Because atomic coherence can enhance optical nonlinearity, the four-wave mixing process in hot rubidium atomic vapor is a promising technology to generate squeezed states and entangled states. Among the optical parametric amplifier, the gain of the phase-sensitive amplifier varies with the phase of the input signal, and the phase-sensitive amplification process has low noise characteristics. Therefore, the phase-sensitive four-wave mixing process in hot rubidium atomic vapor has potential application value in practical application. The theoretical scheme of phase-sensitive four-wave mixing process in hot rubidium atomic vapor is introduced. The quantum characteristics of the output field of the system are summarized. The experimental applications of phase-sensitive four-wave mixing process in hot rubidium atomic vapor are reviewed, including proving the quantum squeezing enhancement in a two-mode phase-sensitive amplifier which is induced by its interference effect, realizing two types of entanglement by manipulating the phase of a two-mode phase sensitive amplifier, introducing the direct intensity detection method to measure the phase sensitivity of the bright-seeded SU(1,1) interferometer.

In order to put forward a more convenient, effective, and practical method of diagonalizing Hamiltonian, the exact diagonalization of the Hamiltonian of the quantum boson model can be achieved through a general computation process in linear algebra, namely the N-ary quadratic form. Taking the Dicke model and the optomechanical hybrid system for example, the accuracy and effectiveness of this proposed method are verified through the linear algebra calculation process from the ordinary quadratic form to the standard quadratic one. Although this method is constrained by the second-order configuration for dealing with the Hamiltonian diagonalization, it is also believed to be able to provide another basic and effective method for studying the quantum phase transitions for some special systems.

The transportable optical clock can be widely used in clock comparison, timekeeping, precision measurement, dark matter detection, and so on. The development of transportable optical clocks is a significant direction of optical clocks. It is an important piece of equipment for the comparison of different types of optical clocks and the measurement of gravitational redshift. It is the crucial step to develop a transportable cooling light source for the preparationof cold atoms. This letter mainly introduces the development and application of the transportable light source for the second-stage cooling. Firstly, we simulated and analyzed a transportable cubic ultra-stable cavity with a length of 25 mm and the fineness is measured to be 1.94×105. Then we locked a 689 nm semiconductor laser on the cubic ultra-stable cavity through the Pound-Drever-Hall frequency stabilization technology to obtain a narrow linewidth frequency stabilized laser source at the wavelength of 689 nm. After beating with a reference laser, the linewidth of the frequency stabilized laser is measured to be less than 263 Hz, and the frequency stability is better than 1.56×10-14 at one second, showing the laser source can be used for the second-stage cooling in a strontium optical clock. In addition, to verify the application of the laser source, two slave lasers with the same performance were prepared by injection locking technique, which was used as cooling and stirring light respectively in the second-stage cooling. All optical parts are integrated on a 0.56m2 optical breadboard to achieve transportability and fiber-coupled on the vacuum system so as to be easily moved. At last, we applied the transportable system tothe preparation of a cold strontium cloud. As a result, 2×106 atoms with a temperature of 5.3K are prepared in the second-stage cooling experimentally, which laid the foundation for optical lattice loading and clock transition spectrum detection of cold atoms in the transportable strontium optical clock.

In this paper, a quantum hybrid interferometer consisting of a nonlinear beam splitter and a linear beam splitter is proposed in theory. The nonlinear beam splitter is composed of a nonlinear parametric process combined with linear coherent feedback of a quantum squeezed source to improve the phase sensitivity of the quantum hybrid interferometer. Specifically, the optical parametric amplifier (OPA) output beam and the quantum squeezed source are linearly fed back to the input end to obtain a pair of dual-mode squeezed beams with controllable entanglement. Combined with the linear beam combination, the feedback quantum hybrid interferometer is constructed. The results show that compared with the non-feedback quantum hybrid interferometer, the scheme can increase the number of phase-sensitive photons by adjusting the feedback intensity to an appropriate range to improve the interference signal intensity and enhance the degree of entanglement between the two interference arms to reduce the interference noise. In addition, the squeezed vacuum state as the input state of the feedback loop structure can further reduce the noise of the detection signal. Therefore, this scheme can effectively improve the measurement sensitivity of the hybrid interferometer from the above three aspects. Even in the case of lossy, the feedback quantum hybrid interferometer compared with without feedback still has better phase sensitivity and anti-damage ability, which is expected to be applied in the field of quantum metrology.

