results for au:Deng_L in:quant-ph

- Jan 16 2018 quant-ph arXiv:1801.04418v1We perform decoy-state quantum key distribution between a low-Earth-orbit satellite and multiple ground stations located in Xinglong, Nanshan, and Graz, which establish satellite-to-ground secure keys with ~kHz rate per passage of the satellite Micius over a ground station. The satellite thus establishes a secure key between itself and, say, Xinglong, and another key between itself and, say, Graz. Then, upon request from the ground command, Micius acts as a trusted relay. It performs bitwise exclusive OR operations between the two keys and relays the result to one of the ground stations. That way, a secret key is created between China and Europe at locations separated by 7600 km on Earth. These keys are then used for intercontinental quantum-secured communication. This was on the one hand the transmission of images in a one-time pad configuration from China to Austria as well as from Austria to China. Also, a videoconference was performed between the Austrian Academy of Sciences and the Chinese Academy of Sciences, which also included a 280 km optical ground connection between Xinglong and Beijing. Our work points towards an efficient solution for an ultralong-distance global quantum network, laying the groundwork for a future quantum internet.
- Quantum key distribution (QKD) uses individual light quanta in quantum superposition states to guarantee unconditional communication security between distant parties. In practice, the achievable distance for QKD has been limited to a few hundred kilometers, due to the channel loss of fibers or terrestrial free space that exponentially reduced the photon rate. Satellite-based QKD promises to establish a global-scale quantum network by exploiting the negligible photon loss and decoherence in the empty out space. Here, we develop and launch a low-Earth-orbit satellite to implement decoy-state QKD with over kHz key rate from the satellite to ground over a distance up to 1200 km, which is up to 20 orders of magnitudes more efficient than that expected using an optical fiber (with 0.2 dB/km loss) of the same length. The establishment of a reliable and efficient space-to-ground link for faithful quantum state transmission constitutes a key milestone for global-scale quantum networks.
- We present experimental evidence supporting the postulation that the secondary effects of light-assisted collisions are the main reason that the superradiant light scattering efficiency in condensates is asymmetric with respect to the sign of the pump-laser detuning. Contrary to the recent experimental study, however, we observe severe and comparable heating with all three pump-laser polarizations. We also perform two-color, double-pulse measurements to directly study the degradation of condensate coherence and the resulting impact on the superradiant scattering efficiency.
- Apr 01 2011 quant-ph cond-mat.quant-gas arXiv:1103.6240v2We present the first experimental evidence supporting the postulation that an optical-dipole potential in a condensate undergoing superradiant scattering modifies the structure factor of the system and significantly impacts the scattering. Several consequences of this new detuning-dependent mechanism are discussed and verified experimentally. Our experiments indicate that whenever the generation and propagation growth of a new field are significant, the dynamic response of the condensate can have a profound impact on the scattering process.
- Nov 02 2010 cond-mat.quant-gas quant-ph arXiv:1011.0205v1The Comment by Wolfgang Ketterle (Ref.[1]) purports to present a viable model of superradiance in condensates. However, Ref.[1] is not able to explain the red/blue pump detuning asymmetry that was first observed recently by us (Ref.[2]). It is clear from our original paper (Ref.[3]) that the rate-equation-based theories of Ref.[1] are incomplete since they only model the final growth stage of the process when a red-detuned pump is used. Our theoretical framework (Ref.[3]), on the other hand, also treats the initial growth stage of superradiance and is therefore also capable of explaining the genesis of the red/blue detuning asymmetry (Ref.[2]). This is the key message of our response, which we frame in terms of reference to the specific points raised in Ref. [1].
- Jun 24 2010 cond-mat.quant-gas quant-ph arXiv:1006.4619v1We study a highly efficient, matter-wave amplification mechanism in a longitudinally-excited, Bose-Einstein condensate and reveal a very large enhancement due to nonlinear gain from a sixmatter- optical, wave-mixing process involving four photons. Under suitable conditions this opticallydegenerate, four-photon process can be stronger than the usual two-photon inelastic light scattering mechanism, leading to nonlinear growth of the observed matter-wave scattering independent of any enhancement from bosonic stimulation. Our theoretical framework can be extended to encompass even higher-order, nonlinear superradiant processes that result in higher-order momentum transfer.
- Jun 18 2010 cond-mat.quant-gas quant-ph arXiv:1006.3466v1An analytical perturbation theory of short-pulse, matter-wave superradiant scatterings is presented. We show that Bragg resonant enhancement is incapacitated and both positive and negative order scatterings contribute equally. We further show that propagation gain is small and scattering events primarily occur at the end of the condensate where the generated field has maximum strength, thereby explaining the apparent ``asymmetry" in the scattered components with respect to the condensate center. In addition, the generated field travels near the speed of light in a vacuum, resulting in significant spontaneous emission when the one-photon detuning is not sufficiently large. Finally, we show that when the excitation rate increases, the generated-field front-edge-steepening and peak forward-shifting effects are due to depletion of the ground state matter wave.
