We describe a quantum repeater protocol for long-distance quantum communication. In this scheme, entanglement is created between qubits at intermediate stations of the channel by using a weak dispersive light-matter interaction and distributing the outgoing bright coherent light pulses among the stations. Noisy entangled pairs of electronic spin are then prepared with high success probability via homodyne detection and postselection. The local gates for entanglement purification and swapping are deterministic and measurement-free, based upon the same coherent-light resources and weak interactions as for the initial entanglement distribution. Finally, the entanglement is stored in a nuclear-spin-based quantum memory. With our system, qubit-communication rates approaching 100 Hz over 1280 km with fidelities near 99% are possible for reasonable local gate errors.PACS numbers: 03.67. Hk, 03.67.Mn, 42.50.Pq In a quantum repeater, long-distance entanglement is created by distributing entangled states over sufficiently short segments of a channel such that the noisy entangled states in each segment can be purified and connected via entanglement swapping [1,2]. The resulting entanglement between the qubits at distant stations can then be used, for example, to teleport quantum information [3] or transmit secret classical information [4]. Existing approaches to quantum repeaters generate entanglement using postselection with single-photon detection [5,6,7]. In these schemes, high-fidelity entanglement is created and the subsequent entanglement purification is needed primarily to compensate the degrading effect of connecting the imperfect entangled pairs via swapping. However, due to their rather low success probabilities in the initial entanglement distribution, these protocols feature very low communication rates.More efficient schemes, compatible with existing classical optical communication networks, would involve bright multi-photon signals. In this Letter, we propose such a scheme that operates in a regime of modest initial fidelities, but creates entangled states at high speed. The high rate in the generation of entangled pairs is mainly due to the near-unit efficiencies for the homodyne detection of bright signals, as opposed to the low efficiencies of single-photon detectors. In our scheme, the resulting entangled pairs will be discrete atomic qubit states, but the probe system we use is a bright light pulse described and measured via a continuous phase observable; hence, our quantum repeater is "hybrid" not only because it employs matter signals and light probes (as in other schemes), but more distinctly, by utilizing both discrete and continuous quantum variables.In general, in order to realize universal quantum computation or, more relevant to us here, long-distance quantum communication, a nonlinear element is needed for the implementation. Optically, this nonlinear element may be introduced in at least two possible ways. The first method uses only linear transformations, but a measurement-induced nonlinear...

We demonstrate quantum interference between photons generated by the radiative decay processes of excitons that are bound to isolated fluorine donor impurities in ZnSe/ZnMgSe quantum-well nanostructures. The ability to generate single photons from these devices is confirmed by autocorrelation experiments, and the indistinguishability of photons emitted from two independent nanostructures is confirmed via a Hong-Ou-Mandel dip. These results indicate that donor impurities in appropriately engineered semiconductor structures can portray atomlike homogeneity and coherence properties, potentially enabling scalable technologies for future large-scale optical quantum computers and quantum communication networks.

Unprecedented optical nonlinearities can be generated probabilistically in simple linear-optical networks conditioned on specific measurement outcomes. We describe a highly controllable quantum filter for photon number states, which takes advantage of such a measurement-induced amplitude nonlinearity. The basis for this filter is multiphoton nonclassical interference which we demonstrate for one- and two-photon states over a wide range of beam splitter reflectivities. Specifically, we show that the transmission probability, conditional on a specific measurement outcome, can be larger for a two-photon state than a one-photon state; this is not possible with linear optics alone.