Quantum gates are essential for realizing quantum computing. How to achieve high-fidelity quantum gate operations, in the meanwhile maintaining high robustness against multiple control errors has been an important issue. Among various methods to realize quantum gates, using the geometric phase to construct the gates is an effective way, as the geometric phase has built-in noise-resilient characteristics. A general theoretical scheme to realize non-adiabatic non-abelian geometric quantum gates is to force the system to evolve along one of the time-dependent basis, then obtain the constrained conditions for the pulses. However, there might be some singular points in the constructed Rabi frequency, which are not preferred in certain quantum systems such as superconducting quantum circuits. In this work, we proposed a general condition to eliminate such singular points and developed smooth pulses in the three-level system with a Λ configuration. For the purpose of optimizing the gate performance, we introduced a new parameter k, which serves as an additional degree of freedom and is then used to modulate the shape of the pulses. The influence of parameter k on the gate performance has been numerically investigated in various aspects utilizing the Lindblad master equation, where the decoherence effects are considered. We taking superconducting qubit gates as an example, studied the robustness of pulses produced by different k values by introducing the detuning error and the Rabi frequency error. We found that the pulses with larger k values are more robust againstthe detuning error, while the Rabi error rate is not sensitive to k. The reason is that a larger k generates a higher instantaneous amplitude in the Rabi frequency when the gate duration is fixed, so that the perturbative effect is relatively smaller. On the other hand, the Rabi error is proportional to the Rabi frequency by definition, so its robustness is irrelevant to the pulse envelope. These results show that the theoretical scheme in this work provides us with a simple and feasible way for constructing the pulses for implementing the non-adiabatic geometric quantum gates.

It is significant to study the influence of microwave phase noise on the microwave mixing signals with cesium Rydberg atomic antenna. In this paper, the influence of microwave phase noise on the microwave mixing signals based on Rydberg atom is studied theoretically and experimentally. Theoretically, the function of the change of the microwave mixing signal intensity after adding phase noise to the signal field is obtained by summarizing the phase noise term detected by the transmission of the probe laser through the medium. We perform a theoretical diagram that simulates the transmission spectrum of the probe laser as a function of input phase noise. From the theoretical simulation diagram, it can be seen that the microwave mixing signal based on the Rydberg atomic antenna increases with increasing power of the signal field. For the same signal field strength, the microwave mixing signal gradually decreases with the phase noise added increasing. Experimentally, we build an atomic antenna based on the Rydberg atom, which manipulates the Rydberg atom through a reference field, and the Rydberg atom transmits the frequency of the microwave mixing signal directly to the probe laser. Microwave field frequencies of 13.806 057 GHz and 13.806 000 GHz are mixed based on the Rydberg atomic electric dipole transition of 64S1/2→64P1/2 under electromagnetically induced transparency (EIT) spectroscopy of Rydberg atoms in a cesium atomic gas chamber at room temperature. The parameter dependence of the microwave mixing signal intensity was studied by using the Rydberg-EIT spectroscopy. The experimental results show that the microwave mixing signal intensity based on the Rydberg atomic antenna is related to the power of the reference field, the microwave mixing signals enhancement of about 20 dB can be achieved in the reference field under the condition of optimizing the power parameters. When the reference field manipulates the Rydberg atoms to the maximum microwave mixing efficiency, phase noise is added to the signal field. It has been found that when phase noise is introduced into the signal field, it leads to the mixing signal significantly reduced. The experimental results of the influences of the microwave field phase noise upon the microwave mixing signal with cesium Rydberg atomic antenna are the same as the theoretical analysis. This shows that the construction of atomic antennas based on Rydberg atoms to achieve microwave mixing signal opens up the application of different approaches such as communication applications, near-field antenna measurements and radar, and also opens up a wider application of Rydberg atomic sensors in current technology.

