Quantum information science attempts to exploit capabilities from the quantum realm to accomplish tasks that are otherwise impossible in the classical domain [1]. Although sufficient conditions have been formulated for the physical resources required to achieve quantum computation and communication [2], there is an evolving understanding of the power of quantum measurement combined with conditional evolution for accomplishing diverse tasks in quantum information science [3-5]. In this regard, a significant advance is the invention of a protocol by Duan, Lukin, Cirac, and Zoller (DLCZ) [6] for the realization of scalable long distance quantum communication and the distribution of entanglement over quantum networks. Here, we report the first enabling step in the realization of the protocol of DLCZ, namely the observation of quantum correlations for photon pairs generated in the collective emission from an atomic ensemble. The nonclassical character of the fields is evidenced by the violation of a Cauchy-Schwarz inequality for the two fields (1,2). As compared to prior investigations of nonclassical correlations for photon pairs produced in atomic cascades [7] and in parametric down conversion [8], our experiment is distinct in that the correlated (1,2) photons are separated by a programmable time interval delta t, with delta t ~ 400 nsec in our initial experiments.
A bipartite quantum system in a mixed state can exhibit nonclassical correlations, which can go beyond quantum entanglement. While quantum discord is the standard measure of quantifying such general quantum correlations, the nonclassicality can be determined by simpler means via the measurement of witness operators. We experimentally construct a positive map to witness nonclassicality of two qubits in an NMR system. The map can be decomposed so that a single run of an experiment on an ensemble of spins suffices to detect the nonclassicality in the state, if present. We let the state evolve in …
The possibility of observing nonclassical features in a physical system comprised of a cavity with two ensembles of two-level atoms has been investigated by considering different configurations of the ensembles with respect to the Node and Antinode of the cavity field under the framework of open quantum systems. The study reveals the strong presence of nonclassical characters in the physical system by establishing the existence of many facets of nonclassicality, such as the sub-Poissonian boson statistics and squeezing in single modes, intermodal squeezing, intermodal entanglement, antibunchin…
The incoherent dynamical properties of open quantum systems are generically attributed to an ongoing correlation between the system and its environment. Here, we propose a novel way to assess the nature of these system-environment correlations by examining the system dynamics alone. Our approach is based on the possibility or impossibility to simulate open-system dynamics with Hamiltonian ensembles. As we show, such (im)possibility to simulate is closely linked to the system-environment correlations. We thus define the nonclassicality of open-system dynamics in terms of the nonexistence of a H…
In this work we experimentally demonstrate for the first time a recently proposed criterion adressed to detect nonclassical behavior in the fluorescence emission of ensembles of single-photon emitters. In particular, we apply the method to study clusters of NV centres in diamond observed via single-photon-sensitive confocal microscopy. Theoretical considerations on the behavior of the parameter at any arbitrary order in presence of poissonian noise are presented and, finally, the opportunity of detecting manifold coincidences is discussed.
Nonclassical quantum effects gradually reach domains of physics of large systems previously considered as purely classical. We derive a hierarchy of operational criteria suitable for a reliable detection of nonclassicality of light from an arbitrarily large ensemble of independent single-photon emitters. We show, that such large ensemble can always emit nonclassical light without any phase reference and under realistic experimental conditions including incoherent background noise. The nonclassical light from the large ensemble of the emitters can be witnessed much better than light coming from…
The vast majority of physical objects we are dealing with are almost exclusively made of atoms. Due to their discrete level structure, single atoms have proved to be emitters of light which is incompatible with the classical description of electromagnetic waves. We demonstrate this incompatibility for atomic fluorescence when scaling up the size of the source ensemble, which consists of trapped atomic ions, by several orders of magnitude. The presented measurements of nonclassical statistics on light unconditionally emitted from ensembles containing up to more than a thousand ions promise furt…
