qutip

SKILL.md

QuTiP: Quantum Toolbox in Python

Overview

QuTiP provides comprehensive tools for simulating and analyzing quantum mechanical systems. It handles both closed (unitary) and open (dissipative) quantum systems with multiple solvers optimized for different scenarios.

Installation

uv pip install qutip

Optional packages for additional functionality:

# Quantum information processing (circuits, gates)
uv pip install qutip-qip

Quantum trajectory viewer

uv pip install qutip-qtrl

## Quick Start

```python
from qutip import *
import numpy as np
import matplotlib.pyplot as plt

# Create quantum state
psi = basis(2, 0)  # |0⟩ state

# Create operator
H = sigmaz()  # Hamiltonian

# Time evolution
tlist = np.linspace(0, 10, 100)
result = sesolve(H, psi, tlist, e_ops=[sigmaz()])

# Plot results
plt.plot(tlist, result.expect[0])
plt.xlabel('Time')
plt.ylabel('⟨σz⟩')
plt.show()

Core Capabilities

1. Quantum Objects and States

Create and manipulate quantum states and operators:

# States
psi = basis(N, n)  # Fock state |n⟩
psi = coherent(N, alpha)  # Coherent state |α⟩
rho = thermal_dm(N, n_avg)  # Thermal density matrix

# Operators
a = destroy(N)  # Annihilation operator
H = num(N)  # Number operator
sx, sy, sz = sigmax(), sigmay(), sigmaz()  # Pauli matrices

# Composite systems
psi_AB = tensor(psi_A, psi_B)  # Tensor product

See references/core_concepts.md for comprehensive coverage of quantum objects, states, operators, and tensor products.

2. Time Evolution and Dynamics

Multiple solvers for different scenarios:

# Closed systems (unitary evolution)
result = sesolve(H, psi0, tlist, e_ops=[num(N)])

# Open systems (dissipation)
c_ops = [np.sqrt(0.1) * destroy(N)]  # Collapse operators
result = mesolve(H, psi0, tlist, c_ops, e_ops=[num(N)])

# Quantum trajectories (Monte Carlo)
result = mcsolve(H, psi0, tlist, c_ops, ntraj=500, e_ops=[num(N)])

Solver selection guide:

sesolve: Pure states, unitary evolution

mesolve: Mixed states, dissipation, general open systems

mcsolve: Quantum jumps, photon counting, individual trajectories

brmesolve: Weak system-bath coupling

fmmesolve: Time-periodic Hamiltonians (Floquet)

See references/time_evolution.md for detailed solver documentation, time-dependent Hamiltonians, and advanced options.

3. Analysis and Measurement

Compute physical quantities:

# Expectation values
n_avg = expect(num(N), psi)

# Entropy measures
S = entropy_vn(rho)  # Von Neumann entropy
C = concurrence(rho)  # Entanglement (two qubits)

# Fidelity and distance
F = fidelity(psi1, psi2)
D = tracedist(rho1, rho2)

# Correlation functions
corr = correlation_2op_1t(H, rho0, taulist, c_ops, A, B)
w, S = spectrum_correlation_fft(taulist, corr)

# Steady states
rho_ss = steadystate(H, c_ops)

See references/analysis.md for entropy, fidelity, measurements, correlation functions, and steady state calculations.

4. Visualization

Visualize quantum states and dynamics:

# Bloch sphere
b = Bloch()
b.add_states(psi)
b.show()

# Wigner function (phase space)
xvec = np.linspace(-5, 5, 200)
W = wigner(psi, xvec, xvec)
plt.contourf(xvec, xvec, W, 100, cmap='RdBu')

# Fock distribution
plot_fock_distribution(psi)

# Matrix visualization
hinton(rho)  # Hinton diagram
matrix_histogram(H.full())  # 3D bars

See references/visualization.md for Bloch sphere animations, Wigner functions, Q-functions, and matrix visualizations.

5. Advanced Methods

Specialized techniques for complex scenarios:

# Floquet theory (periodic Hamiltonians)
T = 2 * np.pi / w_drive
f_modes, f_energies = floquet_modes(H, T, args)
result = fmmesolve(H, psi0, tlist, c_ops, T=T, args=args)

# HEOM (non-Markovian, strong coupling)
from qutip.nonmarkov.heom import HEOMSolver, BosonicBath
bath = BosonicBath(Q, ck_real, vk_real)
hsolver = HEOMSolver(H_sys, [bath], max_depth=5)
result = hsolver.run(rho0, tlist)

# Permutational invariance (identical particles)
psi = dicke(N, j, m)  # Dicke states
Jz = jspin(N, 'z')  # Collective operators

See references/advanced.md for Floquet theory, HEOM, permutational invariance, stochastic solvers, superoperators, and performance optimization.

