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What is the general form of a classical-quantum state?

Quantum Computing Asked by Polya on March 23, 2021

In the literature, one comes across the following situation: Alice holds two registers $X$ and $A$ and it is given that $X$ is a classical register.

What is the most general way to write down Alice’s state? Is it just $sigma_{XA} = sum_i p_i vert iranglelangle ivert_X otimesrho^i_A$, with each $rho^i_A$ being a quantum state (positive semi-definite and trace one matrix)?

Sorry that this is a yes/no question because if yes, then there is not much to add. But if not, what would be the most general way to write Alice’s state?

One Answer

Your description has X as a mixed state (a quantum state with classical uncertainty) and not a classical state. For example you can apply quantum gates to X but that shouldn’t be allowed if X was a classical state. However we can think of that mixed state as a classical state (see comments) and even use it as such.

I’m not sure if there is some notation for writing classical states rather than mixed quantum states.

Answered by user3717194 on March 23, 2021

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