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Cirq

What is it​

Cirq is an open-source quantum programming framework maintained by Google Quantum AI. It is written in Python and is designed for the NISQ (Noisy Intermediate-Scale Quantum) era, giving you explicit control over qubits, gates, moments, and timing. Where some frameworks hide hardware details, Cirq deliberately exposes them, which makes it popular for research on near-term algorithms and on Google's own processors.

A defining idea in Cirq is the Moment: a circuit is a sequence of moments, and each moment is a set of operations that act simultaneously. This makes circuit timing and parallelism explicit.

Install​

pip install cirq

This includes Cirq's built-in simulators. Vendor integrations (for example IonQ or Azure) are available as separate cirq-* packages.

Hello Quantum​

A Bell state 12(∣00⟩+∣11⟩)\frac{1}{\sqrt{2}}\left( \lvert 00 \rangle + \lvert 11 \rangle \right) on Cirq's local Simulator:

import cirq

# Allocate two named qubits
q0, q1 = cirq.LineQubit.range(2)

circuit = cirq.Circuit(
    cirq.H(q0),            # superposition
    cirq.CNOT(q0, q1),     # entangle
    cirq.measure(q0, q1, key="result"),
)

print(circuit)

# Simulate
simulator = cirq.Simulator()
result = simulator.run(circuit, repetitions=1024)
print(result.histogram(key="result"))
# e.g. Counter({0: 507, 3: 517})  -> 0b00 and 0b11

The histogram keys 0 (00) and 3 (11) dominate, confirming entanglement.

Strengths & use cases​

  • Explicit control. Direct manipulation of qubits, gates, and moments — ideal for hardware-aware NISQ research.
  • Google hardware. The native path for Google Quantum AI processors and Google-led research code.
  • Precise circuit timing. The moment model makes parallelism and scheduling first-class.
  • Foundation for higher-level tools. Cirq is the circuit engine under TensorFlow Quantum and OpenFermion.

Ecosystem​

  • qsim / qsimcirq — high-performance state-vector simulator for larger circuits.
  • OpenFermion — quantum chemistry and fermionic simulation built on Cirq.
  • TensorFlow Quantum — hybrid quantum-classical ML layered on Cirq (see its page).
  • cirq-ionq, cirq-aqt, cirq-pasqal, cirq-rigetti — third-party hardware integrations.

Resources​

For hands-on practice, try the labs; to plan your learning, see the roadmaps.