单光子技术理论与应用的若干问题

Several Problems on the Theory and Application of Single Photon Technology

为什么存在光子?经典物理给出了推导,但解释不了光子的本质和奇异特性。量子理论表明光子是一种非局域性粒子,并与通常的微观粒子有重大区别。然而任何理论都不能提供光子的具体形象,这就在单光子定义上发生困难。光子可由光脉冲获取,其中的光子数服从Poisson分布。另外也可以从光功率出发而定义单光子,光子实际上是一种最弱光源。本文论述了单光子理论与实验的进展,给出历史评论和对某些矛盾悖论的探讨。例如,光子是能量子,具有物质属性,但却给不出光子的尺寸和体积;狭义相对论实际上赋予光子点粒子形象。又如,量子力学(例如Schrodinger方程)用波函数Ψ(r,t)描写电子,空间定位为几率性分布,︱Ψ︱~2是几率密度;但对光子却无法定义一个自洽的波函数,也不能写出相应的波方程。而且众所周知,经典Maxwell波方程并不能满意地描写光子。本文认为"光子无静质量"假说造成了理论自洽性的缺失。对有质(量)光子,可用1936年提出的Proca方程组取代Maxwell方程组。我们推导了新的电磁波和光子的波方程,称为Proca波方程(PWE)。在PWE中有包含粒子质量参数(m)的项,这与Schrdinger波方程、Dirac波方程一致。这使理论关系改善,而有质(量)光子与点粒子划清了界限。量子信息学的迅猛发展迫使科学家考虑少数(例如1万个或更少)光子的行为特点。最近测量光动量成功,使得可能用实验确定1个激光束中包含的光子数。量子保密通信要求理想单光子源(PSPS),其中1个光脉冲仅有1个光子。然而迄今并未制成这种源,所用均为近似PSPS,故通信不会绝对安全。由于白天的光干扰,卫星量子通信及量子雷达难以工作得好。如用微波频段会有好效果,但微波单光子能量极小,实现起来非常难。

Why the photon exists,the theoretical derivation is derived by the classical physics,but it can’t explain the essences and the odd characters of the photon. Quantum theory shows that the photon is a non-localizable particle,and there have the important differences between the photons and the common micrographic particles. But any theory can’t give the concrete shape of the photon,this situation made difficulties in understanding the definition of single photon. The photon can be obtained by the signal of light pulse,and the number of photons obeys the Poisson’s equation. Another method used as defining the single photon is to compute the power of light. Actually,the photon is the weakest light source. This paper is devoted to a survey of developments on theoretical and experimental study of the single photon technology. The historical notes and the exploration of some paradoxes of this subject are discussed. For example,the definition of photon is the particle of energy,which has the character of matter,but we can’t give the size and the volume of the photon. In fact,the Special Relativity define the photon is the image of dot-particle. In addition,Quantum Mechanics(such as the Schr dinger equation)prescribes that,the localization of an electron described by the wave function Ψ(r,t) is probability distributed in space,|Ψ|2 is the probability density. But we can’t define a consistent wave function for the photon case. Then,we also can’t write down a wave equation for the photon. Everyone knows that the classical Maxwell’s wave equation can’t satisfying describe the photon. The hypothesis of photon’s zero rest mass create an in-consistently theoretical situation. For the photon with mass,the Maxwell equations can be replaced by Proca equations invented in 1936. In this paper,we derive the new wave equation for the electro-magnetic waves and the photon,and we call it the Proca’s Wave Equation(PWE). One item of the PWE contains the mass parameter(m)of the particle,which keep consistent with the Schr dinger wave equation and the Dirac wave equation. Then the theoretical relations get some improvements. Moreover,the photon with mass draws a clear line at the dot-particle. The rapid development of Quantum Information Technology compelled scientists to consider the characteristic of a small number(such as ten thousand or smaller quantity)of the photon. Recently,the successful measurement of the light momentum give a way could confirm the photon’s number in a laser light beam by experiment. The quantum security communication need the perfect single photon source(PSPS)which gives the situation that one pulse has one photon only. But the perfect single photon source is not discovered. Actually,the single photon source found in lab is the approximate solution of PSPS,the absolute security does not exist. Due to interference of the light in daytime,the satellite quantum communication and the quantum radar can’t operate well.If we employ the microwave frequency band,the quantum radar or quantum communication will work well. However,the energy of single microwave photon is very small,the application of technology is very difficult.