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## GENERAL

2022, 71 (22): 220201. doi: 10.7498/aps.71.20220765
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## GENERAL

2022, 71 (22): 220301. doi: 10.7498/aps.71.20220871
Abstract +
In the application of quantum communication networks, it is an important task to realize the optimal allocation of resources according to the current situation. For example, We need to select the optimal quantum key distribution (QKD) protocol and parameters. Traditionally, the most commonly implemented method is the local search algorithm (LSA), which costs a lot of resources. Here in this work, we propose a machine learning based scheme, in which the regression machine learning is used to simultaneously select the optimal protocol and corresponding parameters. In addition, we make comparisons among a few machine learning models including random forest (RF), K-nearest neighbor (KNN) and logistic regression. Simulation results show that the new scheme takes much less time than the LSA scheme, and the RF achieves the best performance. In addition, through the RF residual analysis, we find that the machine learning scheme has good robustness. In conclusion, this work may play an important role in promoting the practical application of quantum communication networks.

## GENERAL

2022, 71 (22): 220302. doi: 10.7498/aps.71.20221125
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Quantum communication is a frontier hotspot of current research, and it has ideal information security. In order to enable quantum systems in arid and desertified areas to work almost under all-weather condition, it is necessary to study the attenuation of free-space quantum signal transmission and the influence of the turbulence atmosphere carrying sand and dust on communication performance. Using Mie scattering theory, multiple scattering simulation method, and atmospheric turbulence theory, the attenuation of optical wave transmission in sand and dust turbulent atmospheric channels with different visibility, and the influence of multiple scattering and turbulence on attenuation are studied. The results show that the effect of multiple scattering increases with the decrease of visibility, the turbulence effect gradually strengthens with the increase of distance. According to the quantum amplitude damped channel model, the effects of multiple scattering and turbulence in the sand and dust turbulent atmosphere with different visibility on the quantum channel capacity, fidelity and bit error rate are analyzed. The results show that as the visibility decreases, the multiple scattering effect increases, resulting in the decrease of attenuation and bit error rate, but an increase in channel capacity, fidelity and the boundaries of security key rate. The existence of turbulence in the dust atmosphere will increase the attenuation and bit error rate, but reduce the channel capacity, fidelity and security key rate. It can be seen that the influence of multiple scattering and turbulence on the communication performance, when the visibility of the sand and dust atmosphere are both low, cannot be ignored. In practical applications, the relevant parameters of quantum communication should be adaptively adjusted according to the visibility and turbulence intensity to improve the probability, efficiency and reliability of quantum communication.

## GENERAL

2022, 71 (22): 220303. doi: 10.7498/aps.71.20220760
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All open quantum systems are affected by environmental noises due to their interactions with the external environment and inevitably suffer from decoherence. Hence, it is fundamentally important and necessary to investigate decoherence suppression for open quantum systems via proper control strategies. Inspired by feed-forward control in the classical control theory, this paper proposes a novel decoherence suppression scheme via weak measurement and environment-assisted measurement. We first take the single-qubit system as an example to illustrate steps of the proposed scheme. To be specific, the single-qubit system is transferred to a state that is more robust to environmental noises via pre-weak measurement operators and feed-forward control operators before the decoherence channel, a measurement is performed on the environment coupled to the protected qubit during the decoherence channel, and the initial state is recovered via reversed feed-forward control operators and post-weak measurement operators after the decoherence channel. The optimum post-weak measurement strength is derived by setting the normalized final state equal to the initial state. By considering the optimum post-weak measurement strength, analytical formulas of the total success probability and the total fidelity are deduced. The proposed scheme is applicable for protecting quantum states from arbitrary decoherence channels with at least one invertible Kraus operator although only the amplitude damping channel and the phase damping channel are taken into account. Provided that the decay rate of the amplitude or phase damping channel is completely known, one can always achieve unit fidelity even for heavy damping cases, which is the biggest advantage of the proposed scheme. Influences of several parameters including strengths of weak measurements, the initial state and the decay rate of the decoherence channel on the performance of decoherence suppression are analyzed, and detailed procedures of a single-qubit pure and mixed state protection are presented on the Bloch sphere, respectively. Subsequently, the Kronecker product is employed to construct operators of dimension $2^N \times 2^N$, the proposed scheme is extended to the general N-qubit case, and unified analytical formulas of the total success probability and the total fidelity are deduced. By applying the proposed scheme to the protection of two-qubit entangled states, it is demonstrated that post-weak measurement operators are not necessary sometimes because of the particular structure of two-qubit entangled states. Furthermore, two numerical simulations are designed to enhance the concurrence of two-qubit entangled states and improve the average fidelity of the standard quantum teleportation in a noisy environment. Analytical formulas of the improvement of concurrence and the average teleportation fidelity are deduced, and the superiority of the proposed scheme is highlighted in comparison with unprotected scenarios.