We have constructed an efficient source of photon pairs using a waveguide-type nonlinear device and performed a twophoton interference experiment with an unbalanced Michelson interferometer. Parametric down-converted photons from the nonlinear device are detected by two detectors located at the output ports of the interferometer. Because the interferometer is constructed with two optical paths of different length, photons from the shorter path arrive at the detector earlier than those from the longer path. We find that the difference of arrival time and the time window of the coincidence counter are important parameters which determine the boundary between the classical and quantum regime. When the time window of the coincidence counter is smaller than the arrival time difference, fringes of high visibility (80± 10%) were observed. This result is only explained by quantum theory and is clear evidence for quantum entanglement of the interferometer's optical paths.Two-photon entanglement has attracted considerable interest for studying the nonlocal correlations of quantum theory [1][2][3][4], and many experiments have been performed [5][6][7][8][9]. The contradiction of local realism can be realized more clearly with multi-photon entanglement systems [10], which have been demonstrated experimentally in recent years [11]. We can expect these systems to be used for novel applications such as quantum cryptography [12], and quantum teleportation [13].Multi-photon entanglement systems can be generated by parametric down-conversion. Since the probability of generating multi-photon entangled systems decreases exponentially with the number of entangled photons, it becomes more difficult to conduct experiments with a large number of entangled photons [14]. One of the candidates for solving this difficulty is to make the ultrabright source of polarization-entangled photons proposed by Kwiat et al. [15]. The source is superior to other sources because nearly every pair of photons is polarization entangled. Since the total number of generated photon pairs is limited by the nonlinear susceptibility and phase matching condition of a nonlinear crystal, a remarkable increase in the number of photon pairs can not be expected if one uses bulk crystals. This is to be compared to the drastic improvement of the efficiency to generate photon pairs we present. Our method uses a waveguide type nonlinear device originally developed for type-I quasi-phase-matching frequency doubling. Using the newly developed source of photon pairs, we then perform a two-photon interference experiment and show that photon pairs are in the entangled state for interferometer's optical paths. Parametric down-converted photons from the nonlinear device are detected by two detectors located at the output ports of the interferometer. Because this interferometer is constructed with two optical paths of different length, photons from the shorter path arrive at the detector earlier than those from the longer path. When the time window of the coincidence counter is ...

We have realized the nonlinear sign shift operation for photonic qubits. This operation shifts the phase of two photons reflected by a beam splitter using an extra single photon and measurement. We show that the conditional phase shift is (1.05+/-0.06)pi in clear agreement with theory. Our results show that, by using an ancilla photon and conditional detection, nonlinear optical effects can be implemented using only linear optical elements. This experiment represents an essential step for linear optical implementations of scalable quantum computation.

We demonstrate a Fock-state filter which is capable of preferentially blocking single photons over photon pairs. The large conditional nonlinearities are based on higher-order quantum interference, using linear optics, an ancilla photon, and measurement. We demonstrate that the filter acts coherently by using it to convert unentangled photon pairs to a path-entangled state. We quantify the degree of entanglement by transforming the path information to polarization information; applying quantum state tomography we measure a tangle of T 20 9%. DOI: 10.1103/PhysRevLett.98.203602 PACS numbers: 42.50.Dv, 03.65.Wj, 03.67.Mn, 42.50.Nn In practice it is extremely difficult to make one photon coherently influence the state of another. The optical nonlinearities required are orders of magnitude beyond those commonly achieved with current technology. Strong effective nonlinearities can be induced in linear optical systems by combining quantum interference and projective measurement [1], opening the possibility of scalable linearoptical quantum computation. Such measurement-induced nonlinearities have had high impact in quantum information, notably in optical quantum logic gate experiments [2,3] and in exotic state production [4,5].Most schemes achieve an effective nonlinearity via lowest-order nonclassical interference, with one photon per mode input to a beam splitter. Higher-order nonclassical interference, where more than one photon is allowed per mode, enables additional control [1]. An ancilla photon has been used to conditionally control the phase of a twophoton path-entangled state [2], and to conditionally absorb either one-or two-photon states [6]. Applied to superpositions, higher-order interference is predicted to act as a Fock-state filter [7,8], conditionally absorbing only terms with a specified number of photons. In this Letter, we prove that conditional absorption is coherent by applying it to a superposition, and experimentally generating a pathentangled state. We quantify the entanglement by transforming path information to polarization, and applying quantum state tomography [9].The Fock-state filter uses nonclassical interference at a single, polarization-independent, beam splitter of reflectivity R. Consider the beam splitter in Fig. 1 with n 1 photons incident: n in mode a, and 1 (the ancilla) in mode b. There are n 1 possible ways for there to be one and only one photon in mode d: all the input photons can be reflected, with probability amplitude R p n1 , or there are n ways for a photon from each input to be transmitted and the rest reflected, n1 ÿ R R p nÿ1 . Assuming indistinguishable photons, the probability amplitude for detecting one and only one photon in mode d is An R nÿ1=2 R ÿ n1 ÿ R [6,7,10]. Note that the probability Pn jAnj 2 can be zero for any single choice of n, when R n=n 1; for all other n, P > 0 [6]. HongOu-Mandel interference is the lowest-order case, where P 0 when n 1 and R The Fock-state filter could be tested by creating a number-state superposition, applying the filter, an...

