定义：
真空态的量子噪声，或者通过真空输入引入光学系统的量子噪声。
 

图1：采用分束器衰减光束，真空噪声从开放端口进入光束。
真空噪声是一种量子噪声，与真空态电场的零点涨落有关。即使真空态光子数为0时仍然存在这种涨落。 

尽管非常弱，真空噪声会影响噪声性质，甚至会影响光学装置的主要功能。例如，参量产生过程可看做是在高增益光参量放大器中将真空噪声放大到宏观功率水平。另外，自发辐射过程（在半经典图像中）就被认为是真空噪声受激发而产生的。 

光束的任何线性损耗都会不仅会减小量子噪声和平均振幅，还会引入真空噪声。这在采用分束器进行光衰减的过程中体现的最明显，可以理解为将一部分输入光在不采用的输入端口与真空噪声混合在一起（图1）。其它任何种类的线性噪声都会引入真空噪声，例如，介质的吸收过程。实际上，线性衰减过程使压缩光接近于相干态。

Definition: quantum noise of the vacuum state, or quantum noise introduced into an optical system via a vacuum input

Vacuum noise is quantum noise related to the zero-point fluctuations of the electric field in the vacuum state. Such fluctuations exist even though the vacuum state has a photon number of zero.

Figure 1: When a beam splitter is used to attenuate a beam, vacuum noise enters via the open port.

Despite being fairly weak, vacuum noise can significantly affect the noise properties or even the essential function of optical devices. For example, parametric generation can be considered as the effect of amplifying vacuum noise to macroscopic power levels in a high-gain optical parametric amplifier. Also, the process of spontaneous emission can (in a semiclassical picture) be interpreted as being stimulated by vacuum noise.

Any linear losses acting on a light beam introduce vacuum noise, rather than simply attenuating the quantum noise together with the average amplitude. This is most obvious for attenuation with a beam splitter, which can be understood to mix a portion of the input beam with vacuum noise from the unused input port (Figure 1). However, vacuum noise is also introduced by any other kind of linear attenuation, e.g. via absorption in a medium. In effect, linear attenuation brings, e.g., squeezed light closer to a coherent state.