The interference of quanta lies at the heart of quantum physics. The multipartite generalization of single-quanta interference creates entanglement, the coherent superposition of states shared by several quanta. Entanglement allows non-local correlations between many quanta and hence is a key resource for quantum information technology. Entanglement is typically considered to be essential for creating non-local quantum interference. Here, we show that this is not the case and demonstrate multiphoton non-local quantum interference that does not require entanglement of any intrinsic properties of the photons. We harness the superposition of the physical origin of a four-photon product state, which leads to constructive and destructive interference with the photons' mere existence. With the intrinsic indistinguishability in the generation process of photons, we realize four-photon frustrated quantum interference. This allows us to observe the following noteworthy difference to quantum entanglement: We control the non-local multipartite quantum interference with a photon that we never detect, which does not require quantum entanglement. These non-local properties pave the way for the studies of foundations of quantum physics and potential applications in quantum technologies.
The interference of quanta lies at the heart of quantum physics. The multipartite generalization of single-quanta interference creates entanglement, the coherent superposition of states shared by several quanta. Entanglement allows non-local correlations between many quanta and hence is a key resource for quantum information technology. Entanglement is typically considered to be essential for creating non-local quantum interference. Here, we show that this is not the case and demonstrate multiphoton non-local quantum interference that does not require entanglement of any intrinsic properties of the photons. We harness the superposition of the physical origin of a four-photon product state, which leads to constructive and destructive interference with the photons' mere existence. With the intrinsic indistinguishability in the generation process of photons, we realize four-photon frustrated quantum interference. This allows us to observe the following noteworthy difference to quantum entanglement: We control the non-local multipartite quantum interference with a photon that we never detect, which does not require quantum entanglement. These non-local properties pave the way for the studies of foundations of quantum physics and potential applications in quantum technologies.
Violation of local realism via Bell inequality-a profound and counterintuitive manifestation of quantum theory that conflicts with the prediction of local realism-is viewed to be intimately linked with quantum entanglement. Experimental demonstrations of such a phenomenon using quantum entangled states are among the landmark experiments of modern physics and paved the way for quantum technology. Here, we report the violation of the Bell inequality that cannot be described by quantum entanglement in the system but arises from quantum indistinguishability by path identity, shown by the multiphoton frustrated interference. By analyzing the measurement of four-photon frustrated interference within the standard Bell-test formalism, we find a violation of Bell inequality by more than four SDs. Our work establishes a connection between quantum correlation and quantum indistinguishability, providing insights into the fundamental origin of the counterintuitive characteristics observed in quantum physics.
Quantum mechanically, multiple particles can jointly be in a coherent superposition of two or more different states at the same time. This property is called quantum entanglement, and gives rise to characteristic nonlocal interference and stays at the heart of quantum information process. Here, rather than interference of different intrinsic properties of particles, we experimentally demonstrated coherent superposition of two different birthplaces of a four-photon state. The quantum state is created in four probabilistic photon-pair sources, two combinations of which can create photon quadruplets. Coherent elimination and revival of distributed 4-photons can be fully controlled by tuning a phase. The stringent coherence requirements are met by using a silicon-based integrated photonic chip that contains four spiral waveguides for producing photon pairs via spontaneous four-wave mixing. The experiment gives rise to peculiar nonlocal phenomena without any obvious involvement of entanglement. Besides several potential applications that exploit the new on-chip technology, it opens up the possibility for fundamental studies on nonlocality with spatially separated locations.
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.
