I don’t think we have mid-circuit measurement or density matrix representations implemented in PennyLane yet.
That being said, we can still answer the question manually. For a simple two qubit system we could do something like:
dev = qml.device('default.qubit',wires=2)
bitstrings = [format(i,'02b') for i in range(4)]
for index,bitstring in enumerate(bitstrings):
probsgiven0 /= np.sum(probsgiven0)
>>> [0.01 0.033 0.249 0.708]
>>> ['00', '01', '10', '11']
>>> [0.23255814 0.76744186]
Here we are just taking the probabilities of over all of the qubits and manually calculating the probability that the second qubit is in state 0 or 1 given that the first qubit was measured as 0.
This code goes through the trouble of using bit strings to give intuition about which qubits we are working with. But we could skip the bit strings and just be careful with our array indexing.
Let me know if this helps or if there are further questions!