Hi @bzzhang, and welcome to the Xanadu Discussion Forum!
Unfortunately, there’s no way to initialize a mixed state directly using a density matrix; you would have to create that state yourself using gates.
If you wish to simulate using mixed states you would have to use a mixed state simulator, e.g. default.mixed
or cirq.mixedsimulator
.
Let us know if you have any other questions or issues with this!