# Can there be an interference term in a two-state quantum system?

Say I have a two-state system $$|\psi \rangle=c_1|0\rangle+c_2|1\rangle$$. If the states $$|0\rangle$$ and $$|1\rangle$$ are orthogonal, then the probability is:

\begin{align} \langle \psi |\psi \rangle&=(\langle 0|c_1^* + \langle 1|c_2^* )(c_1|0\rangle+c_2|1\rangle)\\ &=c_1^*c_1\langle 0|0\rangle+c_2c_2^*\langle 1|1\rangle + c_1^*c_2\langle 0|1\rangle+c_1c_2^*\langle 1|0\rangle\\ &=c_1^*c_1+c_2c_2^*=1 \end{align}

Unlike the double split experiment, there are no interference terms:

$$P=|\psi_1|^2+|\psi_2|^2+2|\psi_1||\psi_2|\cos(\theta_1-\theta_2)$$

Can a two-state system produce a similar $$\cos$$-based interference term?

• Look at what happens if you calculate $\langle \psi | \varphi\rangle$ where the 2 states are not the same Jul 16 '20 at 20:16
• @BySymmetry So $|\psi \rangle=c_1|0\rangle+c_2|1\rangle$ and $\phi\rangle=a_1 |0\rangle+a_2 |1\rangle$. Then $\langle \psi | \phi \rangle = c_1^*a_1 + c_2^*a_2$. That is equal to $1$, right? Where is the cos interference term? Jul 16 '20 at 20:51
• Not is it not $=1$ in general. Take $c_1=1/\sqrt{3}, c_2=\sqrt{\frac{2}{3}}$ but $a_1=a_2=\frac{1}{\sqrt{2}}$. Jul 16 '20 at 21:00
• @BySymmetry Is this what you have in mind: $|\psi \rangle =c_1|0\rangle +c_2|1\rangle$ and $|𝜙\rangle =𝑎_1|0\rangle+𝑎_2|1\rangle$? If so, then $\langle 𝜓|𝜙\rangle=𝑐_1^*𝑎_1+𝑐_2^*𝑎_2$, which contains no interference terms. Jul 16 '20 at 21:53
• Your previous statement $\uparrow$ is incorrect, as demonstrated by a full calculation. In fact this is in general a complex number. Jul 16 '20 at 21:57

Take \begin{align} \vert\psi_1\rangle&=e^{-\frac{i \gamma }{2}} \cos \left(\frac{\beta }{2}\right) \vert 0\rangle+ e^{-\frac{i \gamma }{2}} \sin \left(\frac{\beta }{2}\right)\vert 1\rangle\, ,\\ \vert\psi_2\rangle&= e^{\frac{i \gamma }{2}} \sin \left(\frac{\beta }{2}\right)\vert 0\rangle + e^{\frac{i \gamma }{2}} \cos \left(\frac{\beta }{2}\right)\vert 1\rangle \end{align} for which $$\langle \psi_1\vert \psi_2\rangle=e^{-i\gamma}\sin\beta$$.
• Can you explain why $\langle 1 | 0 \rangle$ is not orthogonal? Jul 16 '20 at 20:52
• So the key is to use 2 parameters $\beta$ and $\gamma$ in both wavefunctions , instead of 4 unique parameters? Jul 16 '20 at 21:36
• there are still 4 coefficients. you could write $c_1$ etc as you did before. Your error is in assuming things like $c_1^*a_1+ c_2^* a_2=1$, which is not true in general. If you want take the angle $\beta$ in $\vert \psi_2\rangle$ to be different from the one in $\vert\psi_1\rangle$ to get an even more general overlap. Jul 16 '20 at 21:54
• I think I might have it, but it appears that the interference term of P is the interference of a mixture, not a pure state. If I have $|\psi_1 \rangle = z_1 e^{i\theta_1} | 0\rangle$ and $|\psi_2 \rangle = z_2 e^{i\theta_2} |0\rangle$, then $\langle \psi_1+\psi_2 | \psi_1+\psi_2 \rangle=|z_1|^2+ |z_2|^2 + 2 |z_1||z_2|\cos (\theta_2-\theta_1)$ Jul 17 '20 at 0:06