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SPECIAL TOPIC—Quantum computation and quantum information

Review on quantum advantages of sampling problems
Li Ying, Han Ze-Yao, Li Chao-Jian, Lü Jin, Yuan Xiao, Wu Bu-Jiao
2021, 70 (21): 210201. doi: 10.7498/aps.70.20211428
Abstract +
Exploiting the coherence and entanglement of quantum many-qubit states, quantum computing can significantly surpass classical algorithms, making it possible to factor large numbers, solve linear equations, simulate many-body quantum systems, etc., in a reasonable time. With the rapid development of quantum computing hardware, many attention has been drawn to explore how quantum computers could go beyond the limit of classical computation. Owing to the need of a universal fault-tolerant quantum computer for many existing quantum algorithms, such as Shor’s factoring algorithm, and considering the limit of near-term quantum devices with small qubit numbers and short coherence times, many recent works focused on the exploration of demonstrating quantum advantages using noisy intermediate-scaled quantum devices and shallow circuits, and hence some sampling problems have been proposed as the candidates for quantum advantage demonstration. This review summarizes quantum advantage problems that are realizable on current quantum hardware. We focus on two notable problems—random circuit simulation and boson sampling—and consider recent theoretical and experimental progresses. After the respective demonstrations of these two types of quantum advantages on superconducting and optical quantum platforms, we expect current and near-term quantum devices could be employed for demonstrating quantum advantages in general problems.

SPECIAL TOPIC—Quantum computation and quantum information

Research progress of measurement-based quantum computation
Zhang Shi-Hao, Zhang Xiang-Dong, Li Lü-Zhou
2021, 70 (21): 210301. doi: 10.7498/aps.70.20210923
Abstract +
Compared with the quantum gate circuit model, the measurement-based quantum computing model provides an alternative way to realize universal quantum computation, and relevant contents have been greatly enriched after nearly two decades of research and exploration. In this article, we review the research history and status of the measurement-based quantum computing model. First, we briefly introduce the basic theories of this model, including the concept and working principles of quantum graph states as resource states, the model’s computational universality and classical simulation methods, and relevant applications in the field of quantum information processing such as designing quantum algorithms and fault-tolerant error correction schemes. Then, from the perspective of quantum physical properties, which include the specific roles of quantum entanglement, contextuality, quantum correlations, symmetry-protected topological order, and quantum phases of matter as computing resources, the close relationship between measurement-based quantum computing model and quantum many-body system is presented. For example, a type of measurement-based computing model for exploiting quantum correlations can show a quantum advantage over the classical local hidden variable models, or certain symmetry-protected topological order states enable the universal quantum computation to be conducted by using only the measurements of single-qubit Pauli operators. Next, a variety of different technical routes and experimental progress of realizing the measurement-based quantum computing model are summarized, such as photonic systems, ion traps, superconducting circuits, etc. These achievements in various physical areas lay the foundation for future scalable and fault-tolerant quantum computers. Finally, we discuss and prospect the future research directions in this field thereby inspiring readers to further study and explore the relevant subjects.

SPECIAL TOPIC—Quantum computation and quantum information

Hybrid quantum-classical algorithms: Foundation, design and applications
Chen Ran-Yi-Liu, Zhao Ben-Chi, Song Zhi-Xin, Zhao Xuan-Qiang, Wang Kun, Wang Xin
2021, 70 (21): 210302. doi: 10.7498/aps.70.20210985
Abstract +
Quantum computing, as an emerging computing paradigm, is expected to tackle problems such as quantum chemistry, optimization, quantum chemistry, information security, and artificial intelligence, which are intractable with using classical computing. Quantum computing hardware and software continue to develop rapidly, but they are not expected to realize universal quantum computation in the next few years. Therefore, the use of quantum hardware to solve practical problems in the near term has become a hot topic in the field of quantum computing. Exploration of the applications of near-term quantum hardware is of great significance in understanding the capability of quantum hardware and promoting the practical process of quantum computing. Hybrid quantum-classical algorithm (also known as variational quantum algorithm) is an appropriate model for near-term quantum hardware. In the hybrid quantum-classical algorithm, classical computers are used to maximize the power of quantum devices. By combining quantum computing with machine learning, the hybrid quantum-classical algorithm is expected to achieve the first practical application of quantum computation and play an important role in the studying of quantum computing. In this review, we introduce the framework of hybrid quantum-classical algorithm and its applications in quantum chemistry, quantum information, combinatorial optimization, quantum machine learning, and other fields. We further discuss the challenges and future research directions of the hybrid quantum-classical algorithm.

