Measuring and Storing

For the code:

p = pq.Program()
p.inst(X(0)).measure(0,1)
print qvm.run(p, [1,0,2])

I would think that the program takes 1 (X[0]), invert it, and print the result as [[0,0,0]]. But the result is [[1,0,0]]. How is this happening?

[willzeng]

In Quil all qubits start in the zero state. To change a qubit into the 1 state you can just apply an X gate. For example:

def init_qubits_in_one(qubit_list)
    p = Program()
    for q_index in qubit_list:
        p.inst(X(q_index))
    return p

Will give you a program where all qubits in qubit_list are now in the one state.

[willzeng]

Hi @rasa97 I can tell you a little more about what is happening. This program:

p = pq.Program()
p.inst(X(0)).measure(0,1)

Applies an X gate to the zeroth qubit, placing that qubit in the 1 state. It then measures the state of the zeroth qubit and deposits that bit result in the first classical register. Since the classical registers are initialized in zero, the state of the classical memory at the end of the computation is: [0,1,0] where the first classical register contains the result of the measurement. You’ll get this outcome if you run:

print qvm.run(p, range(3))

to look at the first three bits of classical memory.

When you run:

print qvm.run(p, [1,0,2])

You are asking for the classical memory to be reordered so in the returned list, so that you get the contents of the first bit, the zeroth bit, and the second bit in that order. Thus instead of [0, 1, 0] you get [1, 0, 0] since the classical bit registers have been swapped.

Does that help?