Experimental recognition of vortex beams in oceanic turbulence combining the Gerchberg-Saxton algorithm and convolutional neural network.

In underwater wireless optical communication (UWOC), vortex beams carrying orbital angular momentum (OAM) can improve channel capacity but are vulnerable to oceanic turbulence (OT), leading to recognition errors. To mitigate this issue, we propose what we believe to be a novel method that combines the Gerchberg-Saxton (GS) algorithm-based recovery with convolutional neural network (CNN)-based recognition (GS-CNN). Our experimental results demonstrate that superposed Laguerre-Gaussian (LG) beams with small topological charge are ideal information carriers, and the GS-CNN remains effective even when OT strength C n2 is high up to 10-11 K 2 m -2/3. Furthermore, we use 16 kinds of LG beams to transmit a 256-grayscale digital image, giving rise to an increase in recognition accuracy from 0.75 to 0.93 and a decrease in bit error ratio from 3.98×10-2 to 6.52×10-3 compared to using the CNN alone.

Reliable experimental manipulation of quantum steering direction.

Noise-adding methods have been widely used to manipulate the direction of quantum steering, but all related experimental schemes only worked under the assumption that Gaussian measurements were performed and ideal target states were accurately prepared. Here, we prove, and then experimentally observe, that a class of two-qubit states can be flexibly changed among two-way steerable, one-way steerable and no-way steerable, by adding either phase damping noise or depolarization noise. The steering direction is determined by measuring steering radius and critical radius, each of which represents a necessary and sufficient steering criterion valid for general projective measurements and actually prepared states. Our work provides a more efficient and rigorous way to manipulate the direction of quantum steering, and can also be employed to manipulate other types of quantum correlations.

Investigation of the influence of measurement imperfections on quantum communication complexity superiority for the Clauser-Horne-Shimony-Holt game.

Demonstrating quantum communication complexity superiority non-trivially with currently available experimental systems is of utmost importance in quantum information science. Here, we propose a generalized entanglement-assisted communication complexity reduction protocol and analyze the robustness of its quantum superiority against the measurement imperfections, such as measurement basis deviation and choice probability bias, a common problem rarely studied before. We find that the quantum superiority can be obtained in a specific entangled state in a suitable range of measurement basis and basis choice parameters. And the quantum superiority strengthens with the increase of the entanglement degree of quantum states. By using the maximum entangled state and its corresponding optimal measurement, the result we obtained violated the optimal classical bound by 239 standard deviations. Besides, the robustness of effective measurement basis in dephasing and depolarizing quantum channels is also investigated. These results not only make a step forward in investigating sufficient experimental conditions to unambiguously demonstrate the superiority of quantum communication complexity but also help to develop relevant practical applications.

Effect of oceanic turbulence on the visibility of underwater ghost imaging.

We carry out a detailed study on underwater ghost imaging (GI) in oceanic turbulence. We set up a physical model of GI through oceanic turbulence, which includes light-field transmission, and interaction between light field and oceanic turbulence without considering the effects of water absorption and scattering of light. We obtain theoretical expressions for the impulse response function and the visibility of GI in oceanic turbulence based on the power spectrum of the turbulence and the extended Huygens-Fresnel integral. The results show that the quality of GI under the effects of oceanic turbulence is related to the intensity of turbulence and the propagation distance of light. The quality of GI could be maintained at a relatively small distance in strong oceanic turbulence, whereas the quality is degraded dramatically at a relatively long distance in strong oceanic turbulence. We further analyze the quality of GI under various turbulence conditions and over different propagation distances by numerical calculation. Our results provide guidance for the realization of adaptive underwater optical GI over different length scales under the effect of oceanic turbulence.

Performance of underwater quantum key distribution with polarization encoding.

Underwater quantum key distribution (QKD) has potential applications in absolutely secure underwater communication. However, the performance of underwater QKD is limited by the optical elements, background light, and dark counts of the detector. In this paper, we propose a modified formula for the quantum bit error rate (QBER), which takes into account the effect of detector efficiency on the QBER caused by the background light. Then we calculate the QBER of the polarization encoding BB84 protocol in Jerlov-type seawater by analyzing the effect of the background light and optical components in a more realistic situation. Finally, we further analyze the final key rate and the maximum secure communication distance in three propagation modes, i.e., upward, downward, and horizontal modes. We find that secure QKD can be performed in the clearest Jerlov-type seawater at a distance of hundreds of meters, even in the worst downward propagation mode. Specifically, by optimizing the system parameters, it is possible to securely transmit information with a rate of 67 kbits/s at a distance of 100 m in the seawater channel with an attenuation coefficient of 0.03/m at night. For practical underwater QKD, the performance can also be improved by using decoy states. Our results are useful for long-distance underwater quantum communication.

Channel analysis for single photon underwater free space quantum key distribution.

We investigate the optical absorption and scattering properties of underwater media pertinent to our underwater free space quantum key distribution (QKD) channel model. With the vector radiative transfer theory and Monte Carlo method, we obtain the attenuation of photons, the fidelity of the scattered photons, the quantum bit error rate, and the sifted key generation rate of underwater quantum communication. It can be observed from our simulations that the most secure single photon underwater free space QKD is feasible in the clearest ocean water.

