Simulation in Cirq

This guide covers quantum circuit simulation, including exact and noisy simulations, parameter sweeps, and the Quantum Virtual Machine (QVM).

Exact Simulation

Basic Simulation

python

import cirq
import numpy as np

# Create circuit
q0, q1 = cirq.LineQubit.range(2)
circuit = cirq.Circuit(
    cirq.H(q0),
    cirq.CNOT(q0, q1),
    cirq.measure(q0, q1, key='result')
)

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

# Get measurement results
print(result.histogram(key='result'))

State Vector Simulation

python

# Simulate without measurement to get final state
simulator = cirq.Simulator()
result = simulator.simulate(circuit_without_measurement)

# Access state vector
state_vector = result.final_state_vector
print(f"State vector: {state_vector}")

# Get amplitudes
print(f"Amplitude of |00⟩: {state_vector[0]}")
print(f"Amplitude of |11⟩: {state_vector[3]}")

Density Matrix Simulation

python

# Use density matrix simulator for mixed states
simulator = cirq.DensityMatrixSimulator()
result = simulator.simulate(circuit)

# Access density matrix
density_matrix = result.final_density_matrix
print(f"Density matrix shape: {density_matrix.shape}")

Step-by-Step Simulation

python

# Simulate moment-by-moment
simulator = cirq.Simulator()
for step in simulator.simulate_moment_steps(circuit):
    print(f"State after moment {step.moment}: {step.state_vector()}")

Sampling and Measurements

Run Multiple Shots

python

# Run circuit multiple times
result = simulator.run(circuit, repetitions=10000)

# Access measurement counts
counts = result.histogram(key='result')
print(f"Measurement counts: {counts}")

# Get raw measurements
measurements = result.measurements['result']
print(f"Shape: {measurements.shape}")  # (repetitions, num_qubits)

Expectation Values

python

# Measure observable expectation value
from cirq import PauliString

observable = PauliString({q0: cirq.Z, q1: cirq.Z})
result = simulator.simulate_expectation_values(
    circuit,
    observables=[observable]
)
print(f"⟨ZZ⟩ = {result[0]}")

Parameter Sweeps

Sweep Over Parameters

python

import sympy

# Create parameterized circuit
theta = sympy.Symbol('theta')
q = cirq.LineQubit(0)
circuit = cirq.Circuit(
    cirq.ry(theta)(q),
    cirq.measure(q, key='m')
)

# Define parameter sweep
sweep = cirq.Linspace(key='theta', start=0, stop=2*np.pi, length=50)

# Run sweep
simulator = cirq.Simulator()
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}")

Multiple Parameters

python

# Sweep over multiple parameters
theta = sympy.Symbol('theta')
phi = sympy.Symbol('phi')

circuit = cirq.Circuit(
    cirq.ry(theta)(q0),
    cirq.rz(phi)(q1)
)

# Product sweep (all combinations)
sweep = cirq.Product(
    cirq.Linspace('theta', 0, np.pi, 10),
    cirq.Linspace('phi', 0, 2*np.pi, 10)
)

results = simulator.run_sweep(circuit, params=sweep, repetitions=100)

Zip Sweep (Paired Parameters)

python

# Sweep parameters together
sweep = cirq.Zip(
    cirq.Linspace('theta', 0, np.pi, 20),
    cirq.Linspace('phi', 0, 2*np.pi, 20)
)

results = simulator.run_sweep(circuit, params=sweep, repetitions=100)

Noisy Simulation

Adding Noise Channels

python

# Create noisy circuit
noisy_circuit = circuit.with_noise(cirq.depolarize(p=0.01))

# Simulate noisy circuit
simulator = cirq.DensityMatrixSimulator()
result = simulator.run(noisy_circuit, repetitions=1000)

Custom Noise Models

python

# Apply different noise to different gates
noise_model = cirq.NoiseModel.from_noise_model_like(
    cirq.ConstantQubitNoiseModel(cirq.depolarize(0.01))
)

# Simulate with noise model
result = cirq.DensityMatrixSimulator(noise=noise_model).run(
    circuit, repetitions=1000
)

See noise.md for comprehensive noise modeling details.

