Abstract: For the experimental implementation of an optically pumped atomic magnetometer, the magnetic resonance signal with a narrow linewidth and a high signal-to-noise ratio (SNR) is required for achieving a high sensitivity. Using 795-nm laser as both the pumping and the probe laser, we compare the magnetic resonance signals from different rubidium atomic vapor cells and investigate the variations of magnetic resonance signals with temperature. Optimized magnetic resonance signal is achieved with a paraffin-coated rubidium atomic vapor cell. Then the 780-nm laser at rubidium D2 line is introduced as a repumping laser, and we explore the changes of linewidth and SNR of the magnetic resonance signal under different power of the pumping laser and the repumping laser. Owing to the 780-nm repumping laser beam, the signal amplitude of rubidium-85 magnetic resonance signal is improved remarkably because more rubidium-85 atoms are spin- polarized by the 795-nm pumping laser beam. At the same time, the linewidth of rubidium-85 magnetic resonance signal is roughly not broadened anymore. We realize a closed-loop optically pumped rubidium-85 atomic magnetometer with a bandwidth of ~1.2 kHz, and the sensitivity is calibrated to be ~245.5 pT/Hz〈sup〉1/2〈/sup〉 only with the 795-nm pumping laser beam. Owing to the employment of the 780-nm repumping laser beam, the sensitivity is improved to be ~26.4 pT/Hz〈sup〉1/2〈/sup〉 which is improved roughly by one order of magnitude. We also calibrate the measurement accuracy and deviation of a commercial fluxgate magnetometer by using the enhanced rubidium magnetic resonance signal.

Abstract: 〈sec〉In the case of continuous-variable quantum key distribution (CV-QKD) systems, synchronization is a key technology that ensures that both the transmitter and receiver obtain corresponding data synchronously. By designing an ingenious time sequence for the transmitter and receiver and using the peaking value acquisition technique and time domain heterodyne detection, we experimentally realize a four-state discrete modulation CV-QKD with a repetition rate of 10 MHz, transmitting over a distance of 25 km. With well-designed time sequence of hardware, Alice and Bob can obtain corresponding data automatically without using numerous software calculation methods.〈/sec〉〈sec〉The secure key rates are calculated by using the method proposed by the Lütkenhaus group at the University of Waterloo in Canada. In the calculation, we first estimate the first and the second moment by using the measured quadratures of displaced thermal states, followed by calculating the secret key rate by using the convex optimization method through the reconstruction of the moments. There is no need to assume a linear quantum transmission channel to estimate the excess noise. Finally, secure key rates of 0.0022—0.0091 bit/pulse are achieved, and the excess noise is between 0.016 and 0.103.〈/sec〉〈sec〉In this study, first, we introduce the prepare-and-measure scheme and the entanglement-based scheme of the four-state discrete modulation protocol. The Wigner images of the four coherent states on Alice’s side, and four displaced thermal states on Bob’s side are presented. Second, the design of hardware synchronization time series is introduced comprehensively. Third, the CV-QKD experiment setup is introduced and the time sequence is verified. Finally, the calculation method of secure key rate using the first and the second moment of quadrature is explained in detail. The phase space distribution of quadratures is also presented. The secret key rate ranges between 0.0022 and 0.0091 bits/pulse, and the equivalent excess noise are between 0.016 and 0.103. The average secret key bit rate is 24 kbit/s. During the experiment, the first and the second moment of the quantum state at the receiver end are found to fluctuate owing to the finite-size effect. This effect reduces the value of the secure key rate and limits the transmission distance of the CV-QKD system.〈/sec〉〈sec〉In conclusion, four-state discrete modulation CV-QKD based on hardware synchronization is designed and demonstrated. The proposed hardware synchronization method can effectively reduce the cost, size, and power consumption. In the future, the finite-size effect will be investigated theoretically and experimentally to improve the performance of system.〈/sec〉

