Since a quantum information lecture today I have been wondering what does it rea
ID: 1380814 • Letter: S
Question
Since a quantum information lecture today I have been wondering what does it really mean for a state to be in superposition? Is this something that is answerable?
This is what we learnt (or what I gathered :) )
A classical bit is always in a state 0 or 1. Sometimes there exists a degree of uncertainty and so probabilities are assigned to either state but in reality it still is 0 or 1 right. However a qubit can be in a state 0, 1 or a superposition whereby this superposition is fundamentally different from the probability mixture for a classical bit. But how can this be so? Surely at any given time a system can in reality either be in the 0 or 1? Does it have something to do with the interference properties of the system?
If you could answer my questions and explain the fundamental differences between the qubit and the classical bit I would really appreciate it. Thanks!
Explanation / Answer
Surely at any given time a system can in reality either be in the 0 or 1?
Actually, no, that is not true. A quantum system can be in a state which is neither ?0? nor ?1?; this is not possible with a classical bit. However, this state can be mathematically described as a linear combination of ?0? and ?1?.
Consider the analogy of a traditional magnetic compass.
A traditional compass
When the compass needle points north, that is like a qubit being in the state ?0?, and when the compass needle points east, that is like a qubit being in the state ?1?. But a compass needle can also point northeast. The direction northeast is neither north nor east, but it is a superposition of equal parts north and east: if you add a north-pointing vector and an east-pointing vector of equal magnitude, you will get a vector that points northeast. Similarly, the qubit state 12?(?0?+?1?) is neither ?0? nor ?1?, but it is a superposition of equal parts ?0? and ?1?.