Quantum teleportation and quantum dense coding are two typical examples to exploit nonlocal quantum correlation of entangled states in quantum information to perform otherwise impossible tasks. Quantum teleportation is the disembodied transport of an unknown quantum state from one place to another 1 . Quantum dense coding provide a method by which two bits of classical information can be transmitted by sending one qubit of quantum information 2 . Discrete and continuous variable teleportations have been performed experimentally for single-photon polarization

The unconditional entanglement swapping for continuous variables is experimentally demonstrated. Two initial entangled states are produced from two nondegenerate optical parametric amplifiers operating at de-amplification. Through implementing the direct measurement of the Bell-state between two optical beams from each amplifier the remaining two optical beams, which have never directly interacted with each other, are entangled. The quantum correlation degrees of 1.23 and 1.12 dB below the shot noise limit for the amplitude and phase quadratures resulting from the entanglement swapping are measured straightly.

Abstract:The quantum entanglement of amplitude and phase quadratures between two intense optical beams with the total intensity of 22mW and the frequency difference of 1nm, which are produced from an optical parametric oscillator operating above threshold, is experimentally demonstrated with two sets of unbalanced Mach-Zehnder interferometers.The measured quantum correlations of intensity and phase are in reasonable agreement with the results calculated based on a semi-classical analysis of the noise characteristics given by C. Fabre et al. OCIS codes: 190.4410, 270.6570 In recent years quantum information with continuous variables (CV) where f is the noise frequency, S 0 is the shot noise limit (SNL), B and ξ =T/(T+δ) are the cavity bandwidth and the output coupling efficiency of NOPO respectively (T -the transmission coefficient of the output coupling mirror; δ -extra intracavity losses), η is the detection efficiency,is the pump parameter (P -the pump power, P 0 -the threshold pump power of NOPO). The intensity difference quantum correlations of twin beams were experimentally measured with self-homodyne detectors by different groups and were effectively applied [7][8][9][10][11][12] . However, the phase correlation of the twin beams was not observed for a long time Almost at a parallel period we were also devoting our efforts to measure the quantum entanglement of twin beams from NOPO above threshold. The measurement scheme used by us is basically same with that presented by O. Glockl et al. inRef.[15], where they performed sub-shot-noise measurement of the phase quadratures of intense pulsed light 16 . Considering that the phase correlation will be significantly affected by the phase fluctuation of the pump laser 6 and the restricted condition deducing Eqs. (1) and (2) in Ref. [4] requires the finesse of the NOPO cavity for the pump laser much lower than that for the twin beams, in our design the ratio of the cavity finesses for the pump and the twin beams is 16/164 which is much smaller than that in Refs.[13] and [14]. Due to the lower finesse the resonant peak of the pump laser in the cavity is relatively flat and thus the threshold power is higher (~120mW).At first, using a pair of Mach-Zehnder (M-Z) interferometers with unbalanced arm-lengths we detected the amplitude and phase noise of signal and idler output fields from a NOPO above threshold at a certain analysis frequency (20MHz), respectively. Then, the quantum correlations were denoted by the noise levels of the intensity difference and the phase sum of the photocurrents measured by two unbalanced interferometers. wave plate P1 (P2). Rotating the polarization orientation of P1 (P2) we can conveniently switch between phase and amplitude measurements 15 . In our system, the distance difference of two arms ∆L is 7.5m which matches the analysis frequency of 20MHz to make θ=π. The difference of the 5 dc photocurrents of D1 and D2 (D3 and D4) serves as the error signal and is fed back onto the PZT mounted on one of mirrors of the interferometer to...

We present a protocol for performing entanglement swapping with intense pulsed beams. In a first step, the generation of amplitude correlations between two systems that have never interacted directly is demonstrated. This is verified in direct detection with electronic modulation of the detected photocurrents. The measured correlations are better than expected from a classical reconstruction scheme. In an entanglement swapping process, a four-partite entangled state is generated. We prove experimentally that the amplitudes of the four optical modes are quantum correlated 3 dB below shot noise, which is consistent with the presence of genuine four-party entanglement.