Abstract: Hall tube is an important model to simulate the quantum Hall effect. However it hasn't been realized in superconducting circuits which have emerged as a promising platform for macro-controlling quantum effect. Taking advantage of the fine tunability of superconducting circuits, the three-chain superconducting transmon qubits with periodic boundary condition are designed in this paper. For constructing a synthetic Hall tube, ac magnetic fluxes are introduced to drive each transmon qubit. The gauge field emerged in this synthetic Hall tube can be tuned independently by properly choosing the driving phases. Then the ground-state chiral currents are discovered in this synthetic Hall tube, which are Meissner current on 〈inline-formula〉〈tex-math id="M1"〉\begin{document}$xy$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M1.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M1.png"/〉〈/alternatives〉〈/inline-formula〉 plane (〈inline-formula〉〈tex-math id="M2"〉\begin{document}$xy$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M2.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M2.png"/〉〈/alternatives〉〈/inline-formula〉-M), vortex current on 〈inline-formula〉〈tex-math id="M3"〉\begin{document}$xy$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M3.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M3.png"/〉〈/alternatives〉〈/inline-formula〉 plane (〈inline-formula〉〈tex-math id="M4"〉\begin{document}$xy$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M4.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M4.png"/〉〈/alternatives〉〈/inline-formula〉-V), vortex current on 〈inline-formula〉〈tex-math id="M5"〉\begin{document}$xz$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M5.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M5.png"/〉〈/alternatives〉〈/inline-formula〉 plane (〈inline-formula〉〈tex-math id="M6"〉\begin{document}$xz$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M6.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M6.png"/〉〈/alternatives〉〈/inline-formula〉-V), and vortex current on both 〈inline-formula〉〈tex-math id="M7"〉\begin{document}$xy$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M7.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M7.png"/〉〈/alternatives〉〈/inline-formula〉 and 〈inline-formula〉〈tex-math id="M8"〉\begin{document}$xz$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M8.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M8.png"/〉〈/alternatives〉〈/inline-formula〉 planes (DV). For distinguishing these chiral currents, four order parameters 〈inline-formula〉〈tex-math id="M9"〉\begin{document}$J_{C//}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M9.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M9.png"/〉〈/alternatives〉〈/inline-formula〉, 〈inline-formula〉〈tex-math id="M10"〉\begin{document}$J_{AB}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M10.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M10.png"/〉〈/alternatives〉〈/inline-formula〉 (〈inline-formula〉〈tex-math id="M11"〉\begin{document}$J_{BC}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M11.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M11.png"/〉〈/alternatives〉〈/inline-formula〉), and 〈inline-formula〉〈tex-math id="M12"〉\begin{document}$J_{CA}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M12.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M12.png"/〉〈/alternatives〉〈/inline-formula〉 are defined. Then the ground-state quantum phase diagrams are mapped out. The emergence of the different quantum phases is due to the competition between the coupling strengths 〈inline-formula〉〈tex-math id="M13"〉\begin{document}$\tilde{t}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M13.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M13.png"/〉〈/alternatives〉〈/inline-formula〉 and 〈inline-formula〉〈tex-math id="M14"〉\begin{document}$t_{CA}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M14.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220293_M14.png"/〉〈/alternatives〉〈/inline-formula〉. The Meissner and vortex currents emerging in this synthetic Hall tube also emerge in type II superconductor, which can generate an opposite field to weaken the influence of the applied field. Thus this synthetic Hall tube can be used as a diamagnet. At last we consider the influence of the imperfections in device fabrication. We proof when the strength of the imperfection is not large enough, the quantum phase diagrams shown in this paper remain valid. Moreover, the possible experimental observations of the ground-state chiral currents are addressed. The ground state of this synthetic Hall tube can be generated by applying microwave pulses. Then the corresponding density matrix can be constructed by the quantum state tomography. After constructing the density matrix, the order parameters can be obtained by calculating the trace. These results enrich the quantum currents in Hall tube and provide a new route to explore novel quantum phases.

Abstract: Entanglement manipulation in various systems is one of the important problems in quantum information science. In this paper, the phase sensitivity and entanglement enhancement of the cascade four-wave mixing of hot atomic steam are studied. The results show that the quantum entanglement of the probe light and the conjugate light output at the second level of the cascade four-wave mixing process is significantly stronger than that at the first level, and the maximum increment can reach more than 5 dB, and the perfect entanglement can be achieved by increasing the intensity factor. The relations of quantum correlation type and the size of the entanglement with the pump phase and the nonlinear intensity factor are also discussed in this work. The results show that because of the enhancement of entanglement and the sensitivity of entanglement type to pump phase, the light field noise characteristics can be changed by controlling the phase and intensity factors thus realize the enhancement of entanglement between the probe and coupling light and the quantum manipulation of entanglement extent and quantum entanglement type. The theoretical study is of important significance for guiding the experimental implementation of optical parameter manipulation of entanglement enhancement, compression angle and compression degree of two-mode compression state.

