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Science Advances

Multiport quantum interferometry with universal nonunitary transformations

Chaojie Wang, Yuning Zhang, Yuanyuan Chen, Xiubao Sui, Lixiang Chen

Published Sep 30, 2026• 0 citations
Publisher

Abstract

Quantum interferometry provides a pivotal method for investigating complex photon-photon interaction, enabling various quantum information technologies that beyond the possibilities of classical physics. While such an effective interaction is mostly studied in N × N unitary transformation, m × n nonunitary transformation that relates to lossy multiport quantum interferometry requires a different approach. Here, we experimentally demonstrate a concise yet powerful tool to unravel fundamental features of multiport quantum interferometry with universal nonunitary transformations. To confirm their intriguing unitary and nonunitary properties, Hong-Ou-Mandel (HOM) interference is observed to reveal its nontrivial particle exchange phase, wherein the conventional HOM interference based on unitary transformation and the anomalous HOM interference based on nonunitary transformation can be readily harnessed. These results provide an alternative approach to investigate the quantum light-matter interaction, facilitating the use of complex multiport quantum interference for large-scale quantum information processing tasks such as quantum simulation and quantum computation.

DOI
10.1126/sciadv.aef4828

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Quantum interferometry provides a pivotal method for investigating complex photon-photon interaction, enabling various quantum information technologies that beyond the possibilities of classical physics. While such an effective interaction is mostly studied in N × N unitary transformation, m × n nonunitary transformation that relates to lossy multiport quantum interferometry requires a different approach. Here, we experimentally demonstrate a concise yet powerful tool to unravel fundamental features of multiport quantum interferometry with universal nonunitary transformations. To confirm their intriguing unitary and nonunitary properties, Hong-Ou-Mandel (HOM) interference is observed to reveal its nontrivial particle exchange phase, wherein the conventional HOM interference based on unitary transformation and the anomalous HOM interference based on nonunitary transformation can be readily harnessed. These results provide an alternative approach to investigate the quantum light-matter interaction, facilitating the use of complex multiport quantum interference for large-scale quantum information processing tasks such as quantum simulation and quantum computation.
Quantum entanglement is essential for various quantum information technologies that are beyond the possibilities of classical physics. However, it is susceptible to decoherence that may arise from noisy channels and environmental noise. While this deleterious problem has been explored by using filtering techniques, such as photon ancillas and Rydberg atom blockade, a concise yet efficient approach to entanglement retrieval is still a formidable challenge. Here, we present that nonunitary material provides an alternative tool to flexibly control the anomalous Hong–Ou–Mandel (HOM) interference and enables the selective entanglement filter with coherent perfect absorption of anti-symmetric quantum states. An elaborate Cr–SiN–Cr film is prepared as a nonunitary beam splitter such that its tunability over the effective photon–photon interaction in frequency modes can be readily explored in the experiment, which may introduce sophisticated nonunitary realization into integrated optical platforms. In addition, we confirm its ability for selective filtering of high-dimensional frequency entanglement by observing the output quantum state of the conventional HOM interference with a unitary beam splitter and the anomalous HOM interference with a nonunitary beam splitter. These results may offer new insights into investigating the quantum light–matter interaction and facilitating the use of two-photon interference for addressing central challenges in quantum technologies.
Quantum interference can produce pivotal effective photon-photon interactions, enabling the exploration of various quantum information technologies that are beyond the possibilities of classical physics. While this effective interaction is fundamentally limited to the bosonic nature of photons and the restricted phase responses from commonly used unitary optical elements, loss-induced nonunitary operations provide an alternative degree of freedom to control quantum interference. Here, we propose and experimentally demonstrate a concise yet powerful tool to unravel the essential features of quantum interference based on the phase change material vanadium dioxide. Since the insulator-metal transition in an elaborate vanadium dioxide thin film can create any desired particle exchange phase response, harnessing its phase transition would allow the dynamical control of photon-photon interaction such that the conventional coalescence and the anomalous anti-coalescence of photons can be continuously tuned. These results highlight an alternative approach for investigating quantum light-matter interactions, and facilitate nonunitary quantum interference for various quantum information processing tasks.
