allow us to transport the concept of entanglement from the current problem of physics to quantum bound states in the Lorentz-covariant world. The time separation between the constituent particles is not observable, and is not known in the present form of quantum mechanics. However, this variable gives its effect in the real world by entangling itself with the longitudinal variable.

Two-dimensional Harmonic Oscillators {#2dim}
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The Gaussian form $$\label{wf01}
  \left[\frac{1}{\sqrt{\pi}}\right]^{1/4}\exp{\left(-\frac{x^{2}}{2}\right)}$$ is used for many branches of science. For instance, we can construct this function by throwing dice.

In physics, this is the wave function for the one-dimensional harmonic oscillator in the ground state. This function is also used for the vacuum state in quantum field theory, as well as the zero-photon state in quantum optics. For excited oscillator states, the wave function takes the form $$\label{wf02}
     \chi_{n}(x) = \left[\frac{1}{\sqrt{\pi}2^n n!}\right]^{1/2}
              H_{n}(x) \exp{\left(\frac{-x^{2}}{2}\right)} ,$$ where $H_{n}(x)$ is the Hermite polynomial of the $n^{th}$ degree. The properties of this wave function are well known, and it becomes the Gaussian form of Eq.(\[wf01\]) when $n = 0$.

We can now consider the two-dimensional space with the orthogonal coordinate variables $x$ and $y$, and the same wave function with the $y$ variable: $$\label{wf05}
     \chi_{m}(y) 