Installation and first circuit

This page installs qsengine, builds a two-qubit circuit and reads its measurement counts. It takes a few minutes.

Install

qsengine needs CPython 3.9 or newer. One wheel per platform covers every version, so there is nothing to compile. NumPy is installed with it.

pip install qsengine

Check that it imports and shows its version:

import qsengine
print(qsengine.__version__)

Output

1.1.1

A GPU is optional. With one, large circuits run on it; without one, everything runs on the CPU. See CPU and GPU backends.

A circuit has a fixed number of qubits that all start in |0⟩. You add gates by calling methods on it. Nothing runs until you ask for a result.

import qsengine

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

counts = qc.run(shots=1000, seed=1)
print(counts)

Output

{'00': 513, '11': 487}

Only 00 and 11 appear: the two qubits always agree.

Line by line:

QuantumCircuit(2) makes a circuit on qubits 0 and 1.
qc.h(0) puts qubit 0 into an equal superposition of 0 and 1.
qc.cx(0, 1) flips qubit 1 whenever qubit 0 is 1, which ties the two together.
qc.run(shots=1000, seed=1) simulates the circuit once, measures it 1000 times and returns how often each result appeared.

The keys are bitstrings with qubit 0 on the right. seed makes the counts repeatable; leave it out for fresh randomness.