Normal-mode splitting (NMS) is an evident sign of strong coupling between the cavity field and movable mirror and is a prerequisite for us to observe many quantum behaviors such as optomechanical squeezing and entanglement. The combination of an optical parametric amplifier and coherent feedback can produce a more obvious NMS. However, the impact of coherent phase on the NMS involving above scheme has not been investigated yet. Here, we theoretically analyze how the phase affects the displacement spectrum of the movable mirror and the noise power spectra of the output field. Introducing the phase of the coherent feedback, the quantum Langevin equation of motion is constructed and solved analytically in the Fourier domain, giving the spectrum expressions of movable mirror’s position and output field. With Routh-Hurwit criterion, the conditions of the system stability are obtained and guaranteed in the numerical simulations. The position and linewidth of normal modes are plotted, varying with feedback phase and strength, parametric gain and input laser power. The spectra of movable mirror’s position and output field varying with analytical frequency are also plotted, under different feedback phases. The results show, with other given parameters, the NMS changes with varying phases from - to . Generally, there is no NMS at a phase near - or , and more obvious NMS in a range around zero phase. The mode separation exhibits asymmetric aroundzero phase, and particularly, the largest mode separation occurs in the negative feedback region. The range occurring NMS and corresponding mode separation are both dependent on other parameters such as input laser power, feedback strength, parametric gain, all of which are plotted and compared. This provides us the convenience to optimize the NMS and optomechanical coupling strength by adjusting multiple parameters. These results show a way to realize stronger optomechanical coupling and hence ground state cooling of a macroscopic mechanical oscillator, and have the potential to be applied to the generation of optomechanical quantum states and sensitive detection of a weak force.

In this paper, based on the theoretical model of optical wave propagation in the parity time symmetric waveguide, the propagation characteristics of bright solitons in the Rosenmorse potential are studied numerically. The refractive index distribution of the parity-time symmetric waveguide has a linear focusing effect for beam, and the gain/loss distribution can cause the transverse energy-flow of beam. The results show that when the modulation depth of refractiveindex is positive, the first-order bright solitons can propagate and form a wavy beam in the waveguide. When the modulation depth is negative, the first-order bright soliton splits into two beams. One beam disappears and the other beam travels forward ata certain speed. The modulation depth of gain/loss can affect the propagation behavior of two beams. The second-order bright soliton can propagate and form a stable breathing beam under a lower modulation depth of the gain/loss. The third-order bright soliton appears splitting and converging periodically during the process of transmission. The fourth-order and higher order bright solitons cannot propagate stably in the waveguide. The results can provide a theoretical basis for the application of parity time symmetry in optical waveguides.

Quantum communication holds promise for absolutely secure transmission of secret messages and the faithful transfer of unknown quantum states. Photonic channels appear to be very attractive for the physical implementation of quantum communication. To achieve long-distance transmission of quantum states, quantum repeaters are essential, which contain quantum memories. Atomic ensemble, single atom, trapped ions and solid-state systems are all potential candidates for quantum repeater, in which quantum storage has been actively performed. The majority of these quantum memories operate at visible wavelengths. However, the visible wavelength bands photons have a large transmission loss in the optical fiber and cannot be directly transmitted over long distances. To achieve efficient long-distance quantum communication, the visible band needs to be converted to the communication band. Here we demonstrate a frequency conversion from rubidium D1 line (795 nm) to the telecom L-band (1 621 nm) based on difference frequency generation by using a fiber-coupled waveguide module. In quantum frequency conversion process, the strong pumping beam causes spontaneous Raman scattering (SRS) noise, which typically covers a region of several hundreds of nanometers. The noise suppression is important for improving the signal-noise ratio (SNR) of the converted field, especially for the frequency conversion of quantum state light field. To suppress the broadband noise, we use two cascaded etalons to narrow the noise bandwidth to 256 MHz and the transmission of the converted photons through a 15 km optical fiber. When the input light is strongly coherent continuous light, the maximum external conversion efficiency of the device was measured to be 0.95%. We use a single photon detector to measure the noise level after the filtering system. When the pump power is 400mW, the noise count is 1.5×104s-1. When the 795nm input pulse is attenuated to the single-photon level, westudy the signal-to-noise ratio of converted photons under different input pulse widths and the mean number of photons, we show that the SNR of converted field is 1.5 when the input photon number is 2 under the condition of low external device conversionefficiency (0.84%) and short duration of input pulse (30 ns). This work is a meaningful attempt to realize the quantum interface between quantum memory at 87Rb D1 line and telecom band.

