If I want to run a circuit on a real quantum computer, I have to measure it before running it. For example:


To run on a quantum computer:

#Import IBMQ
from qiskit import IBMQ #different companies(IBM ic)'s quantum provider
from qiskit.providers.ibmq import least_busy

#ask for the least busy quantum computer
provider = IBMQ.get_provider(hub='ibm-q')
backend = least_busy(provider.backends(filters=lambda x: x.configuration().n_qubits >= 2 
                                       and not x.configuration().simulator #dont want a simulator, want a actual machine
                                       and x.status().operational==True)) #want an operational machine
#tell us what the least busy one is
print("least busy backend: ", backend)

# send the job to a quantum computer
job = execute(qc, backend=backend, shots=100)
result = job.result()

#then we plot our histogram as usual
counts = result.get_counts(qc)

I am wondering why do we have to measure the circuit before sending it to the real quantum computer? I know that we shouldn't measure the circuit if we send it to a statevector simulator.

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The simple answer is that you can't get the statevector information out of a real quantum computer; or, more generally, a quantum system. For the real computer to extract information from the qubits, they have to collapse to some basis state $\left(|0\rangle \; \text{or} \; |1\rangle \right)$. And this collapse is performed by measuring the qubits.


Note that you do not measure anything before sending to QC. What you actually send to QC is a sequence of instructions telling QC what to do. The measurement is the last instruction necessary for obtaining results as pointed out in the other answer.


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