SPECIAL TOPIC—Quantum computation and quantum information

A quantum state readout method based on a single ancilla qubit
Ding Chen, Li Tan, Zhang Shuo, Guo Chu, Huang He-Liang, Bao Wan-Su
2021, 70 (21): 210303. doi: 10.7498/aps.70.20211066
Abstract +
Quantum state measurement is essential for reading-out a quantum computing outcome. Meanwhile, the readout results are always affected by the large noise of quantum measurements in physical implementation, which also hinders the large-scale expansion of quantum computing. In light of this, we present an indirect quantum state readout method based on a single ancilla qubit that can avoid the large noise of multiple-qubit measurements. The theoretical analysis and simulations indicate that our method is more robust against the measurement noise and promises to become a method of large-scale quantum error correction and high-fidelity quantum state readout.


Multi-segment lymphatic vessel model based on lattice Boltzmann method
Zhang Qian-Yi, Wei Hua-Jian, Li Hua-Bing
2021, 70 (21): 210501. doi: 10.7498/aps.70.20210514
Abstract +
The lymphatic system plays an important part in the body’s immunity and cell’s internal environment homeostasis. Like a blood circulatory system, the lymphatic system is a piping system throughout the body, which is composed mainly of lymphatic fluid and lymphatic vessels. The spontaneous contraction of the lymphatic vessels drives the flow of lymphatic fluid in the vessels. The spontaneous contraction-relaxation mechanism of lymphatic vessels is determined by the oscillating feedback of Ca2+ concentration and NO concentration. The distribution of NO in the vessels plays an important role in the contraction cycle of lymphatic vessels. The shear force acting on the lymphatic valves due to the flow of fluid is the main source of NO. In a real system, the distribution of NO in a certain section of lymphatic vessel will be affected by other lymphanion connected to it, especially the upstream connecting fragments. Through the lattice Boltzmann method, a multi-segment lymphatic vessel model with valve structure is established, which reproduces the feedback mechanism of Ca2+ and NO, valve change and fluid flow. There are three types of lymphatic vessels in the model, namely the initial lymphatic vessel, the collecting lymphatic vessel, and the outlet lymphatic vessel. The number of lymphatic vessels can be unlimited and inputted by the parameters. The number of lymphatic vessels is 3-5, and there are two pairs of valves in each lymphatic vessel. In this paper studied are the distribution of NO and pressure in multi-segment lymphatic vessel, and the change in the flow of each vessel in the three-segment vessel model over time.


Control of firing mode in nonlinear neuron circuit driven by photocurrent
Xie Ying, Zhu Zhi-Gang, Zhang Xiao-Feng, Ren Guo-Dong
2021, 70 (21): 210502. doi: 10.7498/aps.70.20210676
Abstract +
Firing patterns discern the electrical activities in biological neurons when intracellular and extracellular ions are pumped into cells and exchanged there. Artificial neural circuits can be tamed to reproduce similar firing modes from biological neurons by applying appropriate physical stimuli. Photocurrent generated in the phototube can be used as a signal source, which can stimulate the neural circuits, while the involvement of which branch circuit will be much different because the channel current can control the dynamics of functional neuron to a different degree. In this paper, based on a nonlinear (FitzHugh-Nagumo, FHN) neural circuit composed of one capacitor, induction coil, nonlinear resistor, two ideal resistors and one periodical stimulus, the phototube is incorporated into different branch circuits for changing the channel current and the biophysical role of photocurrent is investigated. The dynamical equations of three types of system are unified, though they fall in different areas in parameter space. The membrane potential can be directly changed and firing modes are switched when photocurrent is activated to change the channel current by connecting the phototube to the capacitor. The induced current across the induction coil is regulated to balance the external stimulus when the phototube is connected to the induction coil in series. The two types of photosensitive neuron models constructed in this paper are compared with the photocurrent driven inductive branch showing that the photocurrent driven capacitive branch can very effectively regulate the membrane potential and greatly improve the photosensitive sensitivity.

