Continuous-Variable Quantum Key Distribution Based on Heralded Hybrid Linear Amplifier with a Local Local Oscillator

An improved continuous variable quantum key distribution (CVQKD) approach based on a heralded hybrid linear amplifier (HLA) is proposed in this study, which includes an ideal deterministic linear amplifier and a probabilistic noiseless linear amplifier. The CVQKD, which is based on an amplifier, enhances the signal-to-noise ratio and provides for fine control between high gain and strong noise reduction. We focus on the impact of two types of optical amplifiers on system performance: phase sensitive amplifiers (PSA) and phase insensitive amplifiers (PIA). The results indicate that employing amplifiers, local local oscillation-based CVQKD systems can enhance key rates and communication distances. In addition, the PIA-based CVQKD system has a broader application than the PSA-based system.     

基于压缩光的量子激光雷达技术

利用Wigner函数对真空态、单光子态、压缩态在相空间的噪声分布进行仿真,并系统分析了基于压缩光的量子相干激光雷达和压缩光注入式量子激光雷达.研究表明,相比经典激光雷达,较高压缩度有利于量子相干激光雷达探测信噪比的提升,理论上8dB的压缩度可以使信噪比提高6.25倍;而压缩光注入式量子激光雷达系统的空间分辨率主要取决于真空压缩光的压缩度和无噪声相敏放大系统的增益.由于压缩光对探测信噪比的提升作用,量子激光雷达在微弱信号探测和高分辨率成像领域具有显著优势.     

Phase-sensitive multimodal optical parametric amplifier for beam angle amplification

The single-mode phase-sensitive parametric amplifier has been proposed as an approximation to an ideal quantum phase amplifier. We demonstrate numerically that a real, multimode parametric amplifier operated in the spatial domain, and realized by the use of three-wave mixing in nonlinear media, behaves similar to a single-mode quantum phase amplifier, but exhibits smaller phase gain. Constraints for operation in this regime are found to be consistent with small departures of phase from the ideal deamplification condition.     

Fundamentals of optical amplifiers

The minimum noise figure of linear phase-insensitive amplifiers is 3 dB, according to the constraint of the uncertainty principle. Laser amplifiers, parametric amplifiers and Raman amplifiers obey the general quantum limit in an ideal case. A degenerate parametric amplifier is a phase-sensitive linear amplifier, and thus is a noiseless amplifier with a noise figure of 0 dB. The noise figure degradation in a practical amplifier and the signal-to-noise ratio design of an optical amplifier system are presented. Several other amplifier characteristics, such as signal gain, frequency bandwidth and saturation output power, are also discussed.     

On the detectabillity limit of coherent optical signals in thermal radiation

The problem of detecting a completely known coherent optical signal in a thermal background radiation is considered. The problem is a quantum mechanical analog of detection of a known signal in Gaussian noise. The quantum detection counterpart is formulated in terms of a pair of density operators and a solution is shown to exist. A perturbation solution is obtained by making use of a reproducing kernel Hilbert space of entire functions. The solution is particularly applicable to optical frequencies, where the effect of thermal radiation is small, and it is shown to converge to known results at zero thermal radiation. Curves are generated showing the detectability limit at optical frequencies. Also considered is the problem of finding an operator that maximizes a signal-to-noise ratio, defined for quantum detection in analogy with the classical theory. For a coherent signal with random phase, the operator that maximizes the signal-to-noise ratio is identicial to the one obtained by applying the Neyman Pearson criterion, thereby establishing a complete analogy with the classical detection theory. For a signal with known phase, however, the analogy breaks down in the limit of zero thermal radiation. In that case, it is shown that an operator that maximizes the classical signal-to-noise ratio does not exist.A part of this paper has been submitted to the University of California, Los Angeles, in partial satisfaction of the Ph.D. requirement.     

Enhanced phase sensitivity with a nonconventional interferometer and nonlinear phase shifter

We propose a theoretical scheme of an enhanced phase sensitivity by introducing a nonlinear phase shifter to the nonconventional interferometer consisting of a balanced beam splitter (BBS) and an optical parameter amplifier (OPA), a modified nonlinear interferometer (MNI). Then we use coherent state and even coherent state as inputs and homodyne detection at one output port of the MNI for phase sensitivity, both without and with photon losses. We find that the nonlinear phase shifter can not only improve phase sensitivity, but also significantly resist the decoherence from photon losses. In comparison to both the BBS+OPA scheme with linear phase shifter and the traditional Mach–Zehnder interferometer with nonlinear one, the phase sensitivity of the MNI scheme shows the best performance. It is interesting that the nonlinear phase shifter can stimulate potential of the OPA, although there is no improvement in signal-to-noise ratio beyond standard quantum limit for the BBS+OPA scheme with a linear phase shifter.     

