Everybody knows how to derive what are the most general operations one can apply to a quantum state. You just need to assume that a quantum operation

- Is linear.
- Maps quantum states to quantum states.
- Still maps quantum states to quantum states when applied to a part of a quantum system.

And you can prove that such quantum operations are the well-known completely positive and trace preserving maps, which can be conveniently represented using the Kraus operators or the Choi-Jamiołkowski isomorphism.

But what if one does not want general quantum operations, but wants to single out *pure* quantum operations? Can one have such an axiomatic description, a derivation from intuitive1 assumptions?

Well, the usual argument one sees in textbooks to show that the evolution of quantum states must be given by a unitary assumes that the evolution

- Is linear.
- Maps pure quantum states to pure quantum states.

From this, you get that a quantum state $\ket{\psi}$ is mapped to a quantum state $U\ket\psi$ for a linear operator $U$, and furthermore since by definition quantum states have 2-norm equal to 1, we need the inner product $\bra\psi U^\dagger U \ket\psi$ to be 1 for all $\ket\psi$, which implies that $U$ must be a unitary matrix.

The only problem with this argument is that it is false, as the map

\[ \mathcal E(\rho) = \ket\psi\bra\psi \operatorname{tr} \rho, \]which simply discards the input $\rho$ and prepares the fixed state $\ket\psi$ instead is linear, maps pure states to pure states, and is not unitary. The textbooks are fine, as they usually go through this argument before density matrices are introduced, and either implicitly or explicitly state that the evolution takes state vectors to state vectors. But this is not good enough for us, as this restriction to state vectors is both unjustified, and does not satisfy our requirement of being an “intuitive assumption”.

Luckily, the fix is easy: we just need to add the analogue of the third assumption used in the derivation of general quantum operations. If we assume that a pure quantum operation

- Is linear.
- Maps pure quantum states to pure quantum states.
- Still maps pure quantum states to pure quantum states when applied to a part of a quantum system.

then we can prove that pure quantum operations are just unitaries2. Since the proof is simple, I’m going to show it in full.

Let $\mathcal F$ be the pure quantum operation we are interested in. If we apply it to the second subsystem of a maximally entangled state, $\ket{\phi^+} = \frac1{\sqrt d}\sum_{i=1}^d \ket{ii}$, by assumption 3 the result will be a pure state, which we call $\ket{\varphi}$. In symbols, we have

\[ \mathcal I \otimes \mathcal F (\ket{\phi^+}\bra{\phi^+}) = \ket{\varphi}\bra{\varphi}, \]where $\mathcal I$ represents doing nothing to the first subsystem. Now the beautiful thing about the maximally entangled state is that if $\mathcal F$ is a linear map then $\mathcal I \otimes \mathcal F (\ket{\phi^+}\bra{\phi^+})$ contains all the information about $\mathcal F$. In fact, if we know $\mathcal I \otimes \mathcal F (\ket{\phi^+}\bra{\phi^+})$ we can know how $\mathcal F$ acts on any matrix $\rho$ via the identity

\[ \mathcal F (\rho) = \operatorname{tr}_\text{in} [(\rho^T \otimes \mathbb I) \mathcal I \otimes \mathcal F (\ket{\phi^+}\bra{\phi^+})]. \]

This is the famous Choi-Jamiołkowski isomorphism3. Now let’s use the fact that the result $\ket{\varphi}\bra{\varphi}$ is a pure state. If we write it down in the computational basis

\[\ket\varphi = \sum_{i,j=1}^d \varphi_{ij} \ket{i j}, \]we see that if we define a matrix $\Phi$ with elements $\Phi_{ij} = \varphi_{ji} \sqrt d$ then $\ket\varphi = \mathbb I \otimes \Phi \ket{\phi^+}$4, so

\[ \mathcal I \otimes \mathcal F (\ket{\phi^+}\bra{\phi^+}) = (\mathbb I \otimes \Phi) \ket{\phi^+}\bra{\phi^+} (\mathbb I \otimes \Phi^\dagger).\]

Using the identity above we have that

\[ \mathcal F(\rho) = \Phi \rho \Phi^\dagger, \]and since $\operatorname{tr}(\mathcal F(\rho)) = 1$ for every $\rho$ we have that $\Phi^\dagger\Phi = \mathbb I$, so $\Phi$ is an isometry. If in addition we demand that $\mathcal F(\rho)$ has the same dimension as $\rho$, then $\Phi$ must be a square matrix, and therefore has a right inverse which is equal to its left inverse, so $\Phi$ is a unitary.

This result is so amazing, so difficult, and so ground-breaking that the referees allowed me to include it as a footnote in my most recent paper without bothering to ask for a proof or a reference. But joking aside, I’d be curious to know if somebody already wrote this down, as a quick search through the textbooks revealed me nothing.

But how about Wigner’s theorem, I hear you screaming. Well, Wigner was not concerned with deriving what were the quantum operations, but what were the symmetry transformations one could apply to quantum states. Because of this he did not assume linearity, which was not relevant to him (and in fact would make his theorem wrong, as one can have perfectly good anti-linear symmetries, such as time reversal). Also, he assumed that symmetry transformations preserve inner products, which is too technical for my purposes.