We demonstrate in theory and experiment the strict equivalence between nonclassical polarization and the entanglement of indistinguishable photons, thereby unifying these two phenomena that appear dissimilar at first sight. This allows us to analyze nonclassicality and multi-photon entanglement within the same framework. We experimentally verify this double-sided form of quantumness and its independence from the polarization basis, contrasting other notions of coherence that are highly basis-dependent. Our findings show how nonclassical polarization turns out to be equally resourceful for quantum protocols as entanglement, emphasizing its importance in practical applications.
We demonstrate in theory and experiment the strict equivalence between nonclassical polarization and the entanglement of indistinguishable photons, thereby unifying these two phenomena that appear dissimilar at first sight. This allows us to analyze nonclassicality and multi-photon entanglement within the same framework. We experimentally verify this double-sided form of quantumness and its independence from the polarization basis, contrasting other notions of coherence that are highly basis-dependent. Our findings show how nonclassical polarization turns out to be equally resourceful for quantum protocols as entanglement, emphasizing its importance in practical applications.
Multiphoton interference reveals strictly nonclassical phenomena. Its applications range from fundamental tests of quantum mechanics to photonic quantum information processing, where a significant fraction of key experiments achieved so far comes from multiphoton state manipulation. The progress, both theoretical and experimental, of this rapidly advancing research is reviewed. The emphasis is given to the creation of photonic entanglement of various forms, tests of the completeness of quantum mechanics (in particular, violations of local realism), quantum information protocols for quantum communication (e.g., quantum teleportation, entanglement purification, and quantum repeater), and quantum computation with linear optics. The scope of the review is limited to ``few-photon'' phenomena involving measurements of discrete observables.
We investigate duality in entanglement of a bipartite multiphoton system generated from a coherent state of light. The system can exhibit polarization entanglement if the two parts are distinguished by their parity, or parity entanglement if the parts are distinguished by polarization. It was shown in Phys. Rev. Lett. 110, 140404 (2013) that this phenomenon can be exploited as a method to test indistinguishability of two particles and it was conjectured that one can also test indistinguishability of macroscopic systems. We propose a setup to test this conjecture. Contrary to the previous studies using two-particle interference effect as in the Hong-Ou- Mandel setup, our setup neither assumes that the tested state is composed of single particles nor requires that the total number of particles be fixed. Consequently, the notion of entanglement duality is shown to be compatible with a broader class of physical systems. Moreover, by observing duality in entanglement in the above system one can confirm that macroscopic systems exhibit quantum behavior. As a practical side, entanglement duality is a useful concept that enables adaptive conversion of entanglement of one degree of freedom (DOF) to that of another DOF according to varying quantum protocols.
It was shown recently that entanglement of identical particles has a feature called dualism (Bose and Home 2013 Phys. Rev. Lett. 110 140404 ), which is fundamentally connected with quantum indistinguishability. Here we report an experiment that observes the entanglement duality for the first time with two identical photons, which manifest polarization entanglement when labeled by different paths or path entanglement when labeled by polarization states. By adjusting the mismatch in frequency or arrival time of the entangled photons, we tune the photon indistinguishability from the quantum to the classical limit and observe that the entanglement duality disappears under the emergence of classical distinguishability, confirming it as a characteristic feature of quantum indistinguishable particles.
Nonlocality is arguably among the most counterintuitive phenomena predicted by quantum theory.In recent years, the development of an abstract theory of nonlocality has brought a much deeper understanding of the subject, revealing a rich and complex phenomenon.In the current work, we present a systematic experimental exploration of the limits of quantum nonlocality.Using a versatile and high-fidelity source of pairs of polarization-entangled photons, we explore the boundary of quantum correlations, demonstrate the counterintuitive effect of more nonlocality with less entanglement, present the most nonlocal correlations ever reported, and achieve quantum correlations requiring the use of complex qubits.All of our results are in remarkable agreement with quantum predictions, and thus represent a thorough test of quantum theory.Pursuing such an approach is nevertheless highly desirable, as any deviation may provide evidence of new physics beyond the quantum model.
Entanglement is a key resource for fundamental tests of physics and emerging quantum technologies. In quantum optics, two perspectives on entanglement coexist. In the continuous-variable framework, entanglement is understood as holding between optical modes. In contrast, discrete-variable quantum optics focuses on quantum correlations in degrees of freedom such as polarization that label fixed numbers of photons. In this paper, we show that entanglement can transcend this separation. Spontaneous parametric down-conversion inherently generates correlations in optical phase space, photon number, and labelling degrees of freedom simultaneously. In polarization, this structure is traditionally described by macroscopic Bell states. Existing witnesses, however, fail to detect the genuine hybrid entanglement of these states, which goes beyond the continuous-discrete-variable categorization. Here, we lay the groundwork for a general framework unifying continuous- and discrete-variable notions of entanglement. In particular, we derive an operational witness providing a sufficient criterion for genuine hybrid number-polarization entanglement and outline its experimental implementation. Finally, we discuss exemplary states which, together with our results on macroscopic Bell states, motivate a broader classification of genuine hybrid quantum correlations.
The phenomenon of quantum superposition manifests in two distinct ways: it either spreads out across non-orthogonal basis states or remains concealed within their overlaps. Despite its profound implications, the resource theory of superposition often neglects the quantum superposition residing within these overlaps. However, this component is intricately linked to a form of state indistinguishability and can give rise to quantum correlations. In this paper, we introduce a pseudo-Hermitian representation of the density operator, wherein its diagonal elements correspond to biorthogonal extensions of Kirkwood-Dirac quasi-probabilities. This representation provides a unified framework for the inter-basis quantum superposition and basis state indistinguishability, giving rise to what we term as \textit{genuine} quantum superposition. Moreover, we propose appropriate generalizations of current superposition measures to quantify genuine quantum superposition that serves as the fundamental notion of nonclassicality from which both quantum coherence and correlations emerge. Finally, we explore potential applications of our theoretical framework, particularly in the quantification of electron delocalization in chemical bonding and aromaticity.