Reduced quantum noise in waveguide coherent Raman scattering spectroscopy

Waveguide regimes of nonlinear-optical interactions allow a reduction of quantum noise in spectra of coherent Raman scattering (CRS). We will find the optimal waveguide length providing the maximum quantum limit for the signal-to-noise ratio in waveguide CRS and assess this ratio for laser intensities right below the optical breakdown threshold. This analysis shows that the quantum-limit signal-to-noise ratio of coherent femtosecond CRS spectroscopy of the gas phase can be increased in the waveguide regime by more than four orders of magnitude relative to the regime of tight focusing.     

高灵敏度的量子迈克耳孙干涉仪

迈克耳孙干涉仪不仅可以用来研究物理学的基本问题,而且能够用于精密测量,比如引力波信号的测量.因此,构建高灵敏度的迈克耳孙干涉仪是实现微弱信号测量的关键.目前,人们利用压缩态可以降低迈克耳孙干涉仪的噪声;通过光学四波混频过程能够放大马赫·曾德尔干涉仪中的相位信号,从而提高干涉仪的信噪比和灵敏度.本文研究了一种用于高灵敏度相位测量的量子迈克耳孙干涉仪.在迈克耳孙干涉仪中,利用非简并光学参量放大器取代干涉仪中的线性光学分束器;并且将压缩态注入干涉仪的真空通道,可以得到高信噪比和高灵敏度的干涉仪.由于存在不可避免的光学损耗,分析了迈克耳孙干涉仪内部和外部的损耗对相位测量灵敏度的影响.通过理论计算研究了干涉仪的相位测量灵敏度随系统参数的变化关系,得到了高灵敏度的相位测量量子迈克耳孙干涉仪的实现条件,为用于精密测量的干涉仪的设计提供了直接参考.     

Majorization theory approach to the Gaussian channel minimum entropy conjecture

A long-standing open problem in quantum information theory is to find the classical capacity of an optical communication link, modeled as a Gaussian bosonic channel. It has been conjectured that this capacity is achieved by a random coding of coherent states using an isotropic Gaussian distribution in phase space. We show that proving a Gaussian minimum entropy conjecture for a quantum-limited amplifier is actually sufficient to confirm this capacity conjecture, and we provide a strong argument towards this proof by exploiting a connection between quantum entanglement and majorization theory.     

基于激光驱动等离子体光源的近红外傅里叶变换光谱系统

近红外傅里叶变换光谱仪作为一种常用的科研级近红外光谱检测仪器,广泛应用于各个科研领域。目前的近红外光谱仪着重于光谱分辨率方面的提升,在光谱信噪比提升方面关注较少。光谱信噪比直接影响光谱线指数测量精度的优劣,光谱信噪比越高,光谱线指数测量精度越高,越有利于对微量物质进行精细光谱比对。因此,提升光谱仪的光谱信噪比是十分必要的。对比常用的钨灯光源,激光驱动等离子体光源(LDLS)不仅在近红外区域具有高光照强度的优点,而且其独特的高频调制输出信号在经锁相放大器调制解调后能够很好的抑制背景信号对干涉光谱所带来的影响。高亮度与辐射调制的结合使得以LDLS作为光源的近红外傅里叶变换光谱系统在光谱信噪比方面获得显著提升。基于上述原因,提出利用新型激光驱动等离子体光源作为光谱信号输出源的近红外傅里叶变换光谱系统,并与含有调制能力的钨灯光源搭建的近红外傅里叶变换光谱系统进行了信噪比的比较实验。首先利用钨灯光源由斩波器高频调制再经过锁相放大器解调的方式,对锁相放大器积分时间进行优化并通过计算干涉光谱信噪比进行评估,分别对比了积分时间为0.5,1,5,10和20 ms的干涉光谱信噪比与对称度,确定后续系统中的锁相放大器最佳积分时间为5 ms,该状态下钨灯光源所实现的干涉光谱信噪比经计算约为90∶1;其次利用激光驱动等离子体光源代替钨灯光源和斩波器,在最佳积分时间下进行干涉光谱信噪比对比评估,结果表明激光驱动等离子体光源的干涉光谱信噪比与传统钨灯光源相比提升111倍;最后,利用近红外标准片对系统进行光谱测量准确性评估,结果表明利用该光源的近红外傅里叶变换光谱系统的近红外吸收峰值误差<0.5 nm,具有高光谱准确性与分辨能力。     

Brillouin-based fiber-optic distributed temperature sensor with optical preamplification

Optical preamplification has been used in a fiber-optic distributed temperature sensor based on spontaneous Brillouin scattering and the use of direct detection, resulting in improved signal-to-noise ratios. The fiber-based optical preamplifier system comprises a three-port circulator, an erbium-doped fiber amplifier with a small-signal gain of 27 dB, and a fiber Bragg grating with 47-GHz bandwidth. An improvement of 17 dB in the optical signal-to-noise ratio for the Brillouin signal is demonstrated in a 23-km sensor. The limit to the signal-to-noise ratio is attributed to spontaneous-spontaneous beat noise generated at the photodetector by amplified spontaneous emission from the optical amplifier.     

