements in bulk setups [1] or optical chips incorporating multiple tunable beam splitters. Here, we suggest and develop experimentally, for the first time to our knowledge, a new concept of quantum-polarization measurements with a single all-dielectric resonant metasurface [2]. The operating principle is presented in Fig. 1(a): A metasurface spatially splits different components of photon polarization states, which then enables full reconstruction of the photon state based on the photon correlations with simple polarization-insensitive single-photon detectors or EMCCD cameras. The subwavelength thin structure provides an ultimate miniaturization, and can facilitate quantum tomography by spatially-resolved imaging without a need for reconfiguration. Such parallel-detection approach promises not only better robustness and scalability, but also the possibility to study the dynamics of quantum states in real-time.
Metasurfaces have recently entered the realm of quantum photonics, enabling manipulation of quantum light using a compact nanophotonic platform. Realizing the full potential of metasurfaces at the deepest quantum level requires the ability to tune coherent light-matter interactions continuously in space and time. Here, we introduce the concept of space-time quantum metasurfaces for arbitrary control of the spectral, spatial, and spin properties of nonclassical light using a compact photonic platform. We show that space-time quantum metasurfaces allow on-demand tailoring of entanglement among all degrees of freedom of a single photon. We also show that spatiotemporal modulation induces asymmetry at the fundamental level of quantum fluctuations, resulting in the generation of steered and vortex photon pairs out of vacuum. Space-time quantum metasurfaces have the potential to enable novel photonic functionalities, such as encoding quantum information into high-dimensional color qudits using designer modulation protocols, sculpting multispectral and multispatial modes in spontaneous emission, and generating reconfigurable hyperentanglement for high-capacity quantum communications.
Quantum state tomography is an essential tool for characterizing quantum systems and underpins nearly every experimental realization of quantum technologies. Conventional tomography relies on performing a sequence of projective measurements on many identical copies of a quantum state, requiring the measurement apparatus to be reconfigured between successive acquisitions. As the Hilbert-space dimension increases, the number of required measurements grows rapidly; in practice, additional overcomplete measurements are often performed to improve robustness to experimental imperfections. Here, we introduce a tomography platform based on structured anisotropic media that performs informationally complete measurements of photonic polarization qubits within a single acquisition. The approach employs three liquid-crystal metasurfaces with spatially varying optic-axis orientations that transform the input polarization into a far-field distribution of discrete transverse-momentum modes. Each diffraction pattern uniquely determines the polarization state, enabling its reconstruction without sequential changes to the measurement apparatus. Unlike previous implementations, our scheme is intrinsically photon-number independent: the same optical device operates identically for arbitrary photon numbers, while the desired photon-number sector can be selected afterwards through post-selection of the corresponding nnn-fold coincidence events. We experimentally demonstrate single-frame quantum state tomography of both single- and two-photon polarization states, providing a simple and scalable route toward efficient quantum-state characterization.