In this paper, the multiple stable states and Dicke phase transition of two two-level atoms in a single mode optical cavity are studied by using the coherent state variational method in the rotating wave approximation. There is no interaction between two atoms for the distance. Firstly, we obtain the eigenstates and eigenenergies of the system, meanwhile, we solve different photon number solutions and boundary conditions by variational method. Figure 1 gives the the limit case that the second atom does not exist and T-C model degrades into J-C model, in which a second-order Dicke quantum phase transition from the normal phase to the superadiant phase occurs. Meanwhile, the completely reversed atomic population exists with the increase of the atom-field coupling strength. The effects of the detuning between the cavity’s frequency and the atomic frequencies on Dicke phase transition are characterized by numerical calculation. Rich phase diagrams are presented by adjusting the parameters, such as the atom-field coupling strengths, the frequency detuning and the non-equilibrium parameter. These phase diagrams visually show Dicke phase transitions. Interestingly, multi-stable excited states are induced by the atomic inversion. Apart from Dicke phase transition from normal phase to superradiant phase, the system shows a new phase transition from the revised normal state by just one atom to stimulated radiation state, while the completely revised normal state by both atoms exists stably in the whole space. In addition, the application of J-C model or T-C model in quantum optics generally considers the driven field, which can at least make the critical quantum phase point or boundary shift. In the future we can add the driven field in Hamiltonian and discuss the effect on Dicke phase transition.

Experiments based on free space platform have demonstrated that the weak value amplification (WVA) technique can provide high sensitivity and precision for optical metrology. To promote this technique for real-world applications, it is more suitable to implement WVA based on optical fiber platform due to the lower cost, smaller scale and higher stability. In contrast to the free space platform, the birefringence in optical fiber is strong enough to cause polarization cross talk, and the amplitude-type noise must be taken into account. In theory, we deduct that the effect of amplitude-type noise will form noise ripples on the detection spectrum and can significantly compromise the sensitivity of WVA. To overcome this problem, a post-processing method is proposed and simulation results show the effectiveness of the method. Our results indicate the feasibility of implementing WVA based on optical fiber, which provide a possible way for designing optical sensors withhigher sensitivity and stability in the future.

With the development of precision measurement, quantum noise has become an important noise source in precision measurement. Using classical means to measure, it can only reach the classical limit or the standard quantum limit accuracy. In order to improve the accuracy and sensitivity of the measurement, researchers have turned their attention to the non-classical light field, using the squeezed state, quantum entanglement and other non-classical characteristics. Thus, the measurement accuracy and sensitivity can exceed the shot noise limit level, in some cases even approach the Heisenberg limit to achieve the enhancement of the signal-to-noise ratio. As a continuous variable quantum state, the squeezed state light field has a noise characteristic lower than the shot noise benchmark, and has been used as a good light source to improve the accuracy of quantum measurement since its inception. This novel quantum resource provides an effective means to further improvethe precision of precision measurement. Signal amplification is the most basic part of signal processing and signal measurement, but limited by the no-go theory. Noise is inevitably introduced during the decisive linear amplification of the quantum state, which will destroy the quantum state and reduce the signal-to-noise ratio of the input state. However, a perfect noiseless linear linear amplifier (NLA) can increase the power of the input signal without changing the noise power, thereby achieving a linear increase in the signal-to-noise ratio. Appropriate post-processing of the measurement results to simulate the linear function of a perfect noiseless linear amplifier is called "measurement-based NLA (MB-NLA)". In this article, combining the advantagesof compressed light and noiseless linear amplification technology, a quantum-based precision measurement enhancement scheme based on noiseless linear amplification is realized. Compared with traditional measurement, the signal-to-noise ratio is increasedto 2.00 times.