State Histograms

Visualize Results

python

import matplotlib.pyplot as plt

# Get histogram
result = simulator.run(circuit, repetitions=1000)
counts = result.histogram(key='result')

# Plot
plt.bar(counts.keys(), counts.values())
plt.xlabel('State')
plt.ylabel('Counts')
plt.title('Measurement Results')
plt.show()

State Probability Distribution

python

# Get state vector
result = simulator.simulate(circuit_without_measurement)
state_vector = result.final_state_vector

# Compute probabilities
probabilities = np.abs(state_vector) ** 2

# Plot
plt.bar(range(len(probabilities)), probabilities)
plt.xlabel('Basis State Index')
plt.ylabel('Probability')
plt.show()

Quantum Virtual Machine (QVM)

QVM simulates realistic quantum hardware with device-specific constraints and noise.

Using Virtual Devices

python

# Use a virtual Google device
import cirq_google

# Get virtual device
device = cirq_google.Sycamore

# Create circuit on device
qubits = device.metadata.qubit_set
circuit = cirq.Circuit(device=device)

# Add operations respecting device constraints
circuit.append(cirq.CZ(qubits[0], qubits[1]))

# Validate circuit against device
device.validate_circuit(circuit)

Noisy Virtual Hardware

python

# Simulate with device noise
processor = cirq_google.get_engine().get_processor('weber')
noise_props = processor.get_device_specification()

# Create realistic noisy simulator
noisy_sim = cirq.DensityMatrixSimulator(
    noise=cirq_google.NoiseModelFromGoogleNoiseProperties(noise_props)
)

result = noisy_sim.run(circuit, repetitions=1000)

Advanced Simulation Techniques

Custom Initial State

python

# Start from custom state
initial_state = np.array([1, 0, 0, 1]) / np.sqrt(2)  # |00⟩ + |11⟩

simulator = cirq.Simulator()
result = simulator.simulate(circuit, initial_state=initial_state)

Partial Trace

python

# Trace out subsystems
result = simulator.simulate(circuit)
full_state = result.final_state_vector

# Compute reduced density matrix for first qubit
from cirq import partial_trace
reduced_dm = partial_trace(result.final_density_matrix, keep_indices=[0])

Intermediate State Access

python

# Get state at specific moment
simulator = cirq.Simulator()
for i, step in enumerate(simulator.simulate_moment_steps(circuit)):
    if i == 5:  # After 5th moment
        state = step.state_vector()
        print(f"State after moment 5: {state}")
        break

Simulation Performance

Optimizing Large Simulations

Use state vector for pure states: Faster than density matrix
Avoid density matrix when possible: Exponentially more expensive
Batch parameter sweeps: More efficient than individual runs
Use appropriate repetitions: Balance accuracy vs computation time

python

# Efficient: Single sweep
results = simulator.run_sweep(circuit, params=sweep, repetitions=100)

# Inefficient: Multiple individual runs
results = [simulator.run(circuit, param_resolver=p, repetitions=100)
           for p in sweep]

Memory Considerations

python

# For large systems, monitor state vector size
n_qubits = 20
state_size = 2**n_qubits * 16  # bytes (complex128)
print(f"State vector size: {state_size / 1e9:.2f} GB")

Stabilizer Simulation

For circuits with only Clifford gates, use efficient stabilizer simulation:

python

# Clifford circuit (H, S, CNOT)
circuit = cirq.Circuit(
    cirq.H(q0),
    cirq.S(q1),
    cirq.CNOT(q0, q1)
)

# Use stabilizer simulator (exponentially faster)
simulator = cirq.CliffordSimulator()
result = simulator.run(circuit, repetitions=1000)

Best Practices

Choose appropriate simulator: Use Simulator for pure states, DensityMatrixSimulator for mixed states
Use parameter sweeps: More efficient than running individual circuits
Validate circuits: Check circuit validity before long simulations
Monitor resource usage: Track memory for large-scale simulations
Use stabilizer simulation: When circuits contain only Clifford gates
Save intermediate results: For long parameter sweeps or optimization runs