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 teleportation 1 , a way to transfer the state of a quantum system from one location to another, is central to quantum communication 2 and plays an important role in a number of quantum computation protocols 3-5 . Previous experimental demonstrations have been implemented with photonic 6-8 or ionic qubits 9,10 . Very recently long-distance teleportation 11,12 and open-destination teleportation 13 have also been realized. Until now, previous experiments 6-13 have only been able to teleport single qubits. However, since teleportation of single qubits is insufficient for a large-scale realization of quantum communication and computation 2-5 , teleportation of a composite system containing two or more qubits has been seen as a long-standing goal in quantum information science.Here, we present the experimental realization of quantum teleportation of a two-qubit composite system. In the experiment, we develop and exploit a six-photon interferometer to teleport an arbitrary polarization state of two photons. The observed teleportation fidelities for different initial states are all

We report an experimental demonstration of entanglement swapping over two quantum stages. By successful realizations of two cascaded photonic entanglement swapping processes, entanglement is generated and distributed between two photons, that originate from independent sources and do not share any common past. In the experiment we use three pairs of polarization entangled photons and conduct two Bell-state measurements (BSMs) one between the first and second pair, and one between the second and third pair. This results in projecting the remaining two outgoing photons from pair 1 and 3 into an entangled state, as characterized by an entanglement witness. The experiment represents an important step towards a full quantum repeater where multiple entanglement swapping is a key ingredient.PACS numbers: 03.67. Bg, 03.67.Mn, 42.50.Dv, 42.50.Xa Entanglement swapping is arguably one of the most important ingredients for quantum repeaters and quantum relays, which lays at the heart of quantum communication [1,2,3,4]. For photonic quantum communication, the distance is largely limited due to decoherence from coupling to the environment and an increasing loss of photons in a quantum channel. This leads to an exponential decay in the fidelity of quantum information. This drawback can eventually be overcome by subdividing larger distances into smaller sections over which entanglement or quantum states can be distributed. The sections are then bridged by entanglement swapping processes [2,3]. The swapping procedure therefore constitutes one of the key elements for a quantum relay [3], and a full quantum repeater [2] if combined with quantum purification [5,6] and quantum memory [7]. As a result, quantum communication becomes feasible despite of realistic noise and imperfections. At the same time, the overhead for the used resources and communication time only increase polynomially with the distance [2,3,4].Experimentally, photonic entanglement swapping has so far been successfully achieved for the case of discrete variables [8,9], and for continuous variable [10], both via a single stage process. However, only after successful multiple swapping, will we be able to have a fully functional quantum repeater. There are additional advantages utilizing a multiple swapping process. For a quantum relay with many segments, it is equivalent to significantly lower the dark-count rate, which is a substantial factor limiting the transmission distance of successful quantum communication [3]. For quantum information carriers possessing mass, multiple swapping processes can speed up the distribution of entanglement by a factor that is proportional to the number of segments used [11]. Moreover, multistage entanglement swapping can improve the protection of quantum states against noise from amplitude errors [11].We report in this letter an experimental demonstration of a multiple entanglement swapping over two stages. This is achieved by utilizing three synchronous spatially independent pairs of polarization entangled photons, and performing...

In recent years, there has been heightened interest in quantum teleportation, which allows for the transfer of unknown quantum states over arbitrary distances. Quantum teleportation not only serves as an essential ingredient in long-distance quantum communication, but also provides enabling technologies for practical quantum computation. Of particular interest is the scheme proposed by D. Gottesman and I. L. Chuang [(1999) Nature 402:390-393], showing that quantum gates can be implemented by teleporting qubits with the help of some special entangled states. Therefore, the construction of a quantum computer can be simply based on some multiparticle entangled states, Bell-state measurements, and single-qubit operations. The feasibility of this scheme relaxes experimental constraints on realizing universal quantum computation. Using two different methods, we demonstrate the smallest nontrivial module in such a scheme-a teleportationbased quantum entangling gate for two different photonic qubits. One uses a high-fidelity six-photon interferometer to realize controlled-NOT gates, and the other uses four-photon hyperentanglement to realize controlled-Phase gates. The results clearly demonstrate the working principles and the entangling capability of the gates. Our experiment represents an important step toward the realization of practical quantum computers and could lead to many further applications in linear optics quantum information processing.I n 2001, Knill, Laflamme, and Milburn (KLM) showed that scalable and efficient quantum computation is possible by using linear optical elements, ancilla photons, and postselection (1). The KLM scheme is based on three principles. First, nondeterministic quantum computation is possible with linear optics. Second, universal quantum gates with the probability approaching one can be implemented based on teleportation (2), a process in which a qubit in an unknown state can be transferred to another qubit (3, 4). Third, the demanding resources can be reduced by quantum coding. The first principle has been demonstrated in many experiments. For example, various approaches for realizing photonic controlled-NOT (C-NOT) gates have been reported (5-11). Recently, a three-qubit Toffoli gate has also been carried out in a photonic architecture (12). Additionally, there have been many efforts aimed at reducing the resource requirements of the KLM protocol (13-16). Nevertheless, the teleportation-based two-qubit entangling gate, which plays an important role in the second principle of the KLM scheme, still remains an experimental challenge.Quantum teleportation is useful for quantum communication (17, 18) because it allows us to use entangled states as perfect quantum channels. The unique scheme proposed by Gottesman and Chuang (GC) in 1999 (2) opens the way for promising applications in realizing quantum computation (1,2,13,14,19). In the GC scheme, qubits are teleported through special gates by simply using multiparticle off-line entangled states, Bell-state measurements (BSM), and single...

Quantum computers promise dramatic speed ups for many computational tasks. For large-scale quantum computation however, the inevitable coupling of physical qubits to the noisy environment imposes a major challenge for a real-life implementation. A scheme introduced by Gottesmann and Chuang can help to overcome this difficulty by performing universal quantum gates in a faulttolerant manner. Here, we report a non-trivial demonstration of this architecture by performing a teleportation-based two-qubit controlled-NOT gate through linear optics with a high-fidelity sixphoton interferometer. The obtained results clearly prove the involved working principles and the entangling capability of the gate. Our experiment represents an important step towards the feasibility of realistic quantum computers and could trigger many further applications in linear optics quantum information processing.