I have no idea how to read Dirac notation.

$$\left|↑↑ \right\rangle \tag{1}$$ $$\left|↑↓\right\rangle+\left|↓↑\right\rangle \tag{2}$$ $$\left|↑↓\right\rangle-\left|↓↑\right\rangle \tag{3}$$ $$\left|↓↓\right\rangle \tag{4}$$

What do the above expressions mean? Can you explain this in terms of the superposition?

Note: I am a sophomore in high school and have no linear algebra background. I am taking a science research course on quantum computing. Obviously, Dirac notation pops up a lot, but I don't know how to read it or tell what it is saying.

  • 1
    $\begingroup$ Dirac notation is a way of expressing linear algebra for quantum mechanics. I see it difficult for you to understand such notation with no linear algebra background. I recommend you to start studying the basis from where QM is constructed (linear algebra for example) before you try to study quantum computation. $\endgroup$ Feb 4, 2019 at 16:10
  • $\begingroup$ Related: How does bra-ket notation work? $\endgroup$ Feb 4, 2019 at 16:37

1 Answer 1


The dirac notation itself, the $|$ and $\rangle$ parts is simply a notation to remind you that you're dealing with quantum states. What you write inside this `ket' is completely arbitrary.

In the examples you give, somebody has (probably) chosen to depict a quantum system of two quantum bits. The up arrow and the down arrow depict two distinguishable states, i.e. we could measure each qubit and ask "is it up or down"? They have also made an implicit connection between an ordering of the labels and the qubits themselves.

So, the first example says that both qubits are "up". The last one says that they are both "down".

The middle two are very similar, so let's just talk about $\left|↑↓\right\rangle+\left|↓↑\right\rangle$. You could effectively read this as saying "if you ask which of the two qubits is up or down, you have a 50:50 chance of the first being up and the second down, or vice versa".

You asked for an example using superposition. Examples 2 and 3 that you gave are using a particular type of superposition called entanglement. Basically, whenever you have more than one ket, you've got superposition. When the superpositions span different physical entities (such as two qubits), that superposition is entanglement.

But we can talk about superposition of a single qubit. If we start from $\left|↑\right\rangle$, i.e. the qubit is "up", we can talk about any superposed state of the form $$ \cos\theta \left|↑\right\rangle+\sin\theta e^{i\phi}\left|↓\right\rangle. $$ If you asked whether it's up or down, you'd get answer "up" with probability $\cos^2\theta$, and "down" with probability $\sin^2\theta$.

The problem is if you only talk about the question "up or down?" you can't really see anything quantum happening. To do that, you have to perform measurements in other bases, and then you really start to need to do the maths properly.

  • $\begingroup$ Thank you so much for this helpful answer. What is the difference between the second and third notations? $\endgroup$
    – Logan
    Feb 4, 2019 at 17:09
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    $\begingroup$ It’s rather subtle, and requires going into a lot more maths. There’s no discernible difference if you only ask the up or down questions. $\endgroup$
    – DaftWullie
    Feb 4, 2019 at 17:54
  • $\begingroup$ Thank you very much. Your explanation was very helpful. $\endgroup$
    – Logan
    Feb 4, 2019 at 18:12
  • $\begingroup$ And, does measurement collapse the superposition AND the entanglement or just the superposition? $\endgroup$
    – a3y3
    Sep 10, 2020 at 23:32
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    $\begingroup$ @SohamDongargaonkar Frankly, entanglement is just superposition, except that it's superposition over multiple qubits. So, what can happen to superposition can also happen to entanglement. $\endgroup$
    – DaftWullie
    Sep 11, 2020 at 7:40

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