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Physical Review A

Phase-unlocked Hong-Ou-Mandel interferometry

Ayman F. Abouraddy, Timothy M. Yarnall, Giovanni Di Giuseppe

Published Jun 11, 2013• 18 citations
Publisher

Abstract

There is a fundamental dimensional mismatch between the Hong-Ou-Mandel (HOM) interferometer and two-photon (2P) states: while the latter are represented using two temporal (or spectral) dimensions, the HOM interferometer allows access to only one temporal dimension. We introduce a linear 2P interferometer containing two independent delays spanning the 2P state. By ``unlocking'' the fixed phase relationship between the interfering 2P probability amplitudes in a HOM interferometer, one of these probability amplitudes now serves as a delay-free 2P reference against which the other beats, thereby resolving ambiguities in 2P state identification typical of HOM interferometry and extending its utility to a large family of 2P states.

DOI
10.1103/physreva.87.062106

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A phase-unlocked Hong-Ou-Mandel interferometer

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There is a fundamental dimensional mismatch between the Hong-Ou-Mandel (HOM) interferometer and two-photon (2P) states: while the latter are represented using two temporal (or spectral) dimensions, the HOM interferometer allows access to only one temporal dimension. We introduce a linear 2P interferometer containing two independent delays spanning the 2P state. By ``unlocking'' the fixed phase relationship between the interfering 2P probability amplitudes in a HOM interferometer, one of these probability amplitudes now serves as a delay-free 2P reference against which the other beats, thereby resolving ambiguities in 2P state identification typical of HOM interferometry and extending its utility to a large family of 2P states.
There exists a fundamental dimensional mismatch between the Hong-Ou-Mandel (HOM) interferometer and two-photon states: while the latter are represented using two temporal (or spectral) dimensions, the HOM interferometer allows access to only one temporal dimension owing to its single delay element. We introduce a linear two-photon interferometer containing two independent delays spanning the two-photon state. By unlocking the fixed phase relationship between the interfering two-photon probability amplitudes in a HOM interferometer, one of these probability amplitudes now serves as a delay-free two-photon reference against which the other beats, thereby resolving ambiguities in two-photon state identification typical of HOM interferometry. We discuss the operation of this phase-unlocked HOM on a variety of input states focusing on instances where this new interferometer outperforms a traditional HOM interferometer: frequency-correlated states and states produced by a pulse doublet pump. Additionally, this interferometer affords the opportunity to synchronize two-photon states in a manner analogous to an HOM interferometer; moreover, it extends that capability to the aforementioned class of states.
Quantum interference forces two indistinguishable input photons to depart a beamsplitter in the same (of two possible) spatial output modes; this is known as the Hong-Ou-Mandel (HOM) effect. It provides a quantitative way of measuring the distinguishability between two photons and also has been utilised to determine their relative temporal delays [1, 2]. HOM interferometry offers numerous advantages over its classical counterpart, most importantly it lacks any dependence on the phase of the photons and therefore can potentially provide reliable measurements in environments where the phase is unstable. To date there have been very few investigations on the precision that is achievable with HOM interferometry. Experiments with collinear geometries have yielded sensitivities as low as 0.1fs however these approaches are limited to measurements of birefringence and other polarisation dependent effects [3, 4], as the precision derives from the inherently stable shared path of the photon pairs. Typical non-shared path HOM interferometers exhibit resolution of roughly 1–3 fs (0.3–1 μm).
The Hong-Ou-Mandel interferometer is a versatile tool for analyzing the joint properties of photon pairs, relying on a truly quantum interference effect between two-photon probability amplitudes. While the theory behind this form of two-photon interferometry is well established, the development of advanced photon sources and exotic two-photon states has highlighted the importance of quantifying precisely what information can and cannot be inferred from features in a Hong-Ou-Mandel interference trace. Here we examine Hong-Ou-Mandel interference with regard to a particular class of states, so-called quantum frequency combs, and place special emphasis on the role spectral phase plays in these measurements. We find that this form of two-photon interferometry is insensitive to the relative phase between different comb line pairs. This is true even when different comb line pairs are mutually coherent at the input of a Hong-Ou-Mandel interferometer and the fringe patterns display sharp temporal features. Consequently, Hong-Ou-Mandel interference cannot speak to the presence of high-dimensional frequency-bin entanglement in two-photon quantum frequency combs.
Hong-Ou-Mandel (HOM) interference offers robustness in high-noise environments due to its phase-insensitive nature, while N00N-state interference provides enhanced phase sensitivity for precision measurements in low-noise settings. However, their realizations have typically required separate experimental platforms. Here, we report a single hybrid quantum interferometer that can dynamically transition between HOM and N00N-state interference by adjusting two time delays, τ1τ_1τ1​ and τ2τ_2τ2​. This tunability enables operation in phase-insensitive, phase-sensitive, or hybrid modes within a unified setup, as confirmed by theoretical analysis and experimental demonstration. Our system offers a versatile platform with potential for quantum sensing applications, paving the way for integrated quantum technologies capable of adapting to diverse noise conditions.