Quantum technologies exploit entanglement to revolutionize computing, measurements, and communications. This has stimulated the research in different areas of physics to engineer and manipulate fragile many-particle entangled states. Progress has been particularly rapid for atoms. Thanks to the large and tunable nonlinearities and the well developed techniques for trapping, controlling and counting, many groundbreaking experiments have demonstrated the generation of entangled states of trapped ions, cold and ultracold gases of neutral atoms. Moreover, atoms can couple strongly to external forc…
Mesoscopic spin ensembles coupled to a cavity offer the exciting prospect of observing complex nonclassical phenomena that pool the microscopic features from a few spins with those of macroscopic spin ensembles. Here, we demonstrate how the collective interactions in an ensemble of as many as hundred spins can be harnessed to obtain a periodic pulse train of nonclassical light. To unravel the full quantum dynamics and photon statistics, we develop a time-adaptive variational renormalization group method that accurately captures the underlying Lindbladian dynamics of the mesoscopic spin-cavity …
Nonclassical character of statistical mixtures of the single-photon and vacuum optical states
我们从理论和实验上证明，真空态的统计混合|0>和单光子Fock态|1>是非经典的根据Vogel准则（W。Vogel，Phys. Rev. Lett. 84，1849（2000）），而与真空分数无关。通过对由参数下转换产生的双光子进行条件测量来合成该系综，并且使用平衡零差检测来测量其正交统计。对各种量子态非经典性判据进行了比较评述。
We report the experimental verification of nonclassical correlations for a four-wave-mixing process in an ensemble of cold two-level atoms, confirming theoretical predictions by Du et al. in 2007 for the violation of a Cauchy-Schwarz inequality in the system, and obtaining $R = (1.98\pm0.03) \nleq 1$. Quantum correlations are observed in a nano-seconds timescale, in the interference between the central exciting frequency and sidebands dislocated by the detuning to the atomic resonance. They prevail without filters over the noise background coming from linear scattering from the same optical tr…
Nonclassical state is an essential resource for quantum-enhanced communication, computing and metrology to outperform their classical counterpart. The nonclassical states that can operate at high bandwidth and room temperature while being compatible with quantum memory are highly desirable to enable the scalability of quantum technologies. Here, we present a direct observation of broadband nonclasscal states in a room-temperature light-matter interface, where the atoms can also be controlled to store and interfere with photons. With a single coupling pulse and far off-resonance configuration, …
Nonclassical photon sources of high brightness are key components of quantum communication technologies. We here demonstrate the generation of narrowband, nonclassical photon pairs by employing spontaneous four-wave mixing in an optically-dense ensemble of cold atoms within a hollow-core fiber. The brightness of our source approaches the limit of achievable generated spectral brightness at which successive photon pairs start to overlap in time. For a generated spectral brightness per pump power of up to $2\times 10^{9} \ \textrm{pairs/(s MHz mW)}$ we observe nonclassical correlations at pump p…
Generation of nonclassical light is an essential tool for quantum optics research and applications in quantum information technology. We present realization of the source of nonclassically correlated photon pairs based on the process of spontaneous four-wave-mixing in warm atomic vapor. Atoms are excited only by a single laser beam in retro-reflected configuration and narrowband frequency filtering is employed for selection of correlated photon pairs. Nonclassicality of generated light fields is proved by analysis of their statistical properties. Measured parameters of the presented source pro…
Targeting at the realization of scalable photonic quantum technologies, the generation of many photons, their propagation in large optical networks, and a subsequent detection and analysis of sophisticated quantum correlations are essential for the understanding of macroscopic quantum systems. In this experimental contribution, we explore the joint operation of all mentioned ingredients. We benchmark our time-multiplexing framework that includes a high-performance source of multiphoton states and a large multiplexing network, together with unique detectors with high photon-number resolution, r…
Many-Body Amplified Nonclassical Photon Emission in Cavity-Coupled Atomic Arrays
School of Physics and Optoelectronic Engineering, Guangdong University of Technology（广东工业大学物理与光电工程学院）；Guangdong Provincial Key Laboratory of Sensing Physics and System Integration Applications, Guangdong University of Technology（广东工业大学感知物理与系统集成应用广东省重点实验室）