Fisher Information of Wavefunctions： Classical and Quantum

A parametric quantum mechanical wavefunction naturally induces parametric probability distributions by taking absolute square, and we can consider its classical Fisher information. On the other hand, it also induces parametric rank-one projections which may be viewed as density operators, and we can talk about its quantum Fisher information. Among many versions of quantum Fisher information, there are two prominent ones. The first, defined via a quantum score function, was introduced by Helstrom in 1967 and is well known. The second, defined via the square root of the density operator, has its origin in the skew information introduced by Wigner and Yanase in 1963 and remains relatively unnoticed. This study is devoted to investigating the relationships between the classical Fisher information and these two versions of quantum Fisher information for wavefunctions. It is shown that the two versions of quantum Fisher information differ by a factor 2 and that they dominate the classical Fisher information. The non-coincidence of these two versions of quantum Fisher information may be interpreted as a manifestation of quantum discord. We further calculate the difference between the Helstrom quantum Fisher information and the classical Fisher information, and show that it is precisely the instantaneous phase fluctuation of the wavefunctions.     

基于光前置放大器的量子密钥分发融合经典通信方案

量子密钥分发融合经典通信方案将连续变量量子密钥分发和经典通信合并到了一起,为将来在现有的光网络上同时进行密钥分发和经典通信提供了一个有效的方法.然而,在量子信号上叠加一个经典信号将会给连续变量量子密钥分发系统引入过噪声从而大大降低系统的性能.本文提出基于光前置放大器的量子密钥分发融合经典通信方案,即在接收端插入光前置放...     

光电检测系统的带宽选择

本文分析光电检测系统的带宽与系统响应时间及信噪比之间的关系。提出在方案设计时根据所要求的响应时间和最佳信噪比初步确定带宽的方法。     

Classical and quantum transport through entropic barriers modeled by hardwall hyperboloidal constrictions

We study the quantum transport through entropic barriers induced by hardwall constrictions of hyperboloidal shape in two and three spatial dimensions. Using the separability of the Schrödinger equation and the classical equations of motion for these geometries, we study in detail the quantum transmission probabilities and the associated quantum resonances, and relate them to the classical phase structures which govern the transport through the constrictions. These classical phase structures are compared to the analogous structures which, as has been shown only recently, govern reaction type dynamics in smooth systems. Although the systems studied in this paper are special due their separability they can be taken as a guide to study entropic barriers resulting from constriction geometries that lead to non-separable dynamics.     

Carrier-envelope phase-controlled quantum interference of injected photocurrents in semiconductors

We demonstrate quantum interference control of injected photocurrents in a semiconductor using the phase stabilized pulse train from a mode-locked Ti:sapphire laser. Measurement of the comb offset frequency via this technique results in a signal-to-noise ratio of 40 dB (10 Hz resolution bandwidth), enabling solid-state detection of carrier-envelope phase shifts of a Ti:sapphire oscillator.     

Assessment of image resolution improvement by phase-sensitive optical parametric amplification

Classical information theory can be used to quantify the resolution performance of optical imaging systems. When an optical parametric amplifier (OPA) operated as a phase-sensitive amplifier (PSA) in the transverse spatial domain is used for point source imaging, the angular resolution improvement can approach the de Broglie resolution (i.e. Heisenberg limit). In this paper, classical information theory is employed to quantify the signal-to-noise ratio (SNR) improvement for both an ideal and a realistic multimode PSA applied to the problem of sub-Rayleigh imaging. When only considering the noise originating from the detector, the SNR improvement is found to scale quadratically as a function of the PSA gain, in the limit of noise power comparable to signal power. Differences in performance of an ideal PSA and a realistic PSA are discussed.     

Quantum particles from coarse grained classical probabilities in phase space

Quantum particles can be obtained from a classical probability distribution in phase space by a suitable coarse graining, whereby simultaneous classical information about position and momentum can be lost. For a suitable time evolution of the classical probabilities and choice of observables all features of a quantum particle in a potential follow from classical statistics. This includes interference, tunneling and the uncertainty relation.