In the field of quantum information science, the development of quantum resources for generating copious entangled photons remains a critical challenge. Leveraging the broad spectral characteristics of the parametric down-conversion process, numerous sideband modes distributed symmetrically around the squeezed light in the frequency domain exhibit quantum noise correlations analogous to einstein-podolsky-rosen (EPR) entanglement, which also termed as sideband entanglement. Owing to their distinct advantages in deterministic entanglement distribution, high-capacity entanglement generation, and high-efficiency detection, continuous-variable entangled sideband modes hold significant promise for applications in quantum information science. In this paper, the development of quantum sideband entanglement and experimental progress over the past two decades are reviewed.
In the field of quantum information science, the development of quantum resources for generating copious entangled photons remains a critical challenge. Leveraging the broad spectral characteristics of the parametric down-conversion process, numerous sideband modes distributed symmetrically around the squeezed light in the frequency domain exhibit quantum noise correlations analogous to einstein-podolsky-rosen (EPR) entanglement, which also termed as sideband entanglement. Owing to their distinct advantages in deterministic entanglement distribution, high-capacity entanglement generation, and high-efficiency detection, continuous-variable entangled sideband modes hold significant promise for applications in quantum information science. In this paper, the development of quantum sideband entanglement and experimental progress over the past two decades are reviewed.
Quantum entanglement is a peculiar phenomenon in quantum information science, characterized by nonclassical correlations between quantum states of subsystems in a quantum system. Since the proposal of the Einstein-Podolsky-Rosen (EPR) paradox by Einstein, Podolsky, and Rosen, quantum entanglement has sparked intense debates on local realism. Bell’s inequality experiment established the nonlocality of quantum mechanics. Currently, high-dimensional quantum entanglement of both deterministic and random states can be realized in systems such as photons and cold atoms. Technologies such as quantum teleportation, quantum teleportation, quantum computing, and others rely on quantum entanglement to achieve effects beyond classical limitations. Current research focuses on the implementation of macroscopic quantum entanglement and its significance in fundamental problems of quantum mechanics. Quantum entanglement opens up a new paradigm for information processing with broad application prospects. It is necessary to conduct in-depth research on the nature of quantum entanglement and its advantages in information processing. This paper reviews the theoretical foundations of quantum entanglement, methods of generation and detection, and research progress in its applications in the field of quantum information. It discusses the important applications of quantum entanglement in quantum communication, computing, and sensing and provides an outlook on the future development prospects of quantum entanglement technologies.
Quantum entanglement is a non-classical correlation between quantum systems that cannot be accounted for by any local hidden-variable theory. Since its articulation in the Einstein-Podolsky-Rosen paper of 1935 and its formal testability through Bell's theorem in 1964, entanglement has evolved from a philosophical puzzle into the central resource of quantum information science. This review surveys the conceptual foundations of entanglement, the experimental record culminating in loophole-free Bell tests, and applications including quantum key distribution, quantum teleportation, and measurement-based quantum computation. We summarise quantitative results of representative Bell experiments, discuss entanglement measures for mixed states, and outline recent developments in long-distance entanglement distribution via satellites and quantum repeaters. Persistent challenges decoherence, certification of high-dimensional entanglement, and scalable distribution are identified as active frontiers of research.
Summary form only given. With the rapid development of spatially-resolving single-photon detectors, spatially structured multidimensional entangled states start to play a key role in modern quantum science. In particular, they find extensive applications in emerging fields such as quantum imaging, holography, computation or quantum-enhanced metrology. Moreover, spatially-multiplexed schemes hold a promise to increase the capacity of quantum channels, essential for quantum key distribution. An appealing perspective is to apply these ideas to light-atom interfaces, that are capable of storing and processing continuous-variable (CV) entanglement, critical for novel quantum cryptography, computation and imaging schemes.Here we generate a hybrid bipartite entangled state of a photon and an atomic spin-wave excitation [1], exhibiting Einstein-Podolsky-Rosen (EPR) correlations in real space of continuous position-momentum variables, as in the original EPR proposal [2]. We verify the entanglement by measuring the coincidence patterns corresponding to modulus-squared spatial wavefunctions of the state. By studying temporal evolution and decoherence we find that the EPR entanglement may be stored for several microseconds. Our approach turns out to be by far more robust than the only hitherto performed experiment, based on quadratures of squeezed light and a slow-light medium, where time-delayed EPR correlations were demonstrated [3]. We achieve two orders of magnitude longer delay time using quantum memory setup and significantly stronger violation of EPR inequality [4], with product of variances 3 times below the EPR bound for the full two-dimensional coincidence distribution.