Balanced homodyne detector (BHD) is a key element in quantum optics and quantum information processing with continuous variables. The noise performances and the signal to noise ratio (SNR) of a BHD based on the transimpedance amplification circuit were theoretically analyzed and experimentally studied. With the typical BHD circuit, the signal gain of the circuit and the contributions of the noise sources to the noise spectra were calculated in detail. Characteristics of the electronic circuit were computational simulated and analyzed based on PSpice. With the printed-circuit-board and low noise operational amplifier of LMH6624, the experimental data on noise spectra were obtained. The experimental data are in good agreement with the theoretical calculation and simulation results. By comparing these data, the parasite capacitances on the PCB can be estimated. Under the transimpedance resistor of 6.8 k Ω and the capacitor of 2 pF, the maximum SNR of the developed BHD is 22 dB at the analysis frequency of 2 MHz, which contributes a measurement loss of 0.6% in the detection system. The BHD can meet the requirements of measuring squeezed state in the quantum optics experiments.

The distribution of quantum States over long distances plays an important role in quantum communication and quantum network. The studies of effectively realizing quantum memory have evoked considerable interest in the scientific community. The DLCZ quantum repeaters relying on spontaneous Raman scattering has been demonstrated as an effective approach to realize long-distance quantum memory. Large multimode capacity, long lifetime and high storage efficiency are three important indicators to achieve high quantum storage. To promote storage efficiency of quantum storage, efforts have been devoted by increasing the optical thickness, using the cavity enhanced storage as well as optimizing the optical pulse shape and so on. Among them, optical cavity possesses the advantage of enhancing interaction force and suppressing spontaneous emission, and thus has evoked considerable interest in quantum memory. some recent research has demonstrated that the efficiency of quantum storage can be improved by using optical cavity in quantum storage based on cold atomic ensemble. However, the storage efficiency in long-lived storage in quantum storage still needs further improving. In this paper, the correlated pairs of photons and atomic spin waves are generated by spontaneous Raman scattering process in the 87 Rb cold atom ensemble. Based on optical cavity to enhance the interaction between light and atom, the generation rate of the correlated pairs of photons and atomic spin waves has been increased by 3.8 times. Meanwhile, the effects of optical cavity on the read field efficiency of atomic spin wave are studied. The results show that the read efficiency of spin wave has been increased by 1.5 times, which is more than that of without cavity, and its corresponding intrinsic read efficiency is 40.6%.The results of this paper provide an experimental basis for the further generation of high recovery efficiency quantum entanglement of photon-atom with high recovery efficiency.

A theoretical model of the fiber-optic Fabry-Pérot sensor based on a pressure-sensitive film is developed by carefully analyzing the operation mechanism of the sensor. The total detection sensitivity of the sensor is obtained by multiplying the optical sensitivity proportional to the transfer function of the Fabry-Pérot cavity, and the sound pressure response of the film. The total measurement noise is obtained by taking into account the dark current noise and thermal noise of the photodetector, the relative intensity noise and phase noise of the probe laser. By employing the model, the dependences of the signal-to-noise ratio (SNR) of the Fabry-Pérot sensor on the coating reflectivities, the Fabry-Pérot cavity length, the thickness and effective radius of the pressure-sensitive film are discussed in detail. For the case that the film is made of pure silicon and the probe light is provided by a commercial continuous wave (cw) single frequency fiber laser operating around 1580 nm, the best SNR as high as 104.9 dB can be achieved when the effective radius and thickness of the film are respectively 150 μ m and 1.5 μ m, the reflectivity of the coatings on the fiber end and film are respectively 0.55 and 0.99, as well as the length of the Fabry-Pérot cavity is 14.9 μ m. This SNR is nearly three times higher than the reported Fabry-Pérot sensors operating in either free space or fiber coupled scheme, and consequently meet the demand of the high resolution photoacoustic imaging application. In the future investigations, tunable cw single frequency solid state laser offering better noise properties, and new pressure-sensitive material with higher Young’s modulus are expected to balancethe pressure detection range and further raising up the SNR of the Fabry-Pérot sensor. Our model is still valid in these cases, thus can be used to guide the design and optimization.

