Ge-Sheng Pan

Observation of eight-photon entanglement

Researchers demonstrate the creation of an eight-photon Schrodinger-cat state with genuine multipartite entanglement by developing noise-reduction multiphoton interferometer and post-selection detection. The ability to control eight individual photons will enable new multiphoton entanglement experiments in previously inaccessible parameter regimes.

Quantum teleportation and entanglement distribution over 100-kilometre free-space channels

Transferring an unknown quantum state over arbitrary distances is essential for large-scale quantum communication and distributed quantum networks. It can be achieved with the help of long-distance quantum teleportation and entanglement distribution. The latter is also important for fundamental tests of the laws of quantum mechanics. Although quantum teleportation and entanglement distribution over moderate distances have been realized using optical fibre links, the huge photon loss and decoherence in fibres necessitate the use of quantum repeaters for larger distances. However, the practical realization of quantum repeaters remains experimentally challenging. Free-space channels, first used for quantum key distribution, offer a more promising approach because photon loss and decoherence are almost negligible in the atmosphere. Furthermore, by using satellites, ultra-long-distance quantum communication and tests of quantum foundations could be achieved on a global scale. Previous experiments have achieved free-space distribution of entangled photon pairs over distances of 600 metres (ref. 14) and 13 kilometres (ref. 15), and transfer of triggered single photons over a 144-kilometre one-link free-space channel. Most recently, following a modified scheme, free-space quantum teleportation over 16 kilometres was demonstrated with a single pair of entangled photons. Here we report quantum teleportation of independent qubits over a 97-kilometre one-link free-space channel with multi-photon entanglement. An average fidelity of 80.4 ± 0.9 per cent is achieved for six distinct states. Furthermore, we demonstrate entanglement distribution over a two-link channel, in which the entangled photons are separated by 101.8 kilometres. Violation of the Clauser–Horne–Shimony–Holt inequality is observed without the locality loophole. Besides being of fundamental interest, our results represent an important step towards a global quantum network. Moreover, the high-frequency and high-accuracy acquiring, pointing and tracking technique developed in our experiment can be directly used for future satellite-based quantum communication and large-scale tests of quantum foundations.

Collective dipole oscillations of a spin-orbit coupled Bose-Einstein condensate.

In this Letter, we present an experimental study of the collective dipole oscillation of a spin-orbit coupled Bose-Einstein condensate in a harmonic trap. The dynamics of the center-of-mass dipole oscillation is studied in a broad parameter region as a function of spin-orbit coupling parameters as well as the oscillation amplitude. The anharmonic properties beyond the effective-mass approximation are revealed, such as the amplitude-dependent frequency and finite oscillation frequency at a place with a divergent effective mass. These anharmonic behaviors agree quantitatively with variational wave-function calculations. Moreover, we experimentally demonstrate a unique feature of the spin-orbit coupled system predicted by a sum-rule approach, stating that spin polarization susceptibility--a static physical quantity--can be measured via the dynamics of dipole oscillation. The divergence of polarization susceptibility is observed at the quantum phase transition that separates the magnetic nonzero-momentum condensate from the nonmagnetic zero-momentum phase. The good agreement between the experimental and theoretical results provides a benchmark for recently developed theoretical approaches.

Direct and full-scale experimental verifications towards ground-satellite quantum key distribution

Full-scale verifications for establishing quantum cryptography communication via satellites are reported. Three independent experiments using a hot-air balloon are performed: on a rapidly moving platform over a distance of 40 km, on a floating platform over a distance of 20 km, and over 96 km in air with a huge loss.

Lower Bound on the Speed of Nonlocal Correlations without Locality and Measurement Choice Loopholes

