Consider Bell state of two qubits A and B, +=21(0A1B+1A0B)=21(01+10), and its corresponding density matrix (+). We can think of 0 and 1 as spin- 1/2 states z+ and z, respectively. - Find density matrices A=trB((+)),B=trA((+)) - Suppose a measurement of z is performed on qubit B. Denote the probabilities of obtaining +1 and 1 as p+and p, respectively. What are p+and p? - If the result of z measurement on qubit B is +1, what is the two qubit state after the measurement? - Image we have a lot of pairs of qubits, each pair is prepared in state +. If observe B measure z of qubit B. However, he does not announce or communicate his results. Another observer A has access to qubit A but not qubit B. Without learning B's results, she would have to treat her qubits as an ensemble A=p+0A0A+p1A1A. Is there an experiment she can perform to tell whether B has made an measurement of