We report on the optical investigation of single electron spins bound to fluorine donor impurities in ZnSe. Measurements of photon antibunching establish the presence of single, isolated optical emitters, and magnetooptical studies are consistent with the presence of an exciton bound to the spin-impurity complex. The isolation of this single donor-bound exciton complex and its potential homogeneity offer promising prospects for a scalable semiconductor qubit with an optical interface.PACS numbers: 78.55. Et, Schemes for quantum information processing and quantum communications rely on scalable, robust qubits. In particular, there are many proposals that require fast, efficient, and homogenous single-photon sources 1-3 and still others that rely on the interaction between matter qubits and flying photonic qubits 4 . The requisites for both types of schemes can be satisfied with semiconductor electron spins, which serve as single photon sources 5 or long lived quantum memories with an optical interface 6-8 . However, optical schemes, particularly those based on entanglement, also require large numbers of homogenous photon emitters 9-14 . Electron spins in self-assembled QDs, unfortunately, suffer from large inhomogenities due to their natural size distribution.Impurity-bound electrons in direct bandgap semiconductors, however, have relatively little inhomogeneous broadening 15-20 , yet still possess strong optical transitions when binding an additional exciton 18-21 and long ground state coherence times 7 .An electron bound to a single fluorine donor in ZnSe (F:ZnSe) may serve as a physical qubit with many potential advantages over previously researched qubits. F:ZnSe is particularly appealing because of its nuclear structure compared to III-V-based bound-exciton or quantum dot systems. Unlike III-V systems, isotopic purification of the ZnSe-host matrix to a nuclear-spin-0 background is possible, eliminating magnetic noise from nuclear spin diffusion 22,23 . Further, the F-impurity has a nuclear spin of 1/2 with 100% abundance. Electronnuclear spin swapping schemes 24,25 can be used, which, in combination with the spin-0 background of the isotopically purified host matrix, could lead to an extremely long-lived qubit. Additionally, the applicability of standard microfabrication techniques 26,27 to ZnSe makes the F:ZnSe system particularly scalable.The F:ZnSe system has already shown promise as a scalable source of single photons in Ref. 20. However, a) Electronic address: kdegreve@stanford.edu b) Currently at HRL Laboratories, LLC, 3011 Malibu Canyon Rd., Malibu, CA 90265.this work did not demonstrate the potential of the donor system as a future quantum memory. Here, we show both statistics for single photon emission, as well as the presence of a three-level optical Λ-system through magnetospectroscopy experiments. This introduces F:ZnSe as a valid candidate for use as a scalable qubit with an optical interface.~ 23 meV TES I 2 FE-LH FE-HH c) pump D 0 X D 0 1s 2s,2p I 2 2s-TES 21 meV b) Zn 0.92 Mg 0.08 Se Zn 0.92 M...

Here we demonstrate optical pumping of a single electron within a semiconductor nanostructure comprised of a single fluorine donor located within a ZnSe/ZnMgSe quantum well. Experiments were performed to detect optical pumping behavior by observing single photons emitted from the nanostructure when the electron changes spin state. These results demonstrate initialization and read-out of the electron spin qubit and open the door for coherent optical manipulation of a spin by taking advantage of an unconventional nanostructure.