In the well-known EPR paper, Einstein et al. called the nonlocal correlation in quantum entanglement as ‘spooky action at a distance’. If the spooky action does exist, what is its speed? All previous experiments along this direction have locality and freedom-of-choice loopholes. Here, we strictly closed the loopholes by observing a 12-hour continuous violation of Bell inequality and concluded that the lower bound speed of ‘spooky action’ was four orders of magnitude of the speed of light if the Earth’s speed in any inertial reference frame was less than 10−3 times of the speed of light. [∗] Author to whom correspondence should be addressed; electronic mail: pcz@ustc.edu.cn, qiangzh@ustc.edu.cn and nlliu@ustc.edu.cn. 1 ar X iv :1 30 3. 06 14 v2 [ qu an tph ] 1 8 Ju n 20 13 In order to test the speed of ‘spooky action at a distance’[1], Eberhard proposed[2] a 12hour continuous space-like Bell inequality [3, 4] measurement over a long east-west oriented distance. Benefited from the Earth self rotation, the measurement would be ergodic over all possible translation frames and as a result, the bound of the speed would be universal[2, 5] . Salart et al.[5] recently report to have achieved the lower bound of ‘spooky action’ through an experiment using Eberhard’s proposal. In that experiment, time-bin entangled photon pairs were distributed over two sites, both equipped with a Franson-type interferometer[6] to analyze the photon pairs. At one site, the phase of the interferometer was kept stable and the phase of the interferometer at the other site was continuously scanned. An interference fringe of 87.6% was achieved, which was high enough for a violation of the CHSH-Bell inequality. However any bipartite Bell test requires at least two settings at each site because if there was only one setting, one could always achieve the same interference fringe with a product state without any entanglement. Moreover, as pointed out by Kofler et al.[7], even if the phase of the first site had a second setting, the experiment still had locality loophole, because the setting choice on one site could not be space-like separated from either the measurement events at the other site or the entanglement source generation event[8–10]. Due to these loopholes, the experiment could be explained by common causes[7] instead of ‘spook action’. Actually, similar loopholes existed in all previous attempts for the speed measurement of ‘spooky action’[11–13]. Here, we distributed entangled photon pairs over two site that were 16-km apart sites via free space optical link and implemented a space-like Bell test [3, 4] to close the locality loophole. Meanwhile, we utilized fast electro optic modulators (EOMs) to address the freedom-of-choice loophole. As almost all the photonic Bell experiment[10, 14–16], we utilized fair sampling assumption to address the detection loophole[17]. This assumption is justified by J. S. Bell himself’s comments[15, 18], “. . . it is hard for me to believe that quantum mechanics works so nicely for inefficient practical set-ups and is yet going to fail badly when sufficient refinements are made. Of more importance, in my opinion, is the complete absence of the vital time factor in existing experiments. The analyzers are not rotated during the flight of the particles.” Our two sites are east-west oriented sites at the same latitude as Eberhard proposed[2]. Suppose two events occur in the two sites, respectively at positions ~ rA and ~ rB at time tA

Experimental quasi-single-photon transmission from satellite to earth

Free-space quantum communication with satellites opens a promising avenue for global secure quantum network and large-scale test of quantum foundations. Recently, numerous experimental efforts have been carried out towards this ambitious goal. However, one essential step--transmitting single photons from the satellite to the ground with high signal-to-noise ratio (SNR) at realistic environments--remains experimental challenging. Here, we report a direct experimental demonstration of the satellite-ground transmission of a quasi-single-photon source. In the experiment, single photons (~0.85 photon per pulse) are generated by reflecting weak laser pulses back to earth with a cube-corner retro-reflector on the satellite CHAMP, collected by a 600-mm diameter telescope at the ground station, and finally detected by single-photon counting modules after 400-km free-space link transmission. With the help of high accuracy time synchronization, narrow receiver field-of-view and high-repetition-rate pulses (76 MHz), a SNR of better than 16:1 is obtained, which is sufficient for a secure quantum key distribution. Our experimental results represent an important step towards satellite-ground quantum communication.

Entanglement-based quantum key distribution with biased basis choice via free space

Quantum key distribution (QKD) [1] is a maturing technology that has evolved from an abstract idea to practical systems that are even commercially available. As with every new technology there are still plenty of new developments and the translation from theory to a practical system is difficult. In a security application, the issue of translating theoretical ideas to a working device is even more critical, because any assumption that is made in the theory needs to be verified in the actual implementation. In QKD, we have learned this the hard way, when it was realized that some devices could actually be hacked [2], not because the theory was wrong, but because a practical device is always much more complicated than even the most elaborate security proof. While keeping the security aspect under control, experimenters want to optimize their systems so that they can deliver the highest secure key rate possible under the conditions of a chosen quantum channel.

Effects of electrode biasing on fluctuations and transport in the KT-5C tokamak

Electrode biasing experiments are carried out on the KT-5C tokamak to investigate the effects of the radial electric field E/sub r/ on turbulence. A modestly enhanced E/sub r/ gradient layer is formed at the plasma edge by the electrode biasing. In the E/sub r/ gradient layer, reductions in the fluctuation amplitude, radial correlation length and turbulent particle flux are observed. These results support the theoretical models on the turbulence suppression by decorrelation due to E /spl times/ B flow shear.

Preparation and storage of frequency-uncorrelated entangled photons from cavity-enhanced SPDC

We report the preparation and storage of frequency-uncorrelated cavity-enhanced SPDC entangled photons. The frequency correlation is eliminated with a suitable pulsed pump. The storage of a single photon entangled with another flying photon is demonstrated.

Observation of spatial intermittency in Tokamak plasma turbulence

A sharp variation at some radial positions superimposed on a slow change in the profiles of the fluctuation levels, fluctuation-driven particle and energy fluxes, which is referred as spatial intermittency, is observed in the core plasma of the Keda Tokamak-5C (KT-5C) [World Survey of Activities in Controlled Fusion Research, Nuclear Fusion Special Supplement (International Atomic Energy Agency, Vienna, 1991), p. 190.]. The peaks in the profiles are located in the vicinity of low-q rational surfaces, and fluctuation spectra perpendicular to the magnetic field become more anisotropy there. The intermittency may be related to the radial variations in the nonlinear mode couplings near the low-q resonant surfaces.