The presence of correlations in physical systems can be a significant characteristic that distinguishes quantum systems from classics. The quantification and utilization of quantum correlations are not only considered as core issues in quantum information theory, but also attract much attention of quantum thermodynamics community. For classical thermodynamics, heat spontaneously flows from hot to cold without showing a strong correlation for the isolated system. The entropy of a closed system tends to increase with time and this phenomenon can be regarded as the thermodynamics arrow of time. As a system advances through time, energy dissipation occurs, and it becomes more statically disordered. However, the aboveconclusions are not always valid in quantum area, due to the correlations between subsystems, even leading to the reverse flow of energy. This means that the formula of the second law of classical thermodynamics needs to be revised and using more physical quantities other than von Neumann entropy to show this non-classical quality. We here propose that quantum correlations between the subsystems can cause the energy to flow backward or accelerate the forward flow of energy. For explaining such phenomenon, we introduce the relative entropy and other thermodynamic parameters to quantitatively characterize the influence of correlations on the thermodynamic properties of quantum systems. These results show that the mutual information that characterizes the system correlations always decreases during evolution and then increases again when there exists a nonzero correlation terms among subsystems. Thus, we propose an experimental scheme based on the optical system to verify the correctness of the theory. tifying

Resonance in subwavelength metal structures is very important in the field of nanosensors. This paper studies the resonance characteristics and sensing applications of nanocavities in metal structures. The related inclined cavity, V-shaped cavity and X-shaped cavity are constructed. The V-shaped cavity is composed of two inclined cavities and the X-shaped cavity is composed of two V-shaped cavities. The finite difference time domain method is used to simulate thecoupling effect of the three cavities and the waveguide, and the coupled mode theory is applied to verify the transmission spectrum. The standing wave theory is used to analyze the physical mechanism of three kinds of intracavity resonance. It is found that there is one resonance circuit in the inclined cavity and the V-shaped cavity, and there are two resonance paths in the X-shaped cavity. Compared with the inclined cavity, there is a connection mode in the V-shaped cavity; there are both symmetrical and antisymmetric connection modes in the X-shaped cavity. When the symmetry of the X-shaped cavity structure is broken, its resonance mode splits. Considering the differential sensing characteristics of the three resonant cavity structures, the average measurement errors of the gas refractive index sensor composed of the inclined cavity, the V-shaped cavity and the X-shaped cavity are respectively 0.003 91, 0.001 85 and 0.000 37, and the X-shaped cavity sensing is the most accurate. The multiple resonance circuits in the X-shaped cavity form multiple resonance modes, which effectively eliminates the interference of the changes of multiple environmental factors on sensing, thereby obtaining higher sensing measurement accuracy, which provides valuable theoretical reference for the application of differential sensors.

Vector magnetometer is an important branch of quantum precision measurement, which has been widely used in basic physics, biomedicine, material science and other fields. As a quantum sensor, the Nitrogen-Vacancy Center has a longer coherency time at room temperature, allowing for nanoscale magnetic field detection. Three different coaxial NV centers of diamond should be selected as magnetic field sensors to realize the reconstruction of vector magnetic field withnanometer resolution. Whether the axial information of NV center can be obtained quickly and accurately directly affects the accuracy and efficiency of vector magnetic field measurement. Therefore, how to obtain the axial information of the NV center is influential. This paper proposes a confocal microscopy imaging system based on machine learning fitting method, combined with an angularly polarized beam instead of a Gaussian beam to excite the NV center to obtain a scanning fluorescence image. Then the identification of the diamond NV center axis and the problem of calculating the azimuth and polar angle of the NV axis in space are solved through the method. This article demonstrates a model based on convolutional neural network to automatically identify and locate NV centers in diamond, and on the basis of object detection, the method of gradient descent fitting is combined with image recognition processing and matching algorithms. The model accelerates extraction of the axial information of the four NV axes, optimizes the extraction process of the NV center axial information, improves the speed and accuracy of fitting, and thereby increases the efficiency of magnetic field vector reconstruction. The research results of this design can be applied to the measurement of the magnetic field vector, and not only can it improve the calculation efficiency of the NV center angle direction in the diamond, but also has a degree of robustness to the noise in the experimental environment.

