Entanglement of two distant macroscopic objects is a key element for implementing large-scale quantum networks consisting of quantum channels and quantum nodes. Entanglement swapping can entangle two spatially separated quantum systems without direct interaction. Here we propose a scheme of deterministically entangling two remote atomic ensembles via continuous-variable entanglement swapping between two independent quantum systems involving light and atoms. Each of two stationary atomic ensembles placed at two remote nodes in a quantum network is prepared to a mixed entangled state of light and atoms respectively. Then, the entanglement swapping is unconditionally implemented between the two prepared quantum systems by means of the balanced homodyne detection of light and the feedback of the measured results. Finally, the established entanglement between two macroscopic atomic ensembles is verified by the inseparability criterion of correlation variances between two anti-Stokes optical beams respectively coming from the two atomic ensembles.
Entanglement of two distant macroscopic objects is a key element for implementing large-scale quantum networks consisting of quantum channels and quantum nodes. Entanglement swapping can entangle two spatially separated quantum systems without direct interaction. Here we propose a scheme of deterministically entangling two remote atomic ensembles via continuous-variable entanglement swapping between two independent quantum systems involving light and atoms. Each of two stationary atomic ensembles placed at two remote nodes in a quantum network is prepared to a mixed entangled state of light and atoms respectively. Then, the entanglement swapping is unconditionally implemented between the two prepared quantum systems by means of the balanced homodyne detection of light and the feedback of the measured results. Finally, the established entanglement between two macroscopic atomic ensembles is verified by the inseparability criterion of correlation variances between two anti-Stokes optical beams respectively coming from the two atomic ensembles.
Quantum entanglement is an essential quantum resource. With the development of quantum information science, quantum network consisting of quantum nodes and quantum channels has attracted extensive attention. The development of quantum information network requires the capability of generating, storing and distributing quantum entanglement among multiple quantum nodes. It is significant to construct the quantum information, and it has very important applications in the distributed quantum computation and quantum internet. Here we propose a simple and feasible scheme to deterministically entangle three distant atomic ensembles via the interference and feedforward network of the light-atom mixed entanglement. Firstly, three atomic ensembles placed at three remote nodes in a quantum network are prepared into the mixed entangled state of light and atomic ensembles via the spontaneous Raman scattering (SRS) process. Then, the first and second Stokes optical field are interfered on an R1∶T1 optical beam splitter (BS1), and one of the output optical fields from the first optical beam splitter is interfered with the third Stokes field on the second R2∶T2 optical beam splitter (BS2). The quantum fluctuations of the amplitude and phase quadratures of these three output optical fields from BS1 and BS2 are detected by three sets of balanced homodyne detectors, respectively. Finally, the detected signals of the amplitude and phase quadratures are fed to the three atomic ensembles via the radio frequency coils to establish the entanglement among three remote atomic ensembles. At the user-controlled time, three read optical pulses can be applied to these three atomic ensembles to convert the stored entangled state from the atomic spin waves into the anti-Stokes optical fields via the SRS process. According to the tripartite inseparability criterion, the correlation variance combinations of these three anti-Stokes optical fields can be used to verify the performance of entanglement of three atomic ensembles. This scheme can be extended to larger-scale quantum information network with different physical systems and more atomic nodes. Moreover, the entanglement distillation can be combined with this scheme to realize the entanglement among longer distance quantum nodes.
The development of quantum network relies on high-quality entanglement between remote quantum nodes. In reality the unavoidable decoherence limits the quality of entangled quantum nodes, however, entanglement distillation can overcome this problem. Here we propose an experimentally feasible scheme of continuous-variable entanglement generation, storage, and distillation between distant quantum nodes, which only requires the atomic ensemble quantum memory and balanced homodyne detection (BHD). Initially one copy of a bipartite entangled state of light, suffering phase fluctuations during the distributions, is stored in two distant atomic ensembles so that the atomic ensembles are entangled. Within the storage lifetime, by distributing and storing another copy of entangled optical modes in these two atomic ensembles, the distillation on entangled atomic spin waves can be implemented based on the post-selection of the BHD result. Our scheme provides a highly entangled state between remote quantum nodes for downstream applications, and enables the extension to multipartite entanglement distillation in a large-scale quantum network.