A new quantum measurement scheme of intensity difference fluctuation between two light beams of equal mean intensity is presented. In this system a beam splitter is used as the coupling device and the twin beams with high quantum correlation are injected into its dark port as the input meter wave instead of the usual vacuum field. A measurement satisfying all the quantum nondemolition criteria is experimentally achieved. The measured sum of the transfer coefficients and the conditional variance are, respectively, T s 1 T m 1.31 and V s͞m 22.1 dB. [S0031-9007(99)08445-8] PACS numbers: 42.50.Dv, 03.65.Bz Quantum nondemolition (QND) measurements have attracted extensive interest [1]. Since 1986, a variety of QND-type measurements have been successfully demonstrated in optical experiments [2][3][4][5][6][7][8][9]. In most experiments measurements of the quadrature phases of the probe field were involved to provide the information of signal observables. In a recent paper Harrison et al. [10] have proposed a QND scheme, in which the signal and probe observables are the intensity difference between twin beams on the left and right hand sides of a double ended nondegenerate optical parametric oscillator; therefore, only the field intensities need to be measured rather than the quadrature phases [10]. So far there is no published experimental realization of this QND-type measurement.It is well known that a beam splitter is the simplest optical coupling device [11]. Recently Bruckmeier et al. [9] have realized a quantum measurement satisfying the quantum nondemolition criteria by injection of a 3.7 dB quadrature squeezed wave into the usual vacuum port of a beam splitter. The good results of signal transfer T s 1 T m 1.29 and the conditional variance V s͞m 21.3 dB were obtained. On the other hand great reductions of quantum fluctuations in the intensity difference between twin beams produced by a nondegenerate parametric oscillator were achieved in several groups [12][13][14]. The above-mentioned successful experiments motivated us to design a quantum measurement scheme using a beam splitter, the dark port of which is filled by quantum correlated twin beams instead of a quadrature squeezed wave as in Ref. [9]. When the twin beams with quantum noise reduction in the intensity difference of 76% below the standard quantum limit (SQL) are injected into the vacuum port of a beam splitter, the measurement of intensity difference fluctuation in the quantum domain is experimentally realized. The measured T s 1 T m 1.31 and V s͞m 22.1 dB fulfill the QND criteria introduced by Holland et al. [15] and Poizat et al. [16].At first we simply present the measurement principle of the proposed scheme. S in and M in are, respectively, the signal and meter input waves incident upon the beam splitter (BS) from opposite sides with small angles of incidence; S out and M out are, respectively, the output signal and meter waves. Both S in and M in consist of two orthogonal polarized modes (S polarization and P polarization) of equal mean intensit...

Ageing population is now a global challenge, where physical deterioration is the common feature in elderly people. In addition, the diseases, such as spinal cord injury, stroke, and injury, could cause a partial or total loss of the ability of human locomotion. Thus, assistance is necessary for them to perform safe activities of daily living. Robotic hip exoskeletons are able to support ambulatory functions in elderly people and provide rehabilitation for the patients with gait impairments. They can also augment human performance during normal walking, loaded walking, and manual handling of heavy-duty tasks by providing assistive force/torque. In this article, a systematic review of robotic hip exoskeletons is presented, where biomechanics of the human hip joint, pathological gait pattern, and common approaches to the design of robotic hip exoskeletons are described. Finally, limitations of the available robotic hip exoskeletons and their possible future directions are discussed, which could serve a useful reference for the engineers and researchers to develop robotic hip exoskeletons with practical and plausible applications in geriatric orthopaedics.The translational potential of this articleThe past decade has witnessed a remarkable progress in research and development of robotic hip exoskeletons. Our aim is to summarize recent developments of robotic hip exoskeletons for the engineers, clinician scientists and rehabilitation personnel to develop efficient robotic hip exoskeletons for practical and plausible applications.

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International audienceWe report on the fabrication and photovoltaic characterization of In0.12Ga0.88N/GaN multi-quantum-well (MQW) solar cells grown by metal-organic vapor phase epitaxy on (0001) sapphire substrates. Increasing the number of MQWs in the active region from 5 to 30 improves a factor of 10 the peak external quantum efficiency of the device at the price of a slight reduction and increase of the shunt and series resistance, respectively. Solar cells with 30 MQWs exhibit an external quantum efficiency of 38% at 380 nm, an open circuit voltage of 2.0 V, a short circuit current density of 0.23 mA/cm2 and a fill factor of 59% under 1 sun of AM1.5G-equivalent solar illumination. Solar cells with the grid spacing of the top p-contact varying from 100 to 200 µm present the same device performance in terms of spectral response and conversion efficiency