Abstract: The atomic polarizability represents the response characteristics of atoms to externally applied electro-magnetic fields. The wavelength (or frequency) at which the dynamic polarizability of an atom is equal to zero is referred to as the tune-out wavelength (or frequency). Spectroscopy technology based on the tune-out effect has potential applications in quantum precision measurement, quantum computation and quantum communication. Related research topics include the measurement of fundamental physical constants and strong interactions. The tune-out wavelengths of atoms in low-lying states primarily fall within the optical band, where the theoretical calculations and experimental measurements have significant progress. However, for Rydberg atoms in highly excited states, theoretical calculations are challenging due to their high density of atomic states. The difficulty of experimental measurement arises from small splitting of adjacent atomic energy levels. In this paper, we demonstrate the tune-out wavelengths measurement for Rydberg atoms in a cesium vapor cell at room temperature. We utilize a two-photon cascade excitation to prepare Rydberg states and employ amplitude-modulation electromagnetically-induced transparency (AM-EIT) spectroscopy to measure the tune-out wavelength. By continuously scanning the microwave frequencies, we obtain AM-EIT signals of Rydberg atoms. At near-resonant microwave transition wavelengths, strong AM-EIT signals are observed due to microwave-atom coupling. Conversely, at tune-out wavelengths, the dynamically polarization-induced destructive interference in neighboring energy states occurs which leads to the weak AM-EIT signals. The AM-EIT provides a spectral resolution of about 10 MHz. We have developed a simplified three-level model to calculate the tune-out wavelength. The results of our theoretical calculations are consistent with the experimental findings within a range of ±90 MHz.

Abstract: Gravitational wave detection plays an important role in exploring the universe and opening up a new chapter for multi-messenger astronomy. As the most common device used for space and ground-based gravitational wave detection, large-scale laser interferometer requires a low-noise laser as a beam source. The noise of the laser can be suppressed by utilizing the optoelectronic negative-feedback noise reduction technology to improve the sensitivity of large-scale laser interferometer. The optoelectronic negative-feedback control system can suppress laser noise by subtracting the photodetector signal from the voltage reference signal, and then calculating the modulated signal by a proportional integral differentiator to control the output power of the pump current driver. Since the photodetector signal is affected by the laser intensity, its output voltage varies within a certain range, which requires that the output voltage of the voltage reference source signal is variable. In addition, the performance of the voltage reference directly affects the overall performance of the feedback control loop, therefore it is the lower limit of laser noise suppression. We develop a high-precision low-noise program-controlled voltage reference by selecting low-noise reference chip and digital-to-analog conversion chip, designing external control circuit, using low-temperature drift coefficient components and using temperature control and electromagnetic shielding. The digital-to-analog conversion is controlled through the FPGA module programming to accurately realize the reference voltage change. The output voltage range of the developed voltage reference source is from negative 10 V to positive 10 V and the minimal precision of the voltage variation is 20 bit. The voltage noise spectral density of the developed voltage reference is below 〈inline-formula〉〈tex-math id="M1"〉\begin{document}$9.6 \times 1{0^{ - 6}}/\sqrt {\rm Hz}$\end{document}〈/tex-math〉〈alternatives〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20222119_M1.jpg"/〉〈graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20222119_M1.png"/〉〈/alternatives〉〈/inline-formula〉 and the noise performance of the reference source is less than the laser intensity noise in the space-based gravitational wave frequency band. The developed voltage reference source provides an important technical support for laser intensity noise suppression in gravitational wave detection.

Abstract: The space-based gravitational wave detection can acquire the gravitational wave source information with larger characteristic mass and scale, forming a complementary detection scheme with ground-based gravitational wave detection, primordial gravitational wave detection, and pulsar gravitational wave detection. The space-based gravitational wave detection is based on a long-distance laser interference device, which mainly detects gravitational wave signals in a frequency range of 0.1 mHz–1 Hz. The noise evaluation and noise suppression of the laser light source system directly affect the detection sensitivity. In this work, based on low-noise photoelectric detection, a very low-frequency laser intensity noise test and evaluation system is constructed with high-precision digital multimeter, software control and algorithm programming of the host computer. The laser intensity noise can be converted into the fluctuation of the current signal by utilizing the photodiode, and the current signal is converted into the voltage signal and amplified by the transimpedance circuit. Thus the high-frequency interference components are filtered out by a passive low-pass filtering, and the extremely low-frequency noise components are retained. According to the definition of shot noise, it can be known that the photocurrent injected into the detector is inversely proportional to the shot noise, so at least 5 mW laser is chosen for photoelectric detection. After controlling the high-precision digital multimeter through LabVIEW software programming, the acquisition is detected. The output voltage signal by the laser is subjected to the fast Fourier transform and logarithmic frequency axis power spectral density estimation algorithm for noise evaluation in the frequency domain, forming a complete laser intensity noise evaluation and measurement system. The 0.1 mHz–1 Hz frequency band laser intensity noise evaluation is finally obtained. The experimental results show that the noise of the high-precision multimeter in a frequency band of 0.1 mHz–1 Hz is lower than 5×10〈sup〉–5〈/sup〉 V/Hz〈sup〉1/2〈/sup〉; the noise of the detector electronics ina frequency band of 0.1 mHz–1 Hz is lower than 4×10〈sup〉–5〈/sup〉 V/Hz〈sup〉1/2〈/sup〉. The electronic noise of the high-precision multimeters and the detectors meet the requirements for space gravitational wave detection. The experimental results show that the 0.1 mHz–1 Hz frequency band laser intensity noise evaluation system we built meets the needs of space-based gravitational wave detection program, and provides an important foundation for building a laser source that meets the needs of space-based gravitational wave detection.