SPECIAL TOPIC—Quantum computation and quantum information

Nanoscale zero-field detection based on single solid-state spins in diamond
Zhao Peng-Ju, Kong Fei, Li Rui, Shi Fa-Zhan, Du Jiang-Feng
2021, 70 (21): 213301. doi: 10.7498/aps.70.20211363
Abstract +
Characterizing the properties of matter at a single-molecule level is highly significant in today’s science, such as biology, chemistry, and materials science. The advent of generalized nanoscale sensors promises to achieve a long-term goal of material science, which is the analysis of single-molecule structures in ambient environments. In recent years, the nitrogen-vacancy (NV) color centers in diamond as solid-state spins have gradually developed as nanoscale sensors with both high spatial resolution and high detection sensitivity. Owing to the nondestructive and non-invasive properties, the NV color centers have excellent performance in single-molecule measurements. So far, the NV centers have achieved high sensitivity in the detection of many physical quantities such as magnetic field, electric field, and temperature, showing their potential applications in versatile quantum sensors. The combination with the cross measurements from multiple perspectives is conducible to deepening the knowledge and understanding the new substances, materials, and phenomena. Starting from the microstructure of NV sensors, several detections under the special magnetic field condition of zero field, including zero-field paramagnetic resonance detection and electric field detection, are introduced in this work.


Theoretical prediction of solution in ScxY1–x Fe2 and order-disorder transitions in V2x Fe2(1–x)Zr
Jiang Yong-Lin, He Chang-Chun, Yang Xiao-Bao
2021, 70 (21): 213601. doi: 10.7498/aps.70.20210998
Abstract +
Alloying is an important way to increase the diversity of material structure and properties. In this paper, we start from Ising model considering nearest neighbor interaction, in which a ferromagnetic system corresponds to a low temperature phase separation and high temperature solid solution of binary alloy, while antiferromagnetic system corresponds to a low temperature ordered solid solution and a high temperature disorder. The high-throughput first-principles calculation based on the structure recognition is realized by the program SAGAR (structures of alloy generation and recognition) developed by our research group. By considering the contribution of structural degeneracy to the partition function, theoretical prediction of alloy materials can be carried out at finite temperature. Taking hydrogen storage alloy (ScxY1–x Fe2 and V2x Fe2(1–x)Zr) for example, the formation energy of ground state (at zero temperature) can be obtained by the first-principles calculations. It is found that the formation energy of ScxY1–x Fe2 is greater than zero, thereby inducing the phase separation at low temperature. The free energy will decrease with the temperature and concentration increasing, where the critical temperature of solid solution of alloy is determined according to the zero point of free energy. The formation energies of V2x Fe2(1–x)Zr are all lower than zero, and the ordered phase occurs at low temperature. The order-disorder transition temperature of V0.5Fe1.5Zr and V1.5Fe0.5Zr are both about 100 K, while the transition temperature of VFeZr is nearly 50 K. The calculation process will effectively improve the high throughput screening efficiency of alloy, and also provide relevant theoretical reference for experimental research.


