Efficient classical simulation of optical quantum information circuits

We identify a broad class of physical processes in an optical quantum circuit that can be efficiently simulated on a classical computer: this class includes unitary transformations, amplification, noise, and measurements. This simulatability result places powerful constraints on the capability to realize exponential quantum speedups as well as on inducing an optical nonlinear transformation via linear optics, photodetection-based measurement, and classical feedforward of measurement results, optimal cloning, and a wide range of other processes.     

Quantum and classical correlations in tripartite Gaussian states of light

We demonstrate an optical scheme involving two interlinked nonlinear interactions taking place simultaneously in a second-order nonlinear crystal. The three-mode output state is fully inseparable and endowed with perfect photon-number correlations among the generated fields. In order to discriminate between classical and quantum correlations we evaluate the quantum noise reduction, i.e. compare the fluctuations of the difference photocurrent between one mode and the sum of the other two with those coming from three uncorrelated coherent states. Preliminary results indicate that our scheme may achieve three-mode sub-shot noise correlations with current technology.     

A new quantum mechanical photon counting distribution formula

By virtue of the density operator's P-representation in the coherent state representation, we derive a new quantum mechanical photon counting distribution formula. As its application, we calculate photon counting distributions for some given light fields. It is found that the pure squeezed state's photon counting distribution is related to the Legendre function, which is a new result.     

自发参量下转换机理及应用研究综述

自发参量下转换(spontaneous param etric down-conversion,SPDC)光场是一种非经典光场,它是由单色泵浦光子流和量子真空噪声对非线性晶体的综合作用而产生的。基于量子理论解释了SPDC光场的产生机理,说明从经典理论如何理解SPDC光场。较全面介绍了SPDC光场的非经典特性及在激光技术、遥感、量子通讯等领域的应用研究进展,展望了它应用研究的发展方向与前景。     

On the Similarity of the Statistical Characteristics of Spontaneous Radiation of a Quantum Amplifier at Different Gain Factors

The transformation laws governing the statistical properties of the quantum noise of an optical amplifier when its gain factor and spectral line profile are varied are considered. The correlation functions of quantum noise both at the amplifier output and after nonlinear transformation normalized to the correlation time of quantum noise at the amplifier output were found to be independent of the amplifier gain provided that it exceeds 10 for the Gaussian spectral line profile and 10³ for the Lorentzian profile. The spectral density of quantum noise transformed in a nonlinear system was shown to possess the same similarity property. It was found that the statistical characteristics possess a similarity property even in the case of variation of the spectral line profile from Gaussian to Lorentzian. It was concluded that such a characteristic of quantum noise of an optical amplifier as the correlation time bears virtually all information on the above-mentioned statistical properties at a reasonably large amplifier gain. The similarity property revealed has an important application. It allows finding the matching condition between the optical signal spectrum and the gain profile of the optical quantum amplifier to achieve the highest sensitivity of signal detection by the amplifier.     

Quantum simulation of the t–J model

Computer simulation of a many-particle quantum system is bound to reach the inevitable limits of its ability as the system size increases. The primary reason for this is that the memory size used in a classical simulator grows polynomially whereas the Hilbert space of the quantum system does so exponentially. Replacing the classical simulator by a quantum simulator would be an effective method of surmounting this obstacle. The prevailing techniques for simulating quantum systems on a quantum computer have been developed for purposes of computing numerical algorithms designed to obtain approximate physical quantities of interest. The method suggested here requires no numerical algorithms; it is a direct isomorphic translation between a quantum simulator and the quantum system to be simulated. In the quantum simulator, physical parameters of the system, which are the fixed parameters of the simulated quantum system, are under the control of the experimenter. A method of simulating a model for high-temperature superconducting oxides, the t–J model, by optical control, as an example of such a quantum simulation, is presented.     

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...     

Partially Classical States of a Boson Field

Employing positive-definiteness arguments we analyse Boson field states, which combine classical and quantum mechanical features (signal and noise), in a constructive manner. Mathematically, they constitute Bauer simplexes within the convex, weak-*-compact state space of the C*-Weyl algebra, defined by a presymplectic test function space (smooth one-Boson wave functions) and are affinely homeomorphic to a state space of a classical field. The regular elements are expressed in terms of weak distributions (probability premeasures) on the dual test function space. The Bauer simplex arising from the bare vacuum is shown to generalize the quantum optical photon field states with positive P-functions.     

低分析频率压缩光的实验制备

基于PPKTP晶体的阈值以下光学参量振荡(OPO)过程,制备了共振于铷原子D1线795 nm的压缩真空态光场,研究了分析频率处于千赫兹范围的主要噪声来源,特别是795 nm激光及其二次谐波397.5 nm激光在晶体内吸收引起的非线性损耗增加和系统热不稳定的问题(397.5 nm激光处于PPKTP晶体透光范围边缘,具有高于其他波长数倍的吸收系数).以795 nm和1064 nm为例,分析了非线性损耗及晶体内热效应对压缩度的影响.受限于以上因素,795 nm压缩光很难得到1064 nm波段同样的压缩度.探测系统中的噪声耦合则限制了压缩频带.实验上对分析频率为千赫兹的经典噪声进行了有效控制,通过使用真空注入的OPO、垂直偏振及反向传输的腔长锁定光、低噪声的平衡零拍探测器、高稳定度的实验系统及量子噪声锁定等方法,最终在2.6—100 kHz的分析频段得到了约2.8 dB的795 nm压缩真空.该压缩光可用作磁场测量系统的探测光以提高测量灵敏度.     

