I have a mixed state with a density matrices corresponding to $\rho=\sum_0^{2^n-1}p_i|i\rangle\langle i|$. How would i represent this in Q#? How would I go about applying Unitary Operations $U$ on this mixed state in Q#?
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It can be helpful to step back and look at what a density matrix describes: a probability distribution over projectors onto pure states. In your example, for instance, $\rho$ represents a distribution in which $|\psi\rangle = |i\rangle$ with probability $p_i$. That is, a density operator describes a probability over an ensemble of state vectors.
From that perspective, we easily write pseudocode for a quantum program that prepares $\rho = \sum_i p_i |\psi_i\rangle\langle\psi_i|$:
- Draw a random $i$ with probability $p_i$.
- Prepare the pure state $|\psi_i\rangle$.
Thinking of state vectors like $|\psi_i\rangle$ as quantum programs that prepare those states, this is already a complete description of how to prepare $\rho$. You can, for instance, use the PrepareArbitraryState operation to prepare each $|\psi_i\rangle$ on a freshly allocated register of qubits.
In practice, however, you'll often have a much more efficient implementation of the state preparation programs for each $|\psi_i\rangle$. For example, there's no need to use PrepareArbitraryState operation to prepare $|+\rangle$, as the H operation does just fine.
This is, for instance, how the PrepareSingleQubitIdentity operation prepares the particular mixed state $\rho = 𝟙 / 2$ on a single qubit.
