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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Multiport Cyclic Interferometer for Genuine n-photon Indistinguishability Assessment

Riccardo Albiero, Mathias Pont, Sarah E. Thomas, Nicolò Spagnolo, Francesco Ceccarelli, Giacomo Corrielli, Alexandre Brieussel, Niccolò Somaschi, Hêlio Huet, Abdelmounaïm Harouri, Aristide Lemaître, Isabelle Sagnes, Nadia Belabas, Fabio Sciarrino, Roberto Osellame, P. Senellart, Andrea Crespi

Published Jun 26, 2023• 0 citations
Publisher

Abstract

The generation of quantum states of many indistinguishable photons is a crucial resource for the realization of photonic computer architectures that can enable a quantum speedup and create fault-tolerant multi-party quantum networks. On the experimental side, the measurement of indistinguishability for a multi-photon state is a challenging task. In the case of a two-photon state, the photon indistinguishability is fully assessed by the visibility of the interference fringe of the Hong-Ou-Mandel (HOM) interference experiment on a balanced beam splitter. However, pairwise HOM experiments are unable to completely determine the multi-photon indistinguishability when more than two photons are involved [1]–[2].

DOI
10.1109/cleo/europe-eqec57999.2023.10232662

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The generation of quantum states of many indistinguishable photons is a crucial resource for the realization of photonic computer architectures that can enable a quantum speedup and create fault-tolerant multi-party quantum networks. On the experimental side, the measurement of indistinguishability for a multi-photon state is a challenging task. In the case of a two-photon state, the photon indistinguishability is fully assessed by the visibility of the interference fringe of the Hong-Ou-Mandel (HOM) interference experiment on a balanced beam splitter. However, pairwise HOM experiments are unable to completely determine the multi-photon indistinguishability when more than two photons are involved [1]–[2].
We report on a universal method to measure the genuine indistinguishability of n photons-a crucial parameter that determines the accuracy of optical quantum computing.Our approach relies on a low-depth cyclic multiport interferometer with N ¼ 2n modes, leading to a quantum interference fringe whose visibility is a direct measurement of the genuine n-photon indistinguishability.We experimentally demonstrate this technique for an eight-mode integrated interferometer fabricated using femtosecond laser micromachining and four photons from a quantum dot single-photon source.We measure a fourphoton indistinguishability up to 0.81 Æ 0.03.This value decreases as we intentionally alter the photon pairwise indistinguishability.The low-depth and low-loss multiport interferometer design provides an original path to evaluate the genuine indistinguishability of resource states of increasing photon number.
Multiphoton interference represents one of the key features in photonic platforms for quantum communication, quantum simulation, quantum computing and quantum sensing. Starting from the first two-photon experiment by Hong-Ou-Mandel [1], it has been shown that multiphoton interference lies at the heart of computational complexity in linear optical interferometers, the most notable example being provided by the Boson Sampling model [2]. This model corresponds to sampling from the evolution of n indistinguishable photons in a multimode linear network, and has been shown to be hard to simulate classicaly thus representing a promising route to experimentally reach the quantum advantage regime. It has been shown, however, that genuine multiparticle interference is necessary for high computational complexity. Hence, appropriate methods for detection of such feature should be developed since the complexity of problem prevents the application of trivial methods.
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.
In this work I demonstrate the interference of three photons, a generalisation of the famous Hong Ou Mandel (HOM) interference. I show that three-photon interference is governed by four parameters and measure three-photon interference independent of two-photon interference. I demonstrate that even when the states of the photons are highly distinguishable they can still exhibit strong quantum interference, challenging our intuition formed by the double slit and HOM interference. This is followed by a demonstration of four-photon interference, where surprisingly we can still observe a "fringe" when the involved particles are pairwise orthogonal. To explain these effects, I present a new framework to describe multi-photon interference in terms of a graph-theoretical approach, which illustrates the origin of different orders of multi-photon interference. My work leads to a more general definition of what we regard as an interference fringe in multi-photon scattering. This study of multi-photon interference is followed by an interdisciplinary work between photonics and solid state physics in the newly developing field of topological photonics. I realise a simulation of the Jackiw-Rossi model as a localised topological mode in a photonic-crystal analogue of the 2D graphene lattice. I succeed in the experimental demonstration of a single such excitation and I am able to study the detailed mode structure for the first time. The mode is a result of a topological defect and is, as such, protected against errors that do not change the topology of the system.
In the original formulation of the Hong–Ou–Mandel (HOM) experiment, when two otherwise indistinguishable photons are incident upon the two input ports of a balanced beam splitter, they coalesce, always leaving via the same output port. It is often interpreted that this interference arises due to the indistinguishability of the single photons at the beam splitter; the situation, however, is often more nuanced. Here, we demonstrate an analog of HOM interference between two photons of completely different color. To do so, we utilize a quantum frequency converter based on sum- and difference-frequency generation as an “active” beam splitter—coupling frequency-entangled red and telecom single photons with an octave-spanning energy difference of 282 THz. We achieve an uncorrected two-photon interference visibility beyond 90%. This work presents the first demonstration of HOM interference between two single photons of distinctly different color, deepening our understanding of what underlies quantum interference. It also suggests a novel approach to interfacing photonic qubits in heterogeneous quantum systems where frequency conversion and quantum interference are unified.