Quantum Gates

Cirq ships the same core gate set you'd expect from any quantum SDK, applied through the operation pattern from lesson 1: gate(qubit).

Single-qubit gates

import cirq

q = cirq.LineQubit(0)
circuit = cirq.Circuit()
circuit.append(cirq.X(q))  # NOT: flips |0> <-> |1>
circuit.append(cirq.Z(q))  # flips the sign of the |1> amplitude
circuit.append(cirq.H(q))  # Hadamard: creates/undoes superposition

print(circuit)

Exactly the same roles as Qiskit's x, z, and h, X for a classical-style flip, H for superposition, Z mostly useful combined with other gates.

A two-qubit gate: CNOT

q0, q1 = cirq.LineQubit.range(2)

circuit = cirq.Circuit()
circuit.append(cirq.H(q0))          # put q0 into superposition
circuit.append(cirq.X(q1))          # flip q1 to |1>
circuit.append(cirq.CNOT(q0, q1))   # flip q1 IF q0 measures as |1>

print(circuit)

cirq.LineQubit.range(2) is a convenience for creating multiple qubits at once, equivalent to [cirq.LineQubit(0), cirq.LineQubit(1)]. cirq.CNOT(control, target) behaves identically to Qiskit's cx, it's the gate that lets one qubit's state influence another, the mechanism behind entanglement.

Chaining operations

Appending accepts a single operation or a list, both build up the circuit the same way:

q0, q1, q2 = cirq.LineQubit.range(3)

circuit = cirq.Circuit([
    cirq.H(q0),
    cirq.CNOT(q0, q1),
    cirq.CNOT(q1, q2),
    cirq.X(q2),
])

print(circuit)

Passing a list directly to cirq.Circuit([...]) is a common shorthand once you know every operation you want up front, rather than calling .append() repeatedly.