Review of partially coherent diffraction imaging
Xu Wen-Hui, Ning Shou-Cong, Zhang Fu-Cai
2021, 70 (21): 214201. doi: 10.7498/aps.70.20211020
Abstract +
Coherent diffraction imaging (CDI), a type of lensless imaging method, relies on the use of light source with high-degree coherence to compute highly resolved complex-valued objects. The coherence of light source consists of temporal coherence and spatial coherence. In practice, it is difficult to obtain a fully coherent source. Spatial decoherence can be generated in the following three scenarios: no synchronization mechanism for the whole radiation source, a finite (non-zero) point spread function of the detector, and the sample variation within exposure time. Partial temporal coherence means that the beam is not quasi-monochromatic, behaving as the energy spread of the illumination. The consequence of reduced degree of temporal and/or spatial coherence in CDI is the decrease of visibility in the measured diffraction intensity. A fundamental assumption of CDI is the full temporal and spatial coherence, and even a relatively small deviation from full coherence can prevent the phase retrieval algorithm from converging accurately. It is necessary to break the barrier of limited coherence by improving the experimental setups directly or optimizing the phase retrieval algorithms to mitigate decoherence. Based on the Wolf’s model of coherence-mode of light and the framework of CDI using partially coherent light proposed by Nugent et al., various methods have been proposed to solve the problems induced by low coherence. Those methods generally experience a similar development process, that is, from the requirement for measuring the spatial (coherent length or complex coherent factor) or temporal (spectrum distribution) coherence properties to without the need for such priori knowledge. Here in this work, the principles of partial coherent CDI, and the major progress of CDI with partial spatial- and temporal-coherent light are reviewed.


High-power narrow-linewidth single-frequency pulsed fiber amplifier based on self-phase modulation suppression via sawtooth-shaped pulses
Sheng Quan, Wang Meng, Shi Chao-Du, Tian Hao, Zhang Jun-Xiang, Liu Jun-Jie, Shi Wei, Yao Jian-Quan
2021, 70 (21): 214202. doi: 10.7498/aps.70.20210496
Abstract +
Fiber laser system in master oscillator power amplifier (MOPA) scheme is a promising technique for high-power narrow-linewidth laser output. With modulation-generated pulsed seed laser, the fiber MOPA benefits the flexible temporal behavior. However, the spectral linewidth broadening induced by self-phase modulation (SPM) is the main obstacle to achieving high-power single-frequency laser output with narrow spectral linewidth, especially for pulsed fiber MOPA in which the kilowatts level peak power results in strong nonlinearity. The SPM induced linewidth broadening is related to the derivative of light intensity with respect to time (dI/dt). Theoretically, if the dI/dt of the laser pulse is a constant, the SPM process will not generate any new frequency components. Hence, the linewidth broadening can be suppressed. In this work, we demonstrate a high-power single-frequency Yb fiber amplifier at 1064 nm, in which a sawtooth laser pulse is employed to suppress the SPM induced linewidth broadening, for obtaining the output with near-transform-limited narrow linewidth. The sawtooth-shaped seed pulse train is generated through using an electro-optic intensity modulator to modulate the continuous-wave (CW) output of a single-frequency fiber laser. After being pre-amplified, the seed laser with a pulse repetition rate of 20 kHz is coupled into the main amplifier, in which a piece of 0.9-m-long Yb-doped silica fiber with core and clad diameters of 35 μm and 250 μm, respectively, is used as a gain medium. The seed laser is enhanced to an average power value of 3.13 W under a launched 976-nm pump power value of 11.3 W before the onset of stimulate Brillouin scattering. The pulse energy 157 μJ and the pulse width 6.5 ns give a peak power of 24 kW. The spectral linewidth measured using a scanning Fabry-Perot interferometer at the maximum power is only 83 MHz, which is quite close to the 76-MHz transform-limited linewidth of the 6.5-ns sawtooth-shaped pulse. For comparison, we also conduct an experiment with a common Gaussian-shaped seed laser, in which the spectral linewidth is broadened significantly with a peak power value of only 1.5 kW. The results here reveal that the using of the sawtooth-shaped pulse is a promising technique to suppress the SPM induced spectral linewidth broadening in high-peak-power fiber amplifiers and acquire near-transform-limited narrow-linewidth laser output.
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