Two-photon interference effects arise because photons are indistinguishable particles. In the wellknown Hong-Ou-Mandel (HOM) effect, the transmission of two photons at a beam splitter interferes destructively with the reflection of both photons, requiring both photons to "bunch up" by leaving the beam splitter on the same side. Here, we show that the interference between locally propagating photons and photons exchanged by a mode swap can be implemented by post-selecting spatially separated photon outputs of a four-path interferometer. Even though the photons detected at spatially separated locations must have travelled along paths that never met up at the same beam splitter, the Hong-Ou-Mandel effect can be observed in correlations between the output ports that originate from the association of detection events with non-local output modes defined by the two single photon inputs. Local phase shifts can be used to map out non-classical correlations between the photons detected at different output locations, clarifying the role of linear optics in generating entanglement between spatially separated photons. Our work thus establishes a fundamental relation between multiphoton interference and entanglement, opening the door to new possibilities in optical quantum technologies.
The Hong-Ou-Mandel (HOM) effect, an effective two-photon interference phenomenon, is a cornerstone of quantum optics and a key tool for linear optical quantum information processing. While the HOM effect has been extensively studied both theoretically and experimentally for various photonic quantum states, particularly in the spectral domain, detailed overviews of its behaviour for structured photons - those with complex spatial profiles - under arbitrary spatial mode measurement schemes are still lacking. This tutorial aims to fill this gap by providing a comprehensive theoretical analysis of the HOM effect for structured photons, including an arbitrary mode projection on quantum interference outcomes. The tutorial also provides analytical, closed-form expressions of the HOM visibility under different measurement conditions.
The investigation of multi-photon quantum interference in symmetric multi-port splitters has both fundamental and applicative interest. Destructive quantum interference in devices with specific symmetry leads to the suppression of a large number of possible output states, generalizing the Hong-Ou-Mandel effect; simple suppression laws have been developed for interferometers implementing the Fourier or the Hadamard transform over the modes. In fact, these enhanced interference features in the output distribution can be used to assess the indistinguishability of single-photon sources, and symmetric interferometers have been envisaged as benchmark or validation devices for Boson-Sampling machines. In this work we devise an innovative approach to implement symmetric multi-mode interferometers that realize the Fourier and Hadamard transform over the optical modes, exploiting integrated waveguide circuits. Our design is based on the optical implementations of the Fast-Fourier and Fast-Hadamard transform algorithms, and exploits a novel three-dimensional layout which is made possible by the unique capabilities of femtosecond laser waveguide writing. We fabricate devices with m = 4 and m = 8 modes and we let two identical photons evolve in the circuit. By characterizing the coincidence output distribution we are able to observe experimentally the known suppression laws for the output states. In particular, we characterize the robustness of this approach to assess the photons' indistinguishability and to rule out alternative non-quantum states of light. The reported results pave the way to the adoption of symmetric multiport interferometers as pivotal tools in the diagnostics and certification of quantum photonic platforms.
Hong-Ou-Mandel (HOM) interference [1] is a bunching phenomenon based on the bosonic nature of photons. When two photons that are perfectly indistinguishable from each other are mixed using a half beam splitter (HBS), they always bunch and exhibit the absence of coincidence resulting in a specific interferogram called HOM dip. The important property of the HOM interference is that the decrease in coincidence reflects the degree of indistinguishability of the input photons, i.e., it provides information regarding the probability of overlap of the input photons. Thus, this property should enable the estimation of a target photon in an unknown quantum state using a probe photon in a known state. In this work, we realize the above concept as the quantum state tomography (QST) applicable to d-dimensional photonic qudits, which we call HOM-QST [2]. Interestingly, we observe that HOM-QST is robust against various imperfections and that the probe photon can be replaced by classical light. Furthermore, we confirm this by demonstrating a proof-of-principle experiment using two-dimensional polarization qubits.