## GENERAL

2022, 71 (22): 220304. doi: 10.7498/aps.71.20221312
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Bose-Einstein condensate (BEC) is essentially a macroscopic quantum effect with quantum volatility, macroscopic quantum coherence and artificial controllability. Owing to its unique controllability, it becomes a new ideal platform for quantum simulations and studies of interacting quantum systems.In this paper, the generation of vortices and the formation of vortex chains, as well as characteristics of vortex chains in rotating two-dimensional BEC in a potential composed of harmonic potential and optical lattice are studied numerically. Firstly, the generation of vortices, the formation and distribution of vortex chains and the effects of different physical parameters on the vortex chains in two-dimensional BEC are investigated by using the multigrid preconditioned conjugate gradient method. Secondly, the evolution of the vortex chains with time is studied by using the time-splitting spectral method. The results show that the generation of vortices in BEC trapped in the compound potential corresponds to the minimum value of the potential. When the depth of the optical lattice increases to a certain value, vortex chains are formed in the BEC. With the further increase of the depth of the optical lattice, the vortex depth in the vortex chain in the BEC decreases continuously, and finally the vortex chain disappears completely. When the interaction strength between atoms increases, the distribution range of the condensate expands, and the number of vortices and the number of vortex chains in the condensate also increase. When the interaction strength between atoms increases to a certain value, the symmetry of the vortex chains is broken. As the rotation frequency of the condensate increases, the distribution range of the condensate expands, and the number of vortices and the number of vortex chains in the condensate also increase. When the rotation frequency is close to the external trapping potential frequency, the linear alignment of the vortex chains is disrupted. It is also found that there are three stages in the evolution of the vortex chains in the BEC: in the first stage, vortex chains rotate together with the condensate, and the original chain distribution keeps unchanged; in the second stage, the phenomenon of vortex space extrusion appears, and the vortex chain is destroyed; in the third stage, the phenomenon of vortex space expansion occurs, and finally the vortex chains disappear. The results above show that the depth of the optical lattice, the interaction strength between atoms, and the rotation frequency of the condensate have important effects on the vortices and vortex chains in the condensate. By adjusting these physical quantities, the number of vortices and the shape of vortex chains in the BEC can be effectively manipulated. This may provide some theoretical reference and guidance for future experiments and applications.

## GENERAL

2022, 71 (22): 220701. doi: 10.7498/aps.71.20220775
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The spin relaxation time of alkali atoms in nuclear magnetic resonance gyroscope is usually on the order of 10–5 s, which is much less than that in atomic magnetometers. The response of electron paramagnetic resonance signals of atoms with short relaxation time is asymmetric in different directions under oscillating magnetic fields, which makes the measurement results of atomic transverse relaxation time and Larmor frequency different. In this work this phenomenon is analyzed based on Bloch equation theory and the theoretical correction is given. The shorter the relaxation time, the greater the differences of the response intensity and resonance frequency of the electron paramagnetic resonance signal under different magnetic field directions will be. Using this property, the transmission delay time of the system can be measured accurately. In this paper proposed is a method of measuring transverse relaxation time based on the difference between signal phases in X-axis direction and Y-axis direction, which can accurately and quickly measure very short transverse relaxation time. The difference between the half-width fitting method and the phase measurement method is compared by measuring the transverse relaxation times of 87Rb atoms under different magnetic field intensities. The half-width fitting method is greatly affected by the transmission delay time and has its measurement limit. The phase measurement method is greatly affected by the angle of the probe light, but the measurement range is wider and the anti-magnetic interference ability is stronger.

## NUCLEAR PHYSICS

2022, 71 (22): 222101. doi: 10.7498/aps.71.20220795
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We investigate the thermodynamical properties of strange quark matter (SQM) at zero/finite temperature and under constant magnetic field within quasiparticle model. The quark matter symmetry energy, energy per baryon, free energy per baryon, anisotropic pressures are also studied and the result indicates that both the effects of temperature and magnetic field can significantly influence the thermodynamical properties of quark matter and proto-quark stars (PQSs). Our result also indicates that the maximum mass and the core temperature of PQSs not only depends on the heating process at the isentropic stages, but also but also the magnetic field strength and orientation distribution inside the magnetar within quasiparticle model.