- Jun 17 2010 cond-mat.quant-gas quant-ph arXiv:1006.3250v2We demonstrate clear collective atomic recoil motion in a dilute, momentum-squeezed, ultra-cold degenerate fermion gas by circumventing the effects of Pauli blocking. Although gain from bosonic stimulation is necessarily absent because the quantum gas obeys Fermi-Dirac statistics, collective atomic recoil motion from the underlying wave-mixing process is clearly visible. With a single pump pulse of the proper polarization, we observe two mutually-perpendicular wave-mixing processes occurring simultaneously. Our experiments also indicate that the red-blue pump detuning asymmetry observed with Bose-Einstein condensates does not occur with fermions.
- Jun 08 2010 cond-mat.quant-gas quant-ph arXiv:1006.1295v3We report the first experimental observations of strong suppression of matter-wave superradiance using blue-detuned pump light and demonstrate a pump-laser detuning asymmetry in the collective atomic recoil motion. In contrast to all previous theoretical frameworks, which predict that the process should be symmetric with respect to the sign of the pump-laser detuning, we find that for condensates the symmetry is broken. With high condensate densities and red-detuned light, the familiar distinctive multi-order, matter-wave scattering pattern is clearly visible, whereas with blue-detuned light superradiance is strongly suppressed. In the limit of a dilute atomic gas, however, symmetry is restored.
- Mar 10 2008 quant-ph arXiv:0803.1010v2A scheme of generating controllable (2+1) photons in a double-$% \Lambda$ atomic system based on active-Raman-gain is presented in this paper. Such (2+1) photons can be a potential candidate to generate a correlated photon pair as the photon `1' acts as a trigger. Compared to other schemes of generating correlated photon pairs, our scheme exhibits a number of advantages due to the exploit of the stimulated Raman process.
- Jan 23 2007 quant-ph arXiv:quant-ph/0701164v1Using a single channel active Raman gain medium we show a $(220\pm 20)$ns advance time for an optical pulse of $\tau_{FWHM}=15.4 \mu$s propagating through a 10 cm medium, a lead time that is comparable to what was reported previously. In addition, we have verified experimentally all the features associated with this single channel Raman gain system. Our results show that the reported gain-assisted superluminal propagation should not be attributed to the interference between the two frequencies of the pump field.
- Apr 11 2005 quant-ph arXiv:quant-ph/0504057v1We propose a scheme to explore two-photon high-dimensional entanglement associated with a transverse pattern by means of two-photon interference in a beamsplitter. We find that the topological symmetry of the angular spectrum of the two-photon state governs the nature of the two-photon interference. We prove that the anti-coalescence interference is the signature of two-photon entanglement. On the basis of this feature, we propose a special Mach-Zehnder interferometer incorporated with two spiral phase plates which can change the interference from a coalescence to an anti-coalescence type only for a two-photon entangled state. The scheme is simple and straightforward compared with the test for a Bell inequality.
- May 27 2002 quant-ph arXiv:quant-ph/0205158v2We report experimental investigationd of optical pulse group velocity reduction and probe pulse regeneration using a Raman scheme. The new scheme which does not rely on the on-one-photon resonance electromagnetically induced transparency (EIT), has many advantages over the conventional method which critically relys on the transparency window created by an EIT process. We demonstrate significant reduction of group velocity, less probe field loss, reduced probe pulse distortion, and high probe pulse regeneration efficiency.
- Mar 26 2002 quant-ph arXiv:quant-ph/0203125v1We present a detailed semiclassical study on the propagation of a pair of optical fields in resonant media with and without adiabatic approximation. In the case of near and on resonance excitation, we show detailed calculation, both analytically and numerically, on the extremely slowly propagating probe pulse and the subsequent regeneration of a pulse via a coupling laser. Further discussions on the adiabatic approximation provide many subtle understandings of the process including the effect on the band width of the regenerated optical field. Indeed, all features of the optical pulse regeneration and most of the intricate details of the process can be obtained with the present treatment without invoke a full field theoretical method. For very far off resonance excitation, we show that the analytical solution is nearly detuning independent, a surprising result that is vigorously tested and compared to numerical calculations with very good agreement.
- Mar 19 2002 quant-ph arXiv:quant-ph/0203076v1We investigate a four-state system interacting with long and short laser pulses in a weak probe beam approximation. We show that when all lasers are tuned to the exact unperturbed resonances, part of the four-wave mixing (FWM) field is strongly absorbed. The part which is not absorbed has the exact intensity required to destructively interfere with the excitation pathway involved in producing the FWM state. We show that with this three-photon destructive interference, the conversion efficiency can still be as high as 25%. Contrary to common belief,our calculation shows that this process, where an ideal one-photon electromagnetically induced transparency is established, is not most suitable for high efficiency conversion. With appropriate phase-matching and propagation distance, and when the three-photon destructive interference does not occur, we show that the photon flux conversion efficiency is independent of probe intensity and can be close to 100%. In addition, we show clearly that the conversion efficiency is not determined by the maximum atomic coherence between two lower excited states, as commonly believed. It is the combination of phase-matching and constructive interference involving the two terms arising in producing the mixing wave that is the key element for the optimized FWM generation. Indeed, in this scheme no appreciable excited state is produced, so that the atomic coherence between states |0> and |2> is always very small.