Reversible operations
You’re composing a letter to the love of your life. You accidentally delete an entire paragraph. So, what do you do? Do you try to recreate the paragraph word by word? No. You use the document editor’s Undo feature to go back to the time when the paragraph appeared.
Some things in life can be undone, whereas others can’t. If you shoot a missile into the air, you can’t get it back onto the launch pad unscathed. If the love of your life reads your letter, you can’t un-write what you wrote. If someone sends bits through a classical AND gate and you examine the gate’s output, you can’t always tell what the input bits were. Figure 3.16 illustrates this point.
Figure 3.16 – Are the inputs 00, 01, or 10?
What about this chapter’s quantum gates? Can they be undone? Pick any starting point on the Bloch sphere and then apply the X gate. When you apply the X gate, you rotate the...
