Your First Circuit
This tutorial builds a Bell state, runs it, and reads the result. It is the same program shown on the home page, explained line by line.
"""Bell state through the Qiskit BackendV2 adapter."""
from qiskit import QuantumCircuit
from menos.qiskit import MENOSBackend
qc = QuantumCircuit(2, 2)
qc.h(0)
qc.cx(0, 1)
qc.measure([0, 1], [0, 1])
backend = MENOSBackend()
result = backend.run(qc, shots=1000, seed_simulator=7).result()
counts = result.get_counts()
print(counts)
# A Bell pair only ever yields 00 or 11, and every shot is accounted for.
assert set(counts) <= {"00", "11"}
assert sum(counts.values()) == 1000What each step does
Build the circuit. QuantumCircuit(2, 2) has two qubits and two classical bits. h(0) puts qubit 0 in superposition, cx(0, 1) entangles it with qubit 1, and measure copies the qubits into the classical bits. Measurements must come last: MENOS does not support operations after a measurement of the same qubit.
Create the backend. MENOSBackend() needs no arguments. It is an ordinary Qiskit BackendV2.
Run. backend.run(qc, shots=1000, seed_simulator=7) executes the circuit. It runs synchronously: the work is finished when run returns, and .result() just hands back the answer.
Read the counts. get_counts() returns a Qiskit Counts object, a dictionary from bitstring to number of shots. Keys are written with the highest classical bit on the left, as everywhere in Qiskit.
Why a seed?
seed_simulator=7 makes the run reproducible: the same circuit, shots and seed give the same counts every time. Leave it out and MENOS draws a fresh seed. See Determinism.
What if I want the exact state, not samples?
That is the next tutorial: Your First Simulation.
Common first mistakes
| You wrote | Result |
|---|---|
backend.run(qc) with no measurements | InvalidCircuitError: shots were requested but the circuit has no Measure |
backend.run(qc, shots=0) | InvalidCircuitError: nothing was requested to produce |
backend.run([qc1, qc2]) | ValueError: one circuit per call |
A gate after a measure on that qubit | UnsupportedFeatureError: non-terminal measurement |
The messages are quoted in Common Errors.