This article develops an expanded critical–propositional analysis of Kaiyi Qian et al.’s 2023 experiment, “Multiphoton Non-local Quantum Interference Controlled by an Undetected Photon,” published in Nature Communications 14, article 1480, DOI https://doi.org/10.1038/s41467-023-37228-y, in systematic dialogue with the Theory of Objectivity (TO) formulated by Vidamor Cabannas and Denivaldo Silva. Qian et al. experimentally realized multiphoton frustrated quantum interference by exploiting the coherent superposition of alternative physical origins of a four-photon product state. Their experiment is especially significant because the phase of a photon that is not detected can control measurable interference patterns involving the remaining detected photons. The reported four-fold interference visibility was V4 = (75.47 ± 2.99)%, three-fold coincidence interference controlled by the undetected photon exhibited a visibility of V3 = (29.84 ± 1.05)%, and even two-fold coincidences displayed phase-dependent interference with a visibility of V2 = (11.42 ± 0.38)%. These results constitute an experimentally controlled demonstration that direct detection of every participating quantum degree of freedom is not required for that degree of freedom to remain physically relevant to the global interference process (Qian et al., 2023). The present analysis examines whether and to what extent these results can be placed in constructive dialogue with the seven Absolute Truths proposed by TO, while carefully distinguishing empirical confirmation, structural compatibility, heuristic analogy, indirect corroboration, and theory-specific corroboration. Particular attention is given to the Fourth Absolute Truth concerning boundaries, the Fifth Absolute Truth concerning relational observation, and the Seventh Absolute Truth concerning a transcendent element identified in later TO formulations with knowledge or information produced in atomic relations and regarded as equivalent to atomic radiation. Qian et al.’s experiment is especially relevant to the Fifth Absolute Truth because the experimentally consequential role of an undetected photon demonstrates that physical participation does not depend on human observation or on the direct registration of every constituent of the relevant quantum process. This point is compatible with TO’s explicit distinction between human observation and ontological relational observation. The article argues that the experiment offers meaningful indirect empirical sup- port for several operational bridges proposed by TO—especially its non-anthropocentric conception of observation, its emphasis on relational determination, and its attribution of physical significance to information-bearing relations. Nevertheless, the experiment does not establish the full modal claims of TO. In particular, Qian et al. do not test the TO requirement that logical existence involves at least two other elements, do not detect the unique individual magnetic field postulated by the Second Absolute Truth, and do not establish that information constitutes a transcendent substance ontologically equivalent to atomic radiation. Nor does the experiment test the TO cosmogonic theorem or its cosmological eras. The analysis therefore proposes a disciplined distinction between compatibility and confirmation. The Qian experiment can be treated as an unusually fertile empirical domain for operationalizing TO, but a stronger scientific claim would require TO to derive quantitative predictions from its axioms, reproduce or constrain the experimen- tally observed phase dependences and visibility bounds, and ideally predict effects that differ from standard quantum theory. The article concludes by proposing a research program based on controlled distinguishability, undetected-photon phase manipulation, boundary transformations, multi-element relational configurations, and searches for theory-specific deviations. Keywords: Theory of Objectivity; multiphoton interference; undetected photons; induced coherence; non-local quantum interference; quantum information; indistin- guishability; relational observation; modal ontology; photon interference; foundations of quantum mechanics.
Multiphoton quantum interference underpins fundamental tests of quantum mechanics and quantum technologies. Consequently, the detrimental effect of photon distinguishability in multiphoton interference experiments can be catastrophic. Here, we employ correlation measurements in the photonic inner modes, time or frequency, to restore quantum interference between photons differing in their colors or injection times in arbitrary linear optical networks, without the need for additional filtering or post selection. Interestingly, we demonstrate how harnessing the multiphoton inner-mode quantum information enables to unravel symmetries of multiphoton networks and states and the generation of an entire class of multipartite entangled states with a fixed interferometer. These results are therefore of profound interest for future applications of universal inner-mode resolved linear optics across fundamental science and quantum technologies with photons with experimentally different spectral properties.
Decoherence is usually deemed detrimental to quantum information processing. Its control and minimization require significant costs and operating overheads, constituting a major hurdle to the practical deployment of quantum technologies. Yet, quantum mechanics provides for counterintuitive, sometimes surprisingly useful, phenomena and effects associated with decoherence, leading to unusual practical utilities. Here we demonstrate such an example of fundamental interest and practical potential, where quantum interference is created among multiple photons through their dissipative coupling to a shared reservoir. On a thin-film lithium niobate chip, we dissipatively link two spontaneous parametric down-converters through a common, highly lossy channel to create phase-sensitive multiphoton states. Our results show that faithful correlations can be established among two, three, and four photons, and tuned by shifting the relative phase between the driving pumps for the converters. This experiment highlights an under-explored territory in quantum science and technology, where loss and decoherence serve as resources, rather than adversaries, for quantum information processing.