Programming a quantum computer

You have built qubits, gates, entanglement, and interference on paper. Now the payoff: this is how you actually run one. A quantum program is a circuit, you run it many times, and you read a histogram of outcomes. That is the entire workflow, and by the end of this lesson you will have driven it yourself.

The workflow in four steps

Programming today, whether on a simulator or real hardware, is always the same loop. There is no magic in the tooling once you have the physics picture you already built.

First, build a circuit: lay gates on wires, left to right. Second, add measurement gates at the end. Third, pick a backend - a simulator on your laptop or a real quantum processor over the cloud - and run shots, say a thousand repetitions. Fourth, collect the results into a histogram: the bar heights approximate the true probabilities $|a_i{|}^2$, sharpening as you add shots.^[1]

Drive it yourself

Here is a live console. Pick a challenge, or start blank and drag gates from the palette onto the wires (CNOT is two clicks: control, then target in the same column). Watch three things move together: the exact state vector, the Qiskit code that would produce it, and - when you press Run shots - the histogram. This is the whole loop in one place.

qc = QuantumCircuit(2)
qc.h(0)
qc.cx(0, 1)

Start with Bell state: an H then a CNOT builds $(|00>+|11>)/\sqrt{2}$. Run the shots and you get roughly half |00> and half |11>, almost never |01> or |10>: the two qubits are entangled, perfectly correlated, yet each one alone looks random. Then try Mini-Grover: a longer circuit that superposes both qubits, flips the phase of |11>, and inverts about the mean. One pass and the shots pile almost entirely onto |11> - interference has amplified the marked answer to near-certainty. The same gates you drew, now doing real work.

The same circuit, written in code

The console shows Qiskit code live as you build. Written by hand, the Bell program is only a few lines - and it makes the shots-and-histogram loop explicit:

from qiskit import QuantumCircuit
from qiskit_aer import AerSimulator

qc = QuantumCircuit(2, 2)
qc.h(0)              # put q0 into an equal superposition
qc.cx(0, 1)          # entangle: q1 follows q0
qc.measure([0, 1], [0, 1])

result = AerSimulator().run(qc, shots=1000).result()
print(result.get_counts())   # ~ {'00': 503, '11': 497} - a histogram, not one answer

Check yourself