Measuring and Storing

[rasa97]

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]

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?

[rasa97]

Thanks for the clarification! One more thing, it appears that I can only create qubits which are initialised as qubit 0. How can I create qubits initialised to qubit 1 so that I can apply Hadamard gate on it?

[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.

[rasa97]

A question on the ordering of multiple qubits... why is it in the reverse direction than in the conventional way? I would think pq.Program().inst(I(0), X(1)) will produce |01>, but it produces |10>.

Also, how do we get the wavefunction of nth qubit in multiple qubits? Like after teleportation, how can we display the wavefunction of qubit[2] ?

[willzeng]

The zeroth qubit is the least significant digit. This makes it easier to convert to binary strings. It just a convention choice.

Do you mean to trace out the other qubits? We don't yet have a function trace out the others, but that would be useful!