central frequency $k_0 > \frac{1}{\Delta T}$ while it remains the same for wave packets with central frequency $k_0 < \frac{1}{\Delta T}$. Therefore Eq. (\[ParticleNumber\]) is not applicable to wave packets with very high central frequency because it does not take into account physical initial conditions.

Squeezing from accelerated mirrors {#squeezing}
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A well known mechanism for generating particles from the vacuum is the two-mode squeezing process. Examples include non-degenerate parametric down conversion [@BachorRalph] and the Unruh effect [@Unruh76]. The two output modes are entangled with each other so that the composite state is a pure state. Another important mechanism is the single-mode squeezing process, for example degenerate parametric down conversion [@BachorRalph]. It is possible that a particle generation process is the combination of the two, which we now show is the case for the uniformly accelerated mirror. Using the quantum circuit model for the uniformly accelerated mirror, it is very easy to show that the wavepacket mode is squeezed at some quadrature phase depending on the central frequency and central position of the wave packet.

The correlations between various output Unruh modes are summarized in Appendix \[appendixB\]. If we consider left-moving and narrow bandwidth Gaussian wave packet modes, using Eqs. 