Quantum information science has recently attracted a lot of attention. Its applications include secure communication, quantum computation, quantum simulation, and quantum metrology. In these applications, photons are one of the most important physical quanta for their tolerance to decoherence. In this manuscript, we review the recent progress in single-photon/entangled-photon emitters and their applications: heralded single-photon sources using parametric downconversion and their application to quantum key distribution, highly indistinguishable heralded single-photon sources, fiber-coupled solid-state single-photon sources, and ultrabroadband-frequency entanglement generation.
Quantum entanglement shared by more than two partites is the essential bases for developing quantum communication networks and quantum computation. The multipartite entanglement of continuous variables can be generated by distributing or combining squeezed states of light using linear optics. We experimentally obtained a tripartite entangled state of bright optical field by means of a nondegenerate optical parametric amplifier (NOPA) and optical beam-splitters. The NOPA is operated in the state of parametric deamplification by locking the relative phase between the pump laser and the injected seed wave to (2n+1)π, so the generated Einstein-Podolsky-Rosen (EPR) beam for continuous variables is an entangled state of light with the quantum correlation of phase quadratures and the quantum anticorrelation of amplitude quadratures [1]. The output EPR beam consists of two frequency-degenerate optical modes with horizontal and vertical polarizations which are distributed to three parties with two linear beam-splitters. We have theoretically and experimentally demonstrated the three parties are in a tripartite entangled state with three-mode “position” (amplitude quadrature) correlation and relative “momentum” (phase quadrature) correlation [2]-[4]. The obtained GHZ-like state is a fully inseparable tripartite continuous-variables state violating the three inequalities used for the criterion of the inseparability [5][6]. The controlled dense coding quantum communication is experimentally accomplished by exploiting the tripartite entanglement. The three entangled modes are sent to a sender (Alice), a receiver (Bob) and a controller (Claire), respectively. The information transmission capacity of the quantum channel between Alice and Bob is controlled by Claire. The channel capacity completed under Claire’s help is always larger than that without her help.
Frequency conversion (FC) and type-II parametric down-conversion (PDC) processes serve as basic building blocks for the implementation of quantum optical experiments: type-II PDC enables the efficient creation of quantum states such as photon-number states and Einstein-Podolsky-Rosen (EPR)-states. FC gives rise to technologies enabling efficient atom-photon coupling, ultrafast pulse gates and enhanced detection schemes. However, despite their widespread deployment, their theoretical treatment remains challenging. Especially the multi-photon components in the high-gain regime as well as the explicit time-dependence of the involved Hamiltonians hamper an efficient theoretical description of these nonlinear optical processes. In this paper, we investigate these effects and put forward two models that enable a full description of FC and type-II PDC in the high-gain regime. We present a rigorous numerical model relying on the solution of coupled integro-differential equations that covers the complete dynamics of the process. As an alternative, we develop a simplified model that, at the expense of neglecting time-ordering effects, enables an analytical solution. While the simplified model approximates the correct solution with high fidelity in a broad parameter range, sufficient for many experimental situations, such as FC with low efficiency, entangled photon-pair generation and the heralding of single photons from type-II PDC, our investigations reveal that the rigorous model predicts a decreased performance for FC processes in quantum pulse gate applications and an enhanced EPR-state generation rate during type-II PDC, when EPR squeezing values above 12 dB are considered.