Quantum memory for the nonclassical state of light and entanglement among multiple remote quantum nodes hold promise for a large-scale quantum network, however, continuous-variable (CV) memory efficiency and entangled degree are limited due to imperfect implementation. Here we propose a scheme to deterministically entangle multiple distant atomic ensembles based on CV cavity-enhanced quantum memory. The memory efficiency can be improved with the help of cavity-enhanced electromagnetically induced transparency dynamics. A high degree of entanglement among multiple atomic ensembles can be obtained by mapping the quantum state from multiple entangled optical modes into a collection of atomic spin waves inside optical cavities. Besides being of interest in terms of unconditional entanglement among multiple macroscopic objects, our scheme paves the way towards the practical application of quantum networks.
It is crucial for the physical realization of quantum information networks to first establish entanglement among multiple space-separated quantum memories and then, at a user-controlled moment, to transfer the stored entanglement to quantum channels for distribution and conveyance of information. Here we present an experimental demonstration on generation, storage, and transfer of deterministic quantum entanglement among three spatially separated atomic ensembles. The off-line prepared multipartite entanglement of optical modes is mapped into three distant atomic ensembles to establish entanglement of atomic spin waves via electromagnetically induced transparency light-matter interaction. Then the stored atomic entanglement is transferred into a tripartite quadrature entangled state of light, which is space-separated and can be dynamically allocated to three quantum channels for conveying quantum information. The existence of entanglement among three released optical modes verifies that the system has the capacity to preserve multipartite entanglement. The presented protocol can be directly extended to larger quantum networks with more nodes.Continuous-variable encoding is a promising approach for quantum information and communication networks. Here, the authors show how to map entanglement from three spatial optical modes to three separated atomic samples via electromagnetically induced transparency, releasing it later on demand.
Atomic ensembles containing a large number of atoms have been proved to be an effective medium for quantum-state (quantum information) engineering and processing via their coupling with multi-photon light pulses. The general mechanism of this coupling, which involves continuous quantum variables for light and atoms, is described. The efficient quantum interface between light and atoms has led to the recent demonstration of an entangled state of two macroscopic atomic objects, more precisely two caesium gas samples. Based on this result, a proposal for teleportation of an entangled state of two atomic samples (entanglement swapping) is presented.
Quantum networks are composed of quantum nodes which coherently interact by way of quantum channels. They offer powerful capabilities for quantum computation, communication, and metrology. A generic requirement for these realizations is the capability to generate and store quantum states among multiple quantum nodes, and to disseminate these resources throughout the network via the quantum channels. In this thesis, I describe a series of experiments whereby single excitations in atomic ensembles are strongly coupled to optical modes and provide efficient means for the coherent control of entangled states between matter and light. By following the seminal proposal by Duan et al., we have generated measurement-induced entanglement of an excitation between two cold atomic ensembles. Using this system, we investigated the relationship for the global bipartite entanglement and local correlations in its subsystems. In addition, we achieved functional quantum nodes for entanglement distribution. Two pairs of remote ensembles at two quantum nodes were prepared into entangled states in a heralded and asynchronous fashion by the conditional controls of the entanglement. The quantum states of the ensembles were then distributed into polarization entangled states of photons. We also prepared an analogous quantum state and transferred the nonlocal coherence between two pairs of heralded entangled atomic ensembles, providing a step towards entanglement connection. Beyond such probabilistic approaches, we demonstrated an experiment where entanglement between two quantum memories is created by the reversible and deterministic mapping of an entangled state of light via dynamic electromagnetically induced transparency. This experiment opens novel prospects of integrating hybrid quantum systems by way of reversible quantum interfaces between light and matter. Then, we extended our work to multipartite quantum systems. We theoretically investigated the characterization of multipartite mode-entangled states by way of quantum uncertainty relations, and introduced theoretical tools to verify the entanglement orders in multipartite systems. In particular, we achieved entanglement for one delocalized photon among multiple optical modes (N > 2). Finally, we have achieved measurement-induced entanglement of spin waves among four quantum memories. The individual atomic components for the entangled W state of the four ensembles were then coherently converted into four propagating entangled beams of light via superradiant emissions. We observed the statistical and dynamic transitions for the multipartite entangled spin waves. Experiments described in this thesis thereby represent significant advances of experimental and theoretical capabilities to generate, store, transfer, and characterize entanglement of matter and light over quantum networks.