Activation of Einstein-Podolsky-Rosen steering sharing with unsharp nonlocal measurements.

Einstein-Podolsky-Rosen (EPR) steering is commonly shared among multiple observers by utilizing unsharp measurements. Nevertheless, their usage is restricted to local measurements and does not encompass all nonlocal measurement-based cases. In this work, a method for finding beneficial local measurement settings has been expanded to include nonlocal measurement cases. This method is applicable for any bipartite state and offers benefits even in scenarios with a high number of measurement settings. Using the Greenberger-Horne-Zeilinger state as an illustration, we show that employing unsharp nonlocal measurements can activate the phenomenon of steering sharing in contrast to using local measurements. Furthermore, our findings demonstrate that nonlocal measurements with unequal strength possess a greater activation capability compared to those with equal strength. Our activation method generates fresh concepts for conservation and recycling quantum resources.

Dynamics of multipartite quantum steering for different types of decoherence channels.

Multipartite quantum steering, a unique resource for asymmetric quantum network information tasks, is very fragile to the inevitable decoherence, which makes it useless for practical purposes. It is thus of importance to understand how it decays in the presence of noise channels. We study the dynamic behaviors of genuine tripartite steering, reduced bipartite steering, and collective steering of a generalized three-qubit W state when only one qubit interacts independently with the amplitude damping channel (ADC), phase damping channel (PDC) or depolarizing channel (DC). Our results provide the region of decoherence strength and state parameters that each type of steering can survive. The results show that these steering correlations decay the slowest in PDC and some non-maximally entangled states more robust than the maximally entangled ones. Unlike entanglement and Bell nonlocality, the thresholds of decoherence strength that reduced bipartite steering and collective steering can survive depend on the steering direction. In addition, we find that not only one party can be steered by a group system, but also two parties can be steered by a single system. There is a trade-off between the monogamy relation involving one steered party and two steered parties. Our work provides comprehensive information about the effect of decoherence on multipartite quantum steering, which will help to realize quantum information processing tasks in the presence of noise environments.

Generation of three-dimensional entangled state between a single atom and a Bose-Einstein condensate via adiabatic passage.

Inspired by a recently experiment by M. Lettner et al. [Phys. Rev. Lett. 106, 210503 (2011)], we propose a robust scheme to prepare three-dimensional entanglement state between a single atom and a Bose-Einstein condensate (BEC) via stimulated Raman adiabatic passage (STIRAP) technique. The atomic spontaneous radiation, the cavity decay, and the fiber loss are efficiently suppressed by the engineering adiabatic passage. Our strictly numerical simulation shows our proposal is good enough to demonstrate the generation of three-dimensional entanglement with high fidelity and within the current experimental technology.

Protecting nonlocal quantum correlations in correlated squeezed generalized amplitude damping channel.

Nonlocal quantum correlations, such as quantum entanglement, quantum steering, and Bell nonlocality, are crucial resources for quantum information tasks. How to protect these quantum resources from decoherence is one of the most urgent problems to be solved. Here, we investigate the evolution of these correlations in the correlated squeezed generalized amplitude damping (SGAD) channel and propose a scheme to protect them with weak measurement (WM) and quantum measurement reversal (QMR). Compared with the results of the uncorrelated SGAD channel, we find that when [Formula: see text], correlation and squeezing effects can prolong the survival time of quantum entanglement, Bell nonlocality, and quantum steering by about 152 times, 207 times, and 10 times, respectively. In addition, local WM and QMR can effectively recover the disappeared nonlocal quantum correlations either in uncorrelated or completely correlated SGAD channels. Moreover, we find that these initial nonlocal quantum correlations could be drastically amplified under the correlated channel. And the steering direction can be flexibly manipulated either by changing the channel parameters or the strength of WM and QMR. These results not only make a step forward in suppressing decoherence and enhancing quantum correlation in noise channels, but also help to develop relevant practical applications.

Clinical outcome of 30 patients with bone marrow metastases.

OBJECTIVE: Cancer patients with bone marrow metastases are rare and dismal. The study was to identify the clinical features and prognostic factors in cancer patients with bone marrow metastases. PATIENTS AND METHODS: A total of 30 patients with bone marrow metastases were reviewed between September 2007 and September 2013. Bone marrow metastases were identified by bone marrow aspiration. RESULTS: The median age was 56.5 years (range, 8-85 years). The two most common primary tumor sites were the stomach (7, 23.3%), breast (5, 16.7%). Bone metastases (27, 90.0%) were the most common concurrent metastases. The most common cause for bone marrow aspiration was anemia and thrombocytopenia (10, 33.3%). The median survival time was 3 months (range, 0.5-82 months). Patients with good performance status (n = 19) had a longer median survival time than patients with poor performance status (n = 11) (8 months vs. 1 months, P = 0.041). Patients with primary unknown origin (n = 5) had a significantly shorter overall survival time than patients with known origin (n = 25) (1 month vs. 6 months = 0.010). The median survival time was 9 months in the systemic therapy group (n = 21) and 1 month in the best supportive care group (n = 9) (P = 0.000). CONCLUSION: To make primary origin clear and start systemic antitumor therapy is beneficial for patients with bone marrow metastases.