Performance analysis of quantum key distribution using polarized coherent-states in free-space channel

In free space channel,continuous-variable quantum key distribution(CV-QKD)using polarized coherent-states can not only make the signal state more stable and less susceptible to interference based on the polarization non-sensitive of the free-space channel,but also reduce the noise introduced by phase interference.However,arbitrary continuous modulation can not be carried out in the past polarization coding,resulting in that the signal state can not obtain arbitrary continuous value in Poincare space,and the security analysis of CV-QKD using polarized coherent-states in free space is not complete.Here we propose a new modulation method to extend the modulation range of signal states with an optical-fiber-based polarization controller.In particular,in terms of the main influence factors in the free-space channel,we utilize the beam extinction and elliptical model when considering the transmittance and adopt the formulation of secret key rate.In addition,the performance of the proposed scheme under foggy weather is also taken into consideration to reveal the influence of severe weather.Numerical simulation shows that the proposed scheme is seriously affected by attenuation under foggy weather.The protocol fails when visibility is less than 1 km.At the same time,the wavelength can affect the performance of the proposed scheme.Specifically,under foggy weather,the longer the wavelength,the smaller the attenuation coefficient,and the better the transmission performance.Our proposed scheme can expand the modulation range of signal state,and supplement the security research of the scheme in the free-space channel,thus can provide theoretical support for subsequent experiments.

Search for topological defect of axionlike model with cesium atomic comagnetometer

Many terrestrial experiments have been designed to detect domain walls composed of axions or axionlike particles(ALPs), which are promising candidates of dark matter. When the domain wall crosses over the Earth, the pseudoscalar field of ALPs could couple to the atomic spins. Such exotic spin-dependent couplings can be searched for by monitoring the transient-in-time change of the atomic spin precession frequency in the presence of a magnetic field. We propose here a single-species cesium atomic comagnetometer, which measures the spin precession frequencies of atoms in different ground-state hyperfine levels, to eliminate the common-mode magnetic-field variations and search for the exotic nonmagnetic couplings solely between protons and ALPs. With the single-species atomic comagnetometer, we experimentally rule out the possibility that the decay constant of the linear pseudoscalar couplings of ALPs to protons is fp■ 3.71 ×107 Ge V. The advanced system has the potential to constrain the constant to be fp■ 10.7 × 109 Ge V, promising to improve astrophysical constraint level by at least one order of magnitude. Our system could provide a sensitive detection method for the global network of optical magnetometers to search for exotic physics.

Rate compatible reconciliation for continuous-variable quantum key distribution using Raptor-like LDPC codes

In the practical continuous-variable quantum key distribution(CV-QKD)system,the postprocessing process,particularly the error correction part,significantly impacts the system performance.Multi-edge type low-density parity-check(MET-LDPC)codes are suitable for CV-QKD systems because of their Shannon-limit-approaching performance at a low signal-to-noise ratio(SNR).However,the process of designing a low-rate MET-LDPC code with good performance is extremely complicated.Thus,we introduce Raptor-like LDPC(RL-LDPC)codes into the CV-QKD system,exhibiting both the rate compatible property of the Raptor code and capacity-approaching performance of MET-LDPC codes.Moreover,this technique can significantly reduce the cost of constructing a new matrix.We design the RL-LDPC matrix with a code rate of 0.02 and easily and effectively adjust this rate from 0.016 to 0.034.Simulation results show that we can achieve more than 98%reconciliation efficiency in a range of code rate variation using only one RL-LDPC code that can support high-speed decoding with an SNR less than-16.45 d B.This code allows the system to maintain a high key extraction rate under various SNRs,paving the way for practical applications of CV-QKD systems with different transmission distances.

Diode laser using narrow bandwidth interference filter at 852 nm and its application in Faraday anomalous dispersion optical filter

We demonstrate an 852-nm external cavity diode laser（ECDL） system whose wavelength is mainly determined by an interference filter instead of other wavelength selective elements. The Lorentzian linewidth measured by the heterodyne beating between two identical lasers is 28.3 k Hz. Moreover, we test the application of the ECDL in the Faraday atomic filter.Besides saturated absorption spectrum, the transmission spectrum of the Faraday atomic filter at 852 nm is measured by using the ECDL. This interference filter ECDL method can also be extended to other wavelengths and widen the application range of diode laser.