## NUCLEAR PHYSICS

2022, 71 (22): 222901. doi: 10.7498/aps.71.20221193
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In order to improve the temporal resolution of photomultiplier tubes, our research group has conducted the in-depth research on photomultiplier tubes based on microchannel plates that are widely used at present. The time resolution of photomultiplier tube based on microchannel plate is limited by the transit time of photoelectric signal in each part, including the transit time of photoelectric signal in the transmission process of photocathode to microchannel plate, the transit time of photoelectric signal in microchannel plate time, the transit time of the photoelectric signal from the microchannel plate to the detector anode, and the transit time of the photoelectric signal on the anode to the electrode port. The transit time of the whole process has a certain degree of influence on the time information measurement of the optoelectronic signal. In this study, various parameters affecting the time resolution of the photomultiplier tube are analyzed, and it is found that the different positions of the photoelectron signal on the anode will bring errors to the measurement of the arrival time of the signal at the anode, and the photoelectric signal is transmitted to the electrode port at the affected point of the anode The spent time will cause the signal measurement time to lag behind the real time, which indirectly affects the time resolution of the system. Therefore, a specific study is carried out on the time measurement error of the signal on the anode, and it is determined that the difference of the photoelectron signal on the anode position is an important factor causing the signal time measurement error, and a simple and effective method of compensating for error is proposed. In the research process, the delay line anode is used, and the positional resolution principle of the photoelectric signal is used to obtain the position information of the photoelectron signal on the anode, and the position information is converted into the time information transmitted from the position to the electrode port. The theoretical value of the transit time on the anode is offset, eliminating unnecessary time in the time-of-arrival measurement of the photoelectron signal. The time measurement error of the optoelectronic signal is compensated for by this time information. The experimental results show that the error compensation method can effectively improve the time measurement accuracy of optoelectronic signals, and provide solutions and theoretical basis for improving the time resolution of photomultiplier tubes based on microchannel plates.

## ATOMIC AND MOLECULAR PHYSICS

2022, 71 (22): 223101. doi: 10.7498/aps.71.20221052
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The electronic structures and single point energy of 14 lowest electronic states of 88Sr79Br molecule are optimized by using the internal contraction multi-reference configuration interaction method and relativistic effective core pseudo-potential basis. Because 88Sr79Br molecule belongs to heavy element system, the single point energy must be corrected to obtain more accurate spectral parameters. Therefore, Davidson is introduced to correct the energy inconsistency, nuclear valence correlation is used to correct the electron correlation effect of inner shell and valence shell, and the relativistic scalar effect is corrected by calculating the third-order Douglas-Kroll-Hess Hamilton single electron integral. According to the single point energy calculated by the modified optimization, the potential energy curves, electric dipole moments, and transition dipole moments of 14 lowest electronic states are obtained. Using the latest LEVEL8.0 program to fit the modified potential energy curve, the spectral constants, molecular constants and vibration energy levels of 5 lowest bound states of 88Sr79Br molecule are given. In order to explain the changing trend of spectral constants of homologous compounds, the spectral parameters of each compound are compared and analyzed in this paper. At the same time, the vibration energy levels and molecular constants of 88Sr81Br molecule are also fitted and calculated for analyzing the influence of isotopes. The comparative analysis shows that the results of 88Sr79Br molecule are in better agreement with the experimental values. Finally, the Franck-Condon factors are gained by fitting the optimized single point energy and transition dipole moment of 88Sr79Br molecule. The transition band with the largest factor and obvious diagonalization is selected by analyzing the Franck-Condon factor of each transition band, and whether it meets the conditions for selecting laser cooling molecular system is judged. The radiation lifetimes of the transitions from the lowest two excited states to the ground state are calculated by combining the transition dipole moment, Franck-Condon factor, single point energy and vibration energy level of each electronic state. The results of this paper are in good agreement with the experimental values, which shows that the method in this paper is reliable. These spectral characteristic parameters provide theoretical support for further experimental measurement and construction of molecular laser cooling scheme of 88Sr79Br molecule.

## ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

2022, 71 (22): 224101. doi: 10.7498/aps.71.20221284
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In this paper, cavity-excited Huygens’ metasurface is proposed for high-efficiency wavefront manipulation. By adjusting the length of electric dipole and magnetic dipole , the proposed Huygens’ metasurface meta unit can provide nearly 360° phase coverage with sufficiently high transmission efficiency. Based on the analysis of the resonance mode of the cavity, the Huygens’ metasurface has successfully performed its function by adopting integrated feeding method. According to the generalized Snell’s law, metasurfaces with different phase gradients are designed. Combined with the cavity structure, one-dimensional Huygens’ metasurfaces excited by cavity is realized, which can directionally emit the electromagnetic waves from the cavity. Both the simulation and experimental results show that the proposed cavity excited metasurfaces can effectively manipulate the direction of the emitted beam. Such a kind of cavity-excited metasurface can flexibly control the emission angle of the electromagnetic wave, reduce the energy loss and improve the efficiency of the electromagnetic wave. These designs have the advantages of compact, light and easy integration.
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