Two very similar proposals have been made recently for witnessing nonclassical features of gravity, by Bose et al. and by Marletto and Vedral. However, while these proposals are asserted to be very general, they are in fact based on a very strong claim: that quantum systems cannot become entangled via a classical intermediary. We point out that the support provided for this claim is only applicable to a very limited class of quantum-classical interaction models, corresponding to Koopman-type dynamics. We show that the claim is also valid for mean-field models, but that it is contradicted by ex…
Characterizing quantum synchronization in the van der Pol oscillator via tomogram and photon correlation
Indian Association for the Cultivation of Science（印度科学文化研究院）；The Institute of Mathematical Sciences（数学科学研究所）；Government Arts and Science College Kozhikode, University of Calicut（卡利卡特大学科泽科德政府文理科学院）；Government College Malappuram, University of Calicut（卡利卡特大学马拉普兰政府学院）
Multiple observers who independently harvest nonclassical correlations from a single physical system share the system's ability to enable quantum correlations. We show that any number of independent observers can share the preparation contextual outcome statistics enabled by state ensembles in quantum theory. Furthermore, we show that even in the presence of any amount of white noise, there exists quantum ensembles that enable such shared preparation contextuality. The findings are experimentally realised by applying sequential unsharp measurements to an optical qubit ensemble which reveals th…
Manipulating the frequency and bandwidth of nonclassical light is essential for implementing frequency-encoded/multiplexed quantum computation, communication, and networking protocols, and for bridging spectral mismatch among various quantum systems. However, quantum spectral control requires a strong nonlinearity mediated by light, microwave, or acoustics, which is challenging to realize with high efficiency, low noise, and on an integrated chip. Here, we demonstrate both frequency shifting and bandwidth compression of nonclassical light using an integrated thin-film lithium niobate (TFLN) ph…
Ensembles of alkali or noble-gas atoms at room temperature and above are widely applied in quantum optics and metrology owing to their long-lived spins. Their collective spin states maintain nonclassical nonlocal correlations, despite the atomic thermal motion in the bulk and at the boundaries. Here we present a stochastic, fully-quantum description of the effect of atomic diffusion in these systems. We employ the Bloch-Heisenberg-Langevin formalism to account for the quantum noise originating from diffusion and from various boundary conditions corresponding to typical wall coatings, thus mode…
Scalable Quantum Computing based on Spin Qubits in CNT QD
我们研究实验证明单电子${}^{12}$ C CNT QD具有显着的自旋轨道相互作用作为一个可扩展的量子计算机候选人。电子自旋和轨道角动量都可以作为用于量子处理的逻辑量子比特。我们引入宏观量子存储系统的形式注入磁性或自旋携带原子系综到纳米管。碳纳米管在有限温度下具有稳定的原子陷阱，在外加磁场中具有一维核自旋晶格。电子通过磁相互作用或超精细相互作用与原子系综耦合。该系统可实现简单的电子和核自旋读出。
Engineering strong single-photon optomechanical couplings is crucial for optomechanical systems. Here, we propose a hybrid quantum system consisting of a nanobeam (phonons) coupled to a spin ensemble and a cavity (photons) to overcome it. Utilizing the critical property of the lower-branch polariton (LBP) formed by the ensemble-phonon interaction, the LBP-cavity coupling can be greatly enhanced by three orders magnitude of the original one, while the upper-branch polariton (UBP)-cavity coupling is fully suppressed. Our proposal breaks through the condition of the coupling strength less than th…
Wavelength-Dependent Electrical Readout of Spin Ensembles in a Thin-Film SiC-on-Insulator Platform
Institute for Quantum Optics, Ulm University（乌尔姆大学量子光学研究所）；Institute for Chemical Research, Kyoto University（京都大学化学研究所）；Center for Spintronics Research Network, ICR, Kyoto University（京都大学自旋电子学研究中心）
An efficient quantum storage is highly desired for quantum information processing. As indicated by certain applications, a universal quantum storage is required to have a storage efficiency above 50% to beat the no-cloning limit. Although significant progress has been achieved in improving various quantum storage, the best storage efficiency of single photons is still below this criteria. By integrating a highly controllable single photon source with an optimized quantum storage, here we demonstrate an optical storage of single photons with storage efficiency of 65% in a cold atomic ensemble b…
Entanglement measures quantify nonclassical correlations present in a quantum system, but can be extremely difficult to calculate, even more so, when information on its state is limited. Here, we consider broad families of entanglement criteria that are based on variances of arbitrary operators and analytically derive the lower bounds these criteria provide for two relevant entanglement measures: the best separable approximation (BSA) and the generalized robustness (GR). This yields a practical method for quantifying entanglement in realistic experimental situations, in particular, when only f…