Spin dynamics of magnetic resonance with parametric modulation in a potassium vapor cell

A typical magnetic-resonance scheme employs a static bias magnetic field and an orthogonal driving magnetic field oscillating at the Larmor frequency, at which the atomic polarization precesses around the static magnetic field. Here we demonstrate both theoretically and experimentally the variations of the resonance condition and the spin precession dynamics resulting from the parametric modulation of the bias field. We show that the driving magnetic field with the frequency detuned by different harmonics of the parametric modulation frequency can lead to resonance as well. Also, a series of frequency sidebands centered at the driving frequency and spaced by the parametric modulation frequency can be observed in the precession of the atomic polarization. We further show that the resonant amplitudes of the sidebands can be controlled by varying the ratio between the amplitude and the frequency of the parametric modulation. These effects could be used in different atomic magnetometry applications.

Negative exponential behavior of image mutual information for pseudo-thermal light ghost imaging： observation, modeling, and verification

When using the image mutual information to assess the quality of reconstructed image in pseudothermal light ghost imaging, a negative exponential behavior with respect to the measurement number is observed. Based on information theory and a few simple and verifiable assumptions, semi-quantitative model of image mutual information under varying measurement numbers is established. It is the Gaussian characteristics of the bucket detector output probability distribution that leads to this negative exponential behavior. Designed experiments verify the model.

Saturated absorption spectrum of cesium micrometric-thin cell with suppressed crossover spectral lines

Micrometric-thin cells(MCs)with alkali vapor atoms have been valuable for research and applications of hyperfine Zeeman splitting and atomic magnetometers under strong magnetic fields.We theoretically and experimentally study the saturated absorption spectra using a 100-μm cesium MC,where the pump and probe beams are linearly polarized with mutually perpendicular polarizations,and the magnetic field is along the pump beam.Because of the distinctive thin chamber of the MC,crossover spectral lines in saturated absorption spectra are largely suppressed leading to clear splittings of hyperfine Zeeman transitions in experiments,and the effect of spatial magnetic field gradient is expected to be reduced.A calculation method is proposed to achieve good agreements between theoretical calculations and experimental results.This method successfully explains the suppression of crossover lines in MCs,as well as the effects of magnetic field direction,propagation and polarization directions of the pump/probe beam on saturated absorption spectrum.The saturated absorption spectrum with suppressed crossover lines is used for laser frequency stabilization,which may provide the potential value of MCs for high spatial resolution strong-field magnetometry with high sensitivity.

Experimental demonstration of a quantum downstream access network in continuous variable quantum key distribution with a local local oscillator

Quantum networks provide opportunities and challenges across a range of intellectual and technical frontiers,including quantum computation,communication,and others.Unlike traditional communication networks,quantum networks utilize quantum bits rather than classical bits to store and transmit information.Quantum key distribution(QKD)relying on the principles of quantum mechanics is a key component in quantum networks and enables two parties to produce a shared random secret key,thereby ensuring the security of data transmission.In this work,we propose a cost-effective quantum downstream access network structure in which each user can get their corresponding key information through terminal distribution.Based on this structure,we demonstrate the first four-end-users quantum downstream access network in continuous variable QKD with a local local oscillator.In contrast to point-to-point continuous variable QKD,the network architecture reevaluates the security of each user and accounts for it accordingly,and each user has a lower tolerance for excess noise as the overall network expands with more users.Hence,the feasibility of the experiment is based on the analysis of the theoretical model,noise analysis,and multiple techniques such as the particle filter and adaptive equalization algorithm used to suppress excess noise.The results show that each user can get a low level of excess noise and can achieve secret key rates of 546 kbps,535 kbps,522.5 kbps,and 512.5 kbps under a transmission distance of 10 km,respectively,with the finite-size block of 1×10~8.This not only verifies the good performance but also provides the foundation for the future multi-user quantum downstream access networks.

Toward practical quantum key distribution using telecom components

The BB84 protocol[1],as the first and the most well-known quantum key distribution(QKD)protocol,has opened up an era of secure commu-nication using quantum methods,by providing information-theoretical secure key distribution[2].In the 2000s,continuous-variable QKD(CV-QKD)was proposed as an alternative approach,and it is now going through a booming period because of its compatibility with the tele-com industry[3].

Faraday anomalous dispersion optical filter at 133Cs weak 459 nm transition

A 459 nm Faraday anomalous dispersion optical filter（FADOF） working at the side wings of the cesium6S1∕2→ 7P1∕2transition with weak oscillator strength is achieved. The transmittance of the higher side wing reaches 98% at a temperature of 179°C and magnetic field above 323 G. The experimental results coincide with the theoretical predictions in 1982 and 1995, which were not realized in experiments for over three decades. Due to its high transmittance, high accuracy, and narrow linewidth, the 459 nm FADOF can be applied in underwater optical communications, the building of active optical clocks, and laser frequency stabilization in active optical clocks.