Due to quantum wells new physical properties under the constrained potential limitation on the electron, there is a confined potential along QW growth directions and it must be a strong confined potential. By adding an asymmetrical semi-exponential potential in the QW’s growth direction, asymmetrical semi-exponential quantum well is formed. In order to deepen the understanding of the structural characteristics of asymmetrical semi-exponential quantum well, an anisotropic parabolic potential perpendicular to the growth direction of the quantum well was added, at the same time, the confinement of the magnetic field to the electrons in the quantum well is increased, the influences of magnetic field on the propertiesof the vibrational frequency of the weak-coupling polaron in a GaAs asymmetrical semi-exponential quantum well is studied. The vibrational frequency of the weak-coupling polaron in a GaAs asymmetrical semi-exponential quantum well varying with the two parameters of the asymmetrical semi-exponential confinement potential, the cyclotron frequency of the magnetic field and the confinement strengths of anisotropic parabolic potential in the x and y directions is derived by using linear combination operator and the unitary transformations methods. The numerical calculated results are indicated for a typical GaAs semiconductor asymmetrical semi-exponential quantum well crystal. It is founded that the vibrational frequency of the weak-coupling polaroa in a asymmetrical semi-exponential quantum well is a lifting function of the parameter U0, whereas it is a decaying one of the other parameter σ, when the anisotropic parabolic potential in the x and y directions and the cyclotron frequency of the magnetic field are fixed, the coupling between electron and phonon is strengthened and the formation of polaron is promoted by the change of parameters. The vibrational frequency of the polaron increases with the confinement strengths of anisotropic parabolic potential in the x and y directions when the cyclotron frequency of the magnetic field and two other parameters are fixed. It demonstrates the peculiar quantum size confinement effect. The vibrational frequency of the polaron increases with the cyclotron frequency of the magnetic field when confinement strengths of anisotropic parabolic potential in y directions and two other parameters are fixed, The cyclotron frequency of the magnetic field increases with the increase of the magnetic field, thepolaron will be more constrained when the cyclotron frequency of the magnetic field increases, so will the vibration frequency be strengthened.

The quantum Hall body is a kind of topological insulating state with insulated inside and conductive edge. It has a unidirectional conductive edge state with zero resistance. The topological structure of the system promise the existence of edge states, which are the essential result of coupling of magnetic moment and momentum of electrons. In the strong magnetic field, electrons can only take the magnetic moment in one direction, so the electron in edge state canonly move in one direction, and cannot be scattered back. Starting form Hamiltonian of one-dimensional directional moving of lattice system, we discussed the low energy excitation and barrier scattering of edge state electrons in quantum Hall system, andits energy eigenstates and eigenvalues were obtained. The results show that: the energy spectrum of the edge state electron is linear, and its quantum tunneling probability is 1, which is independent of the barrier height. It will not be scattered, and its resistance is zero.

Ytterbium (Yb) atomic optical clock is currently the most stable optical clock, and its frequency stability has reached 10-19 level. Based on the high-precision Yb atomic optical clock, scientific and applied research in the fields of general relativity testing and geodesy can be carried out. Laser of multiple wavelengths are used in the Yb atomic optical clock, so the frequency control of these lasers is a key technical issue. A four-channel optical ultrastable cavity is carried out, according to the frequency control requirements of four lasers (expect clock laser) in the Yb optical clock system. Then the finite element analysis method is used to obtain the vibration sensitivity and temperature sensitivityof the ultrastable cavity at the best supporting position. The analysis results show that the vertical vibration sensitivity of the optical cavity at the best supporting position is 4.010-9 /g, and the thermal time constant is 53 h. The temperature sensitivity of 1.310-4 is obtained for the ultrastable cavity, when the ambient temperature varies around room temperature, with a period of 24 h and a amplitude of 1 K. These results show that the four-channel ultrastable optical cavity we design can meet the requirements of the frequency stabilization for four lasers (except clock laser) in the Yb optical clock system. The scheme of using simplified four-channel ultrastable cavity for frequency stabilization provides a new technical attempt forthe future miniaturization and integrated design of ytterbium atomic optical clocks.