cirq

# Cirq - Quantum Computing with Python

Cirq is Google Quantum AI's open-source framework for designing, simulating, and running quantum circuits on quantum computers and simulators.

## When to Use This Skill

Use this skill when:
- Building, simulating, or optimizing NISQ circuits in Python
- Running jobs on Google Quantum AI processors (via `cirq-google`) or partner backends (IonQ, Azure Quantum, AQT, Pasqal)
- Modeling noise, compiling to hardware gatesets, or designing characterization experiments
- Using parameter sweeps, transformers, or the ReCirq experiment patterns

For IBM hardware use **qiskit**; for quantum ML with autodiff use **pennylane**; for physics simulations use **qutip**.

## Installation

Requires Python 3.11+. Current stable release: **1.6.1** (August 2025). Vendor packages share the same version number.

```bash
uv pip install "cirq==1.6.1"
```

For hardware integration (pin matching versions for reproducibility):
```bash
# Google Quantum Engine (requires approved GCP project access)
uv pip install "cirq-google==1.6.1"

# IonQ
uv pip install "cirq-ionq==1.6.1"

# AQT (Alpine Quantum Technologies)
uv pip install "cirq-aqt==1.6.1"

# Pasqal
uv pip install "cirq-pasqal==1.6.1"

# Azure Quantum (IonQ, Honeywell/Quantinuum backends)
uv pip install "azure-quantum[cirq]"
```

For latest features during development, omit version pins; for production or hardware runs, pin all packages to the same Cirq release.

## Quick Start

### Basic Circuit

```python
import cirq
import numpy as np

# Create qubits
q0, q1 = cirq.LineQubit.range(2)

# Build circuit
circuit = cirq.Circuit(
    cirq.H(q0),              # Hadamard on q0
    cirq.CNOT(q0, q1),       # CNOT with q0 control, q1 target
    cirq.measure(q0, q1, key='result')
)

print(circuit)

# Simulate
simulator = cirq.Simulator()
result = simulator.run(circuit, repetitions=1000)

# Display results
print(result.histogram(key='result'))
```

### Parameterized Circuit

```python
import sympy

# Define symbolic parameter
theta = sympy.Symbol('theta')

# Create parameterized circuit
circuit = cirq.Circuit(
    cirq.ry(theta)(q0),
    cirq.measure(q0, key='m')
)

# Sweep over parameter values
sweep = cirq.Linspace('theta', start=0, stop=2*np.pi, length=20)
results = simulator.run_sweep(circuit, params=sweep, repetitions=1000)

# Process results
for params, result in zip(sweep, results):
    theta_val = params['theta']
    counts = result.histogram(key='m')
    print(f"θ={theta_val:.2f}: {counts}")
```

## Core Capabilities

### Circuit Building
For comprehensive information about building quantum circuits, including qubits, gates, operations, custom gates, and circuit patterns, see:
- **[references/building.md](references/building.md)** - Complete guide to circuit construction

Common topics:
- Qubit types (GridQubit, LineQubit, NamedQubit)
- Single and two-qubit gates
- Parameterized gates and operations
- Custom gate decomposition
- Circuit organization with moments
- Standard circuit patterns (Bell states, GHZ, QFT)
- Import/export (OpenQASM, JSON)
- Working with qudits and observables

### Simulation
For detailed information about simulating quantum circuits, including exact simulation, noisy simulation, parameter sweeps, and the Quantum Virtual Machine, see:
- **[references/simulation.md](references/simulation.md)** - Complete guide to quantum simulation

Common topics:
- Exact simulation (state vector, density matrix)
- Sampling and measurements
- Parameter sweeps (single and multiple parameters)
- Noisy simulation
- State histograms and visualization
- Quantum Virtual Machine (QVM)
- Expectation values and observables
- Performance optimization

### Circuit Transformation
For information about optimizing, compiling, and manipulating quantum circuits, see:
- **[references/transformation.md](references/transformation.md)** - Complete guide to circuit transformations

Common topics:
- Transformer framework
- Gate decomposition
- Circuit optimization (merge gates, eject Z gates, drop negligible operations)
- Circuit compilation for hardware
- Qubit routing and SWAP insertion
- Custom transformers
- Transformation pipelines

### Hardware Integration
For information about running circuits on real quantum hardware from various providers, see:
- **[references/hardware.md](references/hardware.md)** - Complete guide to hardware integration

Supported providers:
- **Google Quantum AI** (`cirq-google`) — Sycamore, Weber, Willow processors via Quantum Engine (restricted access; requires approved GCP project)
- **IonQ** (`cirq-ionq`) — trapped-ion QPUs and simulators
- **Azure Quantum** (`azure-quantum[cirq]`) — IonQ and Honeywell/Quantinuum backends
- **AQT** (`cirq-aqt`) — Alpine Quantum Technologies
- **Pasqal** (`cirq-pasqal`) — neutral-atom devices

Topics include device representation, qubit selection, authentication, job management, and circuit optimization for hardware. See [Access and authentication](https://quantumai.google/cirq/google/access) for Google Cloud setup.

### Noise Modeling
For information about modeling noise, noisy simulation, characterization, and error mitigation, see:
- **[references/noise.md](references/noise.md)** - Complete guide to noise modeling

Common topics:
- Noise channels (depolarizing, amplitude damping, phase damping)
- Noise models (constant, gate-specific, qubit-specific, thermal)
- Adding noise to circuits
- Readout noise
- Noise characterization (randomized benchmarking, XEB)
- Noise visualization (heatmaps)
- Error mitigation techniques

### Quantum Experiments
For information about designing experiments, parameter sweeps, data collection, and using the ReCirq framework, see:
- **[references/experiments.md](references/experiments.md)** - Complete guide to quantum experiments

Common topics:
- Experiment design patterns
- Parameter sweeps and data collection
- ReCirq framework structure
- Common algorithms (VQE, QAOA, QPE)
- Data analysis and visualization
- Statistical analysis and fidelity