The heralded generation of entangled states is a long-standing goal in quantum information processing, because it is indispensable for a number of quantum protocols. Polarization entangled photon pairs are usually generated through spontaneous parametric down-conversion, but the emission is probabilistic. Their applications are generally accompanied by post-selection and destructive photon detection. Here, we report a source of entanglement generated in an event-ready manner by conditioned detection of auxiliary photons. This scheme benefits from the stable and robust properties of spontaneous parametric down-conversion and requires only modest experimental efforts. It is flexible and allows the preparation efficiency to be significantly improved by using beamsplitters with different transmission ratios. We have achieved a fidelity better than 87% and a state preparation efficiency of 45% for the source. This could offer promise in essential photonics-based quantum information tasks, and particularly in enabling optical quantum computing by reducing dramatically the computational overhead.Comment: 24 pages, 4 figures, 1 tabl

Quantum memories are regarded as one of the fundamental building blocks of linear-optical quantum computation [1] and long-distance quantum communication [2]. A long standing goal to realize scalable quantum information processing is to build a long-lived and efficient quantum memory. There have been significant efforts distributed towards this goal. However, either efficient but short-lived [3,4] or long-lived but inefficient quantum memories [5][6][7] have been demonstrated so far. Here we report a high-performance quantum memory in which long lifetime and high retrieval efficiency meet for the first time. By placing a ring cavity around an atomic ensemble, employing a pair of clock states, creating a longwavelength spin wave, and arranging the setup in the gravitational direction, we realize a quantum memory with an intrinsic spin wave to photon conversion efficiency of 73(2)% together with a storage lifetime of 3.2(1) ms. This realization provides an essential tool towards scalable linearoptical quantum information processing.A high-performance quantum memory is of crucial importance for large-scale linear-optical quantum computation[1], distributed quantum computing, and long-distance quantum communication [2]. The lifetime and the retrieval efficiency of a quantum memory are two important quantities that determine the scalability of realistic quantum information protocols. For a certain quantum information task, e.g. creating a large-scale cluster state [8] or distributing entanglement through the quantum repeater protocol [9-12], the time overhead T r is inversely proportional to a power law of the retrieval efficiency R, T r ∝ R −n , where n is determined by the scale of the quantum computation or the communication distance. In order to implement one of those tasks, the lifetime of the quantum memory must be larger than this time overhead. To satisfy this condition, one has to improve the lifetime of the quantum memory and reduce the time overhead by improving the retrieval efficiency. Besides, different protocols also set thresholds on the retrieval efficiency and lifetime. For example, in loss-tolerant linear-optical quantum computation the minimum retrieval efficiency required is 50% [13] and in long-distance quantum communication distributing en-tanglement over 1000 km requires a communication time of at least 3.3 ms.Quantum memories for light have been demonstrated with atomic ensembles [14][15][16], solid state systems [17,18], and single atoms [19]. With these quantum memories, the principle of some quantum information protocols have been demonstrated, e.g., functional quantum repeater nodes were realized with atomic ensembles [20,21]. However, due to the low retrieval efficiency and short lifetime, the implementation of further steps is extremely difficult. Therefore, in recent years, many efforts have been devoted towards improving the retrieval efficiency and the lifetime of the quantum memories and significant progress has been achieved. However, an efficient and long-lived quantum memory remai...

Ring exchange is an elementary interaction for modeling unconventional topological matters which hold promise for efficient quantum information processing. We report the observation of fourbody ring-exchange interactions and the topological properties of anyonic excitations within an ultracold atom system. A minimum toric code Hamiltonian in which the ring exchange is the dominant term, was implemented by engineering a Hubbard Hamiltonian that describes atomic spins in disconnected plaquette arrays formed by two orthogonal superlattices. The ring-exchange interactions were resolved from the dynamical evolutions in the spin orders, matching well with the predicted energy gaps between two anyonic excitations of the spin system. A braiding operation was applied to the spins in the plaquettes and an induced phase 1.00(3)π in the four-spin state was observed, confirming 1 2 -anynoic statistics. This work represents an essential step towards studying topological matters with many-body systems and the applications in quantum computation and simulation.Exploiting the laws of quantum mechanics, quantum information processing can be exponentially faster than the classical counterpart [1]. To make this technology a reality, scientists have to solve the crucial problem of decoherence and systematic errors in real quantum systems, which is very difficult due to the request of an extremely small error threshold to enable error corrections [2,3]. A very encouraging solution to this problem is the Kitaev model [4] of fault-tolerant quantum computation by anyons, a sort of topological quasiparticles being neither bosons nor fermions [5]. In this model, anyons are exploited to encode and manipulate information in a manner which is resistant to errors, the so-called topological protection. Unfortunately, except that signatures of anyonic statistics emerged in the fractional quantum Hall systems [6,7], there has been no conclusive observation of anyons in any existing matters. A proposal suggests to solely mimic anyonic statistics with non-interacting qubits [8] and experimental demonstrations were achieved with entangled photons [9, 10] and ions [11]. However, because the background interacting Hamiltonian does not exist in such systems, it is not possible to define anyonic excitations [12]. Therefore, the observation of anyons remains challenging.To construct the appropriate Hamiltonian for studying anyons, a practical scheme [13] was proposed to create artificial topological matters by manipulating ringexchange interactions [14] among ultracold atoms in optical lattices [15,16]. Although a large category of manybody models [17][18][19][20][21] have been realized with optical lattices, implementing the ring-exchange Hamiltonian is notoriously difficult due to its nature of the fourth-order spin interaction, which is greatly suppressed compared to the lower order processes, such as superexchange interactions [19,20]. So, generation and observation of the ring-exchange interactions and the correlated anyonic excitations become the ...

Ultracold atoms in optical lattices offer a great promise to generate entangled states for scalable quantum information processing owing to the inherited long coherence time and controllability over a large number of particles. We report on the generation, manipulation and detection of atomic spin entanglement in an optical superlattice. Employing a spin-dependent superlattice, atomic spins in the left or right sites can be individually addressed and coherently manipulated by microwave pulses with near unitary fidelities. Spin entanglement of the two atoms in the double wells of the superlattice is generated via dynamical evolution governed by spin superexchange. By observing collisional atom loss with in-situ absorption imaging we measure spin correlations of atoms inside the double wells and obtain the lower boundary of entanglement fidelity as 0.79±0.06, and the violation of a Bell's inequality with S = 2.21±0.08. The above results represent an essential step towards scalable quantum computation with ultracold atoms in optical lattices. arXiv:1507.05937v1 [cond-mat.quant-gas] 21 Jul 2015