Due to its potential applications in ultra-sensitive sensing, metamaterials, optical switches, and nonlinear optical devices, Fano resonance in plasmonic metal nanostructures has attracted extensive attention. However, experimental studies on the Fano resonance of metal nanodimers at a single particle scale are still scarce. In this paper, Fano resonance phenomenon in Au-Au nanorod dimers is investigated experimentally using single-particle spectroscopy. The longand short Au nanorods with resonance peaks at 1 060 nm and 700 nm in water are synthesized by the seed growth method, their average lengths are 110 nm and 55 nm, corresponding to average diameters of 17 nm and 20 nm respectively, and the Au-Au nanorod dimers are constructed by electrostatic adsorption self-assembly of L-cysteine molecules. When mixed short and long gold nanorods solution is added with 0.9 mmol/L L-cysteine, the cysteine preferentially binds to the ends of the gold nanorods through the sulfydryl group, and due to the mutual attraction of positive and negative charges, the amphoteric ionic groups assist the two nanorods to interconnect through end-to-end electrostatic adsorption, thus forming a dimer. Scattering spectra of a singleAu-Au nanorod dimer before and after coupling are measured by dark-field microscopy and theoretically simulated by the finite difference time domain (FDTD) method. The near-field electromagnetic distribution images of nanodimers are also analyzed. Dark-field scattering spectrum shows that the destructive interference between the bright dipole mode of a short Au nanorod and the dark quadrupole mode of a long Au nanorod produces an obvious Fano resonance dip at 660 nm, and the theoretical simulated scattering spectrum is in good agreement with the experimental one. The electromagnetic distribution images show that the electric field enhancement of the bright dipole mode of a short Au nanorod is descend at the Fano dip (656 nm), while the dark quadrupole mode of a long Au nanorod is enhanced, which is also the result of the destructive interference of the two Au nanorods. These self-assembled Au-Au nanorod dimers have a broad application prospect in plasmonic sensing and detection.

In terms of the characteristic of solid-state plasma (SSP), which can be switched dynamically between the metallic state and dielectric state through applying an external bias voltage, a reconfigurable bi-functional metamaterial polarization converter based on SSP is designed. Research results show that when SSP is in metallic state, the proposed metamaterial polarization converter can achieve reflective linear-linear polarization conversion within a wider frequency range with polarization conversion efficiency larger than 98%. When SSP is in a dielectric state, the designed converter works in the mode of transmission, and under this circumstance, it can achieve transmissive linear-circular polarization conversion. The background physical mechanism of the polarization conversion is interpreted by the studying the distribution of surface currents under the two states. Furthermore, through changing the structural parameters, changes in polarization conversion performance are studied both for the metallic state SSP and dielectric state SSP, which prove that the proposed SSP-based metamaterial polarization converter not only possesses reconfigurable polarization conversion property, but also can maintain good Robustness to the small change of structural parameters. Both these properties will benefit the experimental verification and real fabrication. Meanwhile, the small change of incident angles will not also bring clear change in the conversion performance, which can ensure real application requirements. As a result, the proposed SSP based reconfigurable metamaterial polarization converter can not only meet the application of different working modes, different conversion characteristics, but also have great potential applications in the area of communication, sensing and detection, imaging, and so on.