Abstract: Chipless radio frequency identification tags have been widely used in many areas, such as vehicle recognition and identification of goods. Near-field measurement of a chipless radio frequency identification tag is important for offering the precise spatial information of the backscattered field of tag. In this paper, we demonstrate the angle discrimination of a line-shape chipless radio-frequency identification tag via the near-field measurements of scattered electric fields in two orthogonal directions. Two laser beams with different frequencies counter propagate and pass through a roomtemperature caesium vapor. A Rydberg ladder-type system is formed in the experiment, which includes three levels, namely 6S1/2, 6P3/2, 51D5/2. The electromagnetically induced transparency of transmission of probe light, which is locked to the transition of 6S1/2↔ 6P3/2, is observed when the frequency of coupling light varies nearby the transition of 6P3/2↔ 51D5/2. When the 5.366 GHz microwave electric field that is resonant with the transition between two adjacent Rydberg states 51D5/2↔ 52P3/2 is applied to the caesium vapor cell by using a standard-gain horn antenna, the transmission signal of probe laser splits into two peaks, which is known as Autler-Townes splitting. The splitting between the transmission peaks is proportional to the microwave electric field strength at the position of laser beam. The spatial distribution of backscattered microwave electric field of the chipless radio-frequency identification tag is obtained through varying the position of the laser beam. The spatial resolution of near-field measurement approximately equals λMW/12, where λMW is the wavelength of the measured microwave electric field. The distributions of the electric field strength in two orthogonal directions show the clarity difference while the angle of radio-frequency identification tag is changed. The scattered electric field strength of the identification tag is strongest when the angle of line-shape tag is the same as that of the polarization of the horn antenna. Moreover, the scattered field strength of identification tag in the incident field direction of the horn antenna increases as the measured position and the identification tag get closer to each other. The scattered electric field distributions in the vertical direction are almost constant at the different angles between the incident electric filed and identification tag. The fluctuation of spatial distribution of the scattered electric field strength is attributed to the Fabry-Pérot effect of microwave electric field in the vapor cell. And the geometry of vapor cell results in the minor asymmetric distribution of scattered field. The simulation results from the electromagnetic simulation software are accordant with the experimental results. The novel approach to near-field measurement of identification tag will contribute to studying and designing the chipless radio-frequency identification tag and complex circuits.

Abstract: In recent years, more than 90% of the signal laser power can be up-converted based on the high-efficiency double resonant external cavity sum-frequency generation (SFG), especially when the whole system runs under the undepleted pump approximation scheme. Therefore it is difficult to directly achieve an error signal with a high signal-to-noise ratio through the signal laser to lock its frequency to the cavity mode. In this paper a novel method, based on the frequency modulation of signal laser and demodulation of the SFG laser, is used to obtain the error signal to realize the cascade frequency locking between the two fundamental lasers and the external cavity. In this experiment, 1064 nm laser is the pump laser and 1583 nm laser is the signal laser. They are coupled into a ring cavity inside which a 5% MgO-doped PPLN (25 mm1 mm0.5 mm) is used to produce the SFG laser of 636 nm. When the pump laser is resonant with the external cavity, a circulating power of 14.3 W is obtained with its input power of 1.3 W. The reflectivity of the input coupling mirror of signal laser is 10% to restrain the impendence mismatch. The temperature of PPLN is set at 68.5 ℃ to reach the optimum SFG temperature. In order to keep the signal laser resonance inside the external cavity, one needs to lock its frequency to the cavity mode. A 28.5 kHz sinusoidal voltage is used to modulate the frequency of the signal laser so that the frequency of 636 nm laser is modulated simultaneously. Then 5% of the output 636 nm laser power is sent into a Si photodiode detector the signal of which is demodulated at the modulation frequency by a lock-in amplifier. Finally the demodulated signal is feedback to the frequency control port of signal laser. Under these conditions, 73% of 1583 nm signal laser power can be converted into 636 nm laser power when the incident power varies from 10 W to demodulation of the transmitted cavity mode of 1583 nm when the incident signal laser power is below 12 mW. When the signal laser power increases from 50 mW to 295 mW, the conversion efficiency linearly drops to 60%, which is mainly caused by depleting the 1064 nm pump laser power. Finally a 440 mW of 636 nm laser is generated with an incident signal laser power of 295 mW. This scheme can realize a high-efficiency SFG with a low input signal laser power or poor single-pass SFG efficiency.