The Quantum Theory of Optical Pulses Amplification

The propagation of optical signals in optical fiber amplifier is treated with fully quantum mechanics method in this paper. The micro-Hamiltonian of the system with nonlinear effect is derived by combining spatial mode and temporal mode, and propagation equation of optical pulses in a optical fiber amplifier is derived with dispersion relation. The characteristic parameters such as gain, output power and noise figure of the amplifier are analyzed and their quantum mechanics formulas are obtained.     

Classical theory of laser noise

The classical theory of laser noise treats light in a classical manner, yet agrees with quantum theory for large particle numbers. The basic concept is that laser noise is caused by atomic jumps between lower and upper levels, and that atoms subjected to classically-prescribed optical fields are independent. The treatment of amplitude noise of single-mode cavities containing resonant three-level atoms is applicable to semiconductor lasers at moderate power. At high power one must account for the dependence of the gain on optical power and for state-occupancy fluctuations. The phasor theory that attributes noise to the beat between the oscillating field and the field spontaneously emitted in the mode by excited-state atoms cannot be understood consistently in semiclassical terms.     

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.     

Measured mixer noise temperature and conversion loss of a cryogenic Schottky diode mixer near 800 GHz

We accurately measured the noise temperature and conversion loss of a cryogenically cooled Schottky diode operating near 800 GHz, using the UCB/MPE Submillimeter Receiver at the James Clerk Maxwell Telescope. The receiver temperature was in the range of the best we now routinely measure, 3150 K (DSB). Without correcting for optical loss or IF mismatch, the raw measurements set upper limits ofT _M=2850 K andL _M=9.1 dB (DSB), constant over at least a 1 GHz IF band centered at 6.4 GHz with an LO frequency of 803 GHz. Correction for estimated optical coupling and mismatch effects yieldsT _M=1600 K andL _M=5.5 dB (DSB) for the mixer diode itself. These values indicate that our receiver noise temperature is dominated by the corner cube antenna's optical efficiency and by mixer noise, but not by conversion loss or IF mismatch. The small fractional IF bandwidth, measured mixer IF band flatness from 2 to 8 GHz, and similarly good receiver temperatures at other IF frequencies imply that these values are representative over a range of frequencies near 800 GHz.     

介观耗散电容耦合电路量子化中的正则变换

针对介观耗散电容耦合RLC电路提出一种一般的正则变换, 并证明了其正确性和合理性. 用这种正则变换研究了双回路介观耗散电容耦合电路的量子化问题, 得出的对角化哈密顿量比文献中多出一非线性项. 这种具有普遍性的一般正则变换可能对研究介观多回路耗散系统的量子涨落、量子噪声等性质具有重要的意义. 关键词： RLC电路')" href="#">RLC电路 量子化 正则变换     

Quantum noise frequency correlations of multiply scattered light

Frequency correlations in multiply scattered light that are present in quantum fluctuations are investigated. The speckle correlations for quantum and classical noise are compared and are found to depend markedly differently on optical frequency, which was confirmed in a recent experiment. Furthermore, novel mesoscopic correlations are predicted that depend on the photon statistics of the incoming light.     

Dynamics of quantum discord in a two-qubit system under classical noise

We study the quantum discord dynamics of two noninteracting qubits that are, respectively, subject to classical noise. The results show that the dynamics of quantum discord are dependent on both the coupling between the qubits and classical noise, and the average switching rate of the classical noise. In the weak-coupling Markovian region, quantum discord exhibits exponent decay without revival, and can be well protected by increasing the average classical noise switching rate. While in the strong-coupling non-Markovian region, quantum discord reveals slowly decayed oscillations with quick revival by decreasing the average switching rate of the classical noise. Thus, our results provide a new method of protecting quantum discord in a two-qubit system by controlling the coupling between the qubits and classical noise, and the average switching rate of the classical noise.     

Classical-Noise-Free Sensing Based on Quantum Correlation Measurement

Quantum sensing,using quantum properties of sensors,can enhance resolution,precision,and sensitivity of imaging,spectroscopy,and detection.An intriguing question is:Can the quantum nature(quantumness)of sensors and targets be exploited to enable schemes that are not possible for classical probes or classical targets?Here we show that measurement of the quantum correlations of a quantum target indeed allows for sensing schemes that have no classical counterparts.As a concrete example,in the case that the second-order classical correlation of a quantum target could be totally concealed by non-stationary classical noise,the higher-order quantum correlations can single out a quantum target from the classical noise background,regardless of the spectrum,statistics,or intensity of the noise.Hence a classical-noise-free sensing scheme is proposed.This finding suggests that the quantumness of sensors and targets is still to be explored to realize the full potential of quantum sensing.New opportunities include sensitivity beyond classical approaches,non-classical correlations as a new approach to quantum many-body physics,loophole-free tests of the quantum foundation,et cetera.     

Quantum optical waveform conversion

Proposals for long-distance quantum communication rely on the entanglement of matter-based quantum nodes through optical communications channels, but the entangling light pulses have poor temporal behavior in current experiments. Here we show that nonlinear mixing of a quantum light pulse with a spectrally tailored classical field can compress the quantum pulse by more than a factor of 100 and flexibly reshape its temporal waveform while preserving all quantum properties, including entanglement. Our scheme paves the way for quantum communication at the full data rate of optical telecommunications.     

Classical and quantum aspects of multimode parametric interactions

Parametric interactions in nonlinear crystals represent a powerful tool in the optical manipulation of information, both in the classical and the quantum regime. Here, we analyze in detail classical and quantum aspects of three-and five-mode parametric interactions in x⁽²⁾ nonlinear crystals. The equations of motion are explicitly derived and then solved within the parametric approximation. We describe several applications, including holography, all-optical gates, generation of entanglement, and telecloning. Experimental results on the photon distributions and correlations of the generated beams are also reported and discussed.