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Entanglement witness on a programmable photonic processor

Reinier van der Meer, Peter Hooijschuur, Chris Toebes, Pim Venderbosch, Pepijn W. H. Pinkse, Jelmer J. Renema

Published Oct 12, 2021• 0 citations
OpenAlex

Abstract

Multiphoton quantum interference is a one of the two technology platforms in which a quantum advantage has been demonstrated [1]. In these systems, a large-scale quantum state forms due to repeated effective interactions of photons (through the generalized Hong-Ou-Mandel effect) at the nodes of a photonic processor, resulting in a state that intrinsically possesses quantum complexity. A key question is how to validate the formation of a large-scale quantum state. One aspect of this is to measure whether the output quantum state is entangled – a necessary but not sufficient criterion for complexity. In this work, we derive and demonstrate an entanglement witness for large-scale optical quantum interference, and experimentally use this to demonstrate the presence of entanglement in our optical processor [2]. [1] Zhong et al., Science 370, 1460 – 1463 (2020) [2] Taballione et al., ArXiv 2012.05673 (2021)

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Multiphoton quantum interference is a one of the two technology platforms in which a quantum advantage has been demonstrated [1]. In these systems, a large-scale quantum state forms due to repeated effective interactions of photons (through the generalized Hong-Ou-Mandel effect) at the nodes of a photonic processor, resulting in a state that intrinsically possesses quantum complexity. A key question is how to validate the formation of a large-scale quantum state. One aspect of this is to measure whether the output quantum state is entangled – a necessary but not sufficient criterion for complexity. In this work, we derive and demonstrate an entanglement witness for large-scale optical quantum interference, and experimentally use this to demonstrate the presence of entanglement in our optical processor [2]. [1] Zhong et al., Science 370, 1460 – 1463 (2020) [2] Taballione et al., ArXiv 2012.05673 (2021)
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.
A core process in many quantum tasks is the generation of entanglement. It is being actively studied in a variety of physical settings-from simple bipartite systems to complex multipartite systems. In this work we experimentally study the generation of bipartite entanglement in a nanophotonic system. Entanglement is generated via the quantum interference of two surface plasmon polaritons in a beamsplitter structure, i.e., utilizing the Hong-Ou-Mandel (HOM) effect, and its presence is verified using quantum state tomography. The amount of entanglement is quantified by the concurrence and we find values of up to 0.77 ± 0.04. Verifying entanglement in the output state from HOM interference is a nontrivial task and cannot be inferred from the visibility alone. The techniques we use to verify entanglement could be applied to other types of photonic system and therefore may be useful for the characterization of a range of different nanophotonic quantum devices.
Abstract The phenomenon of entanglement is the basis of quantum information and quantum communication processes. Entangled systems with a large number of photons are of great interest at present because they provide a platform for streaming technologies based on photonics. In this paper we present a device which operates with four-photons and based on the Hong–Ou–Mandel interference. The presented device allows to maximize the degree of spatial entanglement and generate the highly entangled four-dimensional Bell states. Furthermore, the use of the interferometer in different regimes leads to fast interference fringes in the coincidence probability with period of oscillations twice smaller than the pump wavelength. We have a good agreement between theoretical simulations and experimental results.
In the context of Fock states, demonstrating multiphoton quantum interference, e.g., the generalized Hong-Ou-Mandel (HOM) interference, is at the core of many applications of quantum technologies. However, it is difficult to demonstrate high-visibility quantum interference for a large number of photons. Here, we introduce an experimentally feasible proposal to achieve this goal using phase-randomized weak coherent states and practical photon-number-resolving detectors. Our method can be used to obtain analytically tight bounds of the generalized HOM dip in a beam splitter, and it is robust against practical errors. The simulation shows that for an ideal detector that can resolve any number of photons without an upper bound, the proposed method can bound the generalize HOM interference for a large number of photons, e.g., 100 photons; for a practical detector that can resolve up to 29 photons, the proposed method can bound the generalized HOM interference for 56 photons with visibility 99.499.4%99.4. Our proposal is a general approach for multiphoton quantum interference in optical networks with multiple inputs and outputs.
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.