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Results in Optics

Non-local quantum interference of deterministically separated photons

Kiran Bajar, Vikas S. Bhat, Rounak Chatterjee, Tabish Qureshi, Sushil A. Mujumdar

Published Jun 23, 2025• 3 citations
Publisher

Abstract

Quantum entanglement enables a wide range of fundamental studies and practical applications in quantum optics, computing, and communication. In this work, we investigate two-photon interference using position-momentum entangled photons generated via Type-II Spontaneous Parametric Down-Conversion (SPDC). By employing a double-slit setup, we explore two distinct quantum interference phenomena: the Hanbury Brown and Twiss (HBT) effect, where the photons pass through different slits, and NOON-state interference, where both photons traverse the same slit. We deterministically separate the two photons based on their orthogonal polarizations and demonstrate that both interference effects persist despite their distinguishability at the detectors. This result goes beyond the conventional understanding of two-photon interference and reveals a novel nonlocal HBT effect, where interference occurs even in the absence of spatial overlap, and is rooted in entanglement. Our findings deepen the understanding of quantum interference and open new avenues for applications in quantum optics and precision measurement.

DOI
10.1016/j.rio.2025.100848

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Quantum entanglement enables a wide range of fundamental studies and practical applications in quantum optics, computing, and communication. In this work, we investigate two-photon interference using position-momentum entangled photons generated via Type-II Spontaneous Parametric Down-Conversion (SPDC). By employing a double-slit setup, we explore two distinct quantum interference phenomena: the Hanbury Brown and Twiss (HBT) effect, where the photons pass through different slits, and NOON-state interference, where both photons traverse the same slit. We deterministically separate the two photons based on their orthogonal polarizations and demonstrate that both interference effects persist despite their distinguishability at the detectors. This result goes beyond the conventional understanding of two-photon interference and reveals a novel nonlocal HBT effect, where interference occurs even in the absence of spatial overlap, and is rooted in entanglement. Our findings deepen the understanding of quantum interference and open new avenues for applications in quantum optics and precision measurement.
Simultaneous entanglement in spin and space-time of a two-photon quantum state generated in type-2 spontaneous parametric down-conversion is demonstrated by the observation of quantum interference with 98% visibility in a simple beam-splitter (Hanburry Brown-Twiss) anticorrelation experiment. The nonlocal cancellation of two-photon probability amplitudes as a result of this double entanglement allows us to demonstrate two different types of Bell's inequality violations in one experimental setup.
Usually HBT effect can be interpreted by classical (intensity fluctuation correlation) and quantum (interference of two-photon probability amplitudes) theories properly at the same time. In this manuscript, we report a deliberately designed experiment in which two chaotic light beams has the same intensity fluctuation but mutual-orthogonal polarizations to each other so there will be no interference of two-photon probability amplitudes. Classical and quantum theory give different predictions on if there should be HBT (photon bunching) effect or not in the experiment. The experiment results are used to test the two different predictions. At the end, both the temporal and spatial HBT effects are observed.
The Hanbury Brown–Twiss (HBT) effect, at the quantum level, is essentially an interference of one particle with another, as opposed to interference of a particle with itself. Conventional treatments of identical particles encounter difficulties while dealing with entanglement. A recently introduced label-free approach to indistinguishable particles is described, and is used to analyze the HBT effect. Quantum wave-packets have been used to provide a better understanding of the quantum interpretation of the HBT effect. The effect is demonstrated for two independent particles governed by Bose–Einstein or Fermi–Dirac statistics. The HBT effect is also analyzed for pairs of entangled particles. Surprisingly, entanglement has almost no effect on the interference seen in the HBT effect. In the light of the results, an old quantum optics experiment is reanalyzed, and it is argued that the interference seen in that experiment is not a consequence of non-local correlations between the photons, as is commonly believed. Quanta 2017; 6: 61–69.
Simultaneous entanglement in spin and space-time of a two-photon quantum state generated in type-II spontaneous parametric down-conversion is demonstrated by the observation of quantum interference with 98% visibility in simple beam-splitter (Hanburry Brown-Twiss) anticorrelation and quantum beating experiments. The cancellation of two-photon probability amplitudes as a result of this double entanglement allow us to demonstrate two different types of Bell's inequality violations in one experimental setup.
High-dimensional entanglement in the form of transverse spatial correlation between a pair of photons generated via spontaneous parametric downconversion is not only a valuable resource in many academic and real-life applications but also provides access to several intriguing quantum phenomena. One such non-intuitive phenomenon is phase entanglement, in which the biphoton state is correlated in the complex phase of its wavefunction. This state, which emerges during the propagation of the biphoton wavefunction, exhibits neither position nor momentum correlation, yet retains full entanglement. In this work, we experimentally explore this state in two distinct ways. The first is by tracking the vanishing spatial photon number correlation over propagation distances lying in [0,∞)\left[0,\infty\right)[0,∞), folded into a finite range using single-lens imaging. These observations show excellent agreement with our theoretical predictions based on the Double Gaussian (DG) approximation of the biphoton state. The second approach involves performing a two-photon interference experiment using a double slit and this state, which reveals the correlated phase front. We show, both theoretically and experimentally, that the observed two-photon interference structure is markedly different from that produced by position-correlated photons, as confirmed by computing the joint probability distribution of photons (JPD) and related metrics. Such interference using phase-entangled light has not been attempted before and opens avenues for advanced experiments and applications in the field of spatial entanglement.
From quantum point of view, Hanbury Brown-Twiss effect is a result of constructive-destructive two-photon interference. There should be no Hanbury Brown-Twiss effect if there was no two-photon interference. In this paper, we observed Hanbury Brown- Twiss effect in a specially designed experiment, in which two-photon interference is impossible by keeping only one two-photon probability amplitude in the experimental scheme. However, our experimental results can still be interpreted by Glauber's quantum optical coherence theory. The researches in our paper are helpful to understand the physics of the second-order coherence of light, especially the physics of Hanbury Brown-Twiss effect.