Common Workflows

Simulating a Damped Harmonic Oscillator

# System parameters
N = 20  # Hilbert space dimension
omega = 1.0  # Oscillator frequency
kappa = 0.1  # Decay rate

# Hamiltonian and collapse operators
H = omega * num(N)
c_ops = [np.sqrt(kappa) * destroy(N)]

# Initial state
psi0 = coherent(N, 3.0)

# Time evolution
tlist = np.linspace(0, 50, 200)
result = mesolve(H, psi0, tlist, c_ops, e_ops=[num(N)])

# Visualize
plt.plot(tlist, result.expect[0])
plt.xlabel('Time')
plt.ylabel('⟨n⟩')
plt.title('Photon Number Decay')
plt.show()

Two-Qubit Entanglement Dynamics

# Create Bell state
psi0 = bell_state('00')

# Local dephasing on each qubit
gamma = 0.1
c_ops = [
    np.sqrt(gamma) * tensor(sigmaz(), qeye(2)),
    np.sqrt(gamma) * tensor(qeye(2), sigmaz())
]

# Track entanglement
def compute_concurrence(t, psi):
    rho = ket2dm(psi) if psi.isket else psi
    return concurrence(rho)

tlist = np.linspace(0, 10, 100)
result = mesolve(qeye([2, 2]), psi0, tlist, c_ops)

# Compute concurrence for each state
C_t = [concurrence(state.proj()) for state in result.states]

plt.plot(tlist, C_t)
plt.xlabel('Time')
plt.ylabel('Concurrence')
plt.title('Entanglement Decay')
plt.show()

Jaynes-Cummings Model

# System parameters
N = 10  # Cavity Fock space
wc = 1.0  # Cavity frequency
wa = 1.0  # Atom frequency
g = 0.05  # Coupling strength

# Operators
a = tensor(destroy(N), qeye(2))  # Cavity
sm = tensor(qeye(N), sigmam())  # Atom

# Hamiltonian (RWA)
H = wc * a.dag() * a + wa * sm.dag() * sm + g * (a.dag() * sm + a * sm.dag())

# Initial state: cavity in coherent state, atom in ground state
psi0 = tensor(coherent(N, 2), basis(2, 0))

# Dissipation
kappa = 0.1  # Cavity decay
gamma = 0.05  # Atomic decay
c_ops = [np.sqrt(kappa) * a, np.sqrt(gamma) * sm]

# Observables
n_cav = a.dag() * a
n_atom = sm.dag() * sm

# Evolve
tlist = np.linspace(0, 50, 200)
result = mesolve(H, psi0, tlist, c_ops, e_ops=[n_cav, n_atom])

# Plot
fig, axes = plt.subplots(2, 1, figsize=(8, 6), sharex=True)
axes[0].plot(tlist, result.expect[0])
axes[0].set_ylabel('⟨n_cavity⟩')
axes[1].plot(tlist, result.expect[1])
axes[1].set_ylabel('⟨n_atom⟩')
axes[1].set_xlabel('Time')
plt.tight_layout()
plt.show()

Tips for Efficient Simulations

Truncate Hilbert spaces: Use smallest dimension that captures dynamics

Choose appropriate solver: sesolve for pure states is faster than mesolve

Time-dependent terms: String format (e.g., 'cos(w*t)') is fastest

Store only needed data: Use e_ops instead of storing all states

Adjust tolerances: Balance accuracy with computation time via Options

Parallel trajectories: mcsolve automatically uses multiple CPUs

Check convergence: Vary ntraj, Hilbert space size, and tolerances

Troubleshooting

Memory issues: Reduce Hilbert space dimension, use store_final_state option, or consider Krylov methods

Slow simulations: Use string-based time-dependence, increase tolerances slightly, or try method='bdf' for stiff problems

Numerical instabilities: Decrease time steps (nsteps option), increase tolerances, or check Hamiltonian/operators are properly defined

Import errors: Ensure QuTiP is installed correctly; quantum gates require qutip-qip package

References

This skill includes detailed reference documentation:

references/core_concepts.md: Quantum objects, states, operators, tensor products, composite systems

references/time_evolution.md: All solvers (sesolve, mesolve, mcsolve, brmesolve, etc.), time-dependent Hamiltonians, solver options

references/visualization.md: Bloch sphere, Wigner functions, Q-functions, Fock distributions, matrix plots

references/analysis.md: Expectation values, entropy, fidelity, entanglement measures, correlation functions, steady states

references/advanced.md: Floquet theory, HEOM, permutational invariance, stochastic methods, superoperators, performance tips

External Resources

Documentation: https://qutip.readthedocs.io/

Tutorials: https://qutip.org/qutip-tutorials/

API Reference: https://qutip.readthedocs.io/en/stable/apidoc/apidoc